Multivariate quantitative analysis of FTIR-spectra requires that the spectral line shapes of sample spectrum and spectral reference, and hence, the respective instrumental line shape (ILS) functions, match as closely as possible. In open-path measurements, the ILS' generally differ because of the differences in the optical geometry of the setups, or in the case of synthetic reference spectra, because the ILS cannot exactly be determined from the measuring parameter. In particular, when using large field stop diameters the line shapes can deviate considerably. Therefore, a mathematical method to approximate the ILS based on a parametrical identification has been developed and integrated into an algorithm for quantitative multivariate analysis. This article gives an overview over the parametrical model of the ILS approximation and its integration in the spectral evaluation algorithm, and presents results of their application to transmission spectra of extractive and open-path exhaust gas measurements.