Ion Chromatography- Calibration

Ion chromatography is a method of chromatography that identifies components of a solution based on the difference in ion exchange characteristics between species. It is important to calibrate the ion chromatograph before running unknown samples. From the calibration procedure a linear relationship can be determined between the ion chromatograph’s output and the concentration of the relevant ion. It is only necessary to calibrate the ion chromatograph once because it will give stable responses on different days. According to the study performed by Stover and Brill, using a single calibration curve over several months rather than forming a new calibration curve daily can give equal or better precision.

There are two main methods for creating a calibration curve for ion chromatography, the absolute calibration curve method and the internal standard method. The internal standard method uses output from standards with a constant amount of known internal standard and graded standard component. The ratio of peak heights is used to create a calibration curve. For this project, the absolute calibration curve methodwas performed in which standards only with the component of interest are made; no internal standard is used (see reference 2). Six standards of chloride ion and iodide ion were prepared by dilution, all of different concentrations. An ion chromatography vial was rinsed 3 times with each standard and then filled with about 5 mL of that same standard. The concentrations of ions need to be relatively low (ppm magnitude) and the concentrations of the standards need to encompass the possible concentrations for the unknown.

The peak area and concentration of each ion can be used to construct a calibration curve for each ion. The concentration is the independent variable and is plotted on the x-axis. The peak area is the dependent variable and is plotted on the y-axis. The plot below is a calibration curve created for chloride ion concentration using the absolute calibration method. From the data a linear-least squares analysis can be performed to create an equation giving a relationship between the concentration of ion and the corresponding peak area. The linear relationship should be adjusted to pass through the origin (0 ion concentration corresponds to 0 peak area in the output data). The linear relationship can then be used to identify unknown concentrations of ion by putting the peak area in the calibration curve equation and solving for the concentration.

References

1. Stover, F.; Brill, R. Statistical quality control applied to ion chromatography calibrations.     Journal of Chromatography A. [Online] 1998, 804, 37-43.     http://www.sciencedirect.com/science/article/pii/S0021967398000946
(Accessed Nov 22, 2011)

2. General Tests Ion Chromatography.     http://www.ffcr.or.jp/zaidan/FFCRHOME.nsf/7bd44c20b0dc562649256502001b65e9/146fd852cd5e269049256f32001a133e/$FILE/B18.pdf (Accessed Nov 22, 2011)

3. Lopez-Ruiz, B. Advances in the determination of inorganic anions by ion
Chromatography. Journal of Chromatography A. 2000, 881, 607-627.

Content on this web page authored by Brandon Smith, Davidson Sacolick, Roxanne Demarest & Bethany Wellen