Lesson 7: Ion Chromatography

Suppressed Ion Chromatography

 

Two ion chromatography methods are well known: the suppressed method and the non-suppressed method. The suppressed method is suitable for anion analysis. Sodium carbonate aqueous solution is used as the eluent of Shodex columns for the suppressed method. The suppressor removes sodium ions in the eluent and changes sodium carbonate to carbonic acid with lower conductivity, allowing the background levels to decrease. In addition, the suppressor changes sodium ions (cations), to hydrogen ions with higher conductivity. Therefore the peak response increases and highly sensitive analysis of anions is possible.

Merit of suppressed method.
1. Background conductivity is low.
(anion analysis: about 20μS/cmFS)
2. Sensitivity is high. Detection sensitivity is several ppb and minimum sample concentration is about tens ppb.
3. Free from temperature fluctuation and stable analysis is possible.

Demerit of suppressed method.
The use of a suppressor is essential and the system is rather expensive.

 

It is well known that ion analysis is used with conductivity detector. There are two methods for conductivity detectors: the suppressed conductivity method and the non-suppressed conductivity method. The Non-suppressed conductivity method can be easily performed by adding a conductivity detector and IC columns to any typical HPLC system.

Merit of non-suppressed method.
Since the use of a suppressor is unnecessary, the system is simple and inexpensive.

Demerit of non-suppressed method.
1. Background conductivity is rather high.
(anion analysis: 200 to 250μS/cmFS, cation analysis: 1,000 to 2,500μS/cmFS)
2. Sensitivity is rather low. Detection sensitivity is about tens ppb and minimum sample concentration is about hundreds ppb.
3. The usable eluent is limited to low-conductivity solvents such as phthalic acid.

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