Method 1630

This method is for determination of methyl mercury (CH3Hg) in filtered and unfiltered water by distillation, aqueous ethylation, purge and trap, desorption, and cold-vapor atomic fluorescence spectrometry (CVAFS). This method is for use in EPA's data gathering and monitoring programs associated with the Clean Water Act, the Resource Conservation and Recovery Act, the Comprehensive Environmental Response, Compensation and Liability Act, and the Safe Drinking Water Act. This method is accompanied by Method 1669: Sampling Ambient Water for Determination of Trace Metals at EPA Water Quality Criteria Levels (Sampling Method). The Sampling Method is necessary to preclude contamination during the sampling process. This method is designed for determination of CH3Hg in the range of 0.02-5 ng/L and may be extended to higher levels by selection of a smaller sample size. The ease of contaminating ambient water samples with the metal(s) of interest and interfering substances cannot be overemphasized. This method includes suggestions for improvements in facilities and analytical techniques that should maximize the ability of the laboratory to make reliable trace metal determinations and minimize contamination (Section 4.0). The detection limit and minimum level of quantitation in this method are usually dependent on the level of background elements rather than instrumental limitations. The method detection limit (MDL; 40 CFR 136, Appendix B) for CH3Hg has been determined to be 0.02 ng/L when no background elements or interferences are present. The minimum level (ML) has been established as 0.06 ng/L. An MDL as low as 0.009 ng/L can be achieved for low CH3Hg samples by using extra caution in sample handling and reagent selection, particularly the use of "for ultra-low level only" distillation equipment.


Clean and ultraclean: The terms "clean" and "ultraclean" have been applied to the techniques needed to reduce or eliminate contamination in trace metal determinations. These terms are not used in this method because they lack an exact definition. However, the information provided in this method is consistent with the summary guidance on clean and ultraclean techniques.

Summary of the Method

A 100-2000 mL sample is collected directly into specially cleaned, pretested, fluoropolymer or borosilicate bottle(s) using sample handling techniques specially designed for collection of metals at trace levels. For dissolved CH3Hg, samples are filtered through a 0.45-µm capsule filter. Fresh water samples are preserved by adding 4 mL/L of pretested 11.6 M HCl, while saline samples ([Cl-] > 500 ppm) are preserved with 2 mL/L of 9 M H2SO4 solution, to avoid distillation interferences caused by excess chloride. Prior to analysis, a 45-mL sample aliquot is placed in a specially designed fluoropolymer distillation vessel, and 35 mL of the water is distilled into the receiving vessel at 125°C under N2 flow. After distillation, the sample is adjusted to pH 4.9 with an acetate buffer and ethylated in a closed purge vessel by the addition of sodium tetraethyl borate (NaBEt4). The ethyl analog of CH3Hg, methylethyl mercury (CH3CH3CH2Hg), is separated from solution by purging with N2 onto a graphitic carbon (Carbotrap®) trap. The trapped methylethyl mercury is thermally desorbed from the Carbotrap® trap into an inert gas stream that carries the released methylethyl mercury first through a pyrolytic decomposition column, which converts organo mercury forms to elemental mercury (Hg0), and then into the cell of a cold-vapor atomic fluorescence spectrometer (CVAFS) for detection. Quality is ensured through calibration and testing of the distillation, ethylation, purging, and detection systems.