Identification of Mercury and Dissolved Organic Matter Complexes Using Ultrahigh Resolution Mass Spectrometry

The
chemical speciation and bioavailability of mercury (Hg) is
markedly influenced by its complexation with naturally dissolved organic
matter (DOM) in aquatic environments. To date, however, analytical
methodologies capable of identifying such complexes are scarce. Here,
we utilize ultrahigh resolution Fourier transform ion cyclotron resonance
mass spectrometry (FTICR-MS) coupled with electrospray ionization
to identify individual Hg–DOM complexes. The measurements were
performed by direct infusion of DOM in a 1:1 methanol:water solution
at a Hg to dissolved organic carbon (DOC) molar ratio of 3 ×
10–4. Heteroatomic molecules, especially those containing
multiple S and N atoms, were found to be among the most important
in forming strong complexes with Hg. Major Hg–DOM complexes
of C10H21N2S4Hg+ and C8H17N2S4Hg+ were identified based on both the exact molecular mass and
patterns of Hg stable isotope distributions detected by FTICR-MS.
Density functional theory was used to predict the solution-phase structures
of candidate molecules. These findings represent the first step to
unambiguously identify specific DOM molecules in Hg binding, although
future studies are warranted to further optimize and validate the
methodology so as to explore detailed molecular compositions and structures
of Hg–DOM complexes that affect biological uptake and transformation
of Hg in the environment.