Ion chromatography (IC)

Author: Hans-Jürgen Schwarz
English version by Sandra Leithäuser
back to Analysis of Salts

Abstract

Ion chromatography (or ion-exchange chromatography) is a process that allows the separation of ions and polar molecules based on their charge. It separates the substances as these are preferentially distributed between a stationary and a mobile phase. In ion chromatography, the ions bound to the stationary phase are later released by an eluent in the mobile phase. Quantification is achieved using a suitable detector. Specific columns are used to separate both anions and cations and these can be determined quantitatively even when in low concentrations.

Introduction

Cromatography is one of several methods that can be used for the separation of substances. The method is base on having two phases: a stationary and a mobile one. A piece of filter paper can serve as a stationary phase or a tube or column, as it is usually called, is packed with materials such as alumina, Al₂O₃, activated carbon, ion exchange resins, etc. The mixture to be separated is dissolved in an appropriate solvent, in the case of salts, distilled water is used, that also serves as the mobile phase and fed into the column. Because of the difference in affinity between the substances to be separated in both the mobile and the stationary phase, they are separated as more and more solvent is passed through the column. The origin for the name chromatography stems back to the first separation of pigments found in spinach, such as chlorolphyl, carotenes, etc. so the stationary phase would be colored differently according to the affinity of the pigment for it.

In the case of ion chromatography, ions are also distributed between the stationary and the mobile phase and their separation is dependent on the partition coefficient of each specific ion. The surprisingly high separation efficiency of this relatively simple method is based on the many repetitions, between 100–10000, of individual separation operations during the on going passage of the mobile phase through the stationary one. In order to achieve the separation of chemically similar substances by chromatography, the partition coefficients of the two materials on the stationary phase have to differ at least slightly.

In the broadest sense, ion chromatography includes ion-exchange chromatography, ion exclusion chromatography, ion interaction chromatography (previously ion pair chromatography) and ion suppression chromatography.

Usually, the ion exchange chromatography is used for the detection of salt-forming ions. In general, an exchanger’s affinity for an oppositely charged sample ion is greater, the higher the charge and the smaller the sample ion’s surrounding solvation molecules are. Furthermore, the affinity increases with the polarizability of the ions.

The retention time: The retention times for alkali and alkaline earth metals increase sequentially from lithium to cesium and from magnesium to barium, because their polarizability increases similarly, while solvation, the ability of these cations to form a hydration shell around them, decreases. The same applies to halogen anions from flourine to iodine. For other ions, the situation is not so clear but the bi- or higher valence ions elute after the mono-valence ions. Protons and hydroxyl ions have a large hydration shell and therefore a short retention time. Retention behavior and selectivity of the separation are mainly influenced by pH-value, buffer , eluent ion, organic solvent dissolution agents, and complexing compounds for the case of heavy metals.

In general, an electrical conductivity detector is used for ion chromatography. Nonetheless, there are other detectors that can be used.

Advantage: Ion chromatography can be used for the analysis of cations and anions. However, it is mostly used for detecting anions, because there are better methods for the detection of cations. Ion chromatography provides accurate quantitative analyses.

Disadvantage: Only ion concentrations can be determined, and complete salt-phases are determined by deduction, which is only possible for simple systems.

carrier liquid or mobile phase of a gas

is the time required by each of the individual ions, i.e., the analyte, for passing through the column from injection to detection point

acid-base pair that keeps a relative constant pH value upon dilurion or addition of small amounts of acid or base