Open-path FT-IR spectrometry: is completely unattended operation feasible?

Most protocols used for open-path Fourier transform infrared spectrometry (OP/FT-IR) require that spectra be measured at a resolution of 1 cm−1 and that the concentrations of the analytes be calculated by classical least squares regression (CLS). These specifications were largely developed for monitoring light molecules with easily resolvable rotational fine structure. For most volatile organic compounds in air, the rotational fine structure is not resolvable and better accuracy can be obtained when the spectrum is measured at lower resolution (typically 8 cm−1), provided that the algorithm used for quantification is partial least squares regression (PLS). By measuring the spectrum at low resolution, the need for a liquid-nitrogen-cooled mercury cadmium telluride detector is reduced and a pyroelectric detector operating at ambient temperature can be used instead. By using PLS rather than CLS, spectral features due to water vapor do not have to be compensated and a short-path background spectrum can be used, greatly simplifying field measurements.