Quantitative analysis of individual compounds in complex mixtures using high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) is complicated by differences in the ionization efficiencies of analyte molecules, mainly resulting in signal suppression during ionization. However, the ability to obtain concentration estimates of compounds in environmental samples is important for data interpretation and comparison. We introduce an approach for estimating mass concentrations of analytes observed in a multicomponent mixture by HR-ESI-MS, without prior separation. The approach relies, in part, on a matrix-matched calibration of the instrument using appropriate standards added to the analyte matrix. An illustration of how the proposed calibration can be applied in practice is provided for aqueous extracts of isoprene photooxidation secondary organic aerosol, with multifunctional organic acid standards. We show that the observed ion sensitivities in ESI are positively correlated with the “adjusted mass,” defined as a product of the molecular mass and the H/C ratio in the molecule (adjusted mass = H/C × molecular mass). The correlation of the observed ESI sensitivity with adjusted mass is associated with the trends of the physical and chemical properties of organic compounds that affect ionization in the positive ion mode, i.e., gas-phase basicity, polarizability, and molecular size.
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