Method 1631, Revision E: 

Mercury in Water by Oxidation, Purge and

Trap, and Cold Vapor Atomic Fluorescence

Spectrometry

August 2002 

United States Office of Water 

Environmental Protection 4303 August 2002 

Agency 

EPA-821-R-02-019 

Method 1631, Revision E: 

Mercury in Water by Oxidation, Purge and

Trap, and Cold Vapor Atomic Fluorescence

Spectrometry
Method 1631, Revision E 

Acknowledgments 

This Method was developed under the direction of William A. Telliard and Maria Gomez-Taylor of the 

Engineering and Analysis Division (EAD) within the U.S. Environmental Protection Agency's (EPA's) 

Office of Science and Technology (OST). EPA acknowledges contributions to this method by Frontier 

Geosciences, Inc., Albion Environmental, Battelle Marine Sciences Laboratory, STL-Canton, and Tekran 

Inc. Additional assistance in preparing the Method was provided by DynCorp Environmental and 

Interface, Inc. 

Disclaimer 

This Method has been reviewed and approved for publication by the Statistics and Analytical Support

Branch within EPA’s Engineering and Analysis Division. Mention of trade names or commercial

products does not constitute endorsement or recommendation for use.

Questions concerning this Method or its application should be addressed to:

W.A. Telliard

Statistics and Analytical Support Branch (4303T)

U.S. Environmental Protection Agency

Ariel Rios Building

1200 Pennsylvania Avenue, N.W.

Washington, DC 20460

Phone: 202/566–1061

Fax: 202/566–1053

i 
Method 1631, Revision E 

Table of Contents 

Introduction .......................................................... iii

1.0 Scope and Application ............................................ 1

2.0 Summary of Method .............................................. 2

3.0 Definitions ...................................................... 3

4.0 Contamination and Interferences .................................... 3

5.0 Safety ......................................................... 6

6.0 Apparatus and Materials ........................................... 8

7.0 Reagents and Standards ......................................... 11

8.0 Sample Collection, Preservation, and Storage ......................... 13

9.0 Quality Control ................................................. 14

10.0 Calibration and Standardization .................................... 22

11.0 Procedure ..................................................... 25

12.0 Data Analysis and Calculations .................................... 27

13.0 Method Performance ............................................ 29

14.0 Pollution Prevention ............................................. 29

15.0 Waste Management ............................................. 30

16.0 References .................................................... 30

17.0 Glossary ...................................................... 31

18.0 Tables and Figures .............................................. 34

ii August 2002 
Method 1631, Revision E 

Introduction 

Method 1631 (the "Method") supports technology-based and water quality-based monitoring programs 

authorized under the Clean Water Act (CWA; the "Act"). 

CWA Sections 301 and 306 require EPA to publish effluent standards that restrict the direct discharge of 

pollutants to the nations waters, and CWA Sections 307(b) and (c) require EPA to promulgate nationally 

applicable pretreatement standards which restrict pollutant discharges into sewers flowing to publicly 

owned treatment works (POTWs). The effluent limitations guidelines are published at CFR parts 401-

503. 

CWA Section 303 requires each State to set a water quality standard for each body of water within its 

boundaries. A State water quality standard consists of a designated use or uses of a water body or a 

segment of a water body, the water quality criteria that are necessary to protect the designated use or uses, 

and an antidegradation policy. CWA Section 304(a) requires EPA to publish water quality criteria that 

reflect the latest scientific knowledge concerning the physical fate of pollutants, the effects of pollutants 

on ecological and human health, and the effect of pollutants on biological community diversity, 

productivity, and stability. These water quality standards serve two purposes: (1) they establish the water 

quality goals for a specific water body, and (2) they are the basis for establishing water quality-based 

treatment controls and strategies beyond the technology-based controls required by CWA Sections 301(b) 

and 306. 

In 1987, amendments to the CWA required States to adopt numeric criteria for toxic pollutants 

(designated in Section 307(a) of the Act) based on EPA Section 304(a) criteria or other scientific data, 

when the discharge or presence of those toxic pollutants could reasonably be expected to interfere with 

designated uses. Method 1631 was specifically developed to provide reliable measurements of mercury 

at EPA WQC levels. 

