Anal. Chem. 1983, 5 5 , 1553-1557
tained over 75% of the unknowns among the top five hits. Table VI1 illustrates a comparison of search results obtained from 16 and 32 point search vectors. The 16 point search vectors were calculated from the time domain representations of 8 cm-’ and 512 cm-l spectra while the 32 point search vectors were obtained from the time domain representations of 8 cm-l and 256 cm-l spectra. Since these search vectors were calculated from short segments of time domain representations (beginning 1past the burst), they contained only low resolution information. In both cases search results improved significantly wnth search vectors calculated from the inverse transforms of higher resolution spectra. These superior search results give an indication of the magnitude of leakage and phase error effects induced in calculating absorbance spectra from extremely short interferograms. Since all of the search vectors contained only low resolution information, it also appears that extremely low resolution spectra maintain many of the unique characteristics of highier resolution spectra. The infrared spectral time domain representation can be applied to GC/FT-IR analyses. For sample sizes greater than 400 ng, reliable search results are obtainable from 256 and 512 point interferograms using presently available data bases. With these shorter interferograms, a greater percentage of data collection occurw near the light burst, the region offering the highest interferometrilc signal to noise ratios. Griffiths has shown that a doubling of interferogram length requires a 4-fold increase in measurement time in order to match the signal to noise ratios of the lower resolution spectrum (10). Thus, it appears that the shorter interferograms may yield increased signal to noise ratios, provided, of course, that these shorter interferograms result in sufficiently increased scan rates. Future efforts are being directed toward this and toward
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determining resolution requirements for identifying compounds present in quantities approaching current GC/FT-IR sensitivity limits.
ACKNOWLEDGMENT The authorai wish to thank Leo V. Azarraga of the Environmental Protmtion Agency, Athens, GA, Dan T. Sparks and previous workers at the University of North Carolina, Chapel Hill, NC, and the Research Triangle Institute, Research Triangle Park, NC, for supplying and assisting in the collection of the GC/FT-IR data. Appreciation is also extended to R. B. Lam of Foxboro Analytical, Norwalk, CT, for his many helpful comments. LITERATURE CITED Rasmussen, G. T.; Isenhour, T. L. Appl. Spectrosc. 1979, 33, 37 1-378. Hangac, G.; Wledboldt, R. C.; Lam, R. B.; Isenhour, T. L. Appl. Specfrosc. 1082, 36, 40-47. Lam, R. B.; Foulk, S . J.; Isenhour, T. L. Anal. Chem. 1981, 53, 1679- 1684. Azarraga, L. V.; Williams, R. R . ; de Haseth, J. A. Appl. Spectrosc. 1981, 35, 466-489. Small, G. W.; Rasmussen, G. T.; Isenhour, T. L. Appl. Specfrosc. 1970, 33, 444-450. de Haseth, J. A.; Azarraga, L. V. Anal. Chem. 1981, 53, 2292-2296. Lam, R. B.; Wledboldt, R. C.; Isenhour, T. L. Anal. Chem. 1981, 5 3 , 889A-895A. Crawford, E. F.;Larsen, R. D. Anal. Chem. 1977, 49, 508-510. de Haseth, J. A.; Leclerc, D. F., presented at the 1982 FACSS meeting, paper 496. Grifflths, P. R. Anal. Chem. 1972, 4 4 , 1909-1913.
RECEIVED for review January 13, 1983. Accepted April 22, 1983. This work was supported by the National Science Foundation Gr
