The effects of resolution, spectral window, and background
type on the predictive ability of classical least squares
regression (CLS) on spectra measured by an open-path
Fourier transform (OP/FT-IR) spectrometer were tested in
some detail. It is shown that the most accurate quantitative
results are obtained by using equidistant backgrounds,
reduced spectral windows, and low resolution. The effect
of interfering compounds is shown to be particularly
serious when CLS regression is used to process OP/FT-IR
spectra.