In developing methods for determination of trace metals, EPA found that one of the greatest difficulties 

was precluding sample contamination during collection, transport, and analysis. The degree of difficulty, 

however, is highly dependent on the metal and site-specific conditions. Method 1631 is designed to 

preclude contamination in nearly all situations. It also contains procedures necessary to produce reliable 

results at the lowest WQC levels published by EPA. In recognition of the variety of situations to which 

this Method may be applied, and in recognition of continuing technological advances, Method 1631 is 

performance based. Alternative procedures may be used so long as those procedures are demonstrated to 

yield reliable results. 

Requests for additional copies of this draft Method should be directed to: 

U.S. EPA Sample Control Center 

6101 Stevenson Avenue 

Alexandria, VA 22304-3540 

703/461–2100 

iii 
Method 1631, Revision E 

Note: This Method is performance based. The laboratory is permitted to omit steps or modify 

procedures provided that all performance requirements in this Method are met. The laboratory 

must not omit or modify any procedure defined by the term “shall” or “must” and must perform 

all quality control tests. 

iv 
Method 1631, Revision E 

v
Method 1631, Revision E 

Method 1631, Revision E 

Mercury in Water by Oxidation, Purge and Trap, and Cold Vapor 

Atomic Fluorescence Spectrometry 

1.0 Scope and Application 

1.1 Method 1631, Revision E (the "Method") is for determination of mercury (Hg) in filtered and 

unfiltered water by oxidation, 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. The Method is based on a contractor-developed procedure (Reference 16.1) and on 

peer-reviewed, published procedures for the determination of mercury in aqueous samples, 

ranging from sea water to sewage effluent (References 16.2–16.5). 

1.2 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 

guidance document is recommended to preclude contamination during the sampling process. 

1.3 This Method is for determination of Hg in the range of 0.5–100 ng/L. Application may be 

extended to higher levels by selection of a smaller sample size or by calibration of the analytical 

system across a higher range. For measurement of blank samples, the Method may be extended 

to a lower level by calibration to a lower calibration point. Section 10.4 gives requirements for 

extension of the calibration range. 

1.4 The ease of contaminating ambient water samples with mercury and interfering substances cannot 

be overemphasized. This Method includes suggestions for improvements in facilities and 

analytical techniques that should minimize contamination and maximize the ability of the 

laboratory to make reliable trace metals determinations. Certain sections of this Method contain 

suggestions and other sections contain requirements to minimize contamination. 

1.5 The detection limit and minimum level of quantitation in this Method usually are dependent on 

the level of interferences rather than instrument limitations. The method detection limit (MDL; 

40 CFR 136, Appendix B) for Hg has been determined to be 0.2 ng/L when no interferences are 

present. The minimum level of quantitation (ML) has been established as 0.5 ng/L. An MDL as 

low as 0.05 ng/L can be achieved for low Hg samples by using a larger sample volume, a lower 

BrCl level (0.2%), and extra caution in sample handling. 

1.6 Clean and ultraclean—The terms "clean" and "ultraclean" have been applied to the techniques 

needed to reduce or eliminate contamination in trace metals 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 

(References 16.6-16.7). 

1.7 This Method follows the EPA Environmental Methods Management Council's "Guidelines and 

Format for Methods to Be Proposed at 40 CFR, part 136 or part 141." 

Method 1631, Revision E - August 2002 1 
Method 1631, Revision E 

1.8 This Method is "performance based." The laboratory is permitted to modify the Method to 

overcome interferences or lower the cost of measurements if all performance criteria are met. 

Section 9.1.2.1 gives the requirements for establishing method equivalency. 

1.9 Any modification of this Method, beyond those expressly permitted, shall be considered a major 

modification subject to application and approval of alternate test procedures under 40 CFR 136.4 

and 136.5. 

1.10 This Method should be used only by analysts experienced in the use of CVAFS techniques and 

who are trained thoroughly in the sample handling and instrument techniques described in this 

Method. Each laboratory that uses this Method must demonstrate the ability to generate 

acceptable results using the procedures in Section 9.2. 

1.11 This Method is accompanied by a data verification and validation guidance document, Guidance 

on the Documentation and Evaluation of Trace Metals Data Collected for CWA Compliance 

Monitoring (Reference 16.8), that can be used for verification and validation of the data obtained. 

