Instrumentation
Peter R. Griffiths Department of Chemistry University of California Riverside, Calif. 92521
James A. de Haseth Department of Chemistry University of Georgia Athens, Ga. 30602
Leo V. Azarraga Environmental Research Laboratory U.S. Environmental Protection Agency Athens, Ga. 30613
Capillary GC/FT-IR The multiplex and throughput advantages of Fourier transform infrared (FT-IR) spectrometry allow spectra to be measured either faster or with higher signal-to-noise (S/N) ratio than if a grating monochromator were used for the same measurement. For many years chemists have recognized that one application for which these advantages should be of particular benefit is the on-line measurement of the infrared (IR) spectra of peaks eluting from a gas chromatograph without trapping the sample (GC/FT-IR). The first demonstration of GC/ FT-IR was reported by Low (1 ) in 1966, and the following year Low and Freeman (2) reported more detailed results. They recorded interferograms generated by a small low-resolution (16 cm" 1 ) rapid-scanning Michelson interferometer on an analog tape recorder, while components separated by a gas chromatograph using a semipreparative column were flowing through a gas cell. In 1969, the first medium-resolution minicomputer-controlled mid-infrared F T - I R spectrometer, the FTS-14, was introduced by Digilab. Several GC/ F T - I R measurements were made on this instrument. Most of the early experiments were performed using a small accessory that could be easily installed in the sample compartment of the FTS-14. The flow-through gas cell of this accessory was constructed from stainless steel and had an internal diameter (i.d.) of 6 mm and a length of 5 cm. The entire accessory had a trans0003-2700/83/A351-1361$01.50/0 © 1983 American Chemical Society
mittance of about 25% when installed, which was high enough that spectra of fairly high S/N could be obtained from components injected into the chromatograph at levels of a few tens of micrograms (3). Like many subsequent GC/FT-IR interfaces, this device integrated optics, electronics, and software. A sensor monitored the signal from the GC detector. When this signal exceeded a certain threshold voltage, and after a delay to permit the "peak" to travel from the detector to the gas cell, interferograms were averaged until the GC signal returned below the threshold. Peaks could be trapped in the cell to permit more extensive signal averaging, and the option of interrupting the flow of carrier gas through the chromatograph while each peak was trapped in the cell was also available. The relatively high detection limits afforded by this accessory, however, did not lead to wide acceptance of GC/ F T - I R by analytical chemists. The modern era of GC/FT-IR was heralded by two developments that took place almost a decade ago. The first was the introduction of the narrow-range mercury cadmium telluride (MCT) photodetector. The specific detectivity (D*) of this detector is more than an order of magnitude greater than that of the triglycine sulfate (TGS) pyroelectric bolometer, which is the standard detector supplied with most commercial FT-IR spectrometers. The second breakthrough was the construction of gold1361 A
coated borosilicate glass "light pipes" by Azarraga (4). These tubes typically have i.d.'s of between 1 and 2 mm and lengths up to 80 cm. The effluent from the GC column is fed into one end of the light pipe and out the other. Alkali halide windows are fixed to each end. A schematic of a GC/FT-IR instru ment is given in Figure 1. When a very smooth layer of gold is coated on the inside surface, the transmittance of a 2-mm-i.d., 50-cmlong light pipe at ambient tempera ture can be as great as 25%. The vol ume and transmittance of this light pipe are about the same as that of the original light pipe made by Digilab, but the path length is 10 times longer, yielding a decrease in GC/FT-IR de tection limits of about an order of magnitude. The first routine submicrogram detection limits for GC/FT-
tra can be measured in
