Gas Chromatography - The Modern Gas Chromatograph > Page 3


Figure 1 The Design of a Modern Gas Chromatograph

Most gas chromatographs consist of four chromatography units, supported by three temperature controllers and 2 micro processors systems. In some instruments, a single microprocessor unit is employed to service the entire chromatograph but this tends to restrict the choice available for the different parts of the chromatograph.

The first unit, the gas supply unit, provides all the necessary gas supplies which may involve a number of different gases, depending on the type of detector that is chosen. For example, a flame ionization detector will require hydrogen or some other combustible gas mixture, air or oxygen to support combustion and a mobile phase supply that could be nitrogen, helium or some other appropriately inert gas. Thus, for the detector postulated, a minimum of three different gases would be required which will also involve the use of three flow controllers, three flow monitors and possibly a flow programmer. In addition the gas supply unit would be serviced by a microprocessor to monitor flow rates, adjust individual gas flows and, when and if necessary, program the mobile phase flow rate.

The second unit is the sampling unit which contains an automatic injector which is situated inside a thermostatically controlled enclosure. The injector usually has its own oven, but sometimes shares the column oven for temperature control. The injector oven, if separate from the column oven, is serviced by its own temperature controller which both monitors and controls the temperature. There is normally a separate controller, usually a microprocessor, that controls the injector itself. The injector can range in complexity from a simple sample valve, or mechanically actuated syringe to an automatic multi sampler that is microprocessor controlled. It can have a complex transport system (such as a carousel) that can take samples, wash containers, prepare derivatives and, if necessary, carry out a very complex series of sample preparation procedures before injecting the sample onto the column. Sample preparation is often carried out using a laboratory robot which then becomes part of the sampling unit. If a robot is used it can be programmed to prepare a wide variety of different samples and so software must be written for each type of sample.