Abstract: A method of determining a trace gas concentration in a gas sample utilizing Fourier Transform Infrared Spectroscopy. The method includes the steps of(i) synthetically calibrating a spectrometer, and(ii) determining a spectral window within which to fit a calculated spectral trace to an experimental spectral trace by(a) choosing a series of candidate windows;(b) determining the likely error measure associated with a fitting of the spectral trace for each of the series of candidate windows;(c) utilizing the likely error measure associated with each of the fitting to determine a final window having substantially the lowest likely error measure; and(d) utilizing the final window as the spectral window.The calibration and the spectral window is then utilized to fit a calculated spectral trace to a spectral trace measured by the spectrometer and to determine the concentrations of constituent gases.