The method of spectrometric determination of gas concentration by use of a mask is examined and described. Calibration curves for four gases, NH3, C6H5CH3, NO2, and SO2, are reported in detail, and results are included for C6H6, NO, and SiF4. The calibration curves are shown to be linear in a log-log plot over ranges of practical interest. The slopes are reproducible. The technique used in making masks is described. The proposed manner of making measurements involves the use of a small reference cell containing a sample of the gas being measured. When such a method of measurement is used, several gases of practical importance are concluded to be determined as to concentration to a few parts per billion with a precision of 10% or less, if path lengths of the order of several hundred meters are used. The results appear to be independent of the presence of unknown absorbing gases under many circumstances.
David T. Williams and Byron L. Kolitz, "Molecular Correlation Spectrometry," Appl. Opt. 7, 607-616 (1968)