MS/MS Data Improves Automated Determination of Molecular Formulas by Mass Spectrometry

In theory, the molecular formula of an unknown compound can be calculated from its exact molecular mass. However, even with highly accurate modern mass spectrometers with an accuracy of 1 ppm or lower, it is generally not possible to determine the molecular formula uniquely from measurements. Intensities of isotopic peaks are typically used as additional information to narrow down possible formulas associated with a mass spectrum (MS) peak, but this is not sufficient for larger compounds. Here, we introduce a method that takes information from fragment peak masses of the MS/MS into account to improve the reliability of formula determination. Matchvalues that reflect the consistency with MS isotope peaks and MS/MS fragment patterns are computed for candidate molecular formulas. We demonstrate that these matchvalues outperform methods based on isotope peak intensities alone. In test cases with medium sized organic molecules (< 1000 u) the true molecular formula achieves the highest matchvalues using the MS/MS data.

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