Molecular formula isotope ratio method

As molecules become larger and more complex, the number of possible combinations that yield M + 1 and M+2 peaks grows. For a particular combination of atoms, the intensities of these peaks relative to the intensity of the molecular ion peak are unique. Thus, the isotope ratio method can be used to establish the molecular formula of a compound. Tables of possible combinations of carbon, hydrogen, oxygen, and nitrogen and intensity ratios for the M + 1 and M+2 peaks for each combination have been developed. Appendices 10 an 11 and the book by Beynon (in the reference hst at the end of this chapter) contain extensive tables of this sort. For a given molecular weight, you can examine the tables to find the molecular formula that corresponds to the isotope ratios observed. 

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