Method 1631

Method 1631 allows determination of mercury at a minimum level (ML) of 0.5 ng/L (parts-per-trillion; ppt), approximately 400 times lower than the level achieved by other mercury methods previously approved at 40 CFR Part 136. Determination of mercury at low ppt levels is necessary to support measurements at the ambient water quality criteria for mercury published in the National Toxics Rule as 12 ppt (40 CFR 131.36) and in the Final Water Quality Guidance for the Great Lakes System as 1.3 ppt (60 FR 15366).

Method 1631, Revision B, has four components:
  1. sample pretreatment,
  2. purge and trap,
  3. desorption, and
  4. detection of mercury by atomic fluorescence.

In the sample pretreatment step, bromine monochloride (BrCl) is added to the sample to oxidize all forms of mercury to Hg(ll). After oxidation, the sample is sequentially prereduced with NH2OH HCl to destroy free halogens, then reduced with SnCl2 to convert Hg(ll) to volatile Hg(0). The Hg(0) is purged from the aqueous solution onto a gold-coated sand trap. The trapped mercury is thermally desorbed from the gold trap into a flowing gas stream into the cell of a cold-vapor atomic fluorescence spectrometer. Quality is assured through calibration and testing of the oxidation, purging, and detection systems.

Since Method 1631, Revision B, is performance-based, it encourages advances in analytical technology and may reduce the cost of analysis. The analyst is permitted to modify the method to overcome interferences or lower the cost of measurements, provided that all method equivalency and performance criteria are met.

This performance-based approach is consistent with EPA's streamlining proposal (March 28, 1997; 62 FR 14976) and the Agency's performance-based measurement system policy (October 6, 1997; 62 FR 52098).

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Click here for the entire method (part 2) PDF