Ion Accumulation Time Dependent Molecular Characterization of Natural Organic Matter Using Electrospray Ionization-Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Abstract

Natural organic matter (NOM) is a complex organic mixture and plays a crucial role in environmental processes. By using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS), detailed molecular information on NOM could be achieved. In this paper, ion accumulation time (IAT), a key parameter of FTICR-MS for complex mixture detection, was the focus, and its effect on the molecular characterization of NOM by FTICR-MS was systematically investigated. A notable feature of selective detection of NOM molecules by FTICR-MS with different IAT was observed. Most of the polar molecules with high O/C ratio (O/C ratio >0.5) could be easily detected by FTICR-MS with a short IAT, but extending IAT led to the ion intensities of these molecules decreasing or even disappearing. Meanwhile, a large number of unsaturated and aromatic molecules with low O/C ratio (O/C ratio <0.6) and low polarity, all of which could not be observed with a short IAT, were remarkably detected by extending IAT. Results also revealed that the unsaturated and aromatic molecules, which could only be observed by extending the IAT, were not generated by the fragmentation of molecules in NOM or from the dissociation of NOM aggregations but originally existed in NOM samples. The selective detection of NOM molecules caused by IAT extension was possibly attributed to their different polarity and different stability in the collision cell. On the basis of these results, a novel strategy of combining mass spectrometric data of NOM obtained with different IAT by FTICR-MS was proposed. With this strategy, more than 4715 CHO-molecular formulas were assigned, where about 2000 more formulas were obtained in comparison with using a short IAT (2733 CHO-molecular formulas identified) solely. The strategy is simple and robust and can be used as an alternative method to obtain more molecular information on NOM in the environment.

Molecular Signals of Heterogeneous Terrestrial Environments Identified in Dissolved Organic Matter: A Comparative Analysis of Orbitrap and Ion Cyclotron Resonance Mass Spectrometers

“…Even under these circumstances, the Orbitrap is able to reproduce observed trends in molecular composition and allows separation of ecosystem types based on their DOM fingerprints. Drawbacks in resolving power can be tackled by increasing the specificity for analytes of interest during extraction (Tfaily et al, 2015;Li et al, 2016;Lv et al, 2016;Raeke et al, 2016), ionization (Hertkorn et al, 2008;Hockaday et al, 2009), or instrumental detection (Sleno, 2012;Cao et al, 2016). Although not comprehensive, broad trends in heteroatom content were also captured by the Orbitrap (Figure 6 and Supplementary Table S1).…”

An international laboratory comparison of dissolved organic matter composition by high resolution mass spectrometry: Are we getting the same answer?

“…Even within the data acquisition, the signal response is not uniform across the m/z range. The combination of the user-defined ionization source conditions and HRMS instrument settings for ion optics and detection can often favor low m/z values vs. high m/z values or vice versa-this translates to biases in detection and the potential for variability in relative ion abundance results among instruments (Cao et al 2016;Kew et al 2018a). Such biases may confound molecular-level assessments of DOM chemical assignments and interpretations and limit the ability to utilize datasets generated by different research groups, for example, for meta-analyses of ecosystem trends.…”

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

“…Values of AI n equal to 0.3 and M n equal to 379 Da were observed for laboratory d, which indicates the higher contribution of low molecular mass and relatively saturated compounds, as compared to the other laboratories. The number-averaged values of O/C ratio also indicate higher contributions of reduced compounds in laboratories b and d, with nO/nC below 0.5, compared to laboratories a, c, and e. This could be explained by the different parameters of ion accumulation in ion source and collision cell adjusted in experiments, which are known to affect the proportions of oxidized and reduced components [25].…”