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Chemistry LibreTexts

Chromatograms

The time to elute an analyte is a function of how long the analyte is retained on the column, therefore the output of IC is a graph of conductivity as a function of time, called a chromatogram. Based on the previous discussion of elution, you may expect a chromatogram to look something like:

However, we need to consider another process that impacts shape of the elution peaks. As an analyte flows through the column, some of the analyte molecules may pass by through the length of the stationary phase faster or slower than would be expected due to diffusion processes or the formation of channels. An example of this can be seen in the figure with the path of each ion shown in the figure.

If the two ions are traveling at the same speed, set by the flow of the eluent, what can you say about when they will emerge? Will they emerge at the same time?

From the figure, you should be able to see that the path in red is much shorter than the path in blue. Since the path is shorter, and the ions are traveling at the same speed, the ion following the red path will emerge first. Thus a normal chromatogram peak will have a gaussian distribution, symmetric around the mean, as seen in the figure on the right. You can also peruse a more extensive mathematical modeling of the chromatogram peak here (link takes you to a different website http://www.chem.uoa.gr/applets/AppletChrom/Appl_Chrom2.html ).

Since we are concerned with the concentration of ions present in the solution, how will the chromatogram change as you increase the amount of analyte loaded onto the column?

After considering the prior question: Since we are concerned with the concentration of ions present in the solution, how will the chromatogram change as you increase the amount of analyte loaded onto the column?

The chromatogram peak will increase in height and concamitantly in area. Therefore when quantifying data, the peak height or area is used. In order to determine actual concentrations, a series of standards must be analyzed to calibrate the response between peak area and actual concentration for each ion.

An example chromatogram of Poland Springs bottled water. Each separate peak is due to a different cation.

The area under each peak is used to calculate the concentration of each ion. What information would you need in order to determine the relationship between peak area and concentration?

Just like any other quantitative method, you need to calibrate the response, in this case by analyzing a series of known standards and plotting a calibration curve.

If you notice, the last peak is not completely guassian, there are other factors that do have an impact on peak shapes. For more in-depth information on peak shape, see the following material.