Preparative Chromatography - Preparative Chromatography Apparatus > Fraction Collectors > Page 29
The liquid flows over the plate and the effective path length is the film thickness peculiar to the flowing solvent layer. The UV lamp is situated on one side of the plate and the photo cell on the other side, each facing normal to the plate surface. A reference photo cell (not shown) is placed close to the lamp to compensate for changes in light intensity that may arise from variations in lamp emission. Due to the very short path length (the thickness of the film of column eluent) the detector has the required low sensitivity and low flow impedance. The detector can operate very satisfactorily at concentrations as great as 10-2 g/ml (1% w/w), which is ideal for preparative chromatography. Another advantage of the device for preparative work, is its very low flow impedance and thus can easily cope with the high flow rates used in preparative LC. The film thickness does depend, among other things, on the column flow rate and thus fairly precise flow control is necessary for the satisfactory performance of this detector.
Fraction collection in preparative chromatography is usually achieved using a multi-port valve and a number of collection vessels. The selector valve should be programmable on the basis of time, or be actuated by the detector output signal (preset at the appropriate signal level or signal derivative) or, preferably, both. The valve should have at least six ports, or preferably ten. If the system is to be used solely to separate a binary (e.g., a mixture enantiomer pairs) then a six port valve would usually be adequate. Depending on whether the mobile phase is a gas (as in GC) or a liquid (as in LC) the solute is extracted from the eluent by condensation or adsorption. In the case of LC the sample can be collected as a solution in the mobile phase and recovered by evaporation. In GC, depending on the partial pressure of the eluted solute, the material can be collected by cooling with a mixture of solid CO2 and acetone or, in the extreme, with liquid nitrogen. In cases where the material collected is very valuable, argon can be used as the carrier gas and both the solute and the argon mobile phase condensed in a liquid nitrogen cooled trap. The argon is then carefully allowed to slowly evaporate leaving the collected solute as a residue. To simplify the recovery procedure in LC and reduce the volume of solvent that must be evaporated during product recovery, alternate ports of the multi-port selection valve should be connected to waste.