IMPROVEMENTS IN METHODS FOR SPECTRAL COMBINATION OF GAS-CHROMATOGRAPHY FOURIER-TRANSFORM INFRARED SPECTROSCOPIC DATA

Coaddition of spectra in a single-component peak of a gas chromatograp h (GC) obtained with a Fourier transform infrared spectrometer is the method generally used to improve the signal-to-noise ratio (S/N) of th e spectrum of the eluted analyte. It is commonly thought that coadditi on of spectra to a relative intensity level of 40% of the GC peak will lead to the optimal improvement in S/N of the resulting composite spe ctrum. We have shown that this is not generally the case for either si mulated Gaussian-shaped or experimentally obtained asymmetric GC bands . The optimal intensity level for coaddition is found to be a function of the shape of the GC band and the ratio of the number of background to sample scans used in generating the individual IR spectra. We have also introduced the use of classical least-squares (CLS) techniques a s a superior method to improve the S/N of the composite analyte spectr um. With the use of CLS methods, spectra included in generating the co mposite spectrum can be a small fraction of the maximum intensity in t he GC peak while still resulting in S/N improvements. The theoretical S/N of the composite spectrum with the use of CLS methods is shown to be always as good as or better than that achieved with the coaddition method. The improvements achieved in S/N when CLS methods are used can be more than a factor of two greater than results for the traditional coaddition method for the cases considered in this paper. Furthermore , it is shown that increasing the number of background to sample scans is a very convenient method to improve the S/N of the composite spect rum obtained by either method. The results presented here for GC/FT-IR are also generally applicable to LC/FT-IR, SFC/FT-IR, and TGA/FT-IR f or bands that contain a single analyte.