Column Selectivity Programming and Fast Temperature Programming for High-Speed GC Analysis of Purgeable Organic Compounds

High-speed gas chromatograms are obtained by the use of relatively short lengths of capillary column operated at relatively large carrier gas flow rates. This approach is difficult for more complex mixtures because of the reduced peak capacity available with shorter columns. A solution to this problem is the use of tunable column ensembles consisting of the series (tandem) combination of a polar and a nonpolar column. By adjusting the pressure at the junction point between the columns, the selectivity of the ensemble can be adjusted within the limits imposed by the individual columns. For mixtures representing a relatively large boiling point range and containing more than ～20 components, high-speed, isothermal separations are less effective. These limitations are significantly reduced by combining fast temperature programming with selectivity programming. Selectivity programming is obtained by changing the pressure at the column junction point one or more times during the course of an analysis. In the work described here, the column ensemble temperature and the junction pressure are initially set to give a high-quality separation of the earliest eluting components. After these components have eluted, a linear temperature ramp of ～35 °C/min is initiated. As the temperature increases, the pressure is adjusted to change the selectivity and thus facilitate the separation of groups of components as they migrate through the column ensemble. Using this approach, a mixture of 30 purgeable organic compounds is separated in less than 2.5 min.