Investigating the Ionization of Dissolved Organic Matter by Electrospray

Electrospray
ionization (ESI) operating in the negative mode coupled
to high-resolution mass spectrometry is the most popular technique
for the characterization of dissolved organic matter (DOM). The vast
molecular heterogeneity and the functional group diversity of this
complex mixture prevents the efficient ionization of the organic material
by a single ionization source, so the presence of uncharacterized
material is unavoidable. The extent of this poorly ionizable pool
of carbon is unknown, is presumably variable between samples, and
can only be assessed by the combination of analysis with a uniform
detection method. Charged aerosol detection (CAD), whose response
is proportional to the amount of nonvolatile material and is independent
from the physicochemical properties of the analytes, is a suitable
candidate. In this study, a fulvic acid mixture was fractionated and
analyzed by high-pressure liquid chromatography–mass spectrometry
in order to investigate the polarity and size distributions of highly
and poorly ionizable material in the sample. Additionally, DOM samples
of terrestrial and marine origins were analyzed to evaluate the variability
of these pools across the land–sea aquatic continuum. The relative
response factor values indicated that highly ionizable components
of aquatic DOM mixtures are more hydrophilic and have lower molecular
weight than poorly ionizable components. Additionally, a discrepancy
between the samples of terrestrial and marine origins was found, indicating
that marine samples are better represented by ESI than terrestrial
samples, which have an abundant portion of hydrophobic poorly ionizable
material.