Analyzing Solid-Phase Natural Organic Matter Using Laser Desorption Ionization Ultrahigh Resolution Mass Spectrometry.

Extensive sample preparation procedures are required to analyze natural organic matter (NOM) in soil and sediment samples due to the mineral matrix. The preparation procedure not only requires a large amount of sample (typically more than 50 mg), but NOM extraction is frequently incomplete. In this study, 2-5 μg of solid NOM or 500 μg of unprocessed soil samples were fixed on a metal plate using double-sided adhesive tape and analyzed directly using laser desorption ionization (LDI) and ultrahigh resolution mass spectrometry (UHR-MS). Most of the peaks reported in previous LDI UHR-MS studies using NOM solutions were observed, and an additional ∼2200 unique peaks were found by analyzing the fulvic acids direct solid phase. Differences in the molecular composition of NOM in solid samples were seen clearly with minimum sample preparation. Lignin- and tannin-type molecules were detected in both Elliott soil and topsoil from Kyungpook National University campus. The data presented in this study demonstrate a proof-of-principle that highly sensitive, direct, molecular level analysis of solid-phase NOM from unprocessed soil samples and minimum sample preparation is possible.