1.12 This Method uses either a bubbler or flow-injection system for determination of mercury in 

water. Separate calibration, analysis, and calculation procedures are provided for a bubbler 

system (Sections 10.2, 11.2.1, and 12.2) and for a flow-injection system (Sections 10.3, 11.2.2, 

and 12.3). 

2.0 Summary of Method 

2.1 A 100- to 2000-mL sample is collected directly into a cleaned, pretested, fluoropolymer or glass 

bottle using sample handling techniques designed for collection of mercury at trace levels 

(Reference 16.9). 

2.2 For dissolved Hg, the sample is filtered through a 0.45-:m capsule filter prior to preservation. 

2.3 The sample is preserved by adding either pretested 12N hydrochloric acid (HCl) or bromine 

monochloride (BrCl) solution. If a sample will also be used for the determination of methyl 

mercury, it should be preserved according to procedures in the method that will be used for 

determination of methylmercury. 

2.4 Prior to analysis, all Hg in a 100-mL sample aliquot is oxidized to Hg(II) with BrCl. 

2.5 After oxidation, the sample is sequentially reduced with NH2OH@HCl to destroy the free 

halogens, then reduced with stannous chloride (SnCl2) to convert Hg(II) to volatile Hg(0). 

2.6 The Hg(0) is separated from solution either by purging with nitrogen, helium, or argon, or by 

vapor/liquid separation. The Hg(0) is collected onto a gold trap (Figures 1, 2, and 3). 

2.7 The Hg is thermally desorbed from the gold trap into an inert gas stream that carries the released 

Hg(0) to a second gold (analytical) trap. The Hg is desorbed from the analytical trap into a gas 

stream that carries the Hg into the cell of a cold-vapor atomic fluorescence spectrometer 

(CVAFS) for detection (Figures 2 and 3). 

2.8 Quality is assured through calibration and testing of the oxidation, purging, and detection 

systems. 

2 Method 1631, Revision E - August 2002 
Method 1631, Revision E 

3.0 Definitions 

3.1 Total mercury—all BrCl-oxidizable mercury forms and species found in an unfiltered aqueous 

solution. This includes, but is not limited to, Hg(II), Hg(0), strongly organo-complexed Hg(II) 

compounds, adsorbed particulate Hg, and several tested covalently bound organo-mercurials 

(e.g., CH3HgCl, (CH3)2Hg, and C6H5HgOOCCH3). The recovery of Hg bound within microbial 

cells may require the additional step of UV photo-oxidation. In this Method, total mercury and 

total recoverable mercury are synonymous. 

3.2 Dissolved mercury—all BrCl-oxidizable mercury forms and species found in the filtrate of an 

aqueous solution that has been filtered through a 0.45-:m filter. 

3.3 Apparatus—Throughout this Method, the sample containers, sampling devices, instrumentation, 

and all other materials and devices used in sample collection, sample processing, and sample 

analysis that come in contact with the sample and therefore require careful cleaning will be 

referred to collectively as the Apparatus. 

3.4 Definitions of other terms used in this Method are given in the glossary (Section 17.0). 

4.0 Contamination and Interferences 

4.1 Preventing samples from becoming contaminated during the sampling and analysis process 

constitutes one of the greatest difficulties encountered in trace metals determinations. Over the 

last two decades, marine chemists have come to recognize that much of the historical data on the 

concentrations of dissolved trace metals in seawater are erroneously high because the 

concentrations reflect contamination from sampling and analysis rather than ambient levels. 

Therefore, it is imperative that extreme care be taken to avoid contamination when collecting and 

analyzing samples for trace metals. 

4.2 Samples may become contaminated by numerous routes. Potential sources of trace metals 

contamination during sampling include: metallic or metal-containing labware (e.g., talc gloves 

that contain high levels of zinc), containers, sampling equipment, reagents, and reagent water; 

improperly cleaned or stored equipment, labware, and reagents; and atmospheric inputs such as 

dirt and dust. Even human contact can be a source of trace metals contamination. For example, it 

has been demonstrated that dental work (e.g., mercury amalgam fillings) in the mouths of 

laboratory personnel can contaminate samples directly exposed to exhalation (Reference 16.9). 

