Initial Results From An Interferogram-Based Search System

AbstractSearch algorithms based on the time (or distance) domain representation of the infra­ red absorption spectrum were constructed and demonstrated. Preliminary results indicate significant advantages in speed, reliability, and efficiency of the search systems over the traditional spectrum-based method. A direct interferogram-based search system with a high degree of reliability was demonstrated for the first time and the requirements for an instrument-independent data were delineated.IntroductionOne method of compressing a spectrum is to take its inverse Fourier transform and obtain a time (or distance) domain representation. Fellgett showed that each datum of the time domain contains information about every frequency in the spectrum.1 The resolution of the spectrum also depends upon the size of its time domain representation; i.e., the longer the interferogram, the higher the resolution of the spectrum. Thus, a short interferogram has all peak position, intensity, band-shape and peak width information of a low resolution spectrum.GC-IR data are usually collected with a Fourier transform infrared spectrometer (FT-IR) in the form of interferograms. It seems logical, therefore, to use the interferometric data for direct infrared searches. The usefulness of interferometric data has been demonstrated by the implementation of a gas chromatographic reconstruction algorithm.^ Direct interferometric search systems have been proposed^ and at least one attempt has been made to perform such a search,4 but the results were not suffi­ ciently reliable for it to be used as routine search method. In this work we present a direct interferogram-based search and a search system based on the inverse Fourier trans­ form of the IR absorbance spectrum, named, "Fourier Encoded Data Search for Infrared Spectra (FEDS/IRS)"5, that exhibits high reliability, speed, and efficiency.Exper imentalComputations were carried out on a Nova 3/12 minicomputer with 128 kbytes of memory and a 10-mbyte moving head disk. The minicomputer system is part of a Digilab(R) FTS-14/C/D Fourier Transform Infrared Spectrometer. Some computations were too time con­ suming to be completed on the minicomputer system and these were executed on a Univac 1100/61 mainframe system with 1,048,576 words (36 bits/word) of high speed memory.The data used for the direct inter ferogr am s-earch and FEDS/IRS were from the collec­ tion of 3300 vapor phase infrared spectra collected by the U.S. Environmental Protection Agency, Environmental Research Laboratory, Athens, Georgia. These spectra are in the format specified by the Coblentz Society.6 The sample spectra were obtained from the laboratory's GC/FT-IR system.7 Programs were written for the phase correction of all 6600 interferograms (one sample and one reference per entry) in the laboratory's vapor phase IR library. All programs were written in FORTRAN except for one assembler language subroutine.Results and Discussion Direct interferogram searchIt is not a straightforward operation to calculate the time domain representation of the absorbance spectrum from the background and sample interferograms without going through Fourier transformations. An alternate instrument-independent data representation of the interferograms, therefore, must be found for a direct interferogram based search system. One aspect of experimental FT-IR spectroscopy is that collected interferograms exhibit a phase error. Such errors produce asymmetry (chirping) in the interferogram. The primary causes of the phase error may be found in the frequency dependent dispersive qualities of the beamsplitter (particularly the support medium for the dielectric beam­ splitter film), the sampling of the interferogram, and the recording electronics. In