Absorption Mode Spectral Processing Improves Data Quality of Natural Organic Matter Analysis by Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry.

Natural organic matter (NOM) plays an important role in elemental cycles and ecology. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) is an ultrahigh resolution technique used to molecularly resolve the complexity of NOM mixtures. However, even the very high mass resolution of FT-ICR-MS may result in multiple formula assignments to peaks in an NOM spectrum, especially at the high mass-to-charge ratio (m/z). The absorption mode is one option to process raw FT-ICR-MS data that can further increase the resolution of the peaks and has not been widely applied in NOM studies. In this study, we show the advantages of using the absorption mode for the analysis of NOM samples using a reference sample (Suwannee River fulvic acid). The absorption mode increased the precision of peak detection as well as the number (+23%) and accuracy of formula assignment (by 28%) when compared to the magnitude mode, besides achieving three times higher resolution. The results presented here highlight the potential to reduce the error threshold used during molecular formula assignment. In conclusion, the absorption mode shows advantages in the processing of NOM samples and other complex mixtures and should be promoted in the NOM community.