UV/Visible Absorption Detectors

The majority of compounds absorb in the frequency range of UV and visible light and, thus, over 50% of all capillary electrophoretic separations employ UV/visible detecting systems. The basic detecting system consists of a light source directed through a window formed in the silica capillary  (method of window formation discussed previously) and then onto a suitable sensor. The light source for UV may be a low pressure mercury arc source, a deuterium arc source or a metal vapour source such a cadmium, zinc, iodine or lead etc. For visible light, a tungsten filament lamp is usually employed. The sensor is usually a simple solid-state device and it should be emphasized that the response will not be linearly related to concentration so a linearizing circuit or algorithm will be required for quantitative measurements. Alternatively if a wide band UV source is employed such as the light from a deuterium lamp then a photo diode array sensor can be used. This will respond to all wavelengths of UV light and so the wavelength to which the solute has greatest sensitivity can be chosen to provide the maximum response. An absorption spectrum can also be obtained from a diode array sensor that may help in sample identification or structure elucidation.