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.