Quantification of monomethylmercury in natural waters by direct ethylation: Interference characterization and method optimization

Abstract

AbstractThe accurate analysis of monomethylmercury (MMHg) in natural waters is critical to understanding the biogeochemical cycling of mercury. Direct ethylation has emerged as an effective method for measuring MMHg in seawater without distillation. We evaluated the method's utility in other natural waters, characterized potential interferences, and optimized the method in an attempt to overcome these. The ethylation of MMHg by sodium tetraethylborate (NaBEt4) in pure water was unaffected by pH over the range 2.5–5.5. However, ethylation becomes sensitive to pH with increasing salinity, with the optimal pH narrowing to 4.0–4.1. An increase in NaBEt4 concentration eventually results in a decrease in MMHg ethylation, which occurs at lower NaBEt4 concentrations with increasing salinity. A NaBEt4 concentration of 6 mg L−1 was optimal. There was no interference by environmentally relevant concentrations of major cations, anions, nor many trace metals. However, micromolar concentrations of Cu2+, Zn2+, Pb2+, and Fe2+ decreased MMHg recoveries, but these interferences were eliminated with ethylenediaminetetraacetic acid. Sulfide and thiols at concentrations of only 2 µM depressed MMHg recoveries. Attempts to remove sulfide or overcome the interference it caused were unsuccessful, so the method is currently limited to oxic waters. Addition of 1.2 M chloride eliminated the interference by even 20 µM thiols. Dissolved organic matter (DOM) interferes with direct ethylation, but 1.2 M NaCl increased MMHg recoveries to > 90% even at DOM concentrations > 17 mg OC L−1. The optimized method was used to measure MMHg in hundreds of oxic filtered and unfiltered fresh and saline natural waters; MMHg recoveries averaged 92% ± 6%. The average percent relative standard deviation was < 10% for samples (n = 266) analyzed in triplicate. Daily detection limits for a 30 mL sample are < 25 fM.