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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 T 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.
