3.4 Ion chromatography

Introduction

Ion Chromatography is a method of separating ions based on their distinct retention rates in a given solid phase packing material. Given different retention rates for two anions or two cations, the elution time of each ion will differ, allowing for detection and separation of one ion before the other. Detection methods are separated between electrochemical methods and spectroscopic methods. This guide will cover the principles of retention rates for anions and cations, as well as describing the various types of solid-state packing materials and eluents that can be used.

Principles of ion chromatography

Retention models in anion chromatography

The retention model for anionic chromatography can be split into two distinct models, one for describing eluents with a single anion, and the other for describing eluents with complexing agents present. Given an eluent anion or an analyte anion, two phases are observed, the stationary phase (denoted by S) and the mobile phase (denoted by M). As such, there is equilibrium between the two phases for both the eluent anions and the analyte anions that can be described by [link] .

This yields an equilibrium constant as given in [link] .

Given the activity of the two ions cannot be found in the stationary or mobile phases, the activity coefficients are set to 1. Two new quantities are then introduced. The first is the distribution coefficient, D _A , which is the ratio of analyte concentrations in the stationary phase to the mobile phase, [link] . The second is the retention factor, k ¹ _A , which is the distribution coefficient times the ratio of volume between the two phases, [link] .

Substituting the two quantities from [link] and [link] into [link] , the equilibrium constant can be written as [link] .

Given there is usually a large difference in concentrations between the eluent and the analyte (with magnitudes of 10 greater eluent), equation 4 can be re-written under the assumption that all the solid phase packing material’s functional groups are taken up by E ^y- . As such, the stationary E ^y- can be substituted with the exchange capacity divided by the charge of E ^y- . This yields [link] .

Solving for the retention factor, [link] is developed.

[link] shows the relationship between retention factor and parameters like eluent concentration and the exchange capacity, which allows parameters of the ion chromatography to be manipulated and the retention factors to be determined. [link] only works for a single analyte present, but a relationship for the selectivity between two analytes [A] and [B]can easily be determined.

First the equilibrium between the two analytes is determined as [link] .

The equilibrium constant can be written as [link] (ignoring activity):

The selectivity can then be determined to be [link] .

[link] can then be simplified into a logarithmic form as the following two equations:

When the two charges are the same, it can be seen that the selectivity is only a factor of the selectivity coefficients and the charges. When the two charges are different, it can be seen that the two retention factors are dependent upon each other.