can a gas diffuse in a gas?how about a gas in a liquid?give example?

Question

our topic is about diffusion

Answer 1

diffusion is the movement of paricle from a region of higher conc. to a region of lower conc. therefore a gas can diffuse in a gas if either one is conc. in one point and the same goes for gas in liquid

example: u can smell the fart of someone else even though he's sitting at the other end. that's because his fart diffused into the air you breathe in.

Answer 2

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Answer 3

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Answer 9

Chromatography, a group of methods for separating very small quantities of complex mixtures, with very high resolution, is one of the most important techniques in environmental analysis. The ability of the modern analytical chemist to detect specific compounds at ng/g or lower levels in such complex matrices as natural waters or animal tissues is due in large part to the development of chromatographic methods.

The science of chromatography began early in the twentieth century, with the Russian botanist Mikhail Tswett, who used a column packed with calcium carbonate to separate plant pigments. The method was developed rapidly in the years after World War II, and began to be applied to environmental problems with the invention of the electron capture detector (ECD) in 1960 by James Lovelock. This detector, with its specificity and very high sensitivity toward halogenated organic compounds, was just what was needed to determine traces of pesticides in soils, food and water and halocarbon gases in the atmosphere. This happened at exactly the time when the effect of anthropogenic chemicals on many environmental systems was becoming an issue of public concern. Within a year, it was being applied to pesticide analysis. The pernicious effects of long lived, bioaccumulating pesticides, such as DDT, would have been very difficult to detect without the use of the ECD. The effect of this information on public policy has been far-reaching.

The basis of all types of chromatography is the partition of the sample compounds between a stationary phase and a mobile phase which flows over or through the stationary phase. Different combinations of gaseous or liquid phases give rise to the types of chromatography used in analysis, namely gas chromatography (GC), liquid chromatography (LC), thin layer chromatography (TLC), and supercritical fluid chromatography (SFC).

Chromatography has increased the utility of several types of spectroscopy, by delivering separate components of a complex sample, one at a time, to the spectrometer. This combination of the separating power of chromatography with the identification and quantitation of spectroscopy has been most important in environmental analysis. It has enabled analysts to cope with tremendously complex and extremely dilute samples.

4.1 Principles of Chromatography
All chromatographic systems rely on the fact that a substance placed in contact with two immiscible phases, one moving and one stationary, will equilibrate between them. A reproducible fraction will partition into each phase, depending on the relative affinity of the substance for each phase. A substance which has affinity for the moving or mobile phase will be moved rapidly through the system. A material which has a stronger affinity for the stationary phase, on the other hand, will spend more time immobilized in that phase, and will take a longer time to pass through the system. Therefore, it will be separated from the first substance. By definition, chromatography is a separation technique in which a sample is equilibrated between a mobile and a stationary phase.

Gas chromatography employs an inert gas as the mobile phase, and either a solid adsorbent or a nonvolatile liquid coated on a solid support as the stationary phase. The solid or coated support is packed into a tube, with the gas flowing through it. Separation depends on the relative partial pressures of the sample components above the stationary phase. Liquid chromatography uses similar packed tubular columns and usually a pump to force a liquid mobile phase through the column. Supercritical fluid chromatography occupies a middle ground between gas and liquid chromatography. The mobile phase is a supercritical fluid, i.e., a fluid above its critical temperature and pressure. This allows the use of GC type detectors, since the mobile phase has gas-like properties, but also allows continuous variation in such mobile phase properties as viscosity and density, by changing temperature and pressure. Finally, chromatography may be done on a planar surface. The sample is transported over a solid surface such as cellulose or silica gel, coated on a plate. The sample components are moved over the surface by the mobile phase which is usually allowed to travel through the adsorbent layer by capillary action.

The reason that all molecules of a certain type tend to exit the system at the same time is that they are always re-equilibrating between the phases. Over a large number of such equilibrations, the molecules spend, on average, the same amount of time in each phase. Let us look at one point in the chromatographic column. When the analyte achieves or approaches equilibrium, the mobile phase moves on, leaving the stationary phase with too much of the analyte and the mobile phase with too little. To attempt to reestablish equilibrium, more sample dissolves in the mobile phase and moves along. Figure Chapter 4 .1 shows a mixture of three substances as they move through a chromatographic column.

The movement of analytes in the column can be described mathematically. The basis of chromatography is the equilibrium of each analyte between the mobile and stationary phase. This can be expressed by a simple equilibrium equation, where Kx is partition coefficient,

Kx = [C]s / [C]m ( Chapter 4 .1)

that is: the concentration in the stationary phase ([Cs]) is directly related to the concentration in the mobile phase ([Cm]), at least when the concentrations are low. Chromatographic separations are best done with a small amount of analyte, which keeps either phase from becoming saturated with analyte, so that the concentrations in the two phases are directly proportional. Overloading the column with sample causes one of the phases to become saturated with sample, leading to a loss of column efficiency, and poorly shaped peak profiles.

The quantities in the equilibrium expression for Kx, [C]s and [C]m are not easy to measure. We can define a new constant, the capacity factor, k':

k'x = (moles of X in stationary phase)/(moles of X in mobile phase) ( Chapter 4 .2)

Since the number of moles can be expressed as the concentration multiplied by the volume, Equations 6.1 and 6.2 can be combined and reduced to:

k'x = Kx (Vs/Vm) ( Chapter 4 .3)

All sample molecules spend the same amount of time in the mobile phase. If they were completely unretained by the stationary phase they would exit the column in the time it takes for one volume of mobile phase to pass through the column. This is equal to the void volume of the column. Molecules pass through the column in the time equal to the passage of one void volume, Vm, plus the time spent in the stationary phase, expressed by k'x. Therefore, the volume of eluent which will pass through the column before the sample elutes (the retention volume, Vr) can be expressed as:

Vr = Vm ( 1 + k'x ) ( Chapter 4 .4)

The retention volume, Vr, is related to the column flow Fc, and the retention time, tr. Likewise, the volume of the mobile phase, Vm, is related to the flow and the time the void volume takes to pass through the column, to.

Vr = tr Fc ( Chapter 4 .5)

Vm = to Fc ( Chapter 4 .6 )

Substituting these into equation 6.4 and rearranging gives:

k'x = (tr - to) / to ( Chapter 4 .7)

Values for all these variables can all be obtained from the experimental chromatogram, as shown in Figure Chapter 4 .2. The term (tr - to) is called the adjusted retention time and is often expressed as t’r, so k’x can also be expressed as t’r/ to. This is then the basis for separation of any two analytes. The separation is directly related to the difference in the k’ values for the two substances. If the k’ for the sample components is very small, there is so little retention of the compound that separation is not possible. If the difference between the k’ factors for two compounds is small, separation of them will be difficult. The selectivity, a, of a column for a particular separation, say of substances A and B, is expressed as a ratio of their retention times or retention factors:

( Chapter 4 .8)

One should notice that the sample bands tend to spread out as they move through the column. The narrower the initial band of sample, and the less the individual compounds are spread out as they traverse the column, the more efficient the column is. An efficient column can separate a greater number of individual compounds in a given time.

Column Efficiency
By analogy to the process of distillation, the separating power of a column is expressed in terms of "theoretical plates". This term refers to the length of column in which the analyte equilibrates between the two phases. The more efficient the column is, the smaller the height of the plate will be, and the more equilibrations will occur in the length of the column. An increase in efficiency is most easily seen as a decrease in the width of each sample peak, showing that the bands of sample have not spread much as they passed through the column. A chromatography peak can be approximated to be a normal or Gaussian peak, and “height equivalent of a theoretical plate” H is defined as the variance per unit length. This is a measure of band spreading per unit length of the column. The separation capability of a column is expressed as the number of “theoretical plates” or N, i.e., number of plates in a column. The number of plates contained in the column, is made evident in the peak width of the sample components. Therefore, the number of plates, N, and H can be calculated from a test chromatogram.

N = 16 (tr/w)2 or N = 5.54(tr/w½)2 ( Chapter 4 .9)

where w½ is the peak width at half height, and

H = L/N ( Chapter 4 .10)

where L is the column length.

The efficiency of a column is a function of several parameters. These include the size of the column packing particles, the uniformity of the packing, the flow of eluent, and the rapidity with which equilibrium is established between the two phases.

Two molecules, moving through the column in the eluent flow, may find different paths, especially if there are gaps in the packing which allow eddies or swirls in the eluent flow. Molecules caught in these eddies will be slowed in their movement through the column, and will therefore elute on the back tail of the compound peak. If molecules can take a variety of paths of different lengths through the column, the peak will be broadened. Figure Chapter 4 .3 shows the phenomenon of eddy diffusion.

Band broadening is also due to the fact that a solute in a gas or liquid stream has a natural and unavoidable tendency to diffuse both forward and backward in the stream. The only factor which can change this is the viscosity of the mobile phase, which is not usually very easy to change. A faster flow, however, gives the peak less time to diffuse.

The mass transfer kinetics between the two phases also have an effect on the band width. Equilibrium between the mobile and stationary phases is never quite achieved in a chromatographic system. The further from equilibrium the system is operating, the poorer the efficiency will be. Diffusion of the sample band in the mobile phase and in the stationary phase both have an effect. Solute molecules may be held up excessively when they become trapped in deep pools of stationary phase, or in stagnant portions of mobile phase. If a substantial fraction of the molecules encounters such delays, the result is a spread-out peak. Figure Chapter 4 .4 shows cross sections of support particles coated with a liquid stationary phase, and how molecules may be excessively delayed. These pools and backwaters may be avoided by spreading a very thin film of support on a fairly regular shaped particle. Of course, this thin phase has little volume, which makes k' smaller, and leads to easy overloading of the column. Figure Chapter 4 .5 shows how the maximum concentration of the solute in the mobile phase is slightly ahead of that in the stationary phase. The bigger the difference in the location of these peaks, the wider the final exiting sample band will be. If the flow is too fast for the equilibration rate or vice versa, it will cause band broadening due to mass transfer effects.

The plate height, H, is a function of the flow rate of the mobile phase expressed as linear velocity, v, and can be calculated from the equation:

( Chapter 4 .11)

The B term describes the longitudinal diffusion, and is related to diffusion of the analyte in the mobile phase. This term becomes smaller as the velocity increases, because less time is available for the solute to diffuse. The Cs term is related to mass transfer in the stationary phase, and Cm to mass transfer in the mobile phase. Both of these terms increase with flow, because the solute is less able to reach equilibrium between the two phases as the flow becomes faster. The contribution to the overall H of the column by each of these factors can be plotted against flow velocity. Figure Chapter 4 .6 shows such a plot. The H line shows the combined effect of the factors. The minimum in the curve indicates the flow velocity which will give the maximum efficiency in that particular column. Using a carrier gas flow which is higher than the optimum will cause some loss of efficiency. However, this may be a satisfactory compromise, since it will also reduce the retention times, and so may shorten the time required for an analysis. Using a flow slower than the optimum is, however, never a good idea, since it lengthens the analysis time and also causes a loss in efficiency.

Although information on the optimal flow rate is often supplied by the column manufacturer, the best flow can be determined experimentally. The value of H can be found at several different flows, and a plot constructed for the column.

If there is a difficult separation to be done, there are two approaches. The column material may be changed, which will change both k'A and k'B, in order to produce a larger difference between them. On the other hand, if a more efficient column is used, the same amount of difference in retention time will give a more complete separation, because each peak will be narrower. Figure Chapter 4 .7 shows a poor separation which is improved in one case by increasing the value of a, which moves the peaks apart, and in the second case, by improving the efficiency of the column, which makes the peaks narrower, while not making the retention time difference any greater. Increased efficiency can take the form of either lengthening the column, which has the drawback of also increasing analysis time, or decreasing H by improving the quality of the column itself.

The degree of separation between any two peaks, A and B, can be expressed as the resolution of the peaks. This is defined as the difference between the two retention times divided by their average peak width.

( Chapter 4 .12)

Alternatively, resolution can be expressed in terms of the selectivity factor and the capacity factors of the two substances being separated:

( Chapter 4 .13)

where a and k’ are averages (since the values for these factors are similar, if the retention times are close, as a consequence of Equations 6.7 and 6.8).

From this equation the interaction of the three factors which influence resolution can be seen. A low value of k’ for the compounds being separated will lower resolution, since separation cannot take place efficiently if the analytes are not retained long enough to be separated. If a, the selectivity, is not sufficiently large, the compounds are only slightly different in their retention properties and are therefore very hard to separate. Finally, the resolution depends on N, the number of theoretical plates in the column, a measure of the efficiency of the column, indicating that a more efficient column can accomplish a separation even if a or k’ are lower than one would wish.

The same relationships can be expressed in terms of retention time rather than resolution. The retention time of the second of two peaks, A and B, is given by

( Chapter 4 .14)

where v is the carrier gas velocity.

Example:

Heptane and toluene were separated with retention times of 15.4 and 16.5 min respectively on a 1.0 meter packed column. An unretained species passed through the column in 1.8 min. The peak widths measured at the base were 1.15 for heptane and 1.20 min for toluene.

a) Calculate the resolution between the peaks:



b) Calculate the average number of plates for the column.



c) Calculate the average plate height.



d) Calculate what column length will be necessary to achieve a resolution of 1.5 on this column.

Since k’ and a do not change with column length, Equation 4.13 indicates that the resolution is proportional to . So, and . N2 = 7504.
Since L=NH, the length required to generate this number of plates is :

L = 7504 x 0.34 = 2551 mm or 2.5 meters

e) Calculate the capacity factor for toluene:



The General Elution Problem
There is a problem which arises in all types of chromatography, when samples of widely differing retention properties are present in the same sample. If the elution conditions are correct for the early eluting compounds, the late ones will remain in the column too long. They will be so broadened that it will be difficult to determine their area accurately. Indeed, they may not elute at all. On the other hand, if we set up the system so that the later eluting compounds spend less time in the stationary phase, the early peaks will come out so quickly that they will not have sufficient time in the stationary phase to allow adequate separation. This problem is called the ‘general elution problem’ and is solved in different ways in different types of chromatography. Usually, some type of "programming" is done. This involves a gradual or stepwise change in one of the operating parameters. The best conditions for the separation of the early compounds are set up. Then these are changed during the run to conditions better for the elution of more retained compounds. In gas chromatography, the column temperature is raised over time until a temperature favorable for the separation and elution of the later peaks is reached. This is called temperature programming. In liquid chromatography, where retention is more dependent upon strength of the mobile phase, the composition of the mobile phase is changed as a function of time. This is called gradient programming.

4.2 Quantitation in Chromatography
The amount of each component in the sample is measured as it issues from the column, by passing the effluent through a detector and integrating the detector signal over time. For quantitation in both gas and liquid chromatography, the detector signal is usually fed into a digital or analog recorder. The strip chart recorder produces the typical chromatogram, a plot of signal versus time, as a series of peaks. The peak area is related to the quantity of each material, and the location on the time axis or the retention time is used to identify the compound. Modern chromatographs have high precision in retention time as well as peak area. Consequently, chromatograms provide excellent qualitative and quantitative information. Digital processing of the signal allows the peak areas to be integrated automatically, with the beginning and ending points of peaks indicated. Baselines are usually set automatically and may sometimes be adjusted by the operator. Peak areas obtained on strip chart recorders may be determined graphically by drawing tangent lines to the peak sides and measuring the widths and heights as was indicated in Figure 4.2. The area (A) is calculated from:

A = hmax x w½ or (hmax x wb)/2 ( Chapter 4 .15)

In addition, a peak area may be measured by cutting the peak out of the chart paper with sharp scissors, and weighing it on an analytical balance.

In order to calculate the quantity of sample from the peak area or peak height, the detector must be first calibrated by running standards. Quantitation may be done by either peak height or peak area, with most accurate results being achieved with use of areas, although, when chromatograms are being manually interpreted, heights may be more readily measured. External standards, internal standards or a combination of both can be used.

External Standard Method
A series of standards containing known quantities of the analytes are prepared and run. The peak heights or areas are plotted versus the quantities or concentrations, and the samples are run in the same way. A major source of error in this method is the reproducibility of the injection, especially if manual injections are being made using a syringe. Since most chromatographic samples are only a microliter or less, accurate measurement is difficult. Automatic injectors and sampling valves can reduce this error to a few percent.

The concentration of the sample is calculated from the calibration graph or from:

Cunk/Cstd = Aunk/Astd ( Chapter 4 .16)

where C is the concentration and A is the peak area for the standard (std) and the unknown(unk). This simple calculation is usually good enough if the sample and standard are of the same order of magnitude and the detector response is linear over the range of concentrations covered by the sample and standard. If sample concentrations vary over a wider range, a calibration curve should be constructed over the entire range.

Since it is not always possible to obtain standards over a suitable range or for every compound one needs, detector response factors may be used. These allow calibration with one compound as a surrogate for others.

Cunk / Cstd = funk Aunk / Astd ( Chapter 4 .17)

where funk is the detector response factor which relates the detector response to a quantity of the sample compound to its response to the same amount of the surrogate standard compound. These factors can be determined experimentally. Then the chromatograph can be calibrated daily against the surrogate compound, and the detector response factors used to calibrate for other compounds. It is best to use compounds for standards which resemble the target compounds as much as possible, being of similar molecular weight and polarity, to reduce possible errors.

