Often the resolution of mass spectra is insufficient to conclusively identify and quantify all peaks via least-squares fitting, and we address several practical problems in this area. We first describe an algorithm to identify the most likely composition of individual peaks while using all experimental constraints. For compounds dominated by CHON, bulk chemical properties such as carbon number and elemental ratios can be reliably extracted, even though some ion compositions may remain uncertain. The reason is that the mass defect is related to elemental ratios under our composition assumptions. Second, a statistical simulation approach is used to assess the statistical precision of the retrieved peak intensities. The precision on the fitted peak intensity falls into one of three regimes. In the “counting-limited regime”, above a peak separation χ ~ 2-3 half widths at half-maximum (HWHM), the intensity precision is similar to that due to counting error for an isolated ion. For smaller χ and higher ion counts (~1000 and higher), the intensity precision rapidly degrades as the peak separation is reduced (“calibration-limited regime”). Alternatively for χ &lt; 1.6 but lower ion counts (e.g. 10-100) the intensity precision is dominated by the additional ion count noise from the overlapping ion, and is not affected by the imprecision in the m/Q calibration (“overlap-limited regime”). The transition between the counting and m/Q-limited regimes is shown to be weakly dependent on resolving power and data spacing and can thus be approximated by a simple parameterisation based only on peak intensity ratios and separation. An approximate graphical diagnostic can also be used when evaluating results from fitted spectra containing many ions. Longer integration times can improve the precision in the counting-limited, but a given ion pair can only be moved out of the calibration-limited regime through increased spectrometer resolving power. Third, we propose a method to derive bulk composition parameters directly from the raw spectra without the need of any peak assignment or peak fitting, which shows good agreement with peak fitting results. This method can be applied very rapidly without the need for complex analysis procedures, e.g. as a quick online diagnostic during data acquisition. Real data sets from atmospheric chemistry applications using time-of-flight aerosol mass spectrometers and chemical ionization mass spectrometers are used to illustrate these findings.

Constrained fitting of overlapping peaks in high-resolution mass spectra: Which ions should we fit,  what is the precision of the fitted intensities, can we derive bulk chemical parameters without fitting?

249th National Meeting (2015)

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