Figure 58. The Large Bore Capillary Column Injector

As a result, as the solvent evaporates, the sample is deposited at two or more locations along the tube. Consequently, when the separation is developed, each local concentration of sample acts as a separate injection and causes peak dispersion and, at the extreme, produces a large number of multiple peaks in the chromatogram. A number of solutions have been suggested to solve this problem, one of which is the 'retention gap method' of injection. Stationary phase is removed from the first few centimeters of the column and the sample is injected into this section of the column. If the sample splits and bubbles are formed, the separate droplets will still vaporize in the normal way. As there is no stationary phase present, the solutes will all travel at the same speed as that of the mobile phase until they reach a coated section of the column where they will be absorbed into the stationary phase. Thus, the total sample will accumulate at this point. The retention gap method of injection is usually carried out in conjunction with temperature programming, the program always being started at a fairly low temperature. The lower temperature facilitates solute accumulation at the point where the stationary phase coating begins. After injection, the temperature program is initiated and the solutes are eluted through the column in the normal way. The method will only be successful if there is a significant difference between the boiling point of the sample solvent and those of the components of the sample.

Another procedure used to obviate the production of multiple peaks is called the "solute focusing" method. This procedure is an improvement on the previous method, but involves more complex and expensive equipment. The oven is designed to have two consecutive, independently heated and cooled zones at the column inlet. Before sampling, both zones are cooled, and the sample is then injected into the first zone. Sample splitting almost inevitably occurs, and sample solvent is removed by the carrier gas and is eluted through the column. The first zone is then heated while the second zone is kept cool. The solutes from the first zone are, thus, evaporated and pass through the zone and condense and accumulate at the beginning of the cooled second zone. The total sample is now focused at the beginning of the cooled zone. The second zone is now heated and the separation developed in the usual manner. Sample splitting does not occur in packed columns and it follows that, if the sample is amenable to separation on a packed columns, then the packed column may be the column of choice if high accuracy and precision are required.