The Internal Standard Method
An internal standard is added to the sample before analysis. This is composed of one or more compounds with sufficient similarity to the target analytes, so that they behave similarly, but must surely not appear in the samples. Internal standard compounds must also be readily separated from any of the compounds in the sample. If an internal standard is added before such steps as concentration, extraction, or dilution, it is then not as necessary to make accurate volume measurements, and the injection volume is also not as critical. By examination of the peak area of the internal standard in sequential injections, the reproducibility of the injection volumes can be seen. Small changes in the volume show up as changes in the internal standard peak. These can be compensated for in the calculations, by working with the ratio of the sample peak/internal standard peak, rather than just the peak areas. Again, it is important to keep the concentrations of the internal standard and the target compounds in a similar range, to keep linearity problems from arising. The area of the internal standard (Ais) is used to normalize the areas of all other sample peaks, thus eliminating the effect of differences in injection volumes or dilutions.

Cunk = ( Aunk / A is ) Cis funk ( Chapter 4 .18)

4.3 Gas Chromatography
In gas chromatography, the eluent is an inert gas, often helium, hydrogen or nitrogen. The eluent actually has little effect on the separation process, which is governed more by the volatility of each sample component and its interaction with the stationary phase. Stationary phases are either solids or liquids and are contained in columns which range in internal diameter from 100 micrometers to 4 mm. The chromatographic system consists of three essential elements: an injection system, a temperature controlled column, and a detector. To this basic system, many enhancements can be added. The column oven may be equipped to carry out temperature programming and cryogenic cooling Additional detectors may be added, and an automated injection system, computerized instrumental control and data analysis systems may also be installed. For a chromatograph to be used in environmental analysis, specialized injection systems, such as concentrators or thermal desorbers for air analysis, or purge and trap apparatus for water analyses are often useful. In addition, complex environmental samples often require a detector or array of detectors to assist in identification of the sample components as well as to determine their concentrations.

Injection Devices
An injector which can place a small, narrow, and reproducible band of sample on the head of the column is absolutely essential to good chromatography. The more efficient the column is, the more demand is placed on the injector to keep the initial sample band narrow. The sample band will only become wider as it traverses the column. If it is wide to begin with, the efficiency of the column will be overshadowed by the band broadening which takes place outside the column, and which contributes nothing to the separation. This is known as extra-column band broadening.

A syringe is the traditional injection device in GC. A heated injection port, equipped with a soft polymeric septum, is located at the head of the column. The sample is injected through the septum into the heated carrier gas stream, and vaporizes. The carrier gas flows through the injection port and sweeps the sample onto the column. The injection port must be well designed so that the sample is quickly and efficiently moved onto the column, with all interior areas rapidly swept out by the carrier gas. Unswept areas allow the sample to become trapped, and diffuse slowly back into the gas stream, causing peak tailing. A glass liner is often incorporated into the injection port, so that it may be removed and cleaned or replaced if samples leave a non-volatile residue.

Since a high efficiency column may require a sample of only a few nanograms, a syringe may not produce adequately reproducible samples. A splitting injector port may be used. In this case the sample is injected into a chamber where it is mixed with a volume of carrier gas. Then a small part of this mixture is allowed to flow into the column, and the rest is vented. An injection port and a splitter are shown in Figure Chapter 4 .8.

When the sample to be injected is a gas, a gas sampling valve is often used to inject a measured portion into the column. This valve contains a loop of known volume which is filled in the ‘load position’ by allowing the sample gas to flow through it. When the valve is turned to the ‘inject position’, the carrier gas is diverted through the loop, and the sample is carried onto the column. In Figure Chapter 4 .9, you can see that the carrier gas always flows to the column, whether the valve is in the load or inject position. The sample size is usually changed by installing a different sized loop, although, if a pressure gauge is mounted on the system, the sample pressure may be changed to change the amount injected. Since the sample is a gas, the temperature of the loop will also make a difference, if it is subject to wide variations.

More elaborate injection systems are found in instruments dedicated to a particular determination. A thermal desorber, for instance, is employed for transferring samples from an adsorbent trap into the column. A purge-and-trap apparatus is used for stripping volatile compounds from water or sludge samples, and injecting them into the column. These will be described in the chapters on air and water analysis.

Columns
There is a vast selection of GC columns available. Many of them are so slightly different from others that they are readily interchangeable. The columns differ in physical dimensions. Large diameter packed columns, having a larger volume of stationary phase, are able to accommodate large samples. The smallest columns, open tubular capillary columns, have the highest efficiency, but are limited to tiny samples.

Chromatographic stationary phases may be either solid or liquid. When a solid stationary phase is used, adsorption is the retention mechanism, and the technique is called gas solid chromatography. It is used mainly for low molecular weight volatile organic compounds and gases such as CO, CO2 or SO2 or H2S. The stationary phase may also be a viscous liquid coated or chemically bonded to the surface of a packing material, or on the inside wall of a open tubular capillary column. This is called gas liquid chromatography (GLC). The mechanism of retention in a liquid stationary phase involves partitioning of the analyte between the gas and the stationary phase. Liquid phases must have good solvent characteristics, should be inert and stable at relatively high temperatures.

Table Chapter 4 .1 shows a selection of liquid stationary phases, with the kinds of samples for which they are suited. As gas chromatography developed, hundreds of stationary phases were described in the literature. To characterize these in a quantitative fashion, Rohrschneider developed a series of constants to describe the selectivity of a column toward different types of sample. A suitable phase for a particular sample can be selected on a rational basis by using these constants. This work was extended and refined by McReynolds, and the McReynolds constants for stationary phases are listed in the catalogs of almost every supplier of chromatographic columns.

The McReynolds constants are based on the assumption that different molecular interactions, such as dispersion, orientation, induced dipole, and donor-acceptor complexation, between functional groups on the column material and those of the sample are additive. A series of probe compounds was chosen to represent each type of interaction, and the retention time for each probe is determined on the column in question. Since retention times change with phase loading, temperature, and flow, a retention index for each probe is determined by injecting the probe compound along with a set of normal hydrocarbons which will bracket the probe compound's retention time. The retention index is calculated from the following equation:



( Chapter 4 .19)



where each t'R is an adjusted retention time. The unknown is indicated as x, and z is the number of carbons in the normal hydrocarbon eluting just before the sample. The hydrocarbon, z+1, is that which elutes just after the sample. This index is not entirely independent of column conditions, but is still useful. The McReynolds constant is expressed as the retention index of each probe compound on the tested column minus that on squalane. The probe compounds, and the interactions they indicate, are listed in Table Chapter 4 .2.

These constants are useful in comparing columns, and in selecting a new column to improve a particular separation. For instance, if alcohols are to be separated, one would look for a column which has a large Y constant, indicating good retention and selectivity for alcohols. When a column described in the literature for a particular analysis is not available, a suitable substitute can be chosen, by selecting one with similar McReynolds constants.

Packed Columns
Packed columns vary in internal diameter, with most being constructed from 1/4 or 1/8 inch OD tubing. They also vary in the particle size of the packing, and the amount of stationary phase coated on the support. One of the parameters usually listed is the percent loading. The loading is determined by the mass of stationary phase coated on a given mass of support. Loading ranges from a few percent to 20 or 30%.

Stationary phases are usually coated onto an inert support material, which should be of uniform size and good mechanical strength. The efficiency of the finished column depends on both the particle size and uniformity of the size. Smaller particles give more efficient columns, but if the particles become too small, the flow through the column at reasonable gas pressures is hindered. Usually column packings in the 60-80 mesh or 80-100 mesh ranges are used. These are often made from treated diatomaceous earth, a silica based material.

Supports vary in their surface activity and many treatments have been developed to modify the surface. The support may be extracted with acid to remove surface metal ions which may form active sites on the packing, adsorbing sample molecules too tightly and causing tailing. The support may also be treated with a silanizing agent to further reduce surface activity. For extremely polar compounds like water or sulfur dioxide, an extremely inert support is made from granulated fluorocarbon polymer.

The packing is covered with the stationary phase, generally by mixing the support with a solution of the stationary phase, a high boiling liquid, in a volatile solvent. As the solvent is evaporated the packing is coated with an even layer of stationary phase. When the solvent is dried off, the packing should be free-flowing and easy to pack into the column without gaps or loose spaces.

Stainless steel or glass tubing is used to form the columns. Stainless is considerably easier to handle, as it is not breakable and can be gently bent to fit the instrument at hand. Glass columns are usually deactivated by being silanized, and have a lower surface activity, so may be needed for especially sensitive samples, such as very labile organic compounds. It is important to handle packed columns fairly gently, and bend them carefully. Rough treatment will fracture the packing particles, increasing the non-uniformity of the packing by making smaller particles.

For gas-solid chromatography, several types of solid column packings are available. These may be porous polymeric materials, silica, or carbon based solids. Solid phases may be more durable, and able to stand up to some fairly aggressive samples, since they have no surface coating. They are usually used for small organic molecules and inorganic gases. Molecular sieves are used for gases such as CO, CO2, and O2.Some packings are designed for specific types of samples, such as water and alcohols, amines and nitro compounds, or sulfur gases.

Polymeric packing materials differ in their surface polarity and their pore sizes. They are especially useful for gaseous samples and for aqueous samples. Recently, carbon based materials have become quite popular. These are manufactured specifically for chromatographic separations and vary in such parameters as the pore size, the surface area per unit mass, and the particle size. Table Chapter 4 .3 lists some of the more common types of solid supports.

Open Tubular Columns
Open tubular columns are often used for complex mixtures, where large numbers of components must be separated. These are made of fused silica tubing, coated with a thin film of stationary phase on the interior, and covered with a polyimide coating on the outside surface. The polyimide is necessary to provide mechanical strength as fused silica can break easily if it is scratched. Recently, open tubular columns are also being made with silica lined stainless steel tubing. Choosing the best stationary phase is done in the same way as for packed columns. Selection of column diameter and phase thickness requires consideration of the sample to be separated. The narrowest diameter columns give the highest separating power, but the sample size is very limited. When trace components are to be determined, there may not be enough sample to detect. The thinnest coatings of stationary phases also give highest efficiency, but again limit the size of the sample which can be analyzed.

Many available capillary columns have chemically bonded phases. In these, a chemical reaction takes place between the liquid phase and the surface after the column is coated. The liquid forms chemical bonds to the hydroxyl groups found on the surface of the fused silica. These phases have several advantages. They are mechanically stable, and will not tend to creep towards the lower part of the column, even at high temperatures. This helps to extend the useful life of the column. If a bonded column becomes contaminated, which may happen especially easily when it is used for direct on-column injection, it may be washed out with solvent. This will often return the column to its original efficiency.

Internal diameters of capillary columns range from the highest efficiency ones of 0.15 mm to wide bore columns of 0.53 mm. Typical values for H range from 0.13 mm for a 0.15mm ID column to 0.45 mm for a 0.53 mm ID column. The column length selected again depends on the complexity of the sample and the number of plates needed to effect the separation. The shortest column which will accomplish the separation is best, since a longer column will require a longer analysis time without adding to the information gained. One virtue of capillary columns is the ease of dividing a long column into two shorter ones. Sometimes a long column, which has been shown to have an excess of theoretical plates for the use, may be cut in half.

The usual liquid stationary phase layers are in the range of 0.12um to 0.2 µm. The thickest stationary phase coatings may range up to 5 mm, and find their most usual usage in the analysis of very low boiling substances, especially samples which are gases at room temperature. Figure Chapter 4 .10 shows the effect of film thickness on separation, and Figure Chapter 4 .11 shows the effect of column diameter. The best separation is that which gives an adequate separation in the shortest time.

Porous layer open tubular (PLOT) columns are somewhat of a hybrid between packed and capillary columns. These are open tubular columns with a solid stationary phase layered on the inside surface of the column. These provide a means of doing gas/solid chromatography, with the advantages of an open tubular column.

The considerations which go into the selection of a column for any particular analysis can be summarized as follows:

· Is there a column offered which is designed for this analysis? Suppliers often design and sell columns which have been optimized for a certain commonly done sample. For example columns are made for light hydrocarbons, gasoline, pesticides, fatty acid methyl esters, and amines.

· Is there a column specified in a reference in the literature for a similar analysis? If so, the same column or one with similar McReynolds numbers should be satisfactory.

· A rule of thumb for column selection relies on the fact that the separation takes place in the stationary phase. Therefore, a phase which has a higher affinity for the sample components will be better for the separation. Relying on the maxim "Like dissolves like", a nonpolar column will do best for nonpolar samples and polar samples should be separated on a polar phase.

· Column length, diameter, and film thickness depend on the complexity of the sample, as well as the amount of sample which will be injected and the boiling range of the sample.

Column Temperature
A column has a minimum and maximum use temperature, which depends mostly upon the stationary phase. The minimum temperature is usually the freezing point of the stationary phase, since the solid has quite different properties from the liquid, and does not always partition the sample adequately. The maximum temperature is usually that at which the column bleed becomes intolerable. This is usually due to thermal breakdown of the stationary phase, or to the boiling off of the lower molecular weight fraction of the phase.

The temperature for the analysis must be optimized in the development of the method. The usual method is an educated trial and error approach. Equations exist which attempt to predict retention times based on column temperatures and thermodynamic properties of the sample components, but, in practice, these are seldom used. Generally, a standard or sample containing the components of interest is run on the selected column, at a temperature selected on the basis of the range of boiling points of the sample. It is not necessary to bring the column up to the boiling point of the higher boiling components. Compounds will migrate through the column at temperatures much lower than their boiling point. Thinner phase columns will require lower temperatures to elute peaks than those with thicker films. Referring to Figure 4.10, we can see that the 0.4 mm film column eluted the test peaks much more rapidly than the 1.2 mm film column. If the temperature on column the 1.2 mm column were raised, the chromatogram would begin to appear more like that produced on the 0.4 mm column.

When the last peaks in a chromatogram elute late and are very broad, the problem may be corrected by raising the column temperature. However, this often causes the earliest peaks to be unresolved because they are not retained long enough. In this case, a temperature program must be used. The sample is injected with the temperature optimized for the separation of the earliest peaks. The temperature may be held until the first peaks are eluted, and then the temperature is ramped up to a temperature which will bring out the last peaks, separated, but not excessively broadened. The rate of temperature rise depends on the number of peaks eluting during the ramp. If few peaks are present, then a rapid ramp might be chosen, while a complex sample with many components, requires a slower ramp rate. Figure Chapter 4 .12 shows a standard containing four hydrocarbons. In the isothermal run, the early peaks are sharp and well resolved but the late ones are broad. When a temperature program is used, a better chromatogram is obtained, and the time to resolve the four compounds is halved.

Generally, a few test runs will be needed to determine the best temperature program. Some chromatographic instruments are capable of multiple ramps and pauses during a program. These can be used to fine tune specific parts of a separation. For example, let us suppose that both the early and late parts of a sample are well separated, but there is a pair of peaks in the middle which are overlapping. One might slow the rate of temperature rise before these peaks elute, to retard them a little longer in hopes of improving the separation. After they elute, the oven temperature is raised to the same final temperature as in the original program.

4.4 GC Detectors
The eluent from the column is directed to one or more detectors. These produce signals which are proportional to either the amount of sample present in the detector at any moment, or the concentration of sample in the detector. The detector signal is usually displayed as a plot of signal magnitude versus time, giving the classic chromatogram. Detectors vary in their response to different classes of compound, from the thermal conductivity detector, which is universally responsive to all compounds, to such specialized detectors as the flame photometric detector, which detects only sulfur or phosphorous containing compounds, depending on the way it is set up. The selectivity of a detector is usually expressed by the ratio of the response to the desired analyte divided by that towards an interfering compound. For instance, the selectivity of a sulfur specific detector may be given by its response to a nanogram of sulfur divided by that for a nanogram of hydrocarbon. Since the interfering material may be present in much greater quantity than the analyte, selectivity factors of 104 or more are desirable.

The characteristics sought in a gas chromatographic detector are high sensitivity, a linear dynamic range of 4 orders of magnitude or more, a favorable signal to noise ratio, and good long term stability. A small detector dead volume is also important. If the sample has an opportunity to mix with a volume of carrier gas before the detection process is completed, the peak will be broadened and efficiency lost.

Thermal Conductivity Detector (TCD)
The TCD is a truly universal detector. It consists of a heated sensor in a thermostated chamber, through which the effluent flows. Helium is usually used as a carrier gas, as it has the highest thermal conductivity of any gas, except for hydrogen. As the peaks elute, the thermal conductivity of the gas in the chamber changes. This changes the heat flow from the heated sensor, through the gas, to the walls. Since the sensor is being heated at a constant rate, it becomes hotter as the thermal conductivity of the effluent drops. The change in temperature of the sensing wire filament or thermistor changes its resistance. The sensor is wired into a Wheatstone bridge circuit, and the change in resistance produces an unbalance, which produces a signal. The circuit for the TCD detector is diagrammed in Figure Chapter 4 .13.

The filament is sensitive to oxidation while heated, and therefore must not have current flowing unless the carrier gas is passing through the chamber. The detector is limited by its relatively low sensitivity, compared to other detectors, and usually has a fairly large dead volume. It is, therefore, not very suitable for capillary work. Because of these limitations the TCD is little used in environmental work, except for the determination of major constituents of air.

Flame Ionization Detector (FID)
The FID is a major workhorse of environmental analysis. It is nearly universally sensitive to organic compounds, and shows good sensitivity and excellent linearity. The column effluent is fed into a flame fueled by hydrogen, with a forced air flow. Figure Chapter 4 .14 shows a typical FID. A potential of several hundred volts is imposed between the tip of the flame burner and the collector which surrounds the flame. As the sample components burn, they produce a burst of ions. These produce a tiny current between the flame tip and the collector. The current is amplified by a high impedance electrometer and measured. The background current flowing in the detector is in the region of 2 x 10-14 to 10-13amp. In the presence of organic compounds the current will rise to 10-12 to 10-9amp. The response of the detector will change if the flows of air and hydrogen to the flame change. These flows should be checked for consistency when the detector is calibrated and used.

