How can I determine concentrations using a standard curve?

Question

For example, if you graph a standard curve from the following data:
Concentration of Solution (μg/ml)
1.0
2.0
3.0
5.0
6.0
7.0
Absorbance
0.17
0.48
0.75
1.37
1.69
1.96

What would be the absorbance of a sample whose concentration was 5.4 μg/ml. And how do you determine the answer?

Answer 1

Now that you have the points, graph Absorbance on the Y-axis, and Concentration on the X. you should get a roughly straight line. form the best fit line. then find the point 5.4 on the x-axis, draw a vertical line up to your standard curve (the best fit line), then draw the horizontal line across to the y-axis. The y-value when you do this should be the absorbance at 5.4 micrograms per milliliter.

If you find the slope of the best fit line, you can simple plug in the x value of 5.4 and spit out the y-value you get from solving the equation.

Answer 2

Hi,

As your first answer stated, their is a good correlation for a linear equation to predict the absorbance rates. A quartic equation has a higher correlation that the linear, but both are above an r value of .99.

However, you asked for concentrations to be based on a standard curve. If I worked out the 1-variable stats for the concentrations above, I get a mean of 4 and a standard deviation of 2.16. Based on these as my standard, the z value of a concentration of 5.4 μg/ml would be (5.4 - 4)/2.16 = .648. this indicates that the concentration is .648 standard deviations above the mean, so I would expect that the Absorbance would also be .648 standard deviation units above the mean of the absorbances, based on the absorbance standard deviation.

For the 1-variable stats on absorbances, the mean was 1.07 and the standard deviation was .649. If our concentration of 5.4 μg/ml was to be .648 standard deviation units above the mean of the absorbances, then it is .648 * .649 or .420552 above 1.07 at 1.490552.

For whatever it's worth, the linear equation also gives an absorbance of 1.4905

I hope this helps!! :-)