Determination of Elemental Compositions from Mass Peak Profiles of the Molecular Ion (M) and the M + 1 and M + 2 Ions

The relative abundances of M + 1 and M + 2 ions help to identify the elemental composition of the molecular ion (M). But scan speed, sensitivity, and resolution limitations of mass spectrometers have impeded determination of these abundances. Mass peak profiling from selected ion recording data (MPPSIRD) provided faster sampling and enhanced sensitivity, which permitted use of higher resolution. M + 2 profiles having only a few percent of the ion abundance of M were monitored at 20 000 resolution. The relative abundances, exact masses, and shapes of M, M + 1, and M + 2 mass peak profiles were determined. By applying five criteria based on these quantities, elemental compositions were determined even for ions too large (up to 766 Da) to be uniquely assigned from their exact mass and accuracy limits alone. A profile generation model (PGM) was written to predict these resolution-dependent quantities by considering all M + 1 and M + 2 ions for each candidate composition. The model also provided assurance that no other compositions were possible. Characterization of the M + 1 and M + 2 profiles by MPPSIRD and the PGM greatly expanded the practical ability of high-resolution mass spectrometry to determine elemental compositions.