The FID detector has a number of advantages. The response is roughly proportional to the number of carbon atoms in the flame at any time, although certain substituent atoms, such as chlorine, reduce the response. The detector is insensitive to inorganic gases, water, carbon dioxide, sulfur dioxide, nitrogen oxides and other non-combustible gases. The detector has a very wide linear range, over about 7 orders of magnitude, has a low dead volume of about 1 ml, and is relatively noise free and easy to operate. Its major disadvantage is that it destroys the sample, so it cannot be passed on to another detector. Also, the fact that it requires both compressed air and hydrogen, as well as carrier gas, can be an inconvenience, especially when instrumental portability is an issue.

Electron Capture Detector (ECD)
This is probably the most used of the compound class specific detectors for environmental analyses. It has been used to trace the fate of such pesticides as DDT, as well as the halocarbon gases in the atmosphere. Its response depends on the electron-capturing properties of the sample. The detector is highly sensitive to the presence of electron capturing substituents, such as halogens, peroxides, and nitro- groups, on the sample molecules. When one looks at lists of regulated compounds, it is striking that so many of these are electron capture active.

The detector (Figure Chapter 4 .15) consists of a chamber containing a ß-emitting foil, usually nickel-65. This radioactive source emits electrons which ionize the carrier gas, and form a small current between the electrodes in the chamber. When electron- capturing species are present, they reduce this current. The reduction is detected by the electronics and measured.

The detector response varies widely, depending on the electronegativity of the species being detected. Table Chapter 4 .4 shows the relative sensitivity of the detector towards a variety of substances. We can see that the response to the individual electronegative groups depends upon their number in a molecule as well as their location. The response to chlorinated hydrocarbons increases by about a factor of ten with each additional chlorine atom present in the molecule, for example.

If the carrier gas forms metastable ions, it may cause undesirable collision reactions. Therefore, helium cannot be used with the ECD detector. Nitrogen is suitable, and a mixture of argon with 5 - 10% methane is also sometimes used. If a capillary column is being used, extra gas, called make-up gas must be added to the detector to bring the flow up to that for which the detector is designed. This helps to minimize band broadening. In this case, if nitrogen is used for the make-up gas, helium may be used for the column carrier gas.

Since ECD's contain a radioactive isotope, they are subject to governmental regulation. Depending on the design of the unit, a license may be necessary before one can be purchased. There are tests, called wipe tests, for leakage of radioactivity which must be done at specified intervals, to comply with regulations. The company which sells the unit will supply information on required licensing and routine testing.

The sensitivity of the ECD toward halogenated hydrocarbons and many pesticides is extremely high. The response to a particular compound is usually quite temperature sensitive, and the detector's linearity varies with conditions and analyte. Therefore, a calibration curve should be generated for each compound which is to be quantitated, and the samples should fall within the range of the calibration. Extrapolation is especially risky when the detector may be non-linear, and a large error may easily arise if the calibration line is extended beyond the last measured point.

Photoionization Detector (PID)
A detector which has much the same response as the FID, but which requires no support gases is the photoionization detector. This detector exposes the effluent stream to ultraviolet light, thus ionizing the sample. The ions are collected on an electrode, with the resulting current amplified and measured with an electrometer. Figure Chapter 4 .16 shows the detector. The range of compounds to which the detector is sensitive depends on the wavelength of the lamp used in the detector. Lamps can be purchased with wavelength peaks at 9.5, 10.0, 10.2, 10.7 and 11.7 eV. The 10.2 lamp is the most commonly used. The detector will respond to substances having ionization potentials below the lamp energy, and up to about 0.4 eV above it.

The PID is about 35 times more sensitive to aromatic compounds and somewhat more sensitive to alkanes than is the FID. It has a linear range of about 107. The response toward various compounds seems to be most closely related to their ionization potentials, with those with the lowest potentials giving the highest response. In general, as the carbon number increases, the sensitivity increases.

The chief advantages of the PID are that it is nondestructive, so it can be used in series with other detectors, and does not require support gases, as does the FID. This makes it ideal for portable instruments, which may use air for the carrier gas, and not require any cylinder gases to be carried. A portable PID detector, without a gas chromatograph, is available and can be used for screening for organic emissions, without speciation. The main drawback is a deposit which may form on the window separating the UV lamp from the gas stream. Some sample components react under the UV light and form solid products, which contaminate the lamp window.

Electrolytic Conductivity Detector (ElCD)
The electrolytic conductivity detector (Figure Chapter 4 .17) may be set up to determine compounds containing chlorine, sulfur or nitrogen. The detector is reconfigured, depending which of the elements are to be determined. When configured for chlorine, the effluent from the column is passed over a catalyst which converts any Cl to HCl. In sulfur mode, SO2 is formed, and in nitrogen mode, NH3. To reconfigure the detector, the catalyst is changed and reactor temperature is adjusted.

The reacted gas is scrubbed into a flowing aqueous or alcohol stream, and passed into a conductivity cell. The response of the detector is proportional to the number of Cl, N, or S atoms passing through the cell. The solvent is usually recycled through ion exchange resins. The detector must be watched for dips in the baseline which begin to occur as the solvent becomes exhausted. Detection limits of 10-12 g nitrogen/sec, 5 x 10-13 g chlorine/sec, and 10-12 g sulfur/sec are possible, and the selectivity ranges from 104 to 109 over hydrocarbons. The sensitivity is similar to that of the ECD, and the fact that the response is fairly consistent with the amount of the target heteroatom gives it an advantage over the ECD for some compounds.

Flame Photometric Detector(FPD)
This detector is designed for the specific detection of sulfur and phosphorous. It is similar in construction to the FID, but a cooler flame is produced by altering the hydrogen/air ratio. Instead of measuring the ions formed in the flame, the radiation emitted by the sulfur S2 and phosphorous HPO species formed when the sample components enter the flame is measured. A filter photometer is used to detect the radiation emitted at 394 nm for sulfur or 526 nm for phosphorous. Figure Chapter 4 .18 shows a schematic of this detector. Since the sulfur or phosphorous are measured in the same form for each component the response is governed by the total amount of the element in each sample component.

The detector is subject to negative interferences from hydrocarbons, which quench the emission if they are present in the flame at the same time as the sulfur or phosphorous compound being measured. A good GC separation will reduce this difficulty by eliminating the coelution of the interferent and the analyte.

The square root of the detector response is proportional to the sulfur concentration. For phosphorous, the response is directly proportional to concentration and is linear over 2-3 orders of magnitude.

Thermionic or Nitrogen-Phosphorous Detector (NPD)
The nitrogen-phosphorous detector is a modification of the flame ionization detector. In this detector a bead of a rubidium salt is placed at the tip of the flame. This gives a selectivity of 103 to 104 for N and P compounds over hydrocarbons. Helium carrier gas gives a better response with phosphorous compounds, while nitrogen is better for nitrogen containing compounds. The actual mechanism of the selective response is not entirely clear. It is believed that free radicals, formed in quantity by the nitrogen or phosphorous compounds in the flame, cause the vaporization and ionization of rubidium from the bead, adding to the signal. While the NPD shows selectivity toward the N and P-containing compounds the response to these is not a great deal higher than that of the unmodified FID. This detector gains its selectivity as much by repressing the response to hydrocarbons as by enhancing the response to N and P compounds. Its most common environmental use is in detection of nitro-PAH and other nitrogen-containing compounds in petroleum products.

Mass Selective Detector (MSD)
The mass selective detector, (MSD or GC/MS) is probably the most powerful tool in the hands of the environmental analyst. The detector, essentially a mass spectrometer, is a universal detector, as well as a very specific one. The MS can detect any molecule. Because each effluent component is fragmented and a mass spectrum is generated, plotting the intensity of a single mass fragment or group of fragments will generate a compound-specific chromatogram. These detectors will be discussed in Chapter 5.

Comparison of Detectors
The advantage of a universal detector is obvious, in that it responds to any component of the sample. Class specific detectors are also useful, since they can be used to simplify complex chromatograms. For most environmental work, the mass selective detector is the detector of choice, as it not only identifies but also quantitates the sample. It is mandated in many of the standard methods, but does not match the sensitivity of the ionization detectors. All the ions produced in an ionization detector are collected and measured, while each ion fragment in a mass spectrum is collected for only a small fraction of the time. This makes mass spectrometry inherently less efficient. However, one of the most difficult parts of the analysis with a general purpose detector is the identification of peaks. Because of the difficulty of reproducing the temperature exactly, and because the amount of loading on the column, especially of water vapor, may cause small shifts in retention time, retention times are not always sufficient to determine the identity of individual peaks unequivocally. A mass spectrum, combined with the retention time, is a much better way of making a reliable identification.

4.5 High Performance Liquid Chromatography
When a material is non-volatile or when it is so thermally fragile that it cannot be analyzed by gas chromatography, then liquid chromatography may be appropriate for the analysis. High performance columns are constructed of packing materials with very small particle diameters, on the order of 3-10 micrometers. Therefore, the eluent cannot flow through under gravity flow as was done in early liquid chromatographic analyses. A high pressure pump is necessary to force the eluent through the column at the rate which delivers the maximum number of theoretical plates. High pressure, high performance liquid chromatography is one of the essential tools of environmental analysis. It can readily handle high molecular weight compounds, such as polynuclear aromatic hydrocarbons, highly polar compounds such as phenols or organic acids, and even inorganic ions.

Choice of detector is somewhat limited in HPLC compared to GC. Since the eluent is usually an organic liquid, and ionization detectors, which are so useful in GC, are not applicable. Detection is therefore limited mostly to spectroscopic methods, which are limited by the absorbance characteristics of the analytes.

HPLC can be divided into several related techniques, depending on the separation mechanism and the column type. The most useful types in environmental analysis are reverse phase, normal phase and ion chromatography. Reverse phase liquid chromatography is probably the most frequently used, and the most versatile. It is called 'reverse' because of the comparison with 'normal phase', which is only called normal because it was invented first.

Reverse Phase Liquid Chromatography
Reverse phase columns have a packing composed of solid silica support particles which have an organic coating bonded to their surface. The bonded phase is produced by reacting a halogen substituted organosilane with the surface -OH groups present on the silica support. This leaves hydrocarbon chains, which may contain two, eight, or eighteen carbons, bonded at their ends through Si-O- Si groups to the surface of the support. Figure Chapter 4 .19 shows the bonding of octyl groups to the silica surface. These columns are designated by the carbon number of the chains attached, with the most frequently used column being the bonded octadecyl type, called C18.

Since these coatings are very non-polar in nature, the chief mechanism of retention is dispersion forces. This makes them useful for separation of organic compounds based on slight differences in their backbone or side chain configuration. The mobile phases commonly used are fairly polar in nature, with alcohols and water being common constituents. Since these are weaker eluents than the non-polar solvents which have a strong affinity for the highly non-polar column surface, sample components are retained long enough for good separation to take place. Components are eluted with the most polar ones being least retained and the least polar ones being held the longest.

Figure Chapter 4 .20 shows a chromatogram of a sample containing five compounds. In the first run, an isocratic eluent was used, a mixture of 30% methanol and 70% water. The first peaks are poorly separated while the later ones are too broad and take a long tome to elute. In the second case a gradient elution from 10% methanol to 100% methanol is done. The early peaks are better separated, since the initial eluent was weaker, while the late peaks are moved through the column more rapidly, as the eluent increases in strength. When gradients are done, it is important to begin with a weaker mobile phase, in this case one with a substantial amount of water. This allows the earliest peaks to remain in the column sufficiently long to achieve separation. Then the strength of the eluent is increased, by adding more of the less polar acetonitrile.

Gradients may be linear or curved. Figure Chapter 4 .21 shows some gradient profiles. If one had a situation in which several peaks were eluting close together at the beginning of the chromatogram, one might want to select a concave gradient similar to the one labeled C in the figure. This would allow the eluent to increase in strength very slowly, as these early peaks are being separated. Then, toward the end of the run, the strength is increased rapidly to bring out the later peaks. It is usually better to begin by experimenting with a linear gradient, then see if the beginning or end of the chromatogram could benefit from being stretched out a bit. A curved elution profile might then be tried.

Normal Phase Liquid Chromatography
Normal phase chromatography relies on such column packings as silica and alumina. Modern silica packings with polar bonded coatings are also available and are more reproducible and easy to use then is the bare silica. The difficulty with silica is its high affinity for water. Any trace of water in the solvent will be adsorbed onto the column, thereby changing its characteristics. This makes reproducible chromatography harder to achieve. Characteristically, normal phase columns have a polar surface, and eluents are rather non-polar, to achieve reasonable separation. In contrast to reverse phase separations, the strongest eluents used in this system are the most polar.

Solvent gradients, in this case, would begin with the least polar solvent, and gradually increase in polarity to bring out the most retained, most polar compounds. Samples best separated on normal phase columns are those comprised of different classes of compounds. Homologs are better separated on reverse phase columns.

4.6 HPlc Instrumentation
A complete apparatus consists of a pumping system, either for a single eluent or for a gradient, an injector, a column, one or more detectors and a data handling system. Typical setups are shown in Figure Chapter 4 .22.

Solvent Delivery Systems
Before being fed to the pumping system, the solvents should be filtered and degassed. Any particulate material in the solvent must be removed, because particulates may damage the pumps, and will, in time, collect at the top of the column and cause plugging. Degassing is important, because the dissolved gases may form bubbles when the pressure drops as the solvent enters the detector. Many detectors are severely disrupted by bubbles. Dissolved gases can be removed by purging the solvents with helium, which is quite insoluble in most solvents, or by passing the solvent through a microporous filter under vacuum. Vacuum filtration is the most common technique, since it accomplishes both the filtration and degassing processes at the same time. When solvents have been standing for some time, re-filtration is a good idea.

The characteristics of a HPLC pump which are of highest importance are its ability to deliver a constant, pulse-free flow, over a wide range of different flows. The materials of the pump system must be resistant to attack by the wide variety of mobile phases to be used. An additional desirable feature is the ability to generate a gradient of two or even three solvents, in a reproducible fashion. Pressures of up to 10000 psi are generated by the pump. Another consideration is the ease of changing solvents, which is related to the hold-up volume of the system.

Reciprocating dual piston pumping systems are the most common type. In these, one piston chamber is filling while the other is pumping. The pistons move in small chambers, each of which contains less than half a milliliter of eluent. A system of check valves keeps the solvent flowing in the correct direction. Pulses are kept to a minimum by elaborate design of the piston stroke cycle. As one piston begins to slow at the end of its stroke, the second one, newly filled, begins to deliver solvent, keeping the pressure and flow as constant as possible.

When using reciprocating piston pumps one must be careful to rinse the pump before turning it off. A solvent which contains no solids is pumped for several minutes, to remove any buffer salts which may remain from running an analysis. Salts can dry and crystallize on the surface of the piston when it is idle. Then, when the pump is restarted, the solids will cause abrasion of both the piston rods and the seals through which they pass.

A simpler, less expensive pumping system uses an eluent-filled syringe, driven by a constant speed motor, turning a screw drive. High pressure, pulseless flows can be generated by a syringe pump, but the volume is limited by the capacity of the syringe. This causes some downtime, as the pressure must be brought down, and the flow stopped, each time the syringe is refilled. These pumps are particularly useful for analyses using microbore columns which require very low flows. A constant pressure supplied by a compressed gas cylinder can also be used to drive a pump piston, giving a very smooth, pulseless flow. However, this system suffers from the same inconveniences as does the syringe pump.

Connections between solvent containers, pumps, columns and detectors are usually constructed of narrow bore stainless steel tubing. Extra-column band broadening is highly dependent on the radius of the tubing through which the sample and eluent pass. The smallest bore tubing which is practical without causing undue plugging should be used. Commonly, tubing around 0.01 inch i.d. is used. Increasing either the diameter or length of tubing through which the sample is passed will have a deleterious effect on the separation efficiency. The effect is more serious as smaller diameter columns are used.

When narrow-bore tubing is cut it is important to avoid plugging it, and to produce a smooth, square end for attachment of fittings. If the end of the tubing is cut raggedly or at a slant, a void space will usually occur when the tubing is placed into a compression fitting. Special cutting wheel tools are available to make good cuts.

Solvent Gradient Systems
Solvent gradient systems require a method of mixing a constantly changing amount of solvents from two or three separate reservoirs. The mixing may be done either before the high pressure pump, or after it. The low pressure mixing method uses low pressure metering pumps to deliver the components of the solvent mixture to the inlet of the high pressure pump. One must be careful to have no dissolved gases in the solvents because the gases may have lower solubility in the mixture, and bubbles can form.

High pressure gradient systems use separate high pressure pumps for each component, feeding into a small mixing chamber just before the injector. These are usually controlled by a microprocessor, which gradually increases the speed of one pump, while slowing the other. This is the most commonly used method, although it is somewhat more expensive, since it requires additional high pressure pumps. In addition to the ability to run samples which require gradient elution, another advantage of having a gradient system available is the ability to change the elution mixture easily. When setting up a new method, even one which will be done isocratically, it is faster and easier to make successive runs while having the microprocessor prepare different mixtures for trial runs, than it is to make the solvent mixtures by hand. In a laboratory with several instruments available, the gradient instrument should be used to set up methods, even for isocratic systems.

