The spectral method for determining the concentrations of a substance in a mixture of any number of substances is defined by a chemical image having a plurality of pixels (520). The method includes steps of providing a spectrum for each of the n number of substances in the mixture (530), and obtaining the spectrum for one of the plurality of pixels, and calculating a plurality of estimated concentrations of each substance in the mixture as a function of the spectrum for each substance and the spectrum for the pixel, and calculating a deviation value for each of the plurality of estimated concentrations as a function of the spectrum of each of the number of substances in the mixture, and selecting the estimated concentration with the lowest deviation factor as a most likely concentration of each substance in the mixture (550).