Novel flame photometric detector for gas chromatography based on counter-current gas flows

Abstract

A novel analytical device has been developed for gas chromatography. It is based on optical emission from a counter-current (i.e. counter-flowing) air or oxygen flame, which burns in an opposing stream of hydrogen and column effluent. The flame is typically positioned “upside down” on the upper (air) jet, which faces the lower (hydrogen+effluent) jet. It can also be positioned on the lower jet, be connected to both jets, or be suspended in the gap between them. Excellent stability can be obtained in any of these modes. Overall, this new “counter-current flame photometric detector” (ccFPD) responds to analytes in the manner of a conventional flame photometric detector (FPD); however, it can be operated over a much wider range of gas flows. For instance, the same physical ccFPD burner easily supports stable flames of air flows between 5 and 200 ml/min and corresponding hydrogen flows between 5 and 10,000 ml/min. Visual observation of the counter-current flame, in the presence of sulfur and phosphorus as test analytes, reveals intense, steady luminescence under a wide variety of conditions. Additionally, and in contrast to the commercial FPD, flame conductivity signals can be obtained that are similar in quality to those produced by a conventional flame ionization detector (FID). Thus the ccFPD is a flexible, easily optimized photometric detector. The exceptional flow stability of the ccFPD was used to explore the earlier reported phenomenon of strong signal/noise (S/N) ratios, which had been obtained for hetero-elements of the iron group from a conventional FPD with a small, stoichiometric flame. Results using the ccFPD, which also exhibits this unusual response, indicate that these high S/N ratios are only partly due to the predictable decrease in flame noise with decreasing flame size. Contrary to expectations, the absolute analyte signal often increases as the flame size decreases to the point of extinction. The signal intensity and the magnitude of the observed changes depend to some degree on the flame composition (H 2/O 2 ratio).