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Application of computerized differentiation technique to remote-sensing Fourier transform infrared spectrometry for analysis of toxic vapors.

A differential method was developed for overcoming the difficulty in collection of background spectra when a remote-sensing FTIR is used during field measurements of air contaminants. A background spectrum was generated by introducing a delta lambda interval for every sampled data point of the spectrum. Temporal variations in spectral intensity caused by instability of the interferometer and changing IR beam configurations were greatly reduced through the use of the differential spectrum. Multicomponent analysis was performed by means of using a least-squares fit program with the differential spectrum. Three sets of spectra were evaluated by use of both the differential technique and the traditional method, which employs a clean background spectrum. There was no significant difference found between these two methods when a clean background spectrum could be used. However, in the case of field air monitoring system evaluations (as opposed to laboratory-based instrument evaluations), clean background spectra are frequently not available. Although the differential technique results in an increase in the limit of detection, results demonstrated that this method has sufficient sensitivity to detect and quantify both individual and mixtures of toxic compounds in the workplace.