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

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.

Persistence of dissolved organic matter explained by molecular changes during its passage through soil

“…All measurements were done within days and in random order. In-house reference samples 76 were used to check instrumental stability. External calibration was done every day according to the manufacturer protocol.…”

“…In the Linde soil water sample dataset, only peaks were included detected in more than 10% of samples. In the incubation dataset duplicate measurements were used and only those signals were kept that had been detected in both measurements 76 .…”

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

“…Currently, the research community relies on 'standard' reference materials provided in large quantities by the International Humic Substances Society (IHSS), like Suwannee River fulvic acid (SRFA). These reference materials, described as a collection of humic acids, fulvic acids, and other isolates, are routinely collected and homogenized, and have been used throughout the history of HRMS, setting the foundation to understand complex DOM mixtures (Fievre et al 1997;Stenson 2008;Witt et al 2009;Gaspar et al 2010;Herzsprung et al 2015;Kew et al 2017;Simon et al 2018). These reference materials are often used as measures of instrument performance in studies that analyze newly collected samples, either as a DOM comparison or to control HRMS settings (Koch et al 2005;Mangal et al 2016;Li et al 2017;Hawkes et al 2018b;Solihat et al 2019).…”

“…Previous HRMS studies have defined important metrics with respect to instrument performance and capabilities, such as signal to noise ratio, mass measurement accuracy, and resolving power (Marshall et al 1998;Hawkes et al 2016;Simon et al 2018;Smith et al 2018), have extended optimizations based on IHSS reference materials such as Suwannee River and/or Pony Lake Fulvic Acids (SRFA and PLFA) (Stenson et al 2003;Koch et al 2005;D'Andrilli et al 2013D'Andrilli et al , 2015Mangal et al 2016). Currently, however, laboratories have little access to raw data from other research groups, limiting detailed comparison of instrument performance and results.…”

Spatial gradients in the characteristics of soil-carbon fractions are associated with abiotic features but not microbial communities

“…To develop our second hypothesis we note that actively growing microbial communities are known to enhance biochemical transformations and generate heteroatom-containing organic molecules (sulfur (S), nitrogen (N) and phosphorus (P)) (Guillemette et al, 2018;Koch et al, 2014;Ksionzek et al, 2016); therefore greater heteroatom content and more biochemical transformations are expected with increasing salinity. Our third hypothesis is based on microorganisms adapting to saline conditions through the production or sequestration of osmolytes (Gouffi et al, 1999;Sleator and Hill, 2002), a strategy that may require microbes to break down organic molecules to extract N (i.e., N mining). We therefore hypothesize increases in N-containing biochemical transformation with increasing salinity.…”