Introduction

The term ion chromatography (IC) does not refer to a single, specific chromatographic technique, but rather to the specialized application of a collection of established techniques. When introduced in 1975 IC referred only to the separation of inorganic anions and cations using a specific combination of ion exchange columns coupled to a conductimetric detector. Since that time, the definition of IC has expanded greatly and it can be best categorized in terms of the type of analytes separated rather than the manner in which the separation is achieved. We can therefore define IC to be:

the use of liquid chromatographic methods for the separation of inorganic anions and cations and low molecular weight water-soluble organic acids and bases.

While a range of chromatographic methods (e.g. reversed-phase ion interaction chromatography) can be used to separate these types of analytes, it is true to say that the majority of IC separations are performed by ion exchange using specialized stationary phases. In the interests of brevity, the discussion of IC will therefore be confined to ion exchange methods only. The interested reader seeking a broader coverage of the technique is referred to any of the standard texts listed in the Further Reading section.

IC methods employing ion exchange can be divided somewhat arbitrarily into two main groups, largely on the basis of historical development and commercial marketing influences. These groups of methods are referred to as 'nonsuppressed ion chromatography' and 'suppressed ion chromatography'.

Nonsuppressed IC comprises all those methods in which an ion exchange column is used to separate a mixture of ions, with the separated analytes being passed directly to the detector. The hardware configuration employed is shown schematically in Figure 1A, from which it can be seen that this configuration parallels the hardware used in traditional high performance liquid chromatography (HPLC). Some of the alternative names proposed for this technique are:

1. single-column ion chromatography

2. electronically suppressed ion chromatography.

The first of these names indicates that only a single chromatographic column is employed and that the eluent is not chemically modified prior to entering the detector, whereas the second name pertains to the fact that the background conductance of the eluent can be cancelled electronically by certain types of conductivity detectors. 'Nonsuppressed IC' is the most frequently used term and is recommended.

The second group of ion exchange methods consists of those in which an additional device, called the suppressor, is inserted between the ion exchange separator column and the detector, as shown in Figure 1B. The function of the suppressor is to modify both the eluent and the analyte in order to improve the detectability of the analytes with a conductivity detector. The suppressor often requires a regenerant solution to enable it to operate for extended periods. Methods using this hardware configuration are referred to as:

1. suppressed ion chromatography

2. chemically suppressed ion chromatography

3. eluent-suppressed ion chromatography

4. dual-column ion chromatography.

The last of these names is misleading because modern suppressors are not columns, but rather flow-through membrane devices. The term 'suppressed IC' is recommended.

Ion Chromatography Block Diagram
Figure 1 Block diagram showing the instrumental components used in (A) nonsuppressed and (B) suppressed IC.
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