From sediments to the atmosphere: a mass spectrometry approach revealing structural dissimilarities of common NOM components

Natural organic matter (NOM) is a complex mixture of thousands of organic molecules that reflects environmental conditions and chemical transformations occurring nowadays or in the past. Fourier transform mass spectrometry (FTMS) resolves isobaric constituents and it is widely applied to obtain aquatic, terrigenous and aerosol NOM fingerprints. Traditionally, comparison of mass peak intensities is used to make a distinctive conclusion about samples behavior, but it has the limitation of omitting structural information on the corresponding ions. Due to the stochastic character of NOM synthesis, drastically different samples may appear as resembling, which hampers mechanistic study of NOM dynamics and its attribution to the source. Here we present how implementation of chemical and isotopic tagging in combination with FTMS helps to overcome this issue. The developed approach provides an upper boundary for the presence of specific structural features, e.g. functional groups, in individual NOM components. This facilitates a clear distinction between different NOM samples, which would share isobaric ions, and provides insights on isomeric complexity of these ions. The advantages of the method were demonstrated on two sets of samples. Firstly, we collected permafrost peat cores from different depths in the European Arctic region, which varied in corresponding botanical conditions, peat degradation and oxidation states. Selective deuteromethylation and bromination coupled to FTMS enabled to capture structural differences between shared ions, which differed in carboxylic functionality and aromaticity. Surprisingly, structural differences were found for ions, which abundance positively correlated with peat characteristics and geo-temporal conditions. The second set included aerosol particles collected in marine, rural and urban areas. Application of in-source H/D exchange for FTMS analysis of extracted NOM enabled to enumerate functional groups in shared ions and point molecular constituents with similar and distinct structural features. The observed trends serve to better understand aerosol formation processes and accompanied conventional formula-based statistical analysis including better understanding of Kendrick mass defect series.