spectra that were subsequently digi tized from the hard copy. Computerized identification of com pounds from their IR spectra has been performed in several ways. The most common method involves comparison of the absorbance spectrum of the un known with a library of reference spectra after scaling the absorbance of the most intense peak in the sample and reference spectra to the same val ue. This approach has been very suc cessful for gas-phase spectra, but the level of success for condensed-phase spectra is reduced by the presence of sloping base lines. Condensed-phase spectra have been converted to their first derivatives to reduce the effect of base line slope before starting the search. Another way of improving search re sults is to compare the Fourier trans form (FT) of the spectrum of each un known with the FT of all spectra in the data base. Both the early points and late points in these arrays may be truncated to eliminate the effect of base line slope and high-frequency noise, respectively. Although the effi cacy of this approach has been shown (4), it has still to be implemented on any commercial GC/FT-IR system. The sensitivity and data-processing capabilities of contemporary FT-IR spectrometers are at a very advanced
level. Scan speeds are sufficiently high that several interferograms can be av eraged per second, allowing real-time monitoring of peaks eluting from all types of chromatographs. In the re maining sections of this paper, the present state of the art of GC/FT-IR, HPLC/FT-IR, and SFC/FT-IR inter faces is described. We hope that this treatment shows the common features of these interfaces and provides an in dication of the directions that the de velopment of these devices is taking. Capillary GC/FT-IR
The most important component of most GC/FT-IR interfaces is an inter nally gold-coated glass tube, or light pipe, which serves as a gas cell through which the effluent from a capillary GC column flows. Spectra are measured continuously at inter vals of about 1 s, typically by signalaveraging blocks of 3-10 interfero grams per spectral data file. The di mensions of most light pipes designed for capillary GC/FT-IR are fairly sim ilar as the cell volume should be ap proximately equal to the volume of carrier gas contained between the half-height points of the GC peaks to obtain spectra with the optimum S/N (5). The actual peak volume depends on the internal diameter (i.d.) of the GC column and the thickness of the
stationary phase, but it is usually be tween about 50 and 200 μ\^. A suitable compromise between length and i.d. of the light pipe must be made if maxi mum performance is to be attained. A consensus among most instrument de signers now appears to have been reached, and most light pipes for cap illary GC/FT-IR are constructed from glass tubes with an i.d. of 1 mm and a length between 10 and 20 cm. With GC/FT-IR interfaces based on light pipes, identifiable spectra can usually be measured from injected quantities of strongly absorbing samples in the 5-25-ng range (6-8). For weakly ab sorbing compounds, however, it may be necessary to inject as much as 100 ng to obtain an identifiable spec trum. Although it is possible that the minimum identifiable quantity (MIQ) of strongly absorbing samples being characterized using light-pipe-based GC/FT-IR interfaces could be re duced to about 400 pg in a completely optimized system (9), it is quite un likely that any further decrease can be achieved without changing the funda mental nature of the GC/FT-IR inter face. One way in which the MIQ has been reduced to subnanogram levels has in volved trapping each separated com ponent in some fashion. One of the better ways of trapping GC eluates is
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CIRCLE 35 ON READER SERVICE CARD 1352 A · ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER
1986