Sample Injectors
The requirement for an injector in HPLC is the same as it is for other types of chromatography. It must put a very narrow plug of sample into the eluent stream. One difficulty is that the eluent is under high pressure, which makes the use of a syringe impractical. The usual method is the use of sampling valve, containing a small sampling loop, with a volume of a few microliters. An excess of dissolved sample is flushed through the sampling loop, to fill it completely, with the excess passing out to waste. When the valve is rotated, the eluent flow is diverted through the loop, picking up the sample and moving it on to the column.

Some of these injection valves may also be used in a partial-filling method. In this case a volume of sample, smaller than that of the loop, is injected into the loop, using a syringe to measure the sample. This displaces some, but not all of the eluent in the loop. The accuracy is less than can be achieved by complete flushing of the loop, but it has the advantage that the sample volume can be readily adjusted. Figure Chapter 4 .23 shows a typical injection valve.

HPLC Columns
Precolumns and Guard Columns
The analytical column is expensive and can be damaged by particulate material depositing at the head of the column, as well as by attack of the eluent on the packing itself. Eluents with pH outside the 2 to 7 range may dissolve the silica support of the column. To lengthen the useful column life, guard and precolumns are used.

Precolumns are short segments of tubing packed roughly with similar material to that used in the column. The precolumn will pick up any particulates which are present at the exit of the pump. These particles may arise from poorly filtered eluents, or from wear fragments from the pump. More importantly, if the eluent is aggressive, and is dissolving the silica backbone of the packing, the precolumn serves to saturate the eluent with silica. Since it is located before the injector port, it cannot contribute to band broadening. Therefore, it is not necessary to have this column packed as carefully as an analytical column, or to be concerned about having very low dead volume fittings used to install it. It can be made of rather inexpensive components and packed in the lab.

Another source of particulate matter which may damage columns is the sample. To protect the column from materials in the sample which may deposit at the entrance of the column or which may be irreversibly adsorbed on the column packing, a guard column may be used. Guard columns can contribute to loss of separation efficiency because the sample passes through them. Therefore, they must be packed as carefully as the analytical column, and connected with low dead volume fittings. These columns are placed immediately before the analytical column, and can be considered to be the first part of the analytical column. Guard columns are commercially available, usually in 2-5 cm lengths. Some column systems are available which allow a replaceable cartridge to be placed in the inlet fitting of the analytical column, to serve as a guard column. Figure Chapter 4 .24 shows such a system.

Analytical Columns
Columns are usually constructed of stainless steel tubing with inner diameters of 4 or 5 mm. Microbore columns, with diameters of 1 and 2 mm are also available, as are columns with inside diameters above 10 mm, which are mainly used for preparative scale work. The end fittings on the column contain a frit to hold the packing in place, and a flow distributing plate which spreads the flow from the pump over the end of the column, thus helping to achieve constant flow through the entire cross section of the column.

Installation of columns is achieved by use of threaded fittings, usually using a stainless steel compression ferrule. Many column manufacturers use similar fittings, so that the same threaded nuts and fittings can be used. However, once a steel ferrule is swaged onto a tube, it is often not possible to move this tube to another fitting. The length of tubing which protrudes from the ferrule may be different in each case. If the tubing is too long, the ferrule cannot seat properly, and a good seal will not be achieved. On the other hand, if the tube is too short, a gap will be present inside the fitting and efficiency will be lowered. Figure Chapter 4 .25 shows this problem. Unless a system which uses a ferrule which can be readjusted on the inlet or outlet tube is used, the ferrule should be cut off and a new one fitted whenever columns are changed.

Eluents
The choice of eluent depends on the column and the sample. In reverse phase chromatography, a more polar eluent will move the sample slowly, and allow time for separation. A less polar solvent will elute late peaks more quickly and prevent excessive band broadening. There are several measures of eluent strength, including the polarity index, P’. A higher value of P’ indicates a more polar eluent. Often solvents are mixed to produce an eluent of a suitable strength for a particular separation. For instance, various mixtures of methanol and water are used to produce a variety of different polarities, with an increase in the water content making a less strong, more polar eluent for reverse phase work. The polarity index of a solvent mixture Pm composed of solvents ‘a’ and ‘b’ is computed as:

Pm = Pa * xa + Pb * xb ( Chapter 4 .20)

where Pa and Pb are the polarity indexes of a and b, and xa and xb is their volume fraction. The effect of eluent polarity on the capacity factor k’ of a compound is given by the equation:

( Chapter 4 .21)

where P1’ and P2’ are the polarity indices of the two eluent mixtures.

The eluent must be able to keep the sample components in solution. The viscosity of the eluent is of concern, because a less viscous solvent can be used at a higher flow, without requiring very high pump pressures. Purity of the eluent, as well as its availability, cost and ease of disposal or recycling are other important considerations. Table Chapter 4 .5 lists some common eluent solvents and their physical characteristics important for HPLC.

Example:

In a reverse phase separation of a pesticide, the retention time was 15.5 min, with an eluent composed of methanol/water at a volume ratio of 30:70. An unretained peak eluted at 0.25 min. Calculate k’.



What water/methanol eluent composition will reduce k’ to 5?

Substituting values for methanol and water into Equation 4.20:

P’= 0.3 x 5.1 + 0.7 x 10.2 = 8.7

and so P2’ =6.52

To find the composition, let V = volume fraction of methanol.

6.52 = V x 5.51 + (1-V) x 10.2

V = 0.78. Therefore, the eluent is 78% methanol and 22% water.

4.7 HPLC Detectors
There is no sensitive universal detector available for use in HPLC. The only really universal, bulk property HPLC detector is the refractive index detector, which cannot be used with gradient elution, requires excellent temperature control, and is as much as 103 times less sensitive than other detectors. Therefore, it finds little use in environmental work. The detectors most often used are those such as absorption spectroscopic detectors, which respond to some property of the sample which is not exhibited by the mobile phase.

Ultraviolet Absorption Detectors
Ultra violet detectors are fairly general in application, since most organic compounds absorb some wavelengths in the UV spectrum. However, the spectral region of wavelengths below 210 is usually not useable for analysis because most solvents which would be used as eluents would also absorb in these areas. The response of this detector depends on Beer's Law, and therefore gives a linear response over four to five orders of magnitude. The detection limits vary widely, depending on the sample component and its extinction coefficient at the wavelength being used. In the most favorable cases, 1 ng or less of a compound may be detected.

Fixed wavelength detectors, using filters to isolate a single band of radiation, are inexpensive and stable. Light is passed through the filter then through a flow cell containing the effluent from the column. Finally, it is allowed to impinge on a photocell, where the light is measured. Generally, these are single beam instruments, but dual beam systems are possible. They lack versatility, since the only compounds which can be analyzed are those which absorb at the fixed wavelength. However, for standardized, repetitive analyses, these detectors may be ideal since their reproducibility is often slightly better than that of variable wavelength detectors.

Variable wavelength detectors, are, however, much more versatile. These use a continuum source and a monochromator to select the wavelength desired. A manually adjusted grating disperses the light and passes the target wavelength through the flow cell.

Detectors which can rapidly perform a complete scan over a range of wavelengths can give qualitative as well as quantitative information. This can be done with a rapid scanning instrument, but, more commonly a diode array detector is used. The photo diode array (PDA) detector uses an arrangement of diodes positioned so that each diode intercepts a different band of wavelengths. The signal from each diode is recorded, and a spectrum of the effluent at any moment is obtained. This is very useful in confirming identity of components, and even more, in determining the efficiency of separation. Figure Chapter 4 .26 shows the basic layout of a diode array detector.

The purity of a peak may be determined. Co-elution of components can be confirmed or ruled out by comparing spectra taken on the leading edge, the top, and the trailing edge of a peak. It is difficult to identify a totally unknown compound from the UV spectrum. These are relatively simple spectra, and the solvent, mixed with the sample, has an effect on the spectrum. However, comparison of samples and standards run in the same solvent, gives retention time and spectral information, and strong identification confirmation. Figure Chapter 4 .27 shows part of a chromatogram of a sample of polynuclear aromatic hydrocarbons run using a PDA. The peaks which have spectra and retention times matching those of the standard are identified, and the spectrum of each peak is printed on the report.

The flow cells used for absorption detectors are designed to give the maximum length of sample for the light to pass through, while keeping the volume as small as possible, to insure that the resolution is not compromised. Figure Chapter 4 .28 shows a UV detector cell. The cell has a Z shape to provide the maximum path length with as little cell volume as possible. The principal source of noise in absorbance detectors using a flowing sample is due to slight changes in refractive index. These are due to slight inhomogeneities in the composition of the eluent, changes in temperature, or turbulence in the flow. The change in refractive index diverts some of the light from the path to the detector, momentarily decreasing the signal.

Fluorescence Detectors
Fluorescence detectors are among the most sensitive available. These are most suitable for, but are not limited to, compounds which fluoresce. Non-fluorescent compounds may be derivatized by adding a reagent after the column, which supplies a fluorescent tag to the sample molecules. Alternatively, the eluent may be made fluorescent and the sample peaks detected by the decrease of fluorescence as the peak elutes. This is known as "vacancy chromatography".

Fluorescence detectors need an intense high energy source, either line or continuous, to excite the fluorescence. Mercury lamps are used for line source excitation, and deuterium or xenon arc sources for continuum source. A monochromator is used to select the wavelength for excitation and for emission. The wavelength selection can also be done with filters, at the expense of versatility and sensitivity. The wavelength at the absorbance peak may not be available in a filter instrument, so that the highest sensitivity cannot be achieved. An photomultiplier is used to capture and amplify the weak emission from the fluorescent molecules. Figure Chapter 4 .29 shows a fluorescence detector.

For dilute solutions the equation which relates the emission to the concentration is:

If = Io ff (2.3 abC) ( Chapter 4 .22)

where: If is the measured emission intensity, Io is the excitation beam intensity, ff is the number of photons emitted per photon absorbed (the quantum yield), a is the molar absorption coefficient, b is the cell path length, and C is the sample concentration. The response is linear over about two orders of magnitude. Sensitivity varies widely, depending on the amount of light scattering in the optical system, the intensity of the excitation radiation, and the fluorescence quantum efficiency of the sample. Mobile phase composition is also important since fluorescence is readily quenched. Oxygen is a particularly efficient fluorescence quencher, so solvents must be well degassed. Fluorescence is also temperature dependent, and, at higher concentration, self absorbance can be serious.

Mass Spectrometric Detection
The most informative detector is probably the mass spectrometer. Interfacing between the HPLC and the ion source is even more difficult than it is with GC/MS. The eluent is a liquid, and therefore, must be eliminated in some way before the sample is injected into the vacuum system.

4.8 Ion Chromatography
Ion chromatography is used for separation of ionic species. The stationary phase is an ion exchange resin, and retention of the ionic species occurs as they are exchanged onto and off the resin surface. A cation exchange resin has R-H+ groups on its surface and, a cation such as Zn++ is retained because it exchanges with the hydrogen ions on the resin:

R-H+ + Zn++ Û R-Zn++ + H+

Similarly, an anion exchange resin R-OH-, will exchange OH- ions for anions such as NO3- in the sample:

R-OH- + NO3- Û R-NO3- + OH-

The partition coefficient K for the cation exchange is:

K = [R-Zn++]/[Zn++]

where R-Zn++ is the concentration on the ion exchange resin, and Zn++ is the concentration in the mobile phase. The partition coefficient for the anion exchange is calculated in a similar fashion. The most common anion exchange column incorporates a quaternary amine group, while cation columns usually bear sulfonate groups. The packings are prepared by sulfonating or aminating the surface of the polymer core, with the active sites located close to the surface, to improve the mass transfer between the eluent and the stationary phase.

The instrumentation used for ion chromatography is similar to that used for HPLC and is shown in Figure Chapter 4 .30. The conductivity detector which measures the electrical conductivity of the eluting mobile phase is commonly used.

If the mobile phase has a high ionic strength, the background electrical conductivity will be high and the detector will have low sensitivity. There are two methods used to reduce this difficulty: the suppresser column technique and the single column technique. In the suppresser column method, the a fairly strong eluent is used to move the sample through the analytical column. Then the eluent is passed through a second column, the suppresser column. This neutralizes the eluent and allows easy detection of the sample ions. For instance, in the analysis of cations, a dilute HCl solution may be used as the eluent. The analytical column is a low capacity cation exchange resin, and the suppresser column is a high capacity anion exchange resin. The large excess of H+ ions displaces the sample cations, with each cation establishing its own equilibrium between the eluent and the surface. The suppresser column is an anion exchange resin in the hydroxyl form, and the H+ ions from the mobile phase react with the OH-, forming water. This leaves the sample cations in the eluent stream, with a very low background conductivity, facilitating conductivity detection. The suppresser column eventually becomes exhausted and must be regenerated to replenish the OH- on the surface.

For analysis of anions, the analytical column is an anion exchange resin while the suppresser is a high capacity cation exchange resin. The eluent is, for instance, a dilute solution of NaOH. In the suppresser column, the OH- ions in the eluent are neutralized by the H+ from the column. This leaves only the sample anions in the solution and high sensitivity is obtained.

The single column method, a more recent development, uses a low capacity ion exchange resin designed especially for chromatographic purposes. Since the resin has such low retention, the eluents of very low ionic strength can be used. Buffers of such weak acids as boric acid have very low conductivities and the detection of the sample ions can be done without the use of a suppresser column. This simplifies the system, and allows the usual HPLC equipment, with only a conductivity detector added, to be applied to ion chromatography.

Ion chromatography can be used for the detection and quantitation of many species: Inorganic anions such as chloride, fluoride, sulfate, nitrate and nitrite; cations such as sodium, calcium, copper, lead, ammonium ions; as well as ionizable organic species such as carboxylic acids and amino acids can be determined using this technique. While many metals may be more easily determined by atomic spectroscopy, ion chromatography has the ability to distinguish between species having different oxidation states, such as Fe(II) and Fe(III). This is not possible if atomic absorption spectroscopy is used for the analysis. Figure Chapter 4 .31 shows an ion chromatogram of an extract of anions from an air filter, obtained with a single column system.

4.9 Supercritical Fluid Chromatography
A substance cannot exist in the liquid state at a temperature above its critical temperature. However, if a material is above its critical temperature, and is subjected to sufficiently high pressure, it becomes much more dense than ordinary gases, and takes on some liquid-like properties. This is then referred to as a supercritical fluid. Figure Chapter 4 .32 shows the phase diagram for CO2, a commonly used supercritical fluid. The properties of supercritical fluids can be continuously varied between those of the gas and those of the liquid by changing the temperature and pressure. These fluids can be used as mobile phases in chromatography. Properties which can be varied include the viscosity, solvent properties and diffusivity, all of which are important chromatographic properties.

The properties of these fluids are usually closer to those of liquids than gases. The solubilizing power of a supercritical fluid is much greater than that of a gas. Therefore, nonvolatile and slightly volatile compounds may be separated by supercritical chromatography, while this would be impossible to do with GC. There is also an advantage over HPLC analysis for these compounds, since the solute diffusion coefficients in supercritical fluids are much greater. This means that the eluent velocity required for the maximum column efficiency is 5 to 10 times greater than that for HPLC. Equally efficient separations can therefore be done in much less time than is needed for HPLC. Finally, the viscosity of these fluids is much lower than that of liquids, making them much easier to pump through columns at a faster flow. Both packed and open tubular columns are used.

Any substance stable above its supercritical temperature might be used for eluent in SFC, but only a few are used routinely. Supercritical fluids which have been used are carbon dioxide, nitrous oxide, sulfur hexafluoride, Freon-13, ethane and ammonia. Of these, CO2 is the most common, since its critical temperature, 31oC, is readily attained, it is non-toxic, and is readily available. Between the pressures of 72 and 400 atmospheres, and temperatures of 40 to 140 oC, the density of CO2 can be varied from 0.1 to almost 1 mg/ml. The only practical supercritical fluid which is reasonably polar is ammonia. It is, however, quite reactive and difficult to use. Modifiers are therefore used to improve the separation of more polar substances in nonpolar supercritical fluids such as CO2. Modifiers are added to the eluent to improve peak shape and shorten retention times. These modifiers, including methanol, water and formic acid, are used in low concentrations, below 2% by volume. Modifiers at this low level may be thought of as deactivating silanol groups on the column, rather than increasing the solubility of the sample compounds in the solvent.

Programming in SFC is quite flexible, since temperature, pressure and density all affect the retention of samples. The most common method of programming is density or pressure programming although temperature programming has also been used.

SFC Instrumentation
The components of an SFC system are similar to those of HPLC, since a pump is used to produce the high pressures required, but GC and HPLC detectors can be used. A typical system is shown in Figure Chapter 4 .33.

A syringe pump is the most commonly used pump although reciprocating pumps have been used for packed column work. The mixing of modifiers complicates the system. Cylinders of eluent with modifier already mixed may be purchased, but then the amount of modifier cannot be adjusted. A second pump to add modifier is useful.

Samples are injected into the system using high pressure rotary sampling valves. Sample volumes as small as a few nanoliters are needed for capillary work, so sample splitting techniques may be required. However, the sample is injected at room temperature, where the eluent may not be supercritical. The sample also may not be homogeneously mixed into the eluent quickly enough before the split is made. Therefore, the operation of a splitter is not always simple, and quantitation may be poor. Sample volumes for packed columns are larger, in the microliter range, so injection is a much easier task.

