Adsorption of dissolved organic matter to mineral phases studied by laser desorption ionization mass spectrometry

Abstract

A large fraction of organic matter in soils is associated with mineral phases. This association is mediated by manifold processes (reactive transport, drying-rewetting cycles, biofilm formation, digestion of mineral particles, etc.) and ultimately leads to a stabilization of organic matter in soil [1]. Adsorption describes the mechanism for this association at the molecular level. Ultrahigh resolution mass spectrometry like FT-ICR-MS has gained interest in the study of adsorption processes because of its high sensitivity, resolving power and mass accuracy which allow to study the complex organic mixtures present in soils, and therefore provides molecular insight [2]. For that, supernatant composition can be compared before and after adsorption, but this indirect approach is often not sensitive enough to detect the sorption of small amounts of organic matter, i.e., during initial adsorption to pristine mineral surfaces. An alternative approach is to use laser desorption ionization (LDI) to directly analyze the adsorbed molecules on the mineral surfaces. The method theoretically allows laser spot size of ~ 20-50 µm and even allows imaging of thin sections; methodological advances could therefore improve our understanding of soil organic matter and its spatial heterogeneity [3]. We applied LDI-FT-ICR-MS to study the ionization of individual molecules with and without the presence of DOM (SRFA and pine/ beech litter extracts), and measured adsorption isotherms of individual molecular formulas in dissolved organic matter on quartz, illite and goethite after 24h of contact. Analog to organic matrices used in matrix-assisted LDI, detectability of model compounds improved by factor 2.5 – 40 when spiked into a DOM matrix. In case of sinapic acid, presence of DOM shifted the ionization towards the monomer ion [M-H]- as compared to a mixture of mono-, di- [2M-H]- and trimer [3M-H]- species when analyzed in pure form. These results suggest that soil organic matter and its soluble analogs act as suitable matrices in LDI experiments that ensure proper ionization of the mixture as a whole. In a next step, ion abundance data from sorption experiments was used to model the adsorption process of individual molecular formulas by a Langmuir isotherm approach. We derived estimates of sorption capacity and sorption affinity for each molecular formula, and identified the molecular properties explaining differences in both estimates, as well as their differences between mineral phases. Our data highlight the benefits of LDI-FT-ICR-MS for the study of sorption phenomena in soils, and opens perspectives for resolution of spatial heterogeneity in soils.