Electrospray and photoionization mass spectrometry for the characterization of organic matter in natural waters: a qualitative assessment

Abstract

Fourier‐transform ion cyclotron resonance mass spectrometry (MS) has demonstrated potential to revolutionize the fields of limnology and chemical oceanography by identifying the individual molecular components of organic matter in natural waters. The use of MS for this purpose is made possible by the electrospray technique which successfully ionizes polar, nonvolatile organic molecules. Another recently developed ion source, atmospheric pressure photoionization (APPI), extends MS capabilities to less polar molecules. This article presents early results on the application of APPI MS to natural organic matter. We compare APPI MS and electrospray MS data for dissolved organic matter from Lake Drummond (Virginia, USA). Collectively, electrospray and APPI MS identify more than 6000 molecular species to which we assign unique molecular formulas. Fewer than 1000 molecular species are common to both electrospray and APPI mass spectra, indicating that the techniques are highly complementary in the types of molecules they ionize. Access to a broad range of molecules provided by combining APPI and electrospray has prompted a qualitative analysis. The goal is to assess the extent to which molecular MS data correspond with elemental (CHNOS) and structural characteristics determined by combustion elemental analyses and 13C nuclear magnetic resonance (NMR). Because the data obtained by these different methods are not directly comparable, we propose a novel data analysis procedure that facilitates their comparison. The bulk elemental composition calculated from electrospray MS data are in close agreement (± 15%) with values determined by combustion elemental analysis. APPI and electrospray MS detect protein contributions in agreement with 13C NMR (6 wt %) but underestimate carbohydrates relative to 13C NMR. Nevertheless, MS results agree with NMR on the relative proportions of noncarbohydrate compounds in the organic matter: lignins > lipids > peptides. Finally, we use a molecular mixing model to simulate a 13C NMR spectrum from the MS datasets. The correspondence of the simulated and measured 13C NMR signals (74%) suggests that, collectively, the molecular species identified by APPI and electrospray MS comprise a large portion of the organic matter in Lake Drummond. These results add credibility to electrospray and APPI MS in limnology and oceanography applications, but further characterization of ion source behavior is fundamental to the accurate interpretation of MS data.

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

“…_ 1 ) T D $ F I G ] On the other hand, because in most cases the majority of peaks at even m/z are assigned to the 13 C isotope of peaks in high resolution mass spectra of NOM, only peaks at odd m/z were considered for CHO-molecular formulas assignment in the previous studies [5,19]. However, interestingly, during processing mass spectra in MALDI negative mode in our study, a significant portion of peaks at even m/z can also be assigned as CHOmolecules except for the 13 C isotope peaks.…”

“…However, during ESI analysis, only some polar and moderately polar molecules of NOM can be ionized by ESI, and most of non-polar compounds cannot be detected [11]. Recently, some reports combined ESI and APPI ionization methods to characterize DOM and compared the obtained mass spectra [12,13]. The results suggested that APPI tended to ionize smaller and less polar molecules, whereas ESI produces ions that in general contain more oxygen atoms.…”

Reviewing carbon spiraling approach to understand organic matter movement and transformation in lotic ecosystems

“…Improvements of analytical methods to identify whole classes of compounds such as amino acids, peptides, proteins and carbohydrate-like compounds are being made combining chromatography techniques with highly sensitive detectors that can approach the nanomolar range (Kaplan & Newbold, 2003). Characterization of DOC should also consider recent technics of mass spectrometry in organic molecules such as electrospray and atmospheric pressure photoionization (Hockaday et al, 2009), compounds could be determined by gas chromatography-mass spectrometer (Jaffé et al, 2012) and biodegradable DOC measured with bioreactors (McLaughlin & Kaplan, 2013).…”

Characterization of terrestrial dissolved organic matter fractionated by pH and polarity and their biological effects on plant growth

“…From these diagrams, one can parse the formulae by regions of the van Krevelen diagram that denote molecular similarities with groups of compounds such as lipid, lignin, tannin, or condensed aromatics [42][43][44]. The whole and acid-soluble samples encompass all of these regions, giving van Krevelen diagrams that are very similar to one another (Figure 5a and b).…”