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A computer-driven pressure controller is used to deliver pressure pulses to the junction point of two series-coupled columns using different stationary-phase chemistries. The column ensemble consists of a trifluoropropylmethyl polysiloxane column followed by a dimethyl polysiloxane column. Each pressure pulse causes a differential change in the carrier gas velocities in the two columns, which lasts for the duration of the pulse. A pressure pulse is used to selectively increase the separation of a component pair that is separated by the first column but coelutes from the series-coupled ensemble. If both components are on the same column when the pulse is applied, a small change in the ensemble separation occurs. If one component of the pair is on the first column and the other component is on the second column, a pressure pulse can result in a much larger change in the ensemble separation for the component pair. A model with a spreadsheet algorithm is used to predict the effects of a pressure pulse on the trajectories of component bands on the column ensemble. The effect of the initiation time of a pressure pulse is investigated for a two-component mixture that coelutes from the column ensemble. For the case where the entire pressure pulse occurs when one of the components is on the first column and the other component is on the second column, the peak separation from the ensemble increases nearly linearly with the product of the pressure pulse amplitude and the pulse duration. Peak shape artifacts are observed if the pressure pulse occurs when a solute band is migrating across the column junction point.

A model and a spreadsheet algorithm is described for the prediction of solute-band migration trajectories in a series-coupled combination of two capillary GC columns with pressure-tunable and -programmable selectivity and operated under temperature-programmed conditions. The model takes into account the acceleration of carrier gas in the two columns as a result of decompression effects, the deceleration of carrier gas as a result of the increase in viscosity during temperature programming, the decrease in solute retention factors with increasing temperature during the temperature program, the differences in retention factors for the two columns, and programmed changes in the carrier-gas flow rates in the two columns during selectivity programming. In the model, the 20-meter-long column ensemble is divided into 1-cm-long intervals, and the carrier-gas velocity and column temperature are assummed to be constant in any interval. Migration times for all of the mixture solutes are computed for each column interval, and the solute-band positons in the column ensemble are plotted versus the running sum of these migration times to obtain band trajectory plots. The sum of these migration times for all 2,000 intervals gives the ensemble retention times for the solutes. Isothermal retention factors (k) for all of the mixture components at various column temperatures (Tc) are used as imput to the algorithm. Slope and intercept values of In(k) vs 1/Tc plots are used in the algorithm. General features of the model are tested using a mixture of C12-C24 normal alkanes. A mixture of polar and nonpolar compounds is used to test the utility of the model for the predicition of peak separations and retention times with pressure-tunable and -programmable selectivity. Good agreement is observed in all cases.

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