Effects of resolution, spectral window, and background on multivariate calibrations used for open-path Fourier transform infrared spectrometry

The performance of CLS, PLS-P, and PLS-1 on calibration/validation sets of mixtures of five short-chain alcohols was examined as a function of resolut ion, spectral window, and type of background single beam to determine the f easibility of achieving automated calibrations for open-path Fourier-transf orm infrared (OP-FTIR) spectrometry. The backgrounds used in the study were created by ratioing actual single-beam OP-FTIR spectra. Some backgrounds w ere created by ratioing spectra measured at the same path length, and other s were created by ratioing a long-path sample spectrum against a short-path reference. After conversion to absorbance, these background spectra provid ed a noise term that is representative of what is seen in actual field depl oyment. The absorbance spectra of mixtures of the spectra of up to five sho rt-chain alcohols (chosen because of their spectral similarity) were added to these spectral baselines. CLS was shown to perform well only when using equidistant backgrounds and TO-16 spectral window selection, with average e rrors of 2-20%. CLS calibrations under other conditions tested typically pr oduced average errors of greater then 100%. Under most conditions the measu rement of OP-FTIR spectra at low (8 cm(-1)) resolution produced an increase in the accuracy of the determinations over higher-resolution spectra. Both PLS-I and PLS-P were able to predict the concentrations of the alcohols un der any conditions, including short-path backgrounds, with an error of less than 5%; errors of less than 2% were achieved in most cases. The results i ndicate that an automated calibration system is feasible for OP-FTIR spectr a measured at tow resolution, even when a short-path background is used. (C ) 1999 John Wiley & Sons, Inc.