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

Carsten Simon,Thorsten Dittmar,Gerd Gleixner,Vanessa-Nina Roth

doi:10.3389/feart.2018.00138

Abstract

Terrestrial dissolved organic matter (DOM) interlinks large carbon reservoirs of soils, sediments and marine environments but remains largely uncharacterized on the molecular level. Fourier Transform Mass Spectrometry (FTMS) has proven to be a powerful technique to reveal DOM chemodiversity and potential information encrypted therein. State-of the art FT-ICR MS (Ion Cyclotron Resonance) instruments are yet inaccessible for most researchers. To evaluate the performance of the most recent Orbitrap analyzer as a more accessible alternative, we compared our method to an established 15 Tesla FT-ICR MS on a diverse suite of 17 mainly terrestrial DOM samples regarding 1) ion abundance patterns, 2) differential effects of DOM type on information loss, and 3) derived biogeochemical information. We show that the Orbitrap provides similar information as FT-ICR MS, especially for compound masses below 400 m/z, and is mainly limited by its actual resolving power rather than its sensitivity. Ecosystems that are dominated by inputs of plant-derived material, like DOM from soil, bog, lake and rivers, showed remarkably low average mass to charge ratios, making them also suitable for Orbitrap measurements. The additional information gained from FT-ICR MS was highest in heteroatom-rich (N, S, P) samples from systems dominated by internal cycling, like DOM from groundwater and the deep sea. Here FT-ICR MS detected 37% more molecular formulae and 11% higher ion abundance. However, the overall information content, which was analyzed by multivariate statistical methods, was comparable for both data sets. Mass spectra-derived biogeochemical trends, for example the decrease of DOM aromaticity during the passage through terrestrial environments, were retrieved by both instruments. We demonstrate the growing potential of the Orbitrap as an alternative FTMS analyzer in the context of challenging analyses of DOM complexity, origin and fate.

Highlights

Dissolved organic matter (DOM) links the organic matter pools of terrestrial and marine ecosystems through transport of material derived from biota, degrading plant litter, and soil organic matter (SOM) to the ocean (Marín-Spiotta et al, 2014; Ward et al, 2017)
Both instruments yielded highly similar responses, as can be seen from overall distribution of ion abundance (Table 3), with very similar amounts of information covered by assigned formulae in the mass range m/z 200–650 (70–80%), and a majority of ion abundance being commonly detected (>90%, up to 99% in single cases for the Orbitrap)
Higher numbers of peaks may arise from the difference in concentration (10 ppm, FT-ion cyclotron resonance (ICR) MS; 20 ppm, Orbitrap), which was due to the exchange of the ICR analyzer cell by a more sensitive type (Supplementary Table S2)

Summary

Introduction

Dissolved organic matter (DOM) links the organic matter pools of terrestrial and marine ecosystems through transport of material derived from biota, degrading plant litter, and soil organic matter (SOM) to the ocean (Marín-Spiotta et al, 2014; Ward et al, 2017). Taken together, both pools hold about four times more carbon than the atmospheric carbon pool and minor changes in their oxidation or mobilization rates may have major climatic impacts (Heimann and Reichstein, 2008; Carlson and Hansell, 2014). The advances in our understanding of the molecular DOM “code” are small as the access to ultrahigh resolution mass spectrometry is limited

Methods

Results

Conclusion