Ion Chromatography - The Fixed Wavelength UV Detector
The technique of ion suppression is frequently used in ion exchange chromatography when using the electrical conductivity detector and as already noted when separating cations. A wide variety of different types of ion suppression columns are commercially available but again it should be pointed out that, any suppressor system introduced between the column and the detector, will cause peak spreading and reduce the system resolving power.
Thus, the connecting tubes and suppression column itself must be very carefully designed to eliminate or reduce this dispersion to an absolute minimum. A photograph of an electrical conductivity detector manufactured by Metrohm Ltd. Is shown in figure 22. The Metrohm instrument is available with and without ion suppression facilities as required.
Figure 22. The Metrohm Electrical Conductivity Detector
The Fixed Wavelength UV Detector
The other frequently used detector in ion chromatography is the UV absorption detector, mostly as a diode array detector but also quite frequently as the simple single wavelength UV detector. These detectors can be used when the ions of interest absorb in the UV wavelength or can be made to do so by appropriate derivatization. The UV detector can often selectively detect the solute ion of interest while other components in the mixture give little or no signal. A diagram of the fixed wavelength UV detector is shown in figure 23. The detector sensor consists of a small cylindrical cell (normally 2.0 to 10 ml in volume) through which, the eluent from the column flows. UV light from a UV lamp (usually a mercury discharge lamp emitting UV light mostly at 254 nm), passes through the cell and onto a UV photo electric cell. The type of lamp that is used determines the wavelength of a fixed wavelength detector. There are a number of lamps available that can provide excitation light wavelengths ranging from about 210 nm to 280 nm. Three of the lamps that are commercially available are as follows:-
Figure 23. The Fixed Wavelength UV Absorption Detector
The most popular lamp is the mercury vapor lamp as its emission wavelength allows the detector to sense a wide range of solute types. The detector contains a sample and reference cell and the reference cell output is compared to that from the sample cell The difference signal passes to a non linear amplifier that ensures the signal is linearly related to solute concentration in the sample cell. The fixed wavelength detector is inexpensive and, as the light is emitted at a (largely) specific wavelength(s) it has a high intensity, and consequently, a higher intrinsic sensitivity than the multi-wavelength UV detector