EPA-EAD: 1631:  Mercury in Water Using CVAFS

  • Summary
  • Analytes
  • Revisions
  • Data and Sites
Official Method Name
Mercury in Water by Oxidation, Purge and Trap, and Cold Vapor Atomic Absorption Fluorescence Spectrometry
Current Revision
Revision E, August 2002
Cold Vapor Atomic Fluoresence Spectrophotometer
Method Subcategory
Method Source
Method 1631, Revision E: Mercury in Water by Oxidation, Purge and Trap, and Cold Vapor Atomic Fluorescence Spectrometry. EPA-821-R-02-019. U.S. Environmental Protection Agency, Office of Water, August 2002, 38 p.
Brief Method Summary
This method allows for the determination of mercury at a minimum level of 0.5 parts-per-trillion and supports measurements for mercury published in the National Toxics Rule and in the Final Water Quality Guidance for the Great Lakes System. Includes Guidance for Implementation and Use of Method 1631 and Revision E to the method, which contain additional and revised requirements.

Mercury in a sample is oxidized using BrCl. Oxidized mercury in the sample is then reduced to volatile elemental mercury using SnCl2. Elemental mercury is purged with nitrogen and collected on a gold sand trap. After sufficient mercury has been collected, the trap is heated and desorbed mercury is purged into a cold-vapor atomic fluorescence spectrometer (CVAFS). The concentration of desorbed mercury is measured using the CVAFS system. The use of automated flow-injection systems in analyses is specifically allowed in this version of the method.

This method replaces the previously approved Method 1631 Revision C.
Scope and Application
This method determines mercury in filtered and unfiltered water.
Applicable Concentration Range
0.5 - 100 ng/L, extendable upward
(A) Contamination by carryover: Samples known or suspected to contain the lowest concentration of mercury should be analyzed first followed by samples containing higher levels.

(B) Contamination by samples: Samples known or suspected to contain Hg concentrations greater than 100 ng/L should be diluted prior to bringing them into the clean room or laboratory processing trace metals samples.

(C) Contamination by indirect contact: Every piece of apparatus directly or indirectly used in the collection, processing, and analysis of water samples must be thoroughly cleaned.

(D) Contamination by airborne particulate matter: Samples may be contaminated by airborne dust, dirt, particles, or vapors from unfiltered air supplies; nearby corroded or rusted pipes, wires, or other fixtures; or metal-containing paint.

(E) Contamination from reagents: Use high purity reagents.

(F) Iodide concentrations from 30 to 100 mg/L; at iodide concentrations greater than 3 mg/L, pre-reduce with SnCl2.
Quality Control 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 precision and recovery. Laboratory performance is compared to established performance criteria to determine that the results of analyses meet the performance characteristics of the Method.
Sample Handling
(A) An excess of BrCl should be confirmed.

(B) Samples are collected into rigorously cleaned fluoropolymer bottles with fluoropolymer or fluoropolymer-lined caps. Glass bottles may be used if Hg is the only target analyte. It is critical that the bottles have tightly sealing caps to avoid diffusion of atmospheric Hg through the threads.

(C) Collect samples using the guidance in EPA Method 1669.

(D) For dissolved Hg, a sample is filtered through a 0.45-um capsule filter in a mercury-free clean area prior to preservation. If the sample is filtered, it must be accompanied by a blank that has been filtered under the same conditions.

(E) Samples for mercury are preserved by adding either 5 mL/L of pretested 12N HCl or 5 mL/L BrCl solution to the sample bottle. Preserved samples are stable for up to 90 days of the date of collection.
(1) Samples to be analyzed for total or dissolved Hg only may be shipped to the laboratory unpreserved and unrefrigerated if they are collected in fluoropolymer or glass bottles and capped tightly. Samples must be either preserved or analyzed within 48 hours of collection. If a sample is oxidized in the sample bottle, the time to preservation can be extended to 28 days.
(2) Samples that are acid-preserved may lose Hg to coagulated organic materials in the water or condensed on the walls. The best approach is to add BrCl directly to the sample bottle at least 24 hours before analysis. If other Hg species are to be analyzed, these aliquots must be removed prior to the addition of BrCl. If BrCl cannot be added directly to the sample bottle, the bottle must be shaken vigorously prior to sub-sampling.
(3) Handling of the samples in the laboratory should be undertaken in a mercury-free clean bench, after rinsing the outside of the bottles with reagent water and drying in the clean air hood.

(F) Sample bottles should be stored in clean (new) polyethylene bags until sample analysis.
Maximum Holding Time
90 days - See Sample Handling
Relative Cost
$51 to $200
Sample Preparation Methods