Gas Chromatography - Injection Devices > Page 13

Figure 6 Graph of Retention Time against Pressure Program Rate for a Series of Solutes.


Consider five solutes having actual retention volumes of 150, 300, 600, 900 and 1200 ml eluted under pressure programming conditions where, (g1) is 1.2 and at (p) seconds after the start, gp = g1 + pa, where (a) takes values that range from 0.0025/s (0.0375 psi/s) and 0.025/s (0.15 psi/s) Employing equation (7) the retention time of the solutes can be calculated for the series of different programming rates. The results are shown in figure 6. It is seen that. although the use of pressure programming does indeed reduce the retention time of all solutes, program rates much above 0.2255 psi/s (13.5 psi/min.) provides very little advantage as far as reduction of analysis time is concerned.


Injection Devices

The basic injection devices that are used in chromatography, such as the external loop valve, have been discussed in book 1. In gas chromatography two basic types of sampling system are used, those suitable for packed columns and those designed for open tubular columns. In addition, different sample injectors are necessary that will be appropriate for alternative column configurations. It must be stressed, however, that irrespective of the design of the associated equipment, the precision and accuracy of a GC analysis will only be as good as that provided by the sample injector. The sample injector is a very critical part of the chromatographic equipment and needs to be well designed and well maintained.