Programmable control of column selectivity for high speed GC

A tandem-column ensemble consisting of two capillary columns with different stationary-phase chemistries and computer-controlled carrier-gas pressure at the column junction point is used to change the ensemble selectivity on- the-fly during a separation. When the pressure at the column junction point is changed, differential changes occur in the carrier gas velocities in th e two columns. This changes the relative contribution that each column make s to the ensemble selectivity. This programmable selectivity control is app lied to the problem of high-speed, isothermal GC analysis of volatile organ ic compound (VOC) mixtures. For the ensemble configuration described here, the pressure control system provides over 200 computer-selected junction-po int pressures in 0.1 psi steps. Repeatability typically is +/-0.01 psi. Thi s provides for very stable retention patterns. The relatively large dead vo lume in the controller is managed by the use of a pneumatic vent line. The vent line restriction is set to give pressure set-point transition times of 0.3-3 s. A model is developed to aid in the selection of the pressure prog ram (pressure vs time profile) that will result in a rapid separation of th e target compounds. The model results in plots of the position of component bands in the column ensemble vs time. These sample-band trajectory plots s how discontinuous changes in slope (migration velocity) at pressure set-poi nt change times and when each component band migrates across the column jun ction. The model provides quantitative information on elution peak position s. The model is used to predict the effects of changes in the selectivity p rogram on retention patterns.