Band-Trajectory Model for Temperature-Programmed Series-Coupled Column Ensembles with Pressure-Tunable Selectivity

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Abstract
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 2000 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 ln(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.