Capillary Chromatography - Injection Systems 3
Some stationary phase is removed from the first few centimeters of the column and the sample injected into this section of the column. In general, the sample will split and will vaporize in the normal manner. With no stationary phase present, all solutes will travel at the same speed as the mobile phase (carrier gas) until they reach a coated section of the column. They will then be absorbed onto the stationary phase and all parts of the sample will accumulate at that point. In practice, this technique is used in conjunction with temperature programming, the program being started at a fairly low temperature. A low initial temperature assists the accumulation of all the solutes at one point in the column (i.e., where the stationary phase coating begins). The temperature program is initiated and the separation developed in the normal way. This method will only be successful if there is a significant difference between the boiling points of the sample solvent and those of the components of the sample.
An alternative procedure, "solute focusing method" has been shown to be more effective, but, unfortunately, requires more complicated and expensive equipment. The sample injector is designed to have two consecutive, independently heated and cooled zones at the beginning of the column. A diagram of the solute focusing system is shown in figure 9.
Figure 9. The Solute Focusing Method of Injection
Both zones are initially cooled and the sample is injected into the first zone. Sample splitting almost inevitably occurs and the carrier gas is allowed to remove the solvent and then the first zone is heated and the second zone kept cool. The solutes in the first zone pass through the zone at the higher temperature and accumulate at the beginning of the cooled second zone and are, thus, focused at the beginning of the column. When the focusing is complete the second zone is heated and the separation developed in the usual manner. This technique is more flexible than the previous method but the apparatus and the procedure is more expensive and complex.