Modulation and Manipulation of Gas Chromatographic Bands by Using Novel Thermal Means

Sub-ambient methods have been used widely in gas chromatography to focus or trap migrating analyte peaks. The procedure has been applied to both packed and open-tubular traps. The packed sorbent or the inner wall surface of the open tube upon which the solute condenses merely provides a support phase that physically retains the solute. In this mode, the cold region is able to retain volatile components, however prevention of solute break-through is a major consideration in evaluating the performance of the trap and determining the conditions that it must be operated under. As the vapor phase enters the cold region, a micro-fog or droplets may be produced1, and this will affect the partitioning efficiency of the solute into any absorptive stationary phase material. Too rapid cooling may cause this condition to arise. Higher molecular mass solutes also have reportedly an increased chance of aerosol formation, leading to reduced trapping efficiency.6 The support material of the trap may be an active solid phase, such as alumina or silica, or it may be a regular gas chromatographic liquid stationary phase material, either as a section of coated capillary gas chromatography column, or a coated solid support. It has recently been suggested2 that a coated non-polar phase may be of value in preventing the co-trapping of introduced water, which has deleterious effects since trapped frozen moisture can either cause expansion and fracture of the trap, can cause the column to be plugged, or can result in poor performance when the trap is warmed up and the water is re-volatilized. Since the advent of the flexible fused silica column, cold trapping has been easily implemented by simply placing a 651 ANALYTICAL SCIENCES AUGUST 1998, VOL. 14