4.3 Contamination Control 

4.3.1 Philosophy—The philosophy behind contamination control is to ensure that any object 

or substance that contacts the sample is metal free and free from any material that may 

contain mercury. 

4.3.1.1 The integrity of the results produced cannot be compromised by contamination of 

samples. This Method and the Sampling Method give requirements and 

suggestions for control of sample contamination. 

Method 1631, Revision E - August 2002 3 
Method 1631, Revision E 

4.3.1.2 Substances in a sample cannot be allowed to contaminate the laboratory work 

area or instrumentation used for trace metals measurements. This Method gives 

requirements and suggestions for protecting the laboratory. 

4.3.1.3 Although contamination control is essential, personnel health and safety remain 

the highest priority. The Sampling Method and Section 5 of this Method give 

suggestions and requirements for personnel safety. 

4.3.2 Avoiding contamination—The best way to control contamination is to completely 

avoid exposure of the sample to contamination in the first place. Avoiding exposure 

means performing operations in an area known to be free from contamination. Two of 

the most important factors in avoiding/reducing sample contamination are (1) an 

awareness of potential sources of contamination and (2) strict attention to work being 

done. Therefore, it is imperative that the procedures described in this Method be 

carried out by well-trained, experienced personnel. 

4.3.3 Use a clean environment—The ideal environment for processing samples is a class-100 

clean room. If a clean room is not available, all sample preparation should be 

performed in a class-100 clean bench or a nonmetal glove box fed by mercury-and 

particle-free air or nitrogen. Digestion should be performed in a nonmetal fume hood 

equipped with HEPA filtration and ideally situated in a clean room. 

4.3.4 Minimize exposure—The Apparatus that will contact samples, blanks, or standard 

solutions should be opened or exposed only in a clean room, clean bench, or glove box 

so that exposure to an uncontrolled atmosphere is minimized. When not being used, 

the Apparatus should be covered with clean plastic wrap, stored in the clean bench or in 

a plastic box or glove box, or bagged in clean zip-type bags. Minimizing the time 

between cleaning and use will also minimize contamination. 

4.3.5 Clean work surfaces—Before a given batch of samples is processed, all work surfaces 

in the hood, clean bench, or glove box in which the samples will be processed should 

be cleaned by wiping with a lint-free cloth or wipe soaked with reagent water. 

4.3.6 Wear gloves—Sampling personnel must wear clean, non-talc gloves during all 

operations involving handling of the Apparatus, samples, and blanks. Only clean 

gloves may touch the Apparatus. If another object or substance is touched, the glove(s) 

must be changed before again handling the Apparatus. If it is even suspected that 

gloves have become contaminated, work must be halted, the contaminated gloves 

removed, and a new pair of clean gloves put on. Wearing multiple layers of clean 

gloves will allow the old pair to be quickly stripped with minimal disruption to the 

work activity. 

4.3.7 Use metal-free Apparatus—All Apparatus used for determination of mercury at 

ambient water quality criteria levels must be nonmetallic, free of material that may 

contain metals, or both. 

4.3.7.1 Construction materials—Only fluoropolymer or glass containers must be used for 

collection of samples that will be analyzed for mercury because mercury vapors 

can diffuse in or out of other materials, leading to results that are biased low or 

high. Polyethylene and/or polypropylene labware may be used for digestion and 

other purposes because the time of sample exposure to these materials is 

relatively short. All materials, regardless of construction, that will directly or 

4 Method 1631, Revision E - August 2002 
Method 1631, Revision E 

indirectly contact the sample, must be known to be clean and free of Hg at the 

levels specified in this Method before proceeding. 

4.3.7.2 Serialization—It is recommended that serial numbers be indelibly marked or 

etched on each piece of reusable Apparatus so that contamination can be traced, 

and logbooks should be maintained to track the sample from the container 

through the labware to introduction into the instrument. It may be useful to 

dedicate separate sets of labware to different sample types; e.g., receiving waters 

vs. effluents. However, the Apparatus used for processing blanks and standards 

must be mixed with the Apparatus used to process samples so that contamination 

of all labware can be detected. 

