Soil organic matter (SOM) is a key component of the global biogeochemical cycles as it is responsible for the storage and alteration of various forms of carbon, nitrogen, and other elements essential for all activities on Earth. SOM is formed during the process of humification, which collectively involves the degradation, oxidation, and polymerization of various distinct biopolymers associated mainly with plant litter. These biochemical transformations are commonly evaluated by fractionating SOM into humic and fulvic acids. Understanding the physico-chemical characteristics, chemical reactivity, and structure of humic and fulvic acids is critical for understanding the biogeochemical cycling of SOM.<br/>Non-targeted analysis using electrospray ionization – Fourier transform – ion cyclotron resonance – mass spectrometry (ESI-FT-ICR-MS) has been used to molecularly characterize a wide range of terrestrial, aquatic, and atmospheric samples which has contributed to key findings in the environmental sciences. Humic and fulvic acids are particularly challenging type of samples due to their unique composition and matrix constituents. Several different sample preparation techniques have been previously utilized to overcome these challenges, but rarely sample preparation approaches have been directly compared.<br/>Here, we evaluated sample preparation techniques with which freeze-dried humic and fulvic acids, from both native vegetation and agricultural soils, are solubilized and analyzed using direct-infusion ESI-FT-ICR-MS. Most effective sample preparation technique for humic acids was to solubilize the powdered humic acid in basified water with NaOH followed by cation-exchange and solvent-extraction of the resin. For fulvic acids, the most appropriate technique was to use solid-phase extraction using PPL cartridges. These two approaches yielded the lowest number of salt peaks, the highest number of total and assignable peaks, and the most representative spectra. Here, we show the value of performing a sample preparation comparison study for maximizing the quality of the obtained ESI-FT-ICR-MS spectra enabling us to better understand the composition of humic and fulvic acids and ultimately, SOM.

Comparison of sample preparation techniques for the non-targeted ESI-FT-ICR-MS analysis of soil humic and fulvic acids

ACS Spring 2022

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