Columns used in SFC are usually small bore columns packed with 5 to 10 mm bonded phase particles similar to those used for HPLC. Short capillary columns of 1 to 10 meters in length are also used, with stationary phases which are often more crosslinked than those used in GC. All bonded stationary phases have some contribution from unreacted silanol groups, and these can be a problem in SFC, because of the relatively nonpolar nature of CO2. This is why polar modifiers are effective.

At the end of the column a restrictor is required to keep the fluid in the column at the required pressure. The restrictor is positioned before the detector when the detector is a GC type detector, and after the detector, when an HPLC type detector such as a UV absorption cell, is used. A restrictor may be simply a short length of narrow bore fused silica tubing of 5 to 15 microns i.d. However, the sample may precipitate as fog droplets when the solvent suddenly decompresses at the end of the restrictor. These droplets, when fed into a flame ionization detector, cause signal spikes. This may be avoided by decompressing the eluent more gradually in a tapered or conical restrictor, which is kept warm at the tip so that the sample has a chance to evaporate.

The flame ionization detector is probably the most common detector for SFC, as it is compatible with the usual fluids. While organic modifiers interfere, water and formic acid can be used with FID detectors. The sensitivity of the FID is somewhat lower than its GC counterpart. The UV detector is also often used, especially when wide bore columns or organic modifiers make the FID unsuitable. The volume of the absorbance cell must be very small in SFC, on the order of 50 nanoliters or less, to avoid band broadening when capillary columns are used. Standard HPLC UV cells may be used in packed column work, but modifications may be necessary to allow the cells to be used at the much higher pressures common in SFC.

4.10 Applications of Chromatography in Environmental Analysis
Gas chromatography is the most widely used separation technique. Many of the target pollutants are volatile enough to be analyzed by GC. For semivolatiles such as PAH, PCBs and some pesticides HPLC is widely used. GC has several advantages over HPLC. GC columns provide a larger number of plates and a variety of highly sensitive and selective detectors are available. Since environmental samples are complex, the high separation capability is very important. If there is a choice between GC and HPLC, GC is usually preferred. An example of a difficult separation of some metabolites of benzo-a-pyrene, too non-volatile to be separated easily by GC, is shown in Figure Chapter 4 .34.

Supercritical chromatography has still not become a standard technique in environmental analysis. Most samples can be done by either GC or HPLC, both of which are much more mature techniques. There are some advantages, especially the ability to use high sensitivity GC detectors for relatively nonvolatile samples, but the equipment has not yet reached the sophistication and ease of use of GC or HPLC. Supercritical extraction, on the other hand, will probably develop into a major technique for environmental sample preparation, because it readily replaces toxic solvents.

References and Suggested Readings
1. Principles of Instrumental Analysis; D.A. Skoog and J. J. Leary, Saunders College Publishing, New York, 1992.

2. Instrumental Methods of Analysis; H. H. Willard et. al.; 6th edition, Wadsworth.

3. Gas Chromatographic Environmental Analysis; Fabrizio Bruner, VCH Publishers, Inc, New York, 1993.

4. Chromatography Today, C.F. Poole and S.K.Poole, Elsevier, 1991

5. Ion Chromatography; J. Weiss, 2nd Ed., 1995, VCH Publishers, New York.

6. Introduction to Modern Liquid Chromatography; L. R. Snyder and J. J. Kirkland, 2nd Ed. 1980, John Wiley and Sons, New York.

7. Gas Chromatographic Environmental Analysis: Principles, Techniques and Instrumentation, F. Bruner, VCH Publishers, New York, 1993

Study Questions
1. What are the common factors which exist in all types of chromatography?

2. What is the significance of to in gas chromatography?

3. A gas chromatographic peak from a 25 m long column has an adjusted retention time of 11.2 minutes, and a width at half height of 20 seconds. How many theoretical plates are present in the column? What is the value of H?

4. A sample of pesticide is analyzed by gas chromatography. A 0.1 ml injection of a standard containing 0.234 mg/l gives a peak of area 34873. The same size injection of an unknown sample solution gives an area of 39945. What is the concentration of the pesticide in the sample solution?

5. In gas chromatography predict the effect on the efficiency of a separation when the following changes are made:
a. Particle size of the packing is increased
b. Gas flow is increased
c. Thickness of liquid phase in a capillary column is increased

6. In an HPLC analysis, a reverse phase column is being used with a solvent gradient, starting with solvent A and increasing concentration of solvent B over time. The program starts with 50% A and 50% B and runs linearly, reaching 100%B at 20 minutes. Explain which of the two solvents is more polar.

7. What are some of the properties of a good chromatographic detector.

8. What is the advantage in using a halogen specific detector such as the ECD in environmental analyses?

9. Define the following terms:

Gas-Liquid chromatography.
Bonded phase
Normal phase HPLC
Gradient elution

10. What is the function of the suppresser column in ion chromatography? Under what conditions is it not needed?

11. Predict the elution order of the following in reverse phase and in normal phase chromatography: (a) n-heptane, heptanol, and toluene.
(b) nitrobenzene, benzene and phenol

12. What parameters can be used to improve resolution in GC? in HPLC?

13. What are some advantages of SFC over GC? Over HPLC?

14. In a normal phase LC separation, nitrobenzene had a retention time of 28.0 min, while an unretained compound eluted in 0.9 min. The mobile phase was a 50:50 mixture of chloroform and hexane. What mixture of chloroform and hexane will reduce the k’ to 9.0.

The following data were obtained for a separation using a 0.5 mm id, 10 meter open tubular column:


Retention time (min)
Peak base width (min)

Methane
2.0


Cyclohexane
8.0
0.65

Methylcyclohexane
8.17
0.72

Toluene
10.2
0.95


Draw the chromatogram and label the peaks

b) Calculate the number of peaks for each compound

c) Calculate the capacity factor for toluene

d) Calculate the length of the column needed to separate cyclohexane and methylcyclohexane at a resolution of 1.6.

15) In a reverse phase separation of chlorinated phenols, trichlorophenol has a retention time of 15.0 min, when 30:70 acetonitrile-water mixture is used as the mobile phase. What mixture of these two solvents will reduce the retention time to 12.0.

The state of matter distinguished from the solid and liquid states by relatively low density and viscosity, relatively great expansion and contraction with changes in pressure and temperature, the ability to diffuse readily, and the spontaneous tendency to become distributed uniformly throughout any container.
A substance in the gaseous state.
A gaseous fuel, such as natural gas.
Gasoline.
The speed control of a gasoline engine. Used with the: Step on the gas.
A gaseous asphyxiant, irritant, or poison.
A gaseous anesthetic, such as nitrous oxide.

Flatulence.
Flatus.
Slang. Idle or boastful talk.
Slang. Someone or something exceptionally exciting or entertaining: The party was a gas.

v., gassed, gas·sing, gas·es or gas·ses.

v.tr.
To treat chemically with gas.
To overcome, disable, or kill with poisonous fumes.
v.intr.
To give off gas.
Slang. To talk excessively.
phrasal verb:
gas up

To supply a vehicle with gas or gasoline: gas up a car; gassed up before the trip.

[Dutch, an occult physical principle supposed to be present in all bodies, alteration of Greek khaos, chaos, empty space, coined by Jan Baptista van Helmont (1577–1644), Flemish chemist.]



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Science of Everyday Things
Directory > Science > Science of Everyday Things Gases
Concept

The number of elements that appear ordinarily in the form of a gas is relatively small: oxygen, hydrogen, fluorine, and chlorine in the halogen "family"; and a handful of others, most notably the noble gases in Group 8 of the periodic table. Yet many substances can exist in the form of a gas, depending on the relative attraction and motion of molecules in that substance. A simple example, of course, is water, or H2O, which, though it appears as a liquid at room temperature, begins to vaporize and turn into steam at 212°F (100°C). In general, gases respond more dramatically to changes in pressure and temperature than do most other types of matter, and this allows scientists to predict gas behaviors under certain conditions. These predictions can explain mundane occurrences, such as the fact that an open can of soda will soon lose its fizz, but they also apply to more dramatic, life-and-death situations.

How It Works

Molecular Motion and Phases of Matter

On Earth, three principal phases or states of matter exist: solid, liquid, and gas. The differences between these three are, on the surface at least, easily perceivable. Clearly water is a liquid, just as ice is a solid and steam a vapor or gas. Yet the ways in which various substances convert between phases are often complex, as are the interrelations between these phases. Ultimately, understanding of the phases depends on an awareness of what takes place at the molecular level.

All molecules are in motion, and the rate of that motion determines the attraction between them. The movement of molecules generates kinetic energy, or the energy of movement, which is manifested as thermal energy. In everyday language, thermal energy is what people mean when they say "heat"; but in scientific terms, heat has a different definition.

The force that attracts atoms to atoms, or molecules to molecules, is not the same as gravitational force, which holds the Moon in orbit around Earth, Earth in orbit around the Sun, and so on. By contrast, the force of interatomic and intermolecular attraction is electromagnetic. Just as the north pole of a magnet is attracted to the south pole of another magnet and repelled by that other magnet's north pole, so positive electric charges are attracted to negative charges, and negatives to positives. (In fact, electricity and magnetism are both manifestations of an electromagnetic interaction.)

The electromagnetic attractions between molecules are much more complex than this explanation makes it seem, and they play a highly significant role in chemical bonding. In simple terms, however, one can say that the greater the rate of motion for the molecules in relation to one another, the less the attraction between molecules. In addition, the kinetic energy, and hence the thermal energy, is greater in a substance whose molecules are relatively free to move.

When the molecules in a material move slowly in relation to one another, they exert a strong attraction, and the material is called a solid. Molecules of liquid, by contrast, move at moderate speeds and exert a moderate attraction. A material substance whose molecules move at high speeds, and therefore exert little or no attraction, is known as a gas.

Comparison of Gases to Other Phases of Matter

Water and Air Compared

Gases respond to changes in pressure and temperature in a manner remarkably different from that of solids or liquids. Consider the behavior of liquid water as compared with air—a combination of oxygen (O2), nitrogen (N2), and other gases—in response to experiments involving changes in pressure and temperature.

In the first experiment, both samples are subjected to an increase in pressure from 1 atm (that is, normal atmospheric pressure at sea level) to 2 atm. In the second, both experience an increase in temperature from 32°F (0°C) to 212°F (100°C). The differences in the responses of water and air are striking.

A sample of water that experiences an increase in pressure from 1 to 2 atm will decrease in volume by less than 0.01%, while a temperature increase from the freezing point to the boiling point will result in only a 2% increase in volume. For air, however, an equivalent pressure increase will decrease the volume by a whopping 50%, and an equivalent temperature increase results in a volume increase of 37%.

Air and other gases, by definition, have a boiling point below room temperature. If they did not boil and thus become gas well below ordinary temperatures, they would not be described as substances that are in the gaseous state in most circumstances. The boiling point of water, of course, is higher than room temperature, and that of solids is much higher.

The Arrangement of Particles

Solids possess a definite volume and a definite shape, and are relatively noncompressible: for instance, if one applies extreme pressure to a steel plate, it will bend, but not much. Liquids have a definite volume, but no definite shape, and tend to be noncompressible. Gases, on the other hand, possess no definite volume or shape, and are highly compressible.

At the molecular level, particles of solids tend to be definite in their arrangement and close in proximity—indeed, part of what makes a solid "solid," in the everyday meaning of that term, is the fact that its constituent parts are basically immovable. Liquid molecules, too, are close in proximity, though random in arrangement. Gas molecules are random in arrangement, but tend to be more widely spaced than liquid molecules.

Pressure

There are a number of statements, collectively known as the "gas laws," that describe and predict the behavior of gases in response to changes in temperature, pressure, and volume. Temperature and volume are discussed elsewhere in this book. However, the subject of pressure requires some attention before we can continue with a discussion of the gas laws.

When a force is applied perpendicular to a surface area, it exerts pressure on that surface. Hence the formula for pressure is p = F/A, where p is pressure, F force, and A the area over which the force is applied. The greater the force, and the smaller the area of application, the greater the pressure; conversely, an increase in area—even without a reduction in force—reduces the overall pressure.

Pressure is measured by a number of units in the English and SI systems. Because p = F/A, all units of pressure represent some ratio of force to surface area.

Units of Pressure

The principal SI unit of pressure is called a pascal (Pa), or 1 N/m2. It is named for French mathematician and physicist Blaise Pascal (1623-1662), who is credited with Pascal's principle. The latter holds that the external pressure applied on a fluid—which, in the physical sciences, can mean either a gas or a liquid—is transmitted uniformly throughout the entire body of that fluid.

A newton (N), the SI unit of force, is equal to the force required to accelerate 1 kg of mass at a rate of 1 m/sec2. Thus a Pascal is the pressure of 1 newton over a surface area of 1 m2. In the English or British system, pressure is measured in terms of pounds per square inch, abbreviated as lbs./in2. This is equal to 6.89 · 103 Pa, or 6,890 Pa.

Another important measure of pressure is the atmosphere (atm), which is the average pressure exerted by air at sea level. In English units, this is equal to 14.7 lb/in2, and in SI units, to 1.013 · 105 Pa.

There are two other specialized units of pressure measurement in the SI system: the bar, equal to 105 Pa, and the torr, equal to 133 Pa. Meteorologists, scientists who study weather patterns, use the millibar (mb), which, as its name implies, is equal to 0.001 bars. At sea level, atmospheric pressure is approximately 1,013 mb.

The torr, also known as the millimeter of mercury (mm Hg), is the amount of pressure required to raise a column of mercury (chemical symbol Hg) by 1 mm. It is named for Italian physicist Evangelista Torricelli (1608-1647), who invented the barometer, an instrument for measuring atmospheric pressure.

The Barometer

The barometer constructed by Torricelli in 1643 consisted of a long glass tube filled with mercury. The tube was open at one end, and turned upside down into a dish containing more mercury: the open end was submerged in mercury, while the closed end at the top constituted a vacuum—that is, an area devoid of matter, including air.

The pressure of the surrounding air pushed down on the surface of the mercury in the bowl, while the vacuum at the top of the tube provided an area of virtually no pressure into which the mercury could rise. Thus the height to which the mercury rose in the glass tube represented normal air pressure (that is, 1 atm.) Torricelli discovered that at standard atmospheric pressure, the column of mercury rose to 760 mm (29.92 in).

The value of 1 atm was thus established asequal to the pressure exerted on a column ofmercury 760 mm high at a temperature of 0°C(32°F). In time, Torricelli's invention became afixture both of scientific laboratories and ofhouseholds. Since changes in atmospheric pressure have an effect on weather patterns, manyhome indoor-outdoor thermometers today alsoinclude a barometer.

Real-Life Applications

Introduction to the Gas Laws

English chemist Robert Boyle (1627-1691), who made a number of important contributions to chemistry—including his definition and identification of elements—seems to have been influenced by Torricelli. If so, this is an interesting example of ideas passing from one great thinker to another: Torricelli, a student of Galileo Galilei (1564-1642), was no doubt influenced by Galileo's thermoscope.

Like Torricelli, Boyle conducted tests involving the introduction of mercury to a tube closed at the other end. The tube Boyle used was shaped like the letter J, and it was so long that he had to use the multi-story foyer of his house as a laboratory. At the tip of the curved bottom was an area of trapped gas, and into the top of the tube, Boyle introduced increasing quantities of mercury. He found that the greater the volume of mercury, the greater the pressure on the gas, and the less the volume of gas at the end of the tube. As a result, he formulated the gas law associated with his name.

The gas laws are not a set of government regulations concerning use of heating fuel; rather, they are a series of statements concerning the behavior of gases in response to changes in temperature, pressure, and volume. These were derived, beginning with Boyle's law, during the seventeenth, eighteenth, and nineteenth centuries by scientists whose work is commemorated through the association of their names with the laws they discovered. In addition to Boyle, these men include fellow English chemists John Dalton (1766-1844) and William Henry (1774-1836); French physicists and chemists J. A. C. Charles (1746-1823) and Joseph Gay-Lussac (1778-1850); and Italian physicist Amedeo Avogadro (1776-1856).

There is a close relationship between Boyle's, Charles's, and Gay-Lussac's laws. All of these treat one of three parameters—temperature, pressure, or volume—as fixed quantities in order to explain the relationship between the other two variables. Avogadro's law treats two of the parameters as fixed, thereby establishing a relationship between volume and the number of molecules in a gas. The ideal gas law sums up these four laws, and the kinetic theory of gases constitutes an attempt to predict the behavior of gases based on these laws. Finally, Dalton's and Henry's laws both relate to partial pressure of gases.

Boyle's, Charles's, and Gay-Lussac's Laws

Boyle's and Charles's Laws

Boyle's law holds that in isothermal conditions (that is, a situation in which temperature is kept constant), an inverse relationship exists between the volume and pressure of a gas. (An inverse relationship is a situation involving two variables, in which one of the two increases in direct proportion to the decrease in the other.) In this case, the greater the pressure, the less the volume and vice versa. Therefore, the product of the volume multiplied by the pressure remains constant in all circumstances.

Charles's law also yields a constant, but in this case the temperature and volume are allowed to vary under isobarometric conditions—that is, a situation in which the pressure remains the same. As gas heats up, its volume increases, and when it cools down, its volume reduces accordingly. Hence, Charles established that the ratio of temperature to volume is constant.

Absolute Temperature

In about 1787, Charles made an interesting discovery: that at 0°C (32°F), the volume of gas at constant pressure drops by 1/273 for every Celsius degree drop in temperature. This seemed to suggest that the gas would simply disappear if cooled to −273°C (−459.4°F), which, of course, made no sense. In any case, the gas would most likely become first a liquid, and then a solid, long before it reached that temperature.

