image   Compound Identification

All of the IROA peaks are mathematically calculable and the number of carbons in any mass spectral object is always known. The number of carbon in a biological molecule can be determined by the distance between the two monoisotopic peaks, 12C and 13C (see figure below). The relative height of the M+1 and M-1 provides confirmation of this fact, resulting in a triply redundant quality control check point.

The height of the peaks of an isotopically defined compound may be calculated by the binomial expansion of the expression:

When the IROA approach is combined with accurate mass spectrometry measurement for isotopic abundances across a wide linear dynamic range, this facilitates not only the identification of all “non-carbon” atoms in unknown compounds but also reliable quantitative information (the IROA ratio).


In the few cases in which more than a single formula is returned, the subtraction of the mass of the carbons from the 12C and 13C base peaks, respectively, yields the residual mass of the other elements. Since both the 12C and 13C base peaks share the same formula, the error on this residual value can be minimized by averaging the two residual masses. The method of stating mass used to aid in the identification of molecules of similar chemical structure was suggested in 1963 by the chemist Edward Kendrick. Tables of these residuals can be created to resolve any remaining ambiguities.


A tutorial on IROA Compound Identification can be viewed here.