PRESSURE-TUNABLE SELECTIVITY FOR HIGH-SPEED GAS-CHROMATOGRAPHY

A high-speed GC instrument using a cryofocusing inlet system and pressure-tunable selectivity is described. Tunable selectivity is achieved by the use of the tandem combination of a nonpolar and a polar capillary separation column. Selectivity is tuned by the adjustment of the carrier gas pressure at the midpoint between the columns. When this pressure is changed, differential changes occur in the carrier gas flow rates for the two columns. This results in changes in the relative residence times of all components on the two columns and thus changes the contribution of each column to the overall separation. Relatively short columns are used for high-speed separations. This results in minimal gas compression effects and has allowed the development of a linear tuning model and a reliable tuning procedure. A column bifurcation system is described which obtains independent holdup time measurements for the tandem columns. From these measurements, chromatograms obtained from the tandem column system can be used to estimate capacity factor values for the individual columns. Design features and a statistical analysis of the tuning procedure are presented.