4.3.7.3 The laboratory or cleaning facility is responsible for cleaning the Apparatus used 

by the sampling team. If there are any indications that the Apparatus is not clean 

when received by the sampling team (e.g., ripped storage bags), an assessment of 

the likelihood of contamination must be made. Sampling must not proceed if it is 

possible that the Apparatus is contaminated. If the Apparatus is contaminated, it 

must be returned to the laboratory or cleaning facility for proper cleaning before 

any sampling activity resumes. 

4.3.8 Avoid sources of contamination—Avoid contamination by being aware of potential 

sources and routes of contamination. 

4.3.8.1 Contamination by carryover—Contamination may occur when a sample 

containing a low concentration of mercury is processed immediately after a 

sample containing a relatively high concentration of mercury. The Hg 

concentration at which the analytical system (purge, traps, detector) will carry 

greater than 0.5 ng/L of Hg into a succeeding bubbler or system blank must be 

determined by analyzing calibration solutions containing successively larger 

concentrations of Hg. This test must be run prior to first use of the analytical 

system and whenever a change is made that would increase the amount of 

carryover. When a sample contains 1⁄2 or greater of this determined Hg 

concentration, a bubbler blank (bubbler system) or system blank (flow injection 

system) must be analyzed to demonstrate no carryover at the blank criteria level. 

For the bubbler system, the blank must be run using the same bubbler and sample 

trap used to run the high concentration sample. Samples analyzed following a 

sample that has been determined to result in carryover must be reanalyzed. 

Samples that are known or suspected to contain the lowest concentration of 

mercury should be analyzed first followed by samples containing higher levels. 

4.3.8.2 Contamination by samples—Significant laboratory or instrument contamination 

may result when untreated effluents, in-process waters, landfill leachates, and 

other undiluted samples containing concentrations of mercury greater than 100 

ng/L are processed and analyzed. 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 dedicated for processing trace metals samples. 

4.3.8.3 Contamination by indirect contact—Apparatus that may not directly come in 

contact with the samples may still be a source of contamination. For example, 

clean tubing placed in a dirty plastic bag may pick up contamination from the bag 

and subsequently transfer the contamination to the sample. It is imperative that 

every piece of the Apparatus that is directly or indirectly used in the collection, 

processing, and analysis of water samples be thoroughly cleaned (Section 6.1.2). 

Method 1631, Revision E - August 2002 5 
Method 1631, Revision E 

4.3.8.4 Contamination by airborne particulate matter—Less obvious substances capable 

of contaminating samples include airborne particles. 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 metalcontaining paint. Whenever possible, sample processing and analysis should 

occur as far as possible from sources of airborne contamination. 

4.3.8.5 Contamination from reagents— Contamination can be introduced into samples 

from method reagents used during processing and analysis. Reagent blanks must 

be analyzed for contamination prior to use (see Section 9.4.3). If reagent blanks 

are contaminated, a new batch of reagents must be prepared (see Section 9.4.3.2). 

4.4 Interferences 

4.4.1 At the time of promulgation of this Method, gold and iodide were known interferences. 

At a mercury concentration of 2.5 ng/L and at increasing iodide concentrations from 30 

to 100 mg/L, test data have shown that mercury recovery will be reduced from 100 to 0 

percent. At iodide concentrations greater than 3 mg/L, the sample should be prereduced with SnCl2 (to remove the brown color) and additional or more concentrated 

SnCl2 should be added. To preclude loss of Hg, the additional SnCl2 should be added 

in a closed vessel or analysis should proceed immediately. If samples containing 

iodide concentrations greater than 30 mg/L are analyzed, it may be necessary to clean 

the analytical system with 4N HCl after the analysis (Reference 16.10). 

4.4.2 The potential exists for destruction of the gold traps if free halogens are purged onto 

them, or if they are overheated (>500 °C). When the instructions in this Method are 

followed, neither of these outcomes is likely. 

4.4.3 Water vapor may collect in the gold traps and subsequently condense in the 

fluorescence cell upon desorption, giving a false peak due to scattering of the excitation 

radiation. Condensation can be avoided by predrying the gold trap. Traps that tend to 

absorb large quantities of water vapor should not be used. 

4.4.4 The fluorescent intensity is strongly dependent upon the presence of molecular species 

in the carrier gas that can cause "quenching" of the excited atoms. The dual 

amalgamation technique eliminates quenching due to trace gases, but it remains the 

laboratory's responsibility to ensure high purity inert carrier gas and a leak-free 

analytical train. 

