Liquid Chromatography - The UV Detectors 1

Detectors

A large number of LC detectors have been developed over the past thirty years based on a variety of different sensing principles. However, only about twelve of them can be used effectively for LC analyses and, of those twelve, only four are in common use. The four dominant detectors used in LC analysis are the UV detector (fixed and variable wavelength) the electrical conductivity detector, the fluorescence detector and the refractive index detector. These detectors are employed in over 95% of all LC analytical applications. These four detectors will be described and for those readers requiring more information on detectors are referred to Liquid Chromatography Detectors. The subject of detector specifications will not be discussed here but will also be dealt with in detail there. Detector sensitivities and detector linearity will, however, be given for each of the four detectors.

The UV Detector

The UV detector is by far the most popular and useful LC detector that is available to the analyst at this time. This is particularly true if multi-wavelength technology is included in this class of detectors. Although the UV detector has some definite limitations (particularly for the detection of non polar solutes that do not possess a UV chromaphores) it has the best combination of sensitivity, linearity, versatility and reliability of all the LC detectors so far developed .

Most compounds adsorb UV light in the range of 200-350Å including all substances having one or more double bonds (p electrons) and all substances that have unshared (non bonded) electrons; e.g. all olefins, all aromatics and all substances containing >CO, >CS, -N=O and groups. The relationship between the intensity of UV light transmitted through the detector cell and solute concentration is given by "Beers' Law,

where, (Io) is the intensity of the light entering the cell,

(IT) is the intensity of the transmitted light,