1. A method for determining concentration of a substance in a mixture of n substances defined by a chemical image having a plurality of pixels, the method comprising: (a) providing a spectrum for each of the n substances in the mixture; (b) obtaining a spectrum for one of the plurality of pixels, the spectrum defining the pixel as a function of intensity and wavelength; (c) calculating an estimated concentration for each substance in the mixture as a function of the spectrum for each substance and the pixel spectrum; (d) calculating an estimated pure spectrum for each substance as a function of the estimated concentration for each substance and the pixel spectrum; (e) calculating a deviation value as a function of the estimated pure spectrum and the spectrum provided in step (b); (f) repeating steps (c)-(e) 2 n -1 times with different combinations of n substances to determine m deviation values; and (g) selecting the lowest deviation value from among m deviation values as the most-likely concentration for each substance in the mixture.