Mercury and methyl mercury in freshwater seston: direct determination at picogram per litre levels by dual filtration

Abstract

Here, we describe a method for directly determining sestonic mercury in lake waters at concentrations ranging down to 0.12 ng Hg/L (total Hg) and 0.01 ng MeHg/L (monomethyl mercury) in a 250-mL water sample. Detection limits for the method are 30 pg of Hg and 3.0 pg of MeHg, reported as three times the standard deviation of a procedural blank. The method involves dual filtration using ultraclean 47-mm-diameter, 0.45-µm pore size cellulose nitrate filters in an all-Teflon® filter stack. Lake water is pumped directly through a primary filter that traps suspended seston particles and then through a secondary filter that is used to estimate the integrated reagent, filter, and sorption blank. Both filters are analyzed for Hg or MeHg and the primary filter is corrected by subtracting the secondary blank. Sestonic Hg is determined after acid digestion and wet oxidation of the filters with BrCl, reduction with NH2OH-HCl followed by SnCl2, and purging via N2 onto dual gold traps for detection via cold-vapor atomic fluorescence spectroscopy. Sestonic MeHg is determined after acid distillation of the filters, aqueous-phase ethylation, purging onto Tenax®, isothermal gas chromatography separation, and detection by cold-vapor atomic fluorescence spectroscopy. The method was tested on a variety of lakes in Wisconsin, Florida, and Washington. The blank-corrected mass of sestonic mercury collected from these lakes onto primary filters ranged from 30 to 340 pg Hg and from 3 to 23 pg MeHg. The corresponding concentrations of mercury in the seston were 22-790 ng Hg/g dry weight and 3-75 ng MeHg/g dry weight. The method allowed us to demonstrate a clear negative dependence of sestonic mercury concentrations (weight:weight) on pH in Wisconsin lakes.