helpe me tounderstand better GC?

Question

i work in a aromatic co. i want to know more about aromatics, GC and aromatic plates

Answer 1

You're asking some pretty bold, pretty broad questions....

GC (gas chromatography) operates on the principle of a carrier gas transporting a sample through a column that is coated with a material (solid or liquid-on-solid) with which the sample can interact. Ideally, this interaction involves weak attraction or affinity that permits an equilibrium-driven series of attachments and detachments of the compound to/from the column (substrate) surface. The extent to which this cyclic attachment and release happens determines how fast a given compound migrates through the column via the carrier gas. The more interaction between a target molecule and the substrate, the slower it travels thru the column.

When you have a mixture of compounds in a sample that interact with the column to different extents, you can then use this technique to effect the separation of those compounds. Since a given compound has a specific, often predicable behavior, in a specific column, we can know details about the extent of interaction. Specifically, we can study, and then predict, how fast a given column will travel through that specific column under a given set of conditions. This gives rise to what we call "RETENTION TIME". One can use the retention time and the peak characteristics of a given compound to qualitatively identify compounds, and in many cases, quantify the amount present. (A more elaborate lecture for later...)

As to aromatics, I could give you a nice lecture (as I am about to do tonight) on what aromatics are.... compounds with conjugated ring systems like benzene or benzenoid systems. However, that's usually 2 or 3 chapters out of an Organic text, which we don't have time or space for here. Pick up one and give it a read...or contact me for further help.

Hope this gets you off the ground. Good luck.

Answer 2

Great first answer, I totally concur.

Let me add regarding the plates. This comes from industrial fractional distillation. These distillation columns are not unlike a GC column. A hot tube that volatile substances pass though. The difference is that the big columns are partitioned off into separates compartments by plates with holes in them.

With heating, the vapors pass through the holes and condense on the plate above. With more heat the liquid is vaporized and passes up to the next higher plate. Each plate represents one transition from liquid to gas phase for the substance within that system.

A similar situation occurs in a CG column. The substance is partitioning back and forth between the walls of the column and the mobile gas phase. The faster it does that – the faster it runs thought the column.

Substances that partition really fast between the walls of the column and the mobile phase, spend almost all their time in the mobile phase and no time sticking to the walls of the column. We call these substances un-retained. Now thinking back to the industrial fractionating column, wouldn’t that be like a column with only a single plate? The stuff vaporizes once and it is out of the column.

Likewise a substance that takes a long time to move through the column, spends most of its time stuck to the wall and very little time in the mobile phase. Wouldn’t this situation be similar to an industrial fractionating column with a whole bunch of plates?

Chromatography columns are often rated by a concept called HPTP, Height Per Theoretical Plate for a specific compound. The bigger the number the more retained that compound is in that column.

Best of luck on the new job.