EPA Method 245.7

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EPA Method 245.7:
Mercury in water by cold-vapor atomic fluorescence spectrometry. Official Name: Mercury in water by cold-vapor atomic fluorescence spectrometry

Summary:
A 100- to 2,000-mL sample is collected directly into a specially cleaned, pretested, fluoropolymer bottle using sample handling techniques specially designed for collection of mercury at trace levels.For dissolved Hg, the sample is filtered through a 0.45-um capsule filter prior to preservation.The sample is preserved by adding 5 mL/L of pretested 12N HCl. If a sample also will be used for the determination of methyl mercury, it should be preserved according to procedures in the method that will be used for detection of methyl mercury.Prior to analysis, all Hg in a sample is oxidized by a potassium bromate/potassium bromide reagent.After oxidation, the sample is sequentially pre-reduced with NH2OH.HCl to destroy the excess bromine, then the ionic Hg is reduced with SnCl2 to convert Hg(II) to volatile Hg(0).The Hg(0) is separated from solution by passing the sample through a gas/liquid separator and purging with high purity argon gas.The Hg passes into an inert gas stream that carries the released Hg(0) into the cell of a cold-vapor atomic fluorescence spectrometer for detection. The concentration of Hg is determined by atomic fluorescence spectrometry at 253.7 nm.Quality is assured through calibration and testing of the oxidation, purging, and detection systems.

Scope:
This method is for the determination of mercury in filtered and unfiltered drinking water, surface and ground waters, marine water, and industrial and municipal wastewater.

Citation:
Method 245.7: Mercury in Water by Cold Vapor Atomic Fluorescence Spectrometry, Revision 2.0, February 2005, EPA-821-R-05-001.

Interferences:
Gold, silver and iodide are known interferences. See method for steps to reduce interferences. The use of a brominating digestion coupled with atomic fluorescence detection overcomes many of the chloride, sulfide and molecular absorption interferences. No interferences have been noted for sulfide concentrations below 24 mg/L.High purity argon (99.998%) must be used as the carrier gas. Using nitrogen may reduce the sensitivity by a factor of eight fold, while the use of air may reduce the sensitivity thirty fold.Water vapor may collect in the fluorescence detector cell, resulting in a degradation of the analytical signal or giving a false peak due to scattering of the excitation radiation. The use of a membrane drying tube is required to reduce quenching and to remove any water vapor from the transfer tubing that can contaminate the detector.

QC Requirements:
Each laboratory that uses this method is required to operate a formal quality assurance program. The minimum requirements of this program consist of an initial demonstration of laboratory capability, ongoing analysis of standards and blanks as a test of continued performance, and the analysis of matrix spikes (MS) and matrix spike duplicates (MSD) to assess accuracy and precision. Laboratory performance is compared to established performance criteria to determine that the results of analyses meet the performance characteristics of the method.

Maximum Holding Time:
90 days

Media:
WATER

Subcategory:
Inorganic

Concentration:
5-100 ng/L, may be extended by dilution of the sample

Sample Prep:
Method 1669

Precision:
Precision and recovery was established through blind dublicate samples analyzed as part of an interlaboratory study. This analysis consisted of samples of reagent water, fresh water, marine water, and effluent.

Detection:
The method detection limit (MDL) as determined by 40 CFR 136, Appendix B.

Revision Number:
Revision 2.0, February 2005