The man who solved the quandary raised by Charles's discovery was born a year after Charles died. He was William Thomson, Lord Kelvin (1824-1907); in 1848, he put forward the suggestion that it was molecular translational energy—the energy generated by molecules in motion—and not volume, that would become zero at −273°C. He went on to establish what came to be known as the Kelvin scale of absolute temperature.

Sometimes known as the absolute temperature scale, the Kelvin scale is based not on the freezing point of water, but on absolute zero—the temperature at which molecular motion comes to a virtual stop. This is −273.15°C (−459.67°F). In the Kelvin scale, which uses neither the term nor the symbol for "degree," absolute zero is designated as 0K.

Scientists prefer the Kelvin scale to the Celsius, and certainly to the Fahrenheit, scales. If the Kelvin temperature of an object is doubled, its average molecular translational energy has doubled as well. The same cannot be said if the temperature were doubled from, say, 10°C to 20°C, or from 40°F to 80°F, since neither the Celsius nor the Fahrenheit scale is based on absolute zero.

Gay-Lussac's Law

From Boyle's and Charles's law, a pattern should be emerging: both treat one parameter (temperature in Boyle's, pressure in Charles's) as unvarying, while two other factors are treated as variables. Both, in turn, yield relationships between the two variables: in Boyle's law, pressure and volume are inversely related, whereas in Charles's law, temperature and volume are directly related.

In Gay-Lussac's law, a third parameter, volume, is treated as a constant, and the result is a constant ratio between the variables of pressure and temperature. According to Gay-Lussac's law, the pressure of a gas is directly related to its absolute temperature.

Avogadro's Law

Gay-Lussac also discovered that the ratio in which gases combine to form compounds can be expressed in whole numbers: for instance, water is composed of one part oxygen and two parts hydrogen. In the language of modern chemistry, this is expressed as a relationship between molecules and atoms: one molecule of water contains one oxygen atom and two hydrogen atoms.

In the early nineteenth century, however, scientists had yet to recognize a meaningful distinction between atoms and molecules, and Avogadro was the first to achieve an understanding of the difference. Intrigued by the whole-number relationship discovered by Gay-Lussac, Avogadro reasoned that one liter of any gas must contain the same number of particles as a liter of another gas. He further maintained that gas consists of particles—which he called molecules—that in turn consist of one or more smaller particles.

In order to discuss the behavior of molecules, Avogadro suggested the use of a large quantity as a basic unit, since molecules themselves are very small. Avogadro himself did not calculate the number of molecules that should be used for these comparisons, but when that number was later calculated, it received the name "Avogadro's number" in honor of the man who introduced the idea of the molecule. Equal to 6.022137 · 1023, Avogadro's number designates the quantity of atoms or molecules (depending on whether the substance in question is an element or a compound) in a mole.

Today the mole (abbreviated mol), the SI unit for "amount of substance," is defined precisely as the number of carbon atoms in 12.01 g of carbon. The term "mole" can be used in the same way we use the word "dozen." Just as "a dozen" can refer to twelve cakes or twelve chickens, so "mole" always describes the same number of molecules. The ratio of mass between a mole of Element A and Element B, or Compound A and Compound B, is the same as the ratio between the mass of Atom A and Atom B, or Molecule A and Molecule B. Avogadro's law describes the connection between gas volume and number of moles. According to Avogadro's law, if the volume of gas is increased under isothermal and isobarometric conditions, the number of moles also increases. The ratio between volume and number of moles is therefore a constant.

The Ideal Gas Law

Once again, it is easy to see how Avogadro's law can be related to the laws discussed earlier. Like the other three, this one involves the parameters of temperature, pressure, and volume, but it also introduces a fourth—quantity of molecules (that is, number of moles). In fact, all the laws so far described are brought together in what is known as the ideal gas law, sometimes called the combined gas law.

The ideal gas law can be stated as a formula, pV = nRT, where p stands for pressure, V for volume, n for number of moles, and T for temperature. R is known as the universal gas constant, a figure equal to 0.0821 atm · liter/mole · K. (Like most figures in chemistry, this one is best expressed in metric rather than English units.)

Given the equation pV = nRT and the fact that R is a constant, it is possible to find the value of any one variable—pressure, volume, number of moles, or temperature—as long as one knows the value of the other three. The ideal gas law also makes it possible to discern certain relationships: thus, if a gas is in a relatively cool state, the product of its pressure and volume is proportionately low; and if heated, its pressure and volume product increases correspondingly.

The Kinetic Theory of Gases

From the preceding gas laws, a set of propositions known collectively as the kinetic theory of gases has been derived. Collectively, these put forth the proposition that a gas consists of numerous molecules, relatively far apart in space, which interact by colliding. These collisions are responsible for the production of thermal energy, because when the velocity of the molecules increases—as it does after collision—the temperature increases as well.

There are five basic postulates to the kinetic theory of gases:

1. Gases consist of tiny molecular or atomic particles.
2. The proportion between the size of these particles and the distances between them is so small that the individual particles can be assumed to have negligible volume.
3. These particles experience continual random motion. When placed in a container, their collisions with the walls of the container constitute the pressure exerted by the gas.
4. The particles neither attract nor repel one another.
5. The average kinetic energy of the particles in a gas is directly related to absolute temperature.
These observations may appear to resemble statements made earlier concerning the differences between gases, liquids, and solids in terms of molecular behavior. If so, that is no accident: the kinetic theory constitutes a generally accepted explanation for the reasons why gases behave as they do. Kinetic theories do not work as well for explaining the behaviors of solids and liquids; nonetheless, they do go a long way toward identifying the molecular properties inherent in the various phases of matter.

Laws of Partial Pressure

In addition to all the gas laws so far discussed, two laws address the subject of partial pressure. When two or more gases are present in a container, partial pressure is the pressure that one of them exerts if it alone is in the container.

Dalton's law of partial pressure states that the total pressure of a gas is equal to the sum of its partial pressures. As noted earlier, air is composed mostly of nitrogen and oxygen. Along with these are small components, carbon dioxide, and gases collectively known as the rare or noble gases: argon, helium, krypton, neon, radon, and xenon. Hence, the total pressure of a given quantity of air is equal to the sum of the pressures exerted by each of these gases.

Henry's law states that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of the gas above the surface of the solution. This applies only to gases such as oxygen and hydrogen that do not react chemically to liquids. On the other hand, hydrochloric acid will ionize when introduced to water: one or more of its electrons will be removed, and its atoms will convert to ions, which are either positive or negative in charge.

Applications of Dalton's and Henry's Laws

Partial Pressure: a Matter of Life and Possible Death for Scuba Divers

The gas laws are not just a series of abstract statements. Certainly, they do concern the behavior of ideal as opposed to real gases. Like all scientific models, they remove from the equation all outside factors, and treat specific properties in isolation. Yet, the behaviors of the ideal gases described in the gas laws provide a key to understanding the activities of real gases in the real world. For instance, the concept of partial pressure helps scuba divers avoid a possibly fatal sickness.

Imagine what would happen if a substance were to bubble out of one's blood like carbon dioxide bubbling out of a soda can, as described below. This is exactly what can happen to an undersea diver who returns to the surface too quickly: nitrogen rises up within the body, producing decompression sickness—known colloquially as "the bends." This condition may manifest as itching and other skin problems, joint pain, choking, blindness, seizures, unconsciousness, permanent neurological defects such as paraplegia, and possibly even death.

If a scuba diver descending to a depth of 150 ft (45.72 m) or more were to use ordinary air in his or her tanks, the results would be disastrous. The high pressure exerted by the water at such depths creates a high pressure on the air in the tank, meaning a high partial pressure on the nitrogen component in the air. The result would be a high concentration of nitrogen in the blood, and hence the bends.

Instead, divers use a mixture of helium and oxygen. Helium gas does not dissolve well in blood, and thus it is safer for a diver to inhale this oxygen-helium mixture. At the same time, the oxygen exerts the same pressure that it would normally—in other words, it operates in accordance with Dalton's observations concerning partial pressure.

Opening a Soda Can

Inside a can or bottle of carbonated soda is carbon dioxide gas (CO2), most of which is dissolved in the drink itself. But some of it is in the space (sometimes referred to as "head space") that makes up the difference between the volume of the soft drink and the volume of the container.

At the bottling plant, the soda manufacturer adds high-pressure carbon dioxide (CO2) to the head space in order to ensure that more CO2 will be absorbed into the soda itself. This is in accordance with Henry's law: the amount of gas (in this case CO2) dissolved in the liquid (soda) is directly proportional to the partial pressure of the gas above the surface of the solution—that is, the CO2 in the head space. The higher the pressure of the CO2 in the head space, the greater the amount of CO2 in the drink itself; and the greater the CO2 in the drink, the greater the "fizz" of the soda.

Once the container is opened, the pressure in the head space drops dramatically. Once again, Henry's law indicates that this drop in pressure will be reflected by a corresponding drop in the amount of CO2 dissolved in the soda. Over a period of time, the soda will release that gas, and eventually, it will go "flat."

Fire Extinguishers

A fire extinguisher consists of a long cylinder with an operating lever at the top. Inside the cylinder is a tube of carbon dioxide surrounded by a quantity of water, which creates pressure around the CO2 tube. A siphon tube runs vertically along the length of the extinguisher, with one opening in the water near the bottom. The other end opens in a chamber containing a spring mechanism attached to a release valve in the CO2 tube.

The water and the CO2 do not fill the entire cylinder: as with the soda can, there is "head space," an area filled with air. When the operating lever is depressed, it activates the spring mechanism, which pierces the release valve at the top of the CO2 tube. When the valve opens, the CO2 spills out in the "head space," exerting pressure on the water. This high-pressure mixture of water and carbon dioxide goes rushing out of the siphon tube, which was opened when the release valve was depressed. All of this happens, of course, in a fraction of a second—plenty of time to put out the fire.

Aerosol Cans

Aerosol cans are similar in structure to fire extinguishers, though with one important difference. As with the fire extinguisher, an aerosol can includes a nozzle that depresses a spring mechanism, which in turn allows fluid to escape through a tube. But instead of a gas cartridge surrounded by water, most of the can's interior is made up of the product (for instance, deodorant), mixed with a liquid propellant.

The "head space" of the aerosol can is filled with highly pressurized propellant in gas form, and, in accordance with Henry's law, a corresponding proportion of this propellant is dissolved in the product itself. When the nozzle is depressed, the pressure of the propellant forces the product out through the nozzle.

A propellant, as its name implies, propels the product itself through the spray nozzle when the nozzle is depressed. In the past, chlorofluorocarbons (CFCs)—manufactured compounds containing carbon, chlorine, and fluorine atoms—were the most widely used form of propellant. Concerns over the harmful effects of CFCs on the environment, however, has led to the development of alternative propellants, most notably hydrochlorofluorocarbons (HCFCs), CFC-like compounds that also contain hydrogen atoms.

Applications of Boyle's, Charles's, and Gay-Lussac's Laws

When the Temperature Changes

A number of interesting results occur when gases experience a change in temperature, some of them unfortunate and some potentially lethal. In these instances, it is possible to see the gas laws—particularly Boyle's and Charles's—at work.

There are numerous examples of the disastrous effects that result from an increase in the temperature of combustible gases, including natural gas and petroleum-based products. In addition, the pressure on the gases in aerosol cans makes the cans highly explosive—so much so that discarded cans at a city dump may explode on a hot summer day. Yet, there are other instances when heating a gas can produce positive effects.

A hot-air balloon, for instance, floats because the air inside it is not as dense than the air outside. According to Charles's law, heating a gas will increase its volume, and since gas molecules exert little attraction toward one another, they tend to "spread out" even further with an increase of volume. This, in turn, creates a significant difference in density between the air in the balloon and the air outside, and as a result, the balloon floats.

Although heating a gas can be beneficial, cooling a gas is not always a wise idea. If someone were to put a bag of potato chips into a freezer, thinking this would preserve their flavor, he would be in for a disappointment. Much of what maintains the flavor of the chips is the pressurization of the bag, which ensures a consistent internal environment so that preservative chemicals, added during the manufacture of the chips, can keep them fresh. Placing the bag in the freezer causes a reduction in pressure, as per Gay-Lussac's law, and the bag ends up a limp version of its former self.

Propane tanks and tires offer an example of the pitfalls that may occur by either allowing a gas to heat up or cool down by too much. Because most propane tanks are made according to strict regulations, they are generally safe, but it is not entirely inconceivable that the extreme heat of a summer day could cause a defective tank to burst. An increase in temperature leads to an increase in pressure, in accordance with Gay-Lussac's law, and could lead to an explosion.

Because of the connection between heat and pressure, propane trucks on the highways during the summer are subjected to weight tests to ensure that they are not carrying too much gas. On the other hand, a drastic reduction in temperature could result in a loss in gas pressure. If a propane tank from Florida were transported by truck during the winter to northern Canada, the pressure is dramatically reduced by the time it reaches its destination.

The Internal-Combustion Engine

In operating a car, we experience two applications of the gas laws. One of these is what makes the car run: the combustion of gases in the engine, which illustrates the interrelation of volume, pressure, and temperature expressed in the laws attributed to Boyle, Charles, and Gay-Lussac. The other is, fortunately, a less frequent phenomenon—but it can and does save lives. This is the operation of an airbag, which depends, in part, on the behaviors explained in Charles's law.

When the driver of a modern, fuel-injection automobile pushes down on the accelerator, this activates a throttle valve that sprays droplets of gasoline mixed with air into the engine. The mixture goes into the cylinder, where the piston moves up, compressing the gas and air. While the mixture is still at a high pressure, the electric spark plug produces a flash that ignites the gasoline-air mixture. The heat from this controlled explosion increases the volume of air, which forces the piston down into the cylinder. This opens an outlet valve, causing the piston to rise and release exhaust gases.

As the piston moves back down again, an inlet valve opens, bringing another burst of gasoline-air mixture into the chamber. The piston, whose downward stroke closed the inlet valve, now shoots back up, compressing the gas and air to repeat the cycle. The reactions of the gasoline and air to changes in pressure, temperature, and volume are what move the piston, which turns a crankshaft that causes the wheels to rotate.

The Airbag

So much for moving—what about stopping? Most modern cars are equipped with an airbag, which reacts to sudden impact by inflating. This protects the driver and front-seat passenger, who, even if they are wearing seatbelts, may otherwise be thrown against the steering wheel or dashboard.

In order to perform its function properly, the airbag must deploy within 40 milliseconds (0.04 seconds) of impact. Not only that, but it has to begin deflating before the body hits it. If a person's body, moving forward at speeds typical in an automobile accident, were to smash against a fully inflated airbag, it would feel like hitting concrete—with all the expected results.

The airbag's sensor contains a steel ball attached to a permanent magnet or a stiff spring. The spring or magnet holds the ball in place through minor mishaps when an airbag is not warranted—for instance, if a car were simply to be "tapped" by another in a parking lot. But in a case of sudden deceleration, the magnet or spring releases the ball, sending it down a smooth bore. The ball flips a switch, turning on an electrical circuit. This in turn ignites a pellet of sodium azide, which fills the bag with nitrogen gas.

At this point, the highly pressurized nitrogen gas molecules begin escaping through vents. Thus, as the driver's or rider's body hits the airbag, the deflation of the bag is moving it in the same direction that the body is moving—only much, much more slowly. Two seconds after impact, which is an eternity in terms of the processes involved, the pressure inside the bag has returned to 1 atm.

The chemistry of the airbag is particularly interesting. The bag releases inert, or non-reactive, nitrogen gas, which poses no hazard to human life; yet one of the chemical ingredients in the airbag is so lethal that some environmentalist groups have begun to raise concerns over its presence in airbags. This is sodium azide (NaN3), one of three compounds—along with potassium nitrate (KNO3) and silicon dioxide (SiO2)—present in an airbag prior to inflation.

The sodium azide and potassium nitrate react to one another, producing a burst of hot nitrogen gas in two back-to-back reactions. In the fractions of a second during which this occurs, the airbag becomes like a solid-rocket booster, experiencing a relatively slow detonation known as "deflagration."

The first reaction releases nitrogen gas, which fills the bag, while the second reaction leaves behind the by-products potassium oxide (K2O) and sodium oxide (Na2O). These combine with the silicon dioxide to produce a safe, stable compound known as alkaline silicate. The latter, similar to the sand used for making glass, is all that remains in the airbag after the nitrogen gas has escaped.

Where to Learn More

"Atmospheric Pressure: The Force Exerted by the Weight ofAir" (Web site). (April 7, 2001).

"Chemical Sciences Structure: Structure of Matter: Nature of Gases" University of Alberta Chemistry Department (Web site). (May 12, 2001).

"Chemistry Units: Gas Laws." (February 21, 2001).

"Homework: Science: Chemistry: Gases" Channelone.com (Web site). (May 12, 2001).

"Kinetic Theory of Gases: A Brief Review" University ofVirginia Department of Physics (Web site). (April 15, 2001).

Laws of Gases. New York: Arno Press, 1981.

Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998.

Mebane, Robert C. and Thomas R. Rybolt. Air and OtherGases. Illustrations by Anni Matsick. New York: Twenty-First Century Books, 1995.

"Tutorials—6." Chemistrycoach.com (Web site). (February 21, 2001).

