Abstract: A method and apparatus to determine the constituent ingredients in a sample by a spectroscopic procedure is disclosed. A Fourier transformation infrared spectrometer can secure data of a power spectrum of a sample and data of a power spectrum of a reference. A absorption spectrum of the sample from this respective data can be determined. A set of wave number points across the absorption spectrum of the sample that exhibit a predetermined linear characteristic can be selected and the absorption spectrum values corresponding to the set of wave number points can be used to calculate the concentrations of the constituent ingredients. A plurality of sets of wave number points can also be predetermined for a specific sample and limit values can be stored and compared to pick the most desirable set of wave number points for calculating the concentration of the constituent ingredients.