Natural Organic Matter and the Event Horizon of Mass Spectrometry

Abstract

Soils, sediments, freshwaters, and marine waters contain natural organic matter (NOM), an exceedingly complex mixture of organic compounds that collectively exhibit a nearly continuous range of properties (size-reactivity continuum). NOM is composed mainly of carbon, hydrogen, and oxygen, with minor contributions from heteroatoms such as nitrogen, sulfur, and phosphorus. Suwannee River fulvic acid (SuwFA) is a fraction of NOM that is relatively depleted in heteroatoms. Ultrahigh resolution Fourier transform ion cyclotron (FTICR) mass spectra of SuwFA reveal several thousand molecular formulas, corresponding in turn to several hundred thousand distinct chemical environments of carbon even without accountancy of isomers. The mass difference deltam among adjoining C,H,O-molecules between and within clusters of nominal mass is inversely related to molecular dissimilarity: any decrease of deltam imposes an ever growing mandatory difference in molecular composition. Molecular formulas that are expected for likely biochemical precursor molecules are notably absent from these spectra, indicating that SuwFA is the product of diagenetic reactions that have altered the major components of biomass beyond the point of recognition. The degree of complexity of SuwFA can be brought into sharp focus through comparison with the theoretical limits of chemical complexity, as constrained and quantized by the fundamentals of chemical binding. The theoretical C,H,O-compositional space denotes the isomer-filtered complement of the entire, very vast space of molecular structures composed solely of carbon, hydrogen, and oxygen. The molecular formulas within SuwFA occupy a sizable proportion of the theoretical C,H,O-compositional space. A 100 percent coverage of the theoretically feasible C,H,O-compositional space by SuwFA molecules is attained throughout a sizable range of mass and H/C and O/C elemental ratios. The substantial differences between (and complementarity of) the SuwFA molecular formulas that are observed using six different modes of ionization (APCI, APPI, and ESI in positive and negative modus) imply considerable selectivity of the ionization process and suggest that the observed mass spectra represent simplified projections of still more complex mixtures.

Comprehensive characterization of natural organic matter by MALDI- and ESI-Fourier transform ion cyclotron resonance mass spectrometry

“…This result suggests that constituents of NOM with lower O/C ratio are preferentially ionized in MALDI negative mode, whereas in ESI negative mode, constituents of NOM with higher O/ C ratio (as compared with MALDI uniquely identified compounds) are preferentially ionized. The selectivity of ESI in negative mode for NOM observed in our work coincides with a previous study reported by Hertkorn et al [8].…”

“…As a result, the extent of molecular information about NOM is largely restricted. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), due to its ultrahigh resolution and mass accuracy, can provide molecular elemental information about NOM without chromatographic separation [5,6], and has been used for accurate characterization of NOM [7][8][9]. Electrospray ionization (ESI) is the most commonly used ionization method for FT-ICR-MS characterization of NOM because it is one of the softest ionization methods and can be easily coupled with FT-ICR-MS [10,11].…”

“…The results suggested that APPI tended to ionize smaller and less polar molecules, whereas ESI produces ions that in general contain more oxygen atoms. Moreover, Hertkorn et al used different ionization methods (ESI, APCI and APPI) to characterize Suwannee River fulvic acid (SuwFA) [8]. The results indicated that molecules with lower O/C ratio were preferentially ionized in the negative APCI mode, and molecules with higher O/C ratio were preferentially ionized in the negative ESI and APPI mode.…”

Proposed Guidelines for Solid Phase Extraction of Suwannee River Dissolved Organic Matter

“…The small discrepancies observed likely resulted from ionization selectivity, 45 which probably favored detection of aliphatic carboxylic acids with low O/C ratios, thereby explaining the small gain in H/C ratios and tiny decrease in O/C ratios.…”

Characterization of unknown iodinated disinfection byproducts during chlorination/chloramination using ultrahigh resolution mass spectrometry

“…Recently, ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been successfully used to characterize the previously unknown Cl-DBPs and Br-DBPs (Zhang et al, 2012a;Zhang et al, 2012b;Lavonen et al, 2013;Zhang et al, 2014). The ultrahigh resolution and mass accuracy of FT-ICR MS combined with electrospray ionization (ESI) allows the determination of unambiguous and exact molecular formulas (Stenson et al, 2003;Kim et al, 2006aKim et al, , 2006bHertkorn et al, 2008). It is thus anticipated that the ESI FT-ICR MS method could also be suitable for identifying the molecular formulas of unknown I-DBPs.…”