Preparative Chromatography - Preparative Chromatography Apparatus > Preparative Detectors > Page 27
This is achieved by the use of a sample distribution plate a diagram of which is included in figure 12. It consists of a disc in which is cut a series of radial slots and can be used for both GC and LC. The sample is placed on the top of the disc where it is quickly distributed across the plate surface and then onto the column through the slots. In this way the sample is injected across the top surface of the packing.
Preparative chromatography detectors can have very limited specifications, compared with their analytical counterparts. They need not be particularly sensitive (in fact too great a sensitivity is a distinct disadvantage) as the sample size and the eluent solute concentrations are very large. Preparative chromatography detectors can have large sensor volumes and, as the detector is required only to monitor the separation, they need not have a linear response. They do need to tolerate high flow rates and thus, must have low flow impedance. Analytical detectors can be used for preparative purposes but a portion is usually split from the column eluent, diluted with more mobile phase and then passed through the detector. In practice this becomes a rather clumsy procedure.
The most commonly used detector in preparative GC is the thermal conductivity detector (hot wire detector). Even this detector, however, is often too sensitive and has too high a flow impedance. Under such circumstances, the procedure mentioned above must be employed. The eluent from the preparative column is split and a small portion diverted through the detector (sometimes with further dilution with carrier gas to reduce sensitivity).
In LC, the refractive index detector is probably the most useful of the analytical detectors for preparative work, but even at its lowest sensitivity setting, it may still be too sensitive. The multi-wavelength UV detector can sometimes be used providing its conduits can tolerate the necessary high flow rates. A very short path length cell must be installed, or a variable path length cell can be used. Another option is to set the operating wavelength at that which the solutes have very small extinction coefficients so that the detector has a very weak response. If possible, a detector specifically designed for preparative work should be used, but there are a very limited number of these available.