Factors Affecting Sensitivity in Lightpipe Gas Chromatography Fourier Transform Infrared Interfaces

A variety of experimental parameters are evaluated which are important in maximizing the sensitivity of the combination of gas chromatography and Fourier transform infrared spectrometry (GC/FT-IR) for the detection and identification of mixtures of volatile organic compounds. Basic parameters can be optimized such as the mirror scan velocity and the detector bias current employed to reduce the overall system noise, thus improving the signal-to-noise ratio (S/N) of the resulting spectra. Quantitation of the resulting trade-off between using a lower-noise narrow-spectral-range mercury-cadmium-telluride (MCT) detector versus a wider-spectral-range MCT detector with somewhat higher noise characteristics is examined. An optical design incorporating a variable aperture to reduce the amount of unmodulated infrared radiation from the heated lightpipe which reaches the detector is evaluated. Linearity in sample absorbance is shown to be unaffected by the amount of unmodulated heat reaching the detector despite the nonlinear behavior of the detector observed as interferogram signal loss at elevated detector temperatures. By flowing a fixed concentration of isobutane in helium through the lightpipe, one can monitor the absorbance; it is observed to change significantly from ambient to operating temperature, mostly due to temperature broadening of the absorbance bands. However, no discernible change in the absorbance is observed as a function of the amount of unmodulated light reaching the detector. Finally, lightpipe dimensions are examined as they affect sensitivity. A comparison of the performance of a 1.1-mm i.d. and a 0.6-mm i.d. lightpipe shows increased chromatographic performance of the latter, as expected, but similar signal losses with temperature. Little or no evidence for the counter Jacquinot (sometimes referred to as the Hirschfeld) advantage is found, and reasons for this inconsistency are discussed.