For conversion of spectral information of an FTIR spectrometric instrument for comparison with that of a dispersion instrument, a first standard function is selected for spectral line shape for the first instrument, and a second standard function for line shape is selected for the second instrument. A conversion factor is computed for converting the first standard function to the second standard function. In ordinary operations, first spectral information is obtained with the first instrument for a first sample, and second spectral information is obtained with the second instrument for a second sample. The conversion factor is applied to the first spectral information to effect converted information, and the converted information is compared with the second spectral information. Such conversion also is applied between chromatographic instruments.