Interlaboratory comparison of humic substances compositional space as measured by Fourier transform ion cyclotron resonance mass spectrometry (IUPAC Technical Report)

Abstract

Interlaboratory comparison on the determination of the molecular composition of humic substances (HS) was undertaken in the framework of IUPAC project 2016-015-2-600. The analysis was conducted using high resolution mass spectrometry, nominally, Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) with electrospray ionization. Six samples of HS from freshwater, soil, and leonardite were used for this study, including one sample of humic acids (HA) from coal (leonardite), two samples of soil HA (the sod-podzolic soil and chernozem), two samples of soil fulvic acids (FA) (the sod-podzolic soil and chernozem), and one sample of freshwater humic acids (the Suwannee River). The samples were analyzed on five different FTICR MS instruments using the routine conditions applied in each participating laboratory. The results were collected as mass lists, which were further assigned formulae for the determination of molecular composition. The similarity of the obtained data was evaluated using appropriate statistical metrics. The results have shown that direct comparison of discrete stoichiometries assigned to the mass lists obtained by the different laboratories yielded poor results with low values of the Jaccard similarity score – not exceeding 0.56 (not more than 56 % of the similar peaks). The least similarity was observed for the aromatics-rich HA samples from leonardite (coal) and the chernozem soil, which might be connected to difficulties in their ionization. The reliable similarity among the data obtained in this intercomparison study was achieved only by transforming a singular point (stoichiometry) in van Krevelen diagram into a sizeable pixel (a number of closely located stoichiometries), which can be calculated from the population density distribution. The conclusion was made that, so far, these are descriptors of occupation density distribution, which provide the metrics compliant with the data quality requirements, such as the reproducibility of the data measurements on different instruments.

Molecular Signatures of Glacial Dissolved Organic Matter From Svalbard and Greenland

“…Relative abundance weighted metrics of the mass, AI mod , NOSC, heteroatomic content (i.e., CHO, CHON, CHOS, CHONS), and formulae categories are reported in Table 1. In this study, we focus on changes in these metrics and relative abundance in formula categories, as this has shown to be most robust for comparisons across instruments (Hawkes et al., 2020; Zherebker et al., 2020). Suwanee River Fulvic Acid Standard (2S101F) showed < 2% difference in relative abundance of formula categories across instruments.…”

Molecular Insights into Glacial Cryoconite Dissolved Organic Matter Evolution under Dark Conditions during the Ablation Season on the Tibetan Plateau

“…34,36,43,44 It is worth noting that the number of assigned molecular formulae may be dependent on the sampling method, SPE procedure, ionization mechanism, FT-ICR MS parameters, and data acquisitions, but recent studies have shown robust comparisons across instruments regarding the parameters examined here. 45,46 FT-ICR MS results in our study show that the initial cryoconite DOM solution (d 0 ) is enriched with CHO (containing 3−16 oxygen) (40%) and CHON (containing 1−3 nitrogen and 3−13 oxygen) (49%) elemental formulae, followed by CHOS (containing 1 sulfur and 5−9 oxygen) (7.7%) and CHONS (containing 1 sulfur, 1 nitrogen, and 5−8 oxygen) (3.3%) (Table S1). Classification of the molecular formulae in the d 0 sample by the defined categories in the methods suggested that the majority of the identified formulae correspond to aliphatic (25.9%) and peptide-like (20.1%) assignments, which is highly comparable to data from Himalayan snowpack/ice samples (23.9 and 20.4% aliphatic and peptide-like, respectively).…”

Mass Spectrometry as a Crucial Analytical Basis for Omics Sciences

“…In particular, FTICR-MS made it possible to assign more than 6500 unique gross formulas in the study of compounds of humic origin [ 171 ]. Comparative studies have shown that the absence of a generally accepted protocol for such studies significantly affects the interlaboratory reproducibility of results [ 172 ].…”