Measurement of Trace Level Mercury by EPA Method 1631
Mercury is responsible for over three-quarters of all contaminant-related advisories for threats to human health. During the 1990’s, the number of mercury related fish consumption advisories more than doubled, despite significant decreases in the total mercury emissions over the last 20 years. The increase in advisories is probably the result of more testing rather than more contamination.
While the contamination is showing up in lakes and fish, most mercury does not come from effluent, rather is derived from atmospheric deposition. Atmospheric transport and subsequent bioaccumulation of mercury can affect aquatic ecosystems far from mercury sources. According to EPA estimates, emissions from coal-fired utilities account for 13 to 26 percent of the total (natural plus anthropogenic) airborne emissions of mercury in the United States. Thus, the EPA has begun to regulate emissions from power plant boilers and process heaters.
The impending MACT (Maximum Achievable Control Technologies) regulation (due out in 2004) is prompting affected manufacturers to assess the chloride, mercury and selected metals content of their fuels. The best way to meet the regulations is to burn fuels that are low in chloride, mercury and other metals. Early testing shows that mercury is more of a problem than chloride, particulate matter or selected metals.
Low level mercury measurements are conducted by EPA Method 1631. The analytical technique is very sensitive. In this method, an aqueous sample is oxidized with bromine monochloride and sparged with nitrogen onto a gold trap. The mercury is thermally desorbed from the gold trap into a cold vapor atomic fluorescence spectrometer. CAS can achieve a method detection limit (MDL) of 0.06 ng/L (ppt), which is three orders of magnitude less than the conventional cold vapor mercury method.
While the original method was designed for aqueous samples, CAS has implemented the “Appendix to Method 1631: Total Mercury in Tissue, Sludge, Sediment and Soil by Acid Digestion and BrCl Oxidation.” CAS has achieved an MDL of 0.3 ug/Kg in solid samples.
The clean sampling techniques described in EPA Method 1669 should be used for sampling. Since most mercury contamination comes from the atmosphere, it is very easy to contaminate water samples. Other sources of contamination are metal containers, talc powdered gloves, improperly cleaned and stored equipment, and dust and dirt. By following these clean sampling procedures, it has been shown that much of the historical data for mercury in seawater was erroneously high because of contamination from sampling.
When it is necessary to measure dissolved mercury, field filtering with an in-line filter can be performed as long as care is taken to insure that the filter is clean and free of contamination. It may be more efficient to send an unpreserved sample to the laboratory for filtering under clean conditions. This is particularly true if an in-line filtering device is not available.
Quality assurance is performed at CAS in accordance with the EPA method, ensuring scientifically valid and legally defensible results. Matrix spikes should be at the compliance level of 1 to 5 times the background level, so it is helpful to know the compliance level or the approximate amount of mercury expected in the samples. Besides matrix spikes, the method calls for measuring standards at 5 ng/L from two sources and analyzing many different blanks to assess potential contamination originating either in the field or in the laboratory. The blanks include: equipment, field, bottle, bubbler and reagent blanks, in addition to the usual method blank.
Why choose CAS?
The CAS Kelso laboratory has been performing this method for over six years in our clean laboratory. In addition to a variety of water matrices, CAS has experience analyzing hundreds of samples on several types of solid matrices: fish, coal, oil, sawdust and bark. We have a documented history of freedom from contamination and excellent recovery of ongoing precision and recovery standards and MS/MSDs within the QC acceptance criteria of Method 1631.
Federal Register, Vol. 68, No. 8, January 13, 2003, pg 1666.
Krabbenhoft & Schuster, USGS Fact Sheet FS-051-02, June 2002.