Gas Chromatography - Injection Devices > Sampling by Solute Focusing > Page 18

Sampling by Solute Focusing

Another method of sampling that avoids sample splitting is the 'solute focusing method' which is more effective, but requires more complicated and expensive equipment. The injector is designed so that there are two consecutive, independently heated and cooled zones located at the beginning of the column. A diagram of the solute focusing system is shown in figure 11.

Figure 11 The Solute Focusing Method sampling


Initially the two zones are cooled and the sample is injected onto the first zone. The sample usually splits, but the carrier gas is allowed to remove the solvent, which is eluted through and out of the column. This leaves the sample spread along the first zone in dispersed fragments. The first zone is then heated while the second zone kept cool. The solutes in the first zone are eluted through the zone at the higher temperature and the sample accumulate at the beginning of the cooled second zone. The sample has now been focused as a compact band at the beginning of the column. The second zone is now heated and the separation developed normally. This technique is more flexible than the 'retention gap method' but the apparatus is more expensive and the procedure more complex.