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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Science and Technology Encyclopedia
Directory > Science > Science and Technology Encyclopedia Gas
A phase of matter characterized by relatively low density, high fluidity, and lack of rigidity. A gas expands readily to fill any containing vessel. Usually a small change of pressure or temperature produces a large change in the volume of the gas. The equation of state describes the relation between the pressure, volume, and temperature of the gas. In contrast to a crystal, the molecules in a gas have no long-range order.

At sufficiently high temperatures and sufficiently low pressures, all substances obey the ideal-gas, or perfect-gas, equation of state below, where p is the pressure, T is the absolute temperature, V is the molar volume, and R is the gas constant. Absolute temperature T is expressed on the Kelvin scale. The gas constant is 8.314 joules/(mole K). The molar volume is the molecular weight divided by the gas density.

At lower temperatures and higher pressures, the equation of state of a real gas deviates from that of a perfect gas. Various empirical relations have been proposed to explain the behavior of real gases.

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Thesaurus
Directory > Words > Thesaurus gas

noun

Incessant and usually inconsequential talk: babble, blab, blabber, chat, chatter, chitchat, jabber, palaver, prate, prattle, small talk. Slang gab, yak. See words.
Something or someone uproariously funny or absurd: absurdity. Informal hoot, joke, laugh, scream. Slang howl, panic, riot. Idioms: a laugh a minute. See laughter.
verb

To talk volubly, persistently, and usually inconsequentially: babble, blabber, chatter, chitchat, clack, jabber, palaver, prate, prattle, rattle (on), run on. Informal go on, spiel. Slang gab, jaw, yak. Idioms: run off at the mouth, shoot thebreezebull. See words.


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Idioms
Directory > Words > Idioms gas
In addition to the idiom beginning with gas, also see cook with gas; run out of steam (gas).

Other idioms beginning with gas:
gasket
gasp
gas up



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Antonyms
Directory > Reference > Antonyms gas
n
Definition: something not liquid or solid
Antonyms: liquid, solid


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Hacker Slang
Directory > Science > Hacker Slang gas

[as in ‘gas chamber’]

1. interj. A term of disgust and hatred, implying that gas should be dispensed in generous quantities, thereby exterminating the source of irritation. “Some loser just reloaded the system for no reason! Gas!”

2. interj. A suggestion that someone or something ought to be flushed out of mercy. “The system's getting wedged every few minutes. Gas!”

3. vt. To flush (sense 1). “You should gas that old crufty software.”

4. [IBM] n. Dead space in nonsequentially organized files that was occupied by data that has since been deleted; the compression operation that removes it is called degassing (by analogy, perhaps, with the use of the same term in vacuum technology).

5. [IBM] n. Empty space on a disk that has been clandestinely allocated against future need.


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Encyclopedia of Alternative Medicine
Directory > Health > Encyclopedia of Alternative Medicine Gas
Definition

Gas, or flatus, is produced when naturally occurring bacteria in the gastrointestinal tract begin to break down, or digest, food. When an excess of air builds up in the tract from swallowing air or a disorder that prevents digestion, it is released as gas. Gastrointestinal gases include methane, carbon dioxide, nitrogen, and hydrogen.

Description

Gas production is an essential, normal function of the gastrointesinal tract, and most healthy individuals pass up to 1,200 cc (over 40 oz) of gas each day. However, when gas causes excessive pain and cramping (colic) then evaluation and treatment are appropriate.

Causes & Symptoms

Gastrointestinal gas production can be increased by certain foods, illnesses, and some medications. Common causes of excessive gas include:

Gas-producing foods. Onions, beans, the cabbage family, and other fibrous foods can cause excessive gas or intestinal spasms in some individuals.
Gastrointestinal diseases and disorders. Increased flatulence is a defining symptom of irritable bowel syndrome, diverticulitis, lactose intolerance, malabsorption problems, dysbiosis (digestive problems), and other gastrointestinal disorders.
Air swallowing. Swallowing too much air while eating or chewing gum can introduce extra gas to the gastrointestinal tract.
Medications. Certain prescription and over-the-counter medications may cause gas as a side-effect.
Stress and food allergies can also cause gas.
Symptoms of excessive gas production include:

flatulence
belching, or burping
abdominal cramping, or colic
abdominal pain
Diagnosis

A thorough medical and dietary history and physical examination performed by a healthcare professional can usually identify the cause of gas pains resulting from changes to diet or medication. Gas problems triggered by gastrointestinal disease may be harder to diagnose, and will typically require additional medical testing such

COMMON REMEDIES FOR GAS
Remedy Description
Acupressure Press inward at the point three finger widths below the navel known as Conception Vessel 6.
Exercise Exercise after meals and regularly to increase digestion and expel gas.
Herbal medicine Anise water, peppermint or chamomile tea, and fennel may relieve gas.
Homeopathy Carbo vegetabilis is used to relieve gas. Nux vomica is used to treat gas that accompanies constipation. Chamomilla is used to treat gas in infants.
Diet Increase fiber intake. Do not mix carbohydrates with proteins at the same meal. Avoid beans, peas, cheese, sodas, and alcohol. Do not overeat. Chew food well and eat slowly.
Hydrotherapy Alternate a warm compress with a vigorous cold friction rub on the abdomen.
Yoga The Boat, Bow, Cobra, and Pigeon positions all encourage digestion and help relieve gas pain.

as colonoscopy, barium enema, or an upper and/or lower gastrointestinal (GI) series.

Treatment

For excessive gas caused by a particular food or beverage, adjustments to diet can relieve most symptoms. Gas caused by air swallowing can be alleviated by eating more slowly and avoiding gum chewing.

An herbalist or naturopathic healthcare professional may recommend a preparation of a carminative (gas reducing) herb such as valerian (Valeriana officinalis), or peppermint (Mentha piperita), which may be helpful in eliminating discomfort and gas-related bloating.

Homeopathic remedies for excessive intestinal gas include Carbo vegetabilis, Nux vomica, and Chamomilla. The prescription of a specific homeopathic remedy will depend on an individual's overall symptom picture, mood, and temperament, and should only be prescribed by a qualified homeopathic physician.

Hydrotherapy, acupressure, acupuncture, yoga, reflexology, and mild exercise can also help to relieve the pain and discomfort of excessive gas.

Allopathic Treatment

Over-the-counter preparations of the enzyme alpha-D-galactosidase (Beano) can alleviate gas symptoms caused by ingestion of certain foods in some individuals. These preparations are typically available in liquid or tablet form. Other non-prescription medications such as Gas-X, Phazyme, and Mylanta contain the ingredient simethicone, which can reduce gas bubbles within the gastrointestinal tract.

Expected Results

Mild excess gas is typically easy to treat, especially that triggered by dietary causes. Gas caused by gastrointestinal disease may be more difficult to manage, and successful treatment depends on the type and severity of the disorder.

Prevention

Avoiding fermented foods, drastic increases in fiber intake, and excessive air intake can prevent gas in some individuals. Lactose intolerant individuals should avoid dairy products.

Resources

Books

Hoffman, David. The Complete Illustrated Herbal. New York: Barnes & Noble Books, 1999.

Periodicals

Wu, Olivia. "Miss the Bloat: How to Avoid Bloating." Vegetarian Times (June 2000): 80.

Organizations

The National Institute of Diabetes & Digestive & Kidney Diseases (NIDDK). Office of Communications and Public Liaison. NIDDK, National Institutes of Health, 31 Center Drive, MSC 2560, Bethesda, MD 20892-2560. http://www.niddk.nih.gov/index.htm.

[Article by: Paula Ford-Martin]


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Britannica
Directory > Reference > Britannica Concise gas

One of the three fundamental states of matter, in which matter has no definite shape, is very fluid, and has a density about 0.1% that of liquids. Gas is very compressible but tends to expand indefinitely, and it fills any container. A small change in temperature or pressure produces a substantial change in its volume; these relationships are expressed as equations in the gas laws. The kinetic theory of gases, developed in the 19th century, describes gases as assemblages of tiny particles (atoms or molecules) in constant motion and contributed much to an understanding of their behaviour. The term gas can also mean gasoline, natural gas, or the anesthetic nitrous oxide. See also solid.

For more information on gas, visit Britannica.com.

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Encyclopedia
Directory > Reference > Encyclopedia gas, in physics, one of the three commonly recognized states of matter, the other two being solid and liquid. A substance in the gaseous state has neither definite shape nor definite volume. Like liquids, gases are fluids and assume the shape of their containers. Unlike liquids, they will expand to fill any container, regardless of its size. All gases condense into liquids or solids when sufficiently cooled or compressed (see compression; condensation; liquefaction). Most gases first liquefy, but some pass directly into the solid state (see sublimation); carbon dioxide, for example, can condense into dry ice. Some gases are extremely soluble in certain liquids, the liquid absorbing many times its own volume of gas. Some solids, by a process called adsorption, can take up many times their own volume of certain gases. The behavior of gases under various conditions of pressure, temperature, and volume is described by the various gas laws. Many of the properties of gases can be understood by considering the fact that only a small part of the volume of a gas is occupied by its atoms or molecules, which are in rapid, random motion. See kinetic-molecular theory of gases.

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Legal Encyclopedia
Directory > Legal > Legal Encyclopedia This entry contains information applicable to United States law only.
Gas

Various legal issues arise concerning the use and distribution of gas.

Supply

A municipal corporation does not have the duty to supply gas to its population. In the event that a city assumes the performance of such function, it is acting merely as a business corporation.

The charter of a gas company is a franchise granted by the state. The manufacture of distribution of gas for light, fuel, or power is a business of a public character, and, therefore, a gas company is ordinarily considered to be a public or quasi-public corporation or a business affected with a public interest. A state may regulate gas companies for the protection of the public and may delegate its regulatory powers to municipal corporations in which gas companies operate. In a number of states, gas companies are subject to a public service commission or other such agency. The jurisdiction of the commission ordinarily includes the power to establish rates and to set forth rules and regulations affecting the service, operation, management, and conduct of the business.

Consumer Supply

Upon obtaining a franchise to supply gas to a particular geographic area, a gas company is bound to fulfill its obligation; it cannot withdraw its service from an area merely because it is dissatisfied with the rates permitted there. Once the franchise of a company has expired, it may withdraw the service. A court may, in certain instances, enjoin the discontinuance of service for a reasonable period—to circumvent undue hardship and inconvenience to the residents of the area.

A gas company has the duty to serve all those who are within the franchise area who desire service and subscribe to the reasonable rules that it may set forth. A municipality or corporation supplying gas may make reasonable rules and regulations to secure the payment of bills, such as eliminating service to the consumer. If there is a genuine controversy about the amount owed, a company is not permitted to discontinue service. A gas company may not require the owner or occupant of a building to pay overdue and unpaid bills by a former owner or occupant before it continues service to the building. Some statutes require that gas companies install a meter on the premises, in order to register the consumption of gas by each customer; and where a customer tampers with the meter and uses a significant amount of unmetered gas, the company can discontinue service and refuse to restore it until the customer pays the amount due for the unmetered gas taken.

A gas company that wrongfully refuses to supply a customer with gas is liable for damages. There are also statutory penalties in some states for such wrongful refusal.

Injuries

A gas company is under the obligation to exercise ordinary care in the construction of its works and the conduct of its business in order to protect life and property.

Gas has a highly dangerous and volatile character and tends to escape. A gas company must, therefore, exercise care to avoid harm to others and is liable for its negligence that results in injury to others by reason of the escape or explosion of gas. It must exercise reasonable care in the inspection of its pipes to ensure that leaks may be discovered promptly; and if leaks or defects in the pipes of the company occur due to faulty construction or maintenance, the company is liable for resulting injuries, even though it did not know about the leak.

In the event that the company has taken due care in the inspection of its pipes and a defect or a break occurs through natural causes or by the act of a third person, the gas company must be given notice of the defect and reasonable time to repair it before liability accrues. A gas company subject to notice that gas is escaping is under an obligation to shut off the gas supply until the necessary repairs have been made.

A gas company has a property right in the mains and pipes and other appliances, and where there is unauthorized interference with, or damage to, this property, the company is entitled to recover damages and an injunction if the circumstances so warrant.

Rates

A gas company has a legal obligation to charge reasonable rates. One of the main purposes of the regulation of gas companies is to prescribe fair and reasonable rates for the selling of gas to the public. Rate increases are permitted only following an impartial and complete investigation—with the object of doing justice to the gas company as well as the public. Relief can be sought in the courts if gas rates are unreasonable—to determine whether the rate making body acted beyond the scope of its power or against the weight of the evidence. The courts, however, cannot decide what rates are reasonable, nor can they put those rates into effect.

See: public utilities.




Science
Directory > Science > Science gas

In physics, one of the phases of matter. The atoms or molecules in gases are more widely spaced than in solids or liquids and suffer only occasional collisions with one another.



Medical
Directory > Health > Medical Dictionary gas (găs)
n., pl. gas·es or gas·ses.
The state of matter distinguished from the solid and liquid states by relatively low density and viscosity, relatively great expansion and contraction with changes in pressure and temperature, the ability to diffuse readily, and the spontaneous tendency to become distributed uniformly throughout any container.
A substance in the gaseous state.
A gaseous fuel, such as natural gas.
Gasoline.
A gaseous asphyxiant, an irritant, or a poison.
A gaseous anesthetic, such as nitrous oxide.
Flatulence.
Flatus.
v., gassed, gas·sing, gas·es or gas·ses.
To treat chemically with gas.
To overcome, disable, or kill with poisonous fumes.
To give off gas.



Electronics
Directory > Science > Electronics gas

Any aeriform or completely elastic fluid which is not a solid or a liquid. Gasses are produced by heating a liquid beyond its boiling point.




Word Tutor
Directory > Words > Word Tutor gas

IN BRIEF: A fluid such as hydrogen or air that has no fixed shape and tends to expand without limit.

No steam or gas drives anything until it is confined. No life ever grows great until it is focused, dedicated, disciplined. — Harry Fosdick, (1878-1979), American clergyman.



WordNet
Directory > Words > WordNet Note: click on a word meaning below to see its connections and related words.
The noun gas has 6 meanings:

Meaning #1: the state of matter distinguished from the solid and liquid states by: relatively low density and viscosity; relatively great expansion and contraction with changes in pressure and temperature; the ability to diffuse readily; and the spontaneous tendency to become distributed uniformly throughout any container


Meaning #2: a fluid in the gaseous state having neither independent shape nor volume and being able to expand indefinitely


Meaning #3: a volatile flammable mixture of hydrocarbons (hexane and heptane and octane etc.) derived from petroleum; used mainly as a fuel in internal-combustion engines
Synonyms: gasoline, gasolene, petrol


Meaning #4: a state of excessive gas in the alimentary canal
Synonyms: flatulence, flatulency


Meaning #5: a pedal that controls the throttle valve
Synonyms: accelerator, accelerator pedal, gas pedal, throttle, gun


Meaning #6: a fossil fuel in the gaseous state; used for cooking and heating homes
Synonym: natural gas



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The verb gas has 2 meanings:

Meaning #1: attack with gas; subject to gas fumes


Meaning #2: show off
Synonyms: boast, tout, swash, shoot a line, brag, blow, bluster, vaunt, gasconade


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Wikipedia
Directory > Reference > Wikipedia gas

A gas is one of the four main phases of matter (after solid and liquid, and followed by plasma), that subsequently appear as a solid material is subjected to increasingly higher temperatures. Thus, as energy in the form of heat is added, a solid (e.g. ice) will first melt to become a liquid (e.g. water), which will then boil or evaporate to become a gas (e.g. water vapor). In some circumstances, a solid (e.g. "dry ice") can directly turn into a gas: this is called sublimation. If the gas is further heated, its atoms or molecules can become (wholly or partially) ionized, turning the gas into a plasma.


Physics
In the gas phase, the atoms or molecules constituting the matter basically move independently, with no forces keeping them together or pushing them apart. Their only interactions are rare and random collisions. The particles move in random directions, at tter (after plasma). Because of this high kinetic energy, gas atoms and molecules tend to bounce off of any containing surface and off one another, the more powerfully as the kinetic energy is increased. A common misconception is that the collisions of the molecules with each other is essential to explain gas pressure, but in fact their random velocities are sufficient to define that quantity. Mutual collisions are important only for establishing the Maxwell-Boltzmann distribution.

Gas particles are normally well separated, as opposed to liquid particles, which are in contact. A material particle (say a dust mote) in a gas moves in Brownian Motion. Since it is at the limit of (or beyond) current technology to observe individual gas particles (atoms or molecules), only theoretical calculations give suggestions as to how they move, but their motion is different from Brownian Motion. The reason is that Brownian Motion involves a smooth drag due to the frictional force of many gas molecules, punctuated by violent collisions of an individual (or several) gas molecule(s) with the particle. The particle (generally consisting of millions or billions of atoms) thus moves in a jagged course, yet not so jagged as we would expect to find if we could examine an individual gas molecule.


Some types of gases
Ideal gas, in physics
Various hydrocarbon gases (organic gases) used for heating, lighting, and energy transmission:
Natural gas
Liquefied Petroleum Gas (LPG), including propane and butane
Syngas: various synthetic fuel gases: names include coal gas, water gas, illuminating gas, wood gas, producer gas, holzgas, air gas, blue gas, manufactured gas, town gas, hygas
Gas (chemical warfare), various poison gases used in warfare
Inhalational anaesthetic, including laughing gas (= nitrous oxide)
Trace gas
Toxic gases

Etymology
The word "gas" was invented by Jan Baptist van Helmont as a phonetic spelling of the Dutch pronunciation of the Greek word "chaos".


