Programmable control of column selectivity for temperature-programmed GC

A computer-driven pressure controller connected to the junction point of a series-coupled ensemble of two capillary GC columns having different statio nary-phase selectivity is used to obtain on-the-fly (programmable) changes in ensemble selectivity. Changes in the junction-point pressure result in d ifferential changes in the focal carrier gas velocity in the two columns, a nd this results in changes in the pattern of peaks eluting from the ensembl e. When used with relatively fast temperature programming (30 degreesC/min) , the pattern of eluting peaks can be very sensitive to the time at which a selectivity (junction-point pressure) change is implemented. These elution pattern changes are described for a set of six PCB congeners that elute wi th a small range of retention times. The components are considered as a gro up, and changes in their elution pattern are described for a single junctio n-point pressure change, which is implemented at various times after sample injection. If the pressure change is implemented after the components have migrated across the junction point, the final pressure has relatively litt le impact on the ensemble retention pattern. Pressure changes implemented p rior to the components reaching the junction can have a large effect and us ually result in a pattern of peaks similar to the pattern obtained when the final pressure is used for the entire separation. For pressure changes mad e when the group of components is near the junction point, the observed pea k pattern may be very sensitive to the time of the pressure change. The tim e at which the junction-point pressure change occurs is varied in 1.0-s int ervals. Artifacts such as peak doubling and peak focusing or broadening are observed if a migrating band is crossing the column junction point at the time of the programmed pressure change.