Ion Chromatography - The Diode Array Detector

This detection system is ideal as, by noting the elution time of a particular peak, a spectrum of the solute can be obtained by recalling the output of all the diodes at that particular time. This will provide directly the spectrum of the solute, i.e., a curve relating adsorption against wavelength. The performance of both types of multi-wavelength detectors are very similar and typical values for their more important specifications are as follows.

Typical Specifications for a Multi-Wavelength UV Detector

The UV detector is mostly employed in two ways. Providing the mobile phase does not absorb at the monitoring wavelength it can be used to detect substances eluted in a high conductivity mobile phase without the need to resort to suppressor techniques that can degrade column resolution.

The alternative use of the UV detector in ion chromatography is to pick out the solutes of interest from those that do not absorb at the same wavelength, In fact, the solutes of interest need not necessarily be resolved from other solutes present (providing they do not adsorb UV light at the wavelength selected) so, in effect, the UV detector provides an apparent but useful increase in resolution. In ion chromatography the UV detector can be used explicitly as a selective detector. An example of the former use of the UV absorption detector in ion chromatography is shown in figure 25.

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(Courtesy of Wescan)

Figure 25. The Separation of Metal-Cyano Complexes Employing an Adsorbed Ion Exchanger and UV Detection

In this case the problem is to separate the complex anions ferrocyanide, ferricyanide, argentocyanide, auaurocyanide and the complex nickelcyanide anion. This can be achieved using an absorbed cation exchanger tetrabutylammonium sulphate on a reverse phase column. As discussed elsewhere in this book, the cation exchanger is held by dispersive forces between the butyl groups of the tetrabutylammonium ion and the hydrocarbon chains of the reverse phase. The mobile phase consists of 30%v/v acetoniltrile in water which controls the extent of coverage of the reverse phase with the cation exchanger that contains 0.005M tetrabutylammonium sulphate.. Selective detection was achieved using a wavelength of 210 nm.