Matrix modification to improve the recovery of MMHg from clear water using distillation

Abstract

The use of distillation from aqueous solution dramatically improves the recovery of methyl mercury (MMHg) from complex water samples, as compared to solvent extraction techniques. However, low and irreproducible spike recoveries are often observed when distillation is applied to very clear water samples (i.e., precipitation, double deionized water (DDW), etc.) or those containing high chloride concentrations. Based upon the observation that recoveries and reproducibility are higher for waters containing strong complexing compounds, such as humic matter, we investigated the potential of matrix modification to improve the reproducibility of MMHg by distillation from more difficult matrices. At sample acid concentrations of less than 0.1M hydrochloric acid (HCl), we found that the addition of complexers such as ammonium pyrrolidine dithiocarbamate (APDC), and aqueous humic substances improved distillation spike recoveries and reproducibility. The use of complexers was also found to suppress the co-distillation of divalent mercury (Hg(II), which, in highly contaminated samples can be an interferant with the MMHg determination. At higher HCl concentrations, irreproducibility was dominated by the interference caused by chloride co-distilled with the water. In these cases, neither the addition of complexing agents, or buffers to bind the free hydrogen ions (H⁺) in solution were effective in improving results.