See also
Cooling curve
Gas (chemical warfare), various poison gases used in warfare
Gas chamber
Gas laws
Gas metal arc welding
Ideal gas, in physics
Kinetic theory of gases
Liquefied Petroleum Gas, including propane and butane
List of phases of matter
Natural gas
Category:Pollutants
Vapor
Flatulence is the presence of gas under some degree of pressure, in a confined space. The term is normally used of the presence of gas in the digestive tract of mammals, usually leaving a distinct odor. The non-odorous gases are mainly nitrogen (ingested), carbon dioxide (produced by aerobic microbes or ingested), and hydrogen (produced by some microbes and consumed by others), as well as lesser amounts of oxygen (ingested) and methane (produced by anaerobic microbes)[1]. Odors result from trace amounts of other components (often containing sulphur, see below).

Contents [hide]
1 Composition of flatus gases
2 Causes
3 Mechanism of action
4 Remedies
4.1 Dietary
4.2 Pharmacological
5 Health effects
5.1 Recording flatulence events
6 Environmental impact
7 Social context
8 Literature and the arts
9 Curiosities
10 See also
11 References
11.1 Nontechnical resources
12 External links



[edit] Composition of flatus gases
Nitrogen is the primary gas released. Methane and hydrogen, lesser components, are flammable, and so flatus is susceptible to catching fire. Not all humans produce flatus that contains methane. For example, in one study of the feces of nine adults, only five of the samples contained bacteria capable of producing methane[2]. Similar results are found in samples of gas obtained from within the rectum. The gas released during a flatus event frequently has a foul odor which mainly results from low molecular weight fatty acids such as butyric acid (rancid butter smell) and reduced sulfur compounds such as hydrogen sulfide (rotten egg smell) and carbonyl sulfide that are the result of protein breakdown. The incidence of odoriferous compounds in flatus increases from herbivores, such as cattle, to omnivores to carnivorous species, such as cats. Flatulence odor can also occur when there is a number of bacteria and/or feces in the anus while being expelled. A small amount of solid or liquid fecal matter in fine particulate aerosol form may also be expelled, and included, along with flatulence.


[edit] Causes
Intestinal gas is composed of 90% exogenous sources (air that is ingested through the nose and mouth) and 10% endogenous sources (gas produced within the digestive tract). The exogenous gases are swallowed (aerophagia) when eating or drinking or during times of excessive salivation (as might occur when nauseated or as the result of gastroesophageal reflux disease). The endogenous gases are produced as a by-product of digesting certain types of food.

In the case of those with lactose intolerance, intestinal bacteria feeding on lactose can give rise to excessive gas production when milk or lactose-containing substances have been consumed.

Interest in the causes of flatulence was spurred by high-altitude flight and the space program; the low atmospheric pressure, confined conditions, and stresses peculiar to those endeavours were cause for concern[3].


[edit] Mechanism of action
The noises commonly associated with flatulence are caused by the vibration of the anus. The sound varies depending on the tightness of the sphincter muscle and velocity of the gas being propelled, as well as other factors such as water and body fat. The pitch of the flatulence outburst can also be affected by the anal embouchure. Among humans, sometimes farting happens accidentally, such as incidentally to coughing or sneezing; on other occasions, intentional farting occurs through the tensing and releasing of the anal sphincter.

Flatus is brought to the rectum in the same peristalsis method as feces, causing a similar feeling of urgency and discomfort. Nerve endings in the rectum learn to distinguish between flatus and feces, although loose stool can confuse these nerves, and sometimes results in accidental defecation, colloquially known as "sharting", or "following through".


[edit] Remedies
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You can help Wikipedia by introducing appropriate citations. This article has been tagged since June 2006.

[edit] Dietary
Certain spices counteract the production of intestinal gas, most notably cumin, caraway and the closely related ajwain, turmeric, asafoetida (hing), epazote, and kombu kelp (a Japanese seaweed). Many people report that by reducing intake of most refined carbohydrates (such as rice, pasta, potatoes and bread), the amount of flatulence may decrease significantly. The water-soluble oligosaccharides in beans that contribute to production of intestinal gas can be reduced through a regime of brief boiling followed by a long period of soaking, but at a cost of also leaching out other water-soluble nutrients. Also, gas can be reduced by fermenting the beans, and making them less gas-inducing, by cooking them in the liquor from a previous batch. Lactobacillus casei and Lactobacillus plantarum have recently been proven responsible for this effect.[4] Some legumes also stand up to prolonged cooking, which can help break down the oligosaccharides into simple sugars. Fermentation also breaks down oligosaccharides, which is why fermented bean products such as miso and tofu are less likely to produce as much intestinal gas.

Probiotics (yogurt, kefir, etc.) often reduce flatulence when they are used to restore balance to the normal intestinal flora. Prebiotics, which generally are non-digestible oligosaccharides, such as Fructooligosaccharide, generally increase flatulence in a similar way as described for lactose intolerance.

Medicinal activated charcoal tablets have also been reported as effective in reducing both odor and quantity of flatus when taken immediately before food that is likely to cause flatulence later.


[edit] Pharmacological
Digestive enzyme supplements can significantly reduce the amount of flatulence that is caused by some components of foods not being digested by the body and feeding the microbes in the small and large intestines. It has been shown that alpha-galactosidase enzymes, which can digest complex sugars, are effective in reducing the volume and frequency of flatus[5]. The enzymes alpha-galactosidase (brands Beano, Bean-zyme), lactase (brand Lactaid), amylase, lipase, protease, cellulase, glucoamylase, invertase, malt diastase, pectinase, and bromelain are available, either individually or in combination blends, in commercial products.

The antibiotic rifaximin, often used to treat diarrhea caused by the microorganism E. coli, has been shown to reduce both the production of intestinal gas and the frequency of flatus events[6].

While not affecting the production of the gases themselves, surfactants (agents which lower surface tension) can reduce the disagreeable sensations associated with flatulence, by aiding the dissolution of the gases into liquid and solid fecal matter.

Often it is helpful to ingest small quantities of acidic liquids with meals, such as lemon juice or vinegar, to stimulate the production of hydrochloric acid, which in turn increases enzyme production. This facilitates digestion and may limit gas production.


[edit] Health effects
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You can help Wikipedia by introducing appropriate citations. This article has been tagged since June 2006.
As a normal body function, the action of flatulence is an important signal of normal bowel activity and hence is often documented by nursing staff following surgical or other treatment of patients. However, symptoms of excessive flatulence can indicate the presence of irritable bowel syndrome or some other organic disease. In particular, the sudden occurrence of excessive flatulence together with the onset of new symptoms provide reason for seeking further medical examination.

There is no particular harm to come from holding in flatus. Flatulence is not poisonous; it is a natural component of various intestinal contents. However, discomfort may develop from the build-up of gas pressure. In theory, pathological distension of the bowel, leading to constipation, could result if a person holds in flatus.

Not all flatus is released from the body via the anus. When the partial pressure of any gas component of the intestinal lumen is higher than its partial pressure in the blood, that component enters into the bloodstream of the intestinal wall by the process of diffusion. As the blood passes through the lungs this gas can diffuse back out of the blood and be exhaled. If a person holds in flatus during daytime, it will often be released during sleep when the body is relaxed. Some flatus can become trapped within the feces during its compaction and will exit the body, still contained within the fecal matter, during the process of defecation.


[edit] Recording flatulence events
The term meteorism is defined as the presence of gas within the abdomen or intestines. However, it is sometimes also used to describe the condition of excessive flatulence. Since subjective judgments vary considerably about what constitutes normal and elevated levels of flatulence, medical personnel sometimes instruct a patient complaining about excessive flatulence to maintain a personal flatulence diary. Researchers studying flatulence have also developed what is called a flatulogram. Its horizontal axis represents time (typically 24 hours, with each hour being marked on the time line). The subject is instructed to make a pencil mark on this line at each point in time that they notice flatus passing through the anus. The acoustical volume of the event is indicated by the vertical distance that the pencil mark rises above the time line. Inaudible events are indicated by a short mark that extends only below the time line.


[edit] Environmental impact
Livestock are a significant contributing factor to the greenhouse effect, accounting for around 20% of global methane emissions[7]. Less than 10% of the total greenhouse gas emissions from livestock is produced by animal flatulence; most is produced by animal burping. Livestock in New Zealand account for 60% of the country's greenhouse gas emissions[8]. Livestock in Australia contribute approximately 14% of that country's greenhouse gas emissions [9].


[edit] Social context
In many cultures, excessive human flatulence is regarded as embarrassing and repulsive, even to the point of being a taboo subject. People will often strain to hold in the passing of gas when in polite company, or position themselves to conceal the noise and smell.

Flatulence is a potential source of humor, either due to the foul smell or the sounds produced. Some find humour in flatulence ignition, which is possible due to the presence of flammable gases such as hydrogen and methane, though the process can result in burn injuries to the rectum and anus.

The History of Farting, by Benjamin Bart, is a collection of assorted limericks, facts, and blurbs on farting, while Who Cut the Cheese: A Cultural History of the Fart, by Jim Dawson, gives a more complete cultural discussion of the historical and social significance of farting.


[edit] Literature and the arts
In St. Augustine's The City of God, Augustine makes note of men who "have such command of their bowels, that they can break wind continuously at will, so as to produce the effect of singing." (The City of God Against the Pagans, ed and trans Philip Levine (Cambridge, MA: Harvard University Press, 1966), XIV.24)
In Dante's Divine Comedy, the last line of Inferno Chapter XXI reads: ed elli avea del cul fatto trombetta ("and he made a trumpet of his ***"), in the last example the use of this natural body function underlined a demoniac condition.
In Chaucer's Miller's Tale (one of the Canterbury Tales), the character Nicholas hangs his buttocks out of a window and farts in the face of his rival Absolom. Absolom then sears Nicholas's bum with a red-hot poker ("Nicholas quickly raised the window and thrust his *** far out...At this Nicholas let fly a fart with a noise as great as a clap of thunder, so that Absolom was almost overcome by the force of it. But he was ready with his hot iron and smote Nicholas in the middle of his ***."). (Lines 690–707)
In the translated version of Penguin's 1001 Arabian Nights Tales, a story entitled "The Historic Fart" tells of a man that flees his country from the sheer embarrassment of farting at his wedding.
Friedrich Dedekind's 16th century work, Grobianus et Grobiana, appeared in England in 1605 as The Schoole of Slovenrie: Or, Cato turnd wrong side outward, published by one "R.F.". The "Schoole" taught its students that holding back the desire to urinate, fart, and vomit was bad for one's health; thus, one has to indulge freely in all three activities.
François Rabelais' tales of Gargantua and Pantagruel are laden with acts of flatulence. In Chapter XXVII of the second book, the giant, Pantagruel, releases a fart that "made the earth shake for twenty-nine miles around, and the foul air he blew out created more than fifty-three thousand tiny men, dwarves and creatures of weird shapes, and then he emitted a fat wet fart that turned into just as many tiny stooping women."[10]
Montaigne, in his essay "Of the Force of Imagination", includes a discussion of flatulence. Of "the vessels that serve to discharge the belly", he writes "I myself knew one so rude and ungoverned, as for forty years together made his master vent with one continued and unintermitted outbursting, and 'tis like will do so till he die of it"[11].
Benjamin Franklin, in his open letter "To the Royal Academy of *****", goes on at length in ye olde eloquent style that is his wont about flatulence. He satirically proposes that converting farts into a more agreeable form through science should be a milestone goal of the Royal Academy. http://teachingamericanhistory.org/library/index.asp?document=470
In Mark Twain's 1601, properly named [ Date: 1601.] Conversation, as it was the Social Fireside, in the Time of the Tudors, a cupbearer at Court who's a Diarist reports:
In ye heat of ye talk it befel yt one did breake wind, yielding an exceding mightie and distresfull stink, whereat all did laugh full sore.
The Queen inquires as to the source, and receives various replies. Lady Alice says
"Good your grace, an' I had room for such a thundergust within mine ancient bowels, 'tis not in reason I coulde discharge ye same and live to thank God for yt He did choose handmaid so humble whereby to shew his power. Nay, 'tis not I yt have broughte forth this rich o'ermastering fog, this fragrant gloom, so pray you seeke ye further."[12].
In Emile Zola's La Terre (the 15th volume of the series Les Rougon-Macquart), the eldest Fouan son can fart at will and keeps winning free drinks by betting on his skill.
In James Joyce's Ulysses, the main character Leopold Bloom breaks wind in the "Sirens" chapter of the book.[13]
The Gas We Pass is a popular children's book in the United States about flatulence.
In the cinema, farting has been featured in films intended for adult audiences such as Blazing Saddles. However, this caused some controversy in the United States: when it was run as a movie of the week by ABC the farting sounds were overdubbed with sounds from the surrounding horses, so the scene had cowboys sitting around a campfire standing up and leaning over for no apparent reason (Dawson, 1999, p. 125).
The film Wet Hot American Summer features a boy lighting a fart as an act in a talent show. Additionally, the film's DVD features an optional "fart track" that adds fart noises to the film's audio.
An episode of MythBusters featured myths about flatulence and determined the chemical composition of a typical flatus.
"I fart in your general direction" is a popular phrase from Monty Python and the Holy Grail.
The lighting of flatulence is a plot device in the movie, South Park: Bigger, Longer & Uncut and flatulence in general is featured prominently in many South Park episodes, noteably "Spontaneous Combustion".
Brent Spiner's character in the movie The Master of Disguise suffered from uncontrolled flatulence any time he broke into a fit of evil laughter.

[edit] Curiosities
According to "The Great Fart Survey" [4], 34% of those who participated like the smell of their own farts.
Le Petomane "the Fartiste" a famous French performer in the nineteenth century as well as many professional farters before him did flatulence impressions and held shows. Mel Brooks named his fictional governor (played by himself) William J. LePetomaine in the Western spoof film Blazing Saddles.
An apocryphal story about Edward de Vere, Earl of Oxford is that he farted while swearing loyalty to Queen Elizabeth I and consequently went into self-imposed exile for seven years. After his return, the Queen was reported to have reassured de Vere: "My Lord, I had quite forgotten the fart." (John Aubrey, Brief Lives)
Emperor Claudius passed a law legalizing farting at banquets out of concern for people's health. There was a widespread misconception that a person could be poisoned by retaining flatus.
In August 2005, New Scientist magazine reported that inventors Michael Zanakis and Philip Femano had been awarded a US patent (U.S. Patent 6,055,910) for a "toy gas-fired missile and launcher assembly". The abstract of the patent makes it clear that this is, in fact, a fart-powered rocket:
"A ... missile is composed of a soft head and a tail extending therefrom formed by a piston. The piston is telescoped into the barrel of a launcher having a closed end on which is mounted an electrically activated igniter, the air space between the end of the piston and the closed end of the barrel defining a combustion chamber. Joined to the barrel, and communicating with the chamber therein, is a gas intake tube having a normally closed inlet valve. To operate the assembly, the operator places the inlet tube with its valve open adjacent [to] his anal region, from which a colonic gas is discharged. The piston is then withdrawn to a degree producing a negative pressure to inhale the gas into the combustion chamber to intermix with the air therein to create a combustible mixture. The igniter is then activated to explode the mixture in the chamber and fire the missile into space."
British inventors have also patented fart-related ideas, such as "A fart collecting device," which includes a drawing of the invention deployed and ready for action, with helpful numbers to identify the various components. "It comprises a gas-tight collecting tube 10 for insertion into the rectum of the subject. The tube 10 is connected to a gas-tight collecting bag (not shown). The end of the tube inserted into the subject is apertured and covered with a gauze filter and a gas permeable bladder 28."
Mambo Graphics, an Australian surfwear label, features the iconic "Farting Dog" design [5] in its lineup. Here the flatulence is depicted as a musical note emanating from the dog's backside.

[edit] See also
Professional farter
Borborygmus
Vaginal flatulence
The Gas We Pass
Fart lighting

[edit] References
Wikisource has original text related to this article:
A cure for flatulence from 1872^ Suarez F, Furne J, Springfield J, Levitt M (1997). "Insights into human colonic physiology obtained from the study of flatus composition". Am J Physiol 272(5 Pt 1): G1028–33.
^ Miller TL, Wolin MJ, de Macario EC, Macario AJ (1982). "Isolation of Methanobrevibacter smithii from human feces". Appl Environ Microbiol 43(1): 227–232.
^
^ [1]
^ Ganiats TG, Norcross WA, Halverson AL, Burford PA, Palinkas LA (1994). "Does Beano prevent gas? A double-blind crossover study of oral alpha-galactosidase to treat dietary oligosaccharide intolerance". J Fam Pract 39: 441–445.
^ Di Stefano M, Strocchi A, Malservisi S, Veneto G, Ferrieri A, Corazza GR (2000). "Non-absorbable antibiotics for managing intestinal gas production and gas-related symptoms". Aliment Pharmacol Ther 14: 1001–1008.
^ Nowak, Rachel (September 24, 2004). Burp vaccine cuts greenhouse gas. New Scientist.
^ Fickling, David. "Farmers raise stink over New Zealand 'fart tax'", Guardian Unlimited, September 5, 2003.
^ Marks, Kathy (June 9, 2002). Australian Scientists Looking to Kangaroos to Reduce Greenhouse Gas of Livestock. The Independent (London).
^ François Rabelais, Gargantua and Pantagruel. W.W. Norton & Co. 1990, p.214
^ Michel de Montaigne [1877] (2004-11-01). “Of the Force of Imagination”, William Carew Hazilitt (ed.): The Essays of Montaigne, Volume 3, trans. Charles Cotton, Project Gutenberg.
^ [2]
^ [3]




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