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

The UV Detectors Although over the years a large number of LC detectors have been developed  and described, the vast majority of all contemporary LC analyses are carried out using one of four detectors, the UV detector in one of its forms, the electrical conductivity detector, the fluorescence detector and the refractive index detector. In addition, some form of the UV detector probably accounts for 80% of those analyses. The UV Absorption Detectors UV absorption detectors respond to those substances that absorb light in the range 180 to 350 nm. Many (but not all) substances absorb light in this wavelength range, including those substances having one or more double bonds (p electrons) and substances having unshared (unbonded) electrons, e.g. all olefins, all aromatics and compounds, for example, containing >C=O, >C=S, –N=N– groups. The sensor of a UV detector consists of a short cylindrical cell having a

and the diode array detector. This arrangement would allow a true monochromatic light beam to pass through the detector and then the transmitted beam would itself be dispersed again onto a diode array. Only that diode sensing the wavelength of the incident light would be used for monitoring the transmission. In this way any fluorescent light would strike other diodes, the true absorption would be measured and accurate monochromatic sensing could be obtained. The Multi–Wavelength Dispersive UV Detector A diagram of the multi–wavelength dispersive UV detector is shown in figure 28.   Courtesy of the Perkin Elmer Corporation Figure 28 The Multi–Wavelength Dispersive UV Detector Light from the deuterium lamp is collimated by two curved mirrors onto a holographic diffraction grating. The dispersed light is then focused by means of a curved mirror, onto a plane mirror and light of a specific wavelength is selected by appropriately positioning the angle of the plane

nbsp; As opposed to detectors used for analytical purposes, detectors for preparative work need to have a very low sensitivity as sample sizes are large and consequently the solute concentrations are very high. If analytical detectors are used for preparative work a portion of the eluent is split from the main stream, diluted with more mobile phase and then passed through the detector. In practice, this is a rather awkward procedure. As seen in figure 27 the column eluent passes through a delivery tube and onto a supporting plate that is usually made of fused quartz, so that  adequate UV light can reach the photo cell placed on the other side of the plate. The liquid flows over the plate and the effective path length of the sensor will be the film thickness which will be unique to the particular solvent used as the mobile phase. The UV lamp is situated above the

of two detectors given in units of absorbence the path lengths of the cells in each instrument must be taken into account. UV detectors can be used with gradient elution providing the solvents do not absorb significantly over the wavelength range that is being used for detection. In reversed phase chromatography, the solvents usually employed are water, methanol, acetonitrile and tetrahydrofuran (THF), all of which are transparent to UV light over the total wavelength range normally used by UV detectors. In normal phase operation more care is necessary in solvent selection as many solvents that might be appropriate as the chromatographic phase system are likely to absorb UV light very strongly. The n-paraffins, methylene dichloride,  aliphatic alcohols and THF are useful solvents that are transparent in the UV and can be used with normal distribution systems (e.g. a polar stationary phase such as silica gel). The Fixed Wavelength UV Detector The fixed wavelength UV

The Fixed Wavelength Detector There are two types of UV detector the fixed wavelength detector and the multi-wavelength detector. A diagram of a Fixed Wavelength UV Detector is shown in figure 17.   Figure 17.   The Fixed Wavelength UV Detector The detector consists of a small cylindrical cell (2.0 to 10.0 ml in volume) through which flows the eluent from the column. UV light from an appropriate UV lamp, passes through the cell and falls on a UV photo electric cell. In the fixed wavelength detector the wavelength of the light depend on the type of lamp that is used. There are a number of lamps available the at provide of wavelengths