Development of ultrahigh resolution mass spectrometry techniques to extend the molecular view of soil organic matter in solution and on mineral particles

Soil organic matter plays important roles in soil reactivity and fertility as well as soil physics. Nevertheless, we know relatively little about the individual molecules that make up soil organic matter but ultimately determine its properties. Ultrahigh-resolution mass spectrometry like FT-ICR-MS has revealed an enormous molecular diversity yet it often remains limited to the water-soluble fractions (i.e., dissolved organic matter) analyzed with electrospray ionization (ESI) that represent only a small fraction of the total organic matter contained in soils. To extend the analytical window and leverage the value of non-targeted mass spectrometry, parallel analyses of soluble (via ESI) and particle-associated organic matter (PAOM) via laser-desorption ionization (LDI) and FT-ICR-MS detection is a promising approach, that has yet to prove its full potential. Here, we studied the sensitivity and robustness of the LDI technique based on a combination of dried arable soils, their aqueous DOM extracts, reference DOM samples (Suwannee River Fulvic Acid, SRFA), model compounds (syringic acid, sinapic acid, syringaldehyde, vanillic acid and tannic acid) and model mineral phases (goethite, illite). DOM samples were used to study the effects of a mineral matrix and dilution, while model compounds and SRFA were used to test the effects of laser strength and presence of an organic matrix on intact ionization of analytes. Lastly, non-extracted and extracted soil samples were used to assess if DOM composition trends observed in solution are reproduced in PAOM composition. In general, ESI ionized a very different fraction of the DOM mixture, being more polar and more saturated, while LDI ionized rather small, low-to-mid polar, and less saturated ions. Besides clear differences in PAOM and DOM analytical windows, molecular trends such as aromaticity or nominal oxidation state were well-aligned. Although most insight was gained by combining both types of analyses, our results therefore suggest that direct analysis of soil particles is a fast, reproducible, sensitive and less invasive alternative to routine protocols employing FT-ICR-MS detection, and avoids additional extraction or purification steps.