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 upper side of the plate and the photo cell on the lower side. A reference photo cell (not shown) is situated close to the lamp and the output used to compensate for changes in light intensity from variations in lamp emission. The short path length results in a low sensitivity and the detector can operate satisfactorily at concentrations as high as 10-2 g/ml (1% w/w), which is ideal for preparative chromatography. A particular advantage of this type of sensor is its very low flow impedance and thus can easily accommodate the high flow rates used in preparative LC. The film thickness does depend, among other factors, on the column flow rate, consequently, precise flow control is necessary for the detector to perform satisfactorily.
The Multi–Wavelength UV Detector
The multi–wavelength detector employs a light source that emits light over a wide range of wavelengths. Employing an appropriate optical system (a prism or diffraction grating), light of a specific wavelength can be selected for detection purposes. Light of a specific wave length wavelength might be chosen where a solute has a absorption maximum to provide maximum sensitivity. Alternatively, the absorption spectra of an eluted substances could be obtained for identification purposes by scanning over a range of wavelengths. The latter procedure, however, differs with the type of multi–wavelength detector being used.