5.0 Safety 

5.1 The toxicity or carcinogenicity of each chemical used in this Method has not been precisely 

determined; however, each compound should be treated as a potential health hazard. Exposure to 

these compounds should be reduced to the lowest possible level. 

5.1.1 Chronic mercury exposure may cause kidney damage, muscle tremors, spasms, 

personality changes, depression, irritability and nervousness. Organo-mercurials may 

cause permanent brain damage. Because of the toxicological and physical properties of 

Hg, pure standards should be handled only by highly trained personnel thoroughly 

familiar with handling and cautionary procedures and the associated risks. 

6 Method 1631, Revision E - August 2002 
Method 1631, Revision E 

5.1.2 It is recommended that the laboratory purchase a dilute standard solution of the Hg in 

this Method. If primary solutions are prepared, they shall be prepared in a hood, and a 

NIOSH/MESA-approved toxic gas respirator shall be worn. 

5.2 This Method does not address all safety issues associated with its use. The laboratory is 

responsible for maintaining a current file of OSHA regulations for safe handling of the chemicals 

specified in this Method. OSHA rules require that a reference file of material safety data sheets 

(MSDSs) must be made available to all personnel involved in these analyses (29 CFR 1917.28, 

Appendix E). It also is suggested that the laboratory perform personal hygiene monitoring of 

each analyst who uses this Method and that the results of this monitoring be made available to the 

analyst. Personal hygiene monitoring should be performed using OSHA or NIOSH approved 

personal hygiene monitoring methods. Additional information on laboratory safety can be found 

in References 16.11-16.14. The references and bibliography included in Reference 16.14 are 

particularly comprehensive in dealing with the general subject of laboratory safety. 

5.3 Samples suspected to contain concentrations of Hg at μg/L or higher levels are handled using 

essentially the same techniques employed in handling radioactive or infectious materials. Wellventilated, controlled access laboratories are required. Assistance in evaluating the health hazards 

of particular laboratory conditions may be obtained from certain consulting laboratories and from 

State Departments of Health or Labor, many of which have an industrial health service. Each 

laboratory must develop a safety program for handling Hg. 

5.3.1 Facility—When samples known or suspected of containing high concentrations of 

mercury are handled, all operations (including removal of samples from sample 

containers, weighing, transferring, and mixing) should be performed in a glove box 

demonstrated to be leak-tight or in a fume hood demonstrated to have adequate airflow. 

Gross losses to the laboratory ventilation system must not be allowed. Handling of the 

dilute solutions normally used in analytical and animal work presents no inhalation 

hazards except in an accident. 

5.3.2 Protective equipment—Disposable plastic gloves, apron or lab coat, safety glasses or 

mask, and a glove box or fume hood adequate for radioactive work should be used. 

During analytical operations that may give rise to aerosols or dusts, personnel should 

wear respirators equipped with activated carbon filters. 

5.3.3 Training—Workers must be trained in the proper method of removing contaminated 

gloves and clothing without contacting the exterior surfaces. 

5.3.4 Personal hygiene—Hands and forearms should be washed thoroughly after each 

manipulation and before breaks (coffee, lunch, and shift). 

5.3.5 Confinement—Isolated work areas posted with signs, segregated glassware and tools, 

and plastic absorbent paper on bench tops will aid in confining contamination. 

5.3.6 Effluent vapors—The effluent from the CVAFS should pass through either a column of 

activated charcoal or a trap containing gold or sulfur to amalgamate or react mercury 

vapors. 

5.3.7 Waste handling—Good technique includes minimizing contaminated waste. Plastic 

bag liners should be used in waste cans. Janitors and other personnel must be trained in 

the safe handling of waste. 

5.3.8 Decontamination 

Method 1631, Revision E - August 2002 7 
Method 1631, Revision E 

5.3.8.1 Decontamination of personnel—Use any mild soap with plenty of scrubbing 

action. 

5.3.8.2 Glassware, tools, and surfaces—Sulfur powder will react with Hg to produce 

mercuric sulfide, thereby eliminating the possible volatilization of Hg. 

Satisfactory cleaning may be accomplished by dusting a surface lightly with 

sulfur powder, then washing with any detergent and water. 

