Elemental Composition Determination of Unknown Organometallic Compounds with Mass Spectral Accuracy

Mass Spectral (MS) determination of elemental composition for a small molecule organic compound has been well established, typically through the accurate mass measurement of its monoisotopic peak followed by a search of possible elemental compositions within a given mass tolerance window, e.g., 5ppm for a high resolution MS. For an organometallic compound containing one or more atoms of Sn, Pb, Fe, Hg, Ti, Cr, Mo, Ru, Pd etc., the monoisotopic peak may be too weak or even un-observable in the mass spectrum, creating a unique challenge for the conventional elemental composition determination. In this paper, we propose a generalized approach for elemental composition determination applicable to both conventional organic compounds containing C/H/N/O/S/Br/Cl etc., organometallic compounds, and any other compounds formed by any elements from the periodic table. Instead of relying on the correct identification and accurate measurement of the unknown’s monoisotopic peak, this generalized approach relies on the average or center mass of a whole isotope cluster including all significant isotopes. Searching for possible elemental compositions in a mass tolerance window around the average or center mass, a list of possible elemental compositions will be generated, evaluated, and ranked by spectral accuracy, with the top spectral accuracy hits as the most possible elemental compositions to be considered. Examples from environmental studies and catalytic research including Sn, Ru, and Pd will be presented to showcase the impact and interplay of factors such as computational efficiency, mass accuracy, spectral accuracy, resolving power (unit mass and HiRes including TOF and Orbitrap/FT MS), and spectral interferences and their accommodation and/or compensation.