The isotopic mass defect: a tool for limiting molecular formulas by accurate mass


This paper describes the use of the relative isotopic mass defect, which is the mass defect between the monoisotopic mass of an element and the mass of its A + 1 or its A + 2 isotopic cluster. The relative isotopic mass defect is combined with the intensity of the isotopic cluster and a formula generator to find the correct molecular formula for unknown pesticides, using accurate mass measurements. This paper introduces the concept of the relative mass defect of isotopes and the isotopic mass average (IMA), especially for C, H, N, O, S, Cl, and Br, and how to correlate these measurements to the correct molecular formula of an unknown compound. A heuristic rule of ±3 × 10−3 u (+3 millimass units) is developed as a simple observational tool for viewing accurate mass data with four-decimal-place mass accuracy. This heuristic rule allows one to rapidly scan data “by eye” without the use of sophisticated software, and is a useful and rapid way of examining a molecular formula.

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  1. 1.

    The mass spectrometrist’s definition of mass defect will always be in italics to differentiate it from the correct physical definition of the mass defect.

  2. 2.

    The example described here for the relative isotopic mass defect was further addressed by us in US Patent no. 7462818 by Zweigenbaum, Ferrer, and Thurman on the determination of chemical empirical formulas of unknown compounds using ion mass measurements of all isotopes, issued 9 December 2008.


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The authors acknowledge Agilent Technologies for instrument support and supplies and technical input on this paper. In particular, we acknowledge the help and assistance of Jerry Zweigenbaum of Agilent Technologies.

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Correspondence to E. Michael Thurman.

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Thurman, E.M., Ferrer, I. The isotopic mass defect: a tool for limiting molecular formulas by accurate mass. Anal Bioanal Chem 397, 2807–2816 (2010).

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  • Liquid chromatography/mass spectrometry
  • Accurate mass
  • Mass defect
  • Isotopologs
  • Pesticides
  • Unknown identification