High-speed vacuum outlet capillary gas chromatography with selective detection.

Abstract

A microcavity hollow cathode lamp (HCL) and a commercial photoionization device (PID) were evaluated as selective detectors for high speed, vacuum outlet GC. Because of the need for low pressures, column parameter optimization under vacuum outlet operation was performed and tested with the PID. Element selective detection of C, F, Cl, and Br was obtained with the HCL. A computer model based on the Golay-Giddings equation was developed to select optimal conditions for vacuum outlet operation. It was shown that vacuum outlet operation allowed for the use of higher carrier gas velocities without loss in chromatographic efficiency. This effect was more dramatic with short and wide bore columns. The model also suggested that outlet pressures up to about 10 Torr could be used with less than 10% decrease in chromatographic efficiency. Under optimal conditions, column length values were always shorter for vacuum outlet operation when compared to atmospheric operation. A PID was used to evaluate a GC system designed for vacuum outlet operation. Performance of the PID was dramatically improved when operated at 10 Torr relative to atmospheric pressure. Column efficiency also improved dramatically at reduced pressure and resolution may increase by as much as a factor of two for a 5-s separation with a 3.0-m x 0.32-mm i.d. column. Outlet pressure studies suggested that values up to about 100 Torr can be used but only if dead volume sources are negligible. As the pressure increased the effective dead volume of the detector increased and the efficiency of the chromatogram degraded. The HCL was used as an element specific detector for high speed vacuum outlet GC. A 0.8-mm i.d. microcavity was used to obtain a detector cell volume of about 10 $\mu$L. The effects of discharge current, pressure and cathode cavity diameter on emission intensities from neutral-atom lines of F, Cl, and Br and ions lines of Cl and Br were discussed. Line-to-background ratios generally were largest for smaller cathode diameters and higher pressures values. Element selective chromatograms at the C, F, Cl, and Br lines' wavelength were investigated. Detection limits for these elements are in the low picogram range and are comparable to other commercially available element selective detectors.