The Mercury Group at BRL has developed a method modification to EPA Method 1630 that significantly lowers the methylmercury (MeHg) method detection limit (MDL). At 0.005 ng/L, or parts-per-trillion (ppt), this new low-level MDL is 4 times lower than the standard MDL of 0.02 ng/L. This new low-level MeHg method will be quite useful for applications such as environmental modeling and Total Maximum Daily Load (TMDL) studies where having a true, measurable result, rather than a ‘non-detect’ value, is essential.
An ultra-low MDL can be especially valuable to areas like the San Francisco Bay and Estuary, as well as surrounding tributaries, where MeHg concentrations can often be very low, but it can be critically important to have quantifiable MeHg values for environmental assessments. According to Don Yee of the San Francisco Estuary Institute, the ability to detect low concentrations and small differences is important because bioconcentration from water to tissues begins with uptake by small organisms like phytoplankton and the MeHg can be concentrated by a factor of 100,000 or greater.
To achieve this MDL, rigorous equipment preparation measures are taken prior to distillation followed by ethylation, trap pre-concentration, gas chromatography separation, pyrolytic combustion, and atomic fluorescence spectroscopy (CV-GC-AFS) using a Brooks Rand Instruments MERX-M analyzer. The ultra-sensitive MERX-M can quantify methylmercury at levels less than a tenth of a picogram.