Retention Gap Sampling The first solution to the problem of sample splitting was the 'retention gap method' which is depicted in figure 10. Figure 10. The Retention Gap Method of Sampling In this procedure stationary phase is removed from the first few centimeters of column. The sample is injected into this section and, if the sample becomes split, on commencing development, each split portion will still vaporize in the normal way. However, as there is no stationary phase present, the solutes will all travel at the velocity of the mobile

This problem can be solved in one of two ways. The first solution is the 'retention gap method' which is depicted in figure (4). Figure 4. The Retention Gap method of Injection

dissolved. An alternative procedure, called the 'solute focusing method' is more effective, but requires more involved and expensive equipment. The injector is designed to have two consecutive, independently heated and cooled zones located at the front of the column. A diagram of the solute focusing system is shown in figure (5). The second zone is now heated, using an appropriate temperature program, and the separation developed in the normal manner. This technique is more flexible than the 'retention gap method' but the apparatus and the procedure is more complex. Sample splitting does not occur in packed columns and thus, if the sample is tractable to separation on such columns, then the packed column would be the column of choice if high accuracy and precision are required. LC Sample Valves LC sample valves must operate at high pressures (sometimes as great as 10,000 p.s.i.) which demands high mechanical strength and precise methods of construction to ensure a leak proof

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.  

polymeric material. A 1 ml sample was collected in an LC sample loop and an internal standard added. The sample was then displaced through a short column, 1 cm long, 2 mm I.D., packed with 10 mm particles of PLRP-S polymer (styrene-divinylbenzene copolymer) by pure water. The extraction column was then dried in a stream of nitrogen and the adsorbed materials displaced into the gas chromatograph with 180 ml of ethyl acetate. To remove the solvent, the sample was firstly passed through a short retention gap column before entering the actual analytical column. The oven was maintained at 70ûC so that the ethyl acetate was eluted through the retaining column and then released to waste. At this temperature the solutes of interest were held in the retaining column while the ethyl acetate was removed. After the ethyl acetate had been removed, the column temperature was increased and the components in the residue separated on the analytical column using an appropriate temperature program.

breaks up into discrete segments, due to bubble formation in the first part of the column. As the solvent evaporates the sample is deposited at two or more locations along the column. When development commences, each local concentration of sample acts as a unique injection and a chromatogram containing very wide or multiple peaks is produced. There have been a number of procedures introduced in an attempt to eliminate the sample splitting on the column. The first solution was designated as the 'retention gap method' and is depicted in figure 8.