Detection Methods in Ion Chromatography

Detection methods applied in ion chromatography are divided into electrochemical, spectrometric, nebulization, and others. Conductometric, amperometric, and charge detection are electrochemical methods, while the spectrometric methods include UV/Vis, fluorescence, and refractive index detection. In addition, there are various application forms of these detection methods. Nebulization methods include evaporative light scattering (ELS) and charged aerosol detection (CAD). All of these methods are described in detail in this chapter. 

In most cases, the choice of a suitable detection mode depends on the separation method and the corresponding eluents. If detection is to be carried out by direct measurement of a physical property of the solute ion (e.g., UV absorption), the solute ion must differ substantially in this property from eluent ions that are present in much higher concentration. However, eluent and solute ions often exhibit similar properties, so direct detection is feasible only where selective detection of a limited number of solute ions is desirable. 

A much broader range of applications employ detection methods that measure changes in a certain physical property of the eluent (e.g., conductance) that are caused by the elution of the solute ion. As a prerequisite, the values of this property for eluent and solute ions must differ. Most of the detection methods applied in ion chromatography are based on this technique. In the following discussion, a further subdivision into direct and indirect methods is made. Direct detection methods are those in which eluent ions exhibit a much smaller value than solute ions for the property to be measured. On the other hand, detection methods are called indirect if eluent ions exhibit a much higher value for the property to be measured than do solute ions. 

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While the working potential required for the desired electrochemical reaction may be determined with voltammetric experiments, amperometry is used as the detection method in ion chromatography. A distinction is made between amperometry with constant working potential and pulsed amperometry. 

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