Gas Chromatography - Preparative Gas Chromatography > Page 60

Preparative Gas Chromatography

Gas chromatography has not been used extensively for preparative work although its counterpart, liquid chromatography, has been broadly used in the pharmaceutical industry for the isolation and purification of physiologically active substances. There are a number of unique problems associated with preparative gas chromatography. Firstly, it is difficult to recycle the mobile phase and thus large volume of gas are necessary. Secondly, the sample must be fully vaporized onto the column to ensure radial distribution of the sample across the column. Thirdly, the materials of interest are eluted largely in a very dilute form from the column and therefore must be extracted or condensed from the gas stream which is also difficult to achieve efficiently. Finally, the efficient packing of large GC columns is difficult. Nevertheless, preparative GC has been successfully used in a number of rather special applications; for example the isolation of significant quantities of the trace components of essential oils for organoleptic assessment.

The layout of a preparative gas chromatograph is shown in figure38

Figure 38 Layout of Preparative Gas Chromatograph

Air can not normally be used as the mobile phase due to likely oxidation and so either a gas tank or a gas (e.g., nitrogen) generator must be used. As the flow rates can be large, more than one generator operating in parallel will often be necessary. The sample is usually placed on to the column with a syringe pump and rapidly vaporized in a suitable heater. Passing the gas in vapor form onto the column helps evenly distribute the sample radially across the column. The detector that is used must have specifications that are almost opposite to those of an analytical detector. It should function well at high concentrations of solute, have a generally low sensitivity, if in-line it must be non-destructive and have minimum flow impedance. It need not have a particularly linear response. The katharometer is one of the more popular detectors for preparative GC. The column outlet is passed to a selection valve that diverts the eluent to its appropriate collecting vessel. The collecting vessel may be cooled in ice, solid carbon dioxide or if necessary liquid nitrogen (liquid nitrogen can only be used if a low boiling gas such as helium is employed as the carrier gas). In some cases the solutes contained in the eluent can be extracted into an appropriate liquid or onto the surface of a suitable adsorbent. the desired fractions are then recovered by distillation or desorption.