Direct Analysis of Marine Dissolved Organic Matter Using LC-FT-ICR MS

Marine dissolved organic matter (DOM) is an important
component
of the global carbon cycle, yet its intricate composition and the
sea salt matrix pose major challenges for chemical analysis. We introduce
a direct injection, reversed-phase liquid chromatography ultrahigh
resolution mass spectrometry approach to analyze marine DOM without
the need for solid-phase extraction. Effective separation of salt
and DOM is achieved with a large chromatographic column and an extended
isocratic aqueous step. Postcolumn dilution of the sample flow with
buffer-free solvents and implementing a counter gradient reduced salt
buildup in the ion source and resulted in excellent repeatability.
With this method, over 5,500 unique molecular formulas were detected
from just 5.5 nmol carbon in 100 μL of filtered Arctic Ocean
seawater. We observed a highly linear detector response for variable
sample carbon concentrations and a high robustness against the salt
matrix. Compared to solid-phase extracted DOM, our direct injection
method demonstrated superior sensitivity for heteroatom-containing
DOM. The direct analysis of seawater offers fast and simple sample
preparation and avoids fractionation introduced by extraction. The
method facilitates studies in environments, where only minimal sample
volume is available e.g. in marine sediment pore water, ice cores,
or permafrost soil solution. The small volume requirement also supports
higher spatial (e.g., in soils) or temporal sample resolution (e.g.,
in culture experiments). Chromatographic separation adds further chemical
information to molecular formulas, enhancing our understanding of
marine biogeochemistry, chemodiversity, and ecological processes.