PNNL

Abstract

We report a novel technical approach for subcritical fluid extraction (SFE) for organic matter characterization in complex matrices such as soil. The custom platform combines on-line SFE with micro-solid phase extraction, nano liquid chromatography (LC), electrospray ionization and Fourier transform mass spectrometry (SFE-LC-FTMS). We demonstrated the utility of SFE-LC-FTMS for analysis of complex mixtures of organic compounds in solid matrix by characterizing soil organic matter in peat, a high-carbon soil. For example, in a single experiment, >6,000 molecular formulas can be assigned based upon FTICR MS data from 1-50 µL of soil samples (roughly 1-50 mg of soil, dependent on soil density), nearly twice that typically obtained from direct infusion liquid solvent extraction (LSE) from an order of magnitude larger volume of the same soil. Detected species consisted predominately of lipid-like, lignin-like and protein-like compounds based on their O/C and H/C ratios with predominantly CHO and CHONP molecular composition. These results clearly demonstrate that SFE has the potential to effectively extract a variety of molecular species and could become an important member of a suite of extraction methods for studying SOM and other natural organic matter. This is especially true when comprehensive coverage, minimal sample volumes, and high sensitivity are required or when the presence of organic solvent residue in residual soil is problematic. SFE based extraction protocol could potentially enable spatially resolved characterization of organic matter in soil with resolution of ~1 mm3 to facilitate studies probing spatial heterogeneity of soil.