Understanding molecular formula assignment of Fourier transform ion cyclotron resonance mass spectrometry data of natural organic matter from a chemical point of view

Abstract

Formula assignment is one of the key challenges in evaluation of dissolved organic matter analyses using ultrahigh resolution mass spectrometry (FTICR MS). The number of possible solutions for elemental formulas grows exponentially with increasing nominal mass, especially when non-oxygen heteroatoms like N, S or P are considered. Until now, no definitive solution for finding the correct elemental formula has been given. For that reason an approach from the viewpoint of chemical feasibility was elucidated. To illustrate the new chemical formula assignment principle, a literature data set was used and evaluated by simplified chemical constraints. Only formulas containing a maximum of one sulphur and five nitrogen atoms were selected for further data processing. The resulting data table was then divided into mass peaks with unique component solutions (singlets, representing unequivocal formula assignments) and those with two or more solutions (multiple formula assignments, representing equivocal formula assignments). Based on a [double bond equivalent (DBE) versus the number of oxygen atoms (o)] frequency contour plot and a frequency versus [DBE minus o] diagram, a new assessment and decision strategy was developed to differentiate multiple formula assignments into chemically reliable and less reliable molecular formulas. Using this approach a considerable number of reliable components were identified within the equivocal part of the data set. As a control, a considerable proportion of the assigned formulas deemed to be reliable correspond to those which would have been obtained by CH 2 -based Kendrick mass defect analysis. We conclude that formula assignment in complex mixtures can be improved by group-wise decisions based on the frequency and the [DBE minus o] values of multiple formula assignments.

A review and evaluation of forest canopy epiphyte roles in the partitioning and chemical alteration of precipitation

“…This is unfortunate as absorbance in different UV-visible wavelength ranges shows promise for identifying the leaching of UV-screening metabolites (which can be abundant in bromeliad, bryophyte and lichen species), and fluorescence excitation-emission matrix spectra (EEMS) indicate epiphyte leachates may provide substantial protein-like fluorophores to soil environments below . Connecting chemical formulae to fluorophores from EEMS can be done using Fourier transform ion cyclotron resonance (FTICR or "ultrahigh resolution") mass spectrometry (Herzsprung et al, 2014;Stubbins et al, 2014), yet no studies have, to the authors' knowledge, applied this method to characterize DOM in epiphyte-laden canopy-derived net precipitation fluxes. The authors are also unaware of any studies regarding POM flux or structure in epiphyte-derived net precipitation, although this can represent a large part of the C and N throughfall flux to soils (Lamersdorf and Blank, 1995).…”

Particle size dependence of biogenic secondary organic aerosol molecular composition

“…Data analysis was performed as described previously, targeting closed-shell molecular formulas (Tu et al, 2016). We also performed Kendrick mass defect analysis (Herzsprung et al, 2014;Phungsai et al, 2016) and calculated the RDB-O value (ring and double-bond equivalent of each molecular formula minus the number of oxygen atoms in the formula; the maximum RDB-O distribution resides within the range −10 and 10 for hydrophobic CHO species; Herzsprung et al, 2014) as the updated criteria to help filter the unreasonable assigned formulas. After removing background peaks, unreasonable formulas, and redundancies due to isotopic substitution, hundreds of unique molecular formulas remained for each sample.…”

Comparison of Low Molecular Weight Dissolved Organic Matter Compositions in Lake Inba and Kashima River by Orbitrap Mass Spectrometry

“…Carbon, hydrogen, oxygen, nitrogen and sulfur were considered in the configuration of molecular formula. Additional restrictions were applied in CIA to select chemically plausible formula as the following: conservative elemental ratios (0.2 ≤ H:C ≤ 3.1; O:C ≤ 1.2; N:C ≤ 1.3); nitrogen rule, maximum number of heteroatoms (N ≤ 3 and S ≤ 3); and degree of unsaturation (−10 ≤ DBE-O ≤ 10, see the following section) [27,28].…”