Development and complete validation of a method for the determination of dimethyl mercury in air and other media

The rigorous validation of a previously developed research method for the determination of dimethyl mercury ((CH₃)₂Hg) in environmental samples is presented. During atmospheric analysis, (CH₃)₂Hg was trapped on Carbotrap™ and analyzed by thermal desorption, isothermal GC separation, and cold vapor atomic fluorescence spectrometry (CVAFS). Water samples were analyzed after direct purging of 100mL aliquots onto Carbotrap™, while sediment and tissue samples were digested with 10mL of 25% KOH in methanol at 60°C and diluted to 40mL with methanol. An ambient air-spiking manifold, which allowed simultaneous replicate sampling, was constructed in a room controlled for temperature and humidity. (CH₃)₂Hg was introduced into the feed airflow (0.4m³min^-1) from a well-calibrated diffusion cell, to obtain a concentration of approximately 5.5ngm^-3 as Hg. Samples were collected onto Carbotrap™ columns, and the total volumes quantified by integrating mass flow meters. Trapping efficiency was investigated over a range of sampler flow rates (0.05-0.25Lmin^-1), volumes (2-200L), collection temperatures (15-42°C) and relative humidity levels (10-70%). Method detection limits (MDLs), analytical precision and accuracy were quantified for all media. Carbotrap™ was found to be the best choice as a sampling media, whereas Tenax™ was found to be inadequate due to high breakthrough (>70%). This study verified that the method is sufficiently precise, accurate and robust for field sampling at mercury contaminated sites. No interferences were observed from elevated levels of potential co-contaminants, Hg⁰ (125ngm^-3) and H₂S (1.27ppmv).