5.3.9 Laundry—Clothing known to be contaminated should be collected in plastic bags. 

Persons that convey the bags and launder the clothing should be advised of the hazard 

and trained in proper handling. If the launderer knows of the potential problem, the 

clothing may be put into a washer without contact. The washer should be run through a 

cycle before being used again for other clothing. 

5.3.10 Wipe tests—A useful method of determining cleanliness of work surfaces and tools is 

to wipe the surface with a piece of filter paper. Extraction and analysis by this Method 

can achieve a limit of detection of less than 1 ng per wipe. Less than 0.1 :g per wipe 

indicates acceptable cleanliness; anything higher warrants further cleaning. More than 

10 :g on a wipe constitutes an acute hazard and requires prompt cleaning before 

further use of the equipment or work space, and indicates that unacceptable work 

practices have been employed. 

6.0 Apparatus and Materials 

Disclaimer: The mention of trade names or commercial products in this Method is for 

illustrative purposes only and does not constitute endorsement or recommendation for use by the

Environmental Protection Agency. Equivalent performance may be achievable using apparatus,

materials, or cleaning procedures other than those suggested here. The laboratory is responsible for

demonstrating equivalent performance.

6.1 Sampling equipment 

6.1.1 Sample collection bottles-fluoropolymer or glass, 125- to 1000-mL, with 

fluoropolymer or fluoropolymer-lined cap. 

6.1.2 Cleaning 

6.1.2.1 New bottles are cleaned by heating to 65–75 °C in 4 N HCl or concentrated 

HNO3 for at least 48 h. The bottles are cooled, rinsed three times with reagent 

water, and filled with reagent water containing 1% HCl. These bottles are 

capped and placed in a clean oven at 60-70°C overnight. After cooling, they are 

rinsed three more times with reagent water, filled with reagent water containing 

0.4% (v/v) HCl, and placed in a mercury-free Class-100 clean bench until the 

outside surfaces are dry. The bottles are tightly capped (with a wrench), doublebagged in new polyethylene zip-type bags until needed, and stored in wooden or 

plastic boxes until use. The bottles may be shipped to the sampling site 

containing dilute HCl solution (e.g., 0.04%), containing reagent water, or empty. 

6.1.2.2 Used bottles known not to have contained mercury at high (>100 ng/L) levels are 

cleaned as above, except for only 6–12 h in hot 4 N HCl. 

8 Method 1631, Revision E - August 2002 
Method 1631, Revision E 

6.1.2.3 Bottle blanks must be analyzed as described in Section 9.4.7. To verify the 

effectiveness of the cleaning procedures, bottle blanks must be demonstrated to 

be free of mercury at the ML of this Method. 

6.1.2.4 As an alternative to cleaning by the laboratory, bottles may be purchased from a 

commercial supplier and each lot certified to be clean. Bottles from the lot must 

be tested as bottle blanks (Section 9.4.7) and demonstrated to be free of mercury 

at the ML of this Method. If mercury is present above this level in any bottle, 

either the lot must be rejected or the bottles must be re-cleaned. 

6.1.3 Filtration Apparatus 

6.1.3.1 Filter—0.45-:m, 15-mm diameter capsule filter (Gelman Supor 12175, or 

equivalent) 

6.1.3.2 Peristaltic pump—115-V a.c., 12-V d.c., internal battery, variable-speed, singlehead (Cole-Parmer, portable, "Masterflex L/S," Catalog No. 07570-10 drive with 

Quick Load pump head, Catalog No. 07021-24, or equivalent). 

6.1.3.3 Tubing—styrene/ethylene/butylene/silicone (SEBS) resin for use with peristaltic 

pump, approx 3/8-in ID by approximately 3 ft (Cole-Parmer size 18, Catalog No. 

06424-18, or approximately 1/4-in OD, Cole-Parmer size 17, Catalog No. 06424-

17, or equivalent). Tubing is cleaned by soaking in 5–10% HCl solution for 

8–24 h, rinsing with reagent water in a clean bench in a clean room, and drying 

in the clean bench by purging with metal-free air or nitrogen. After drying, the 

tubing is double-bagged in clear polyethylene bags, serialized with a unique 

number, and stored until use. 

6.2 Equipment for bottle and glassware cleaning 

6.2.1 Vat, 100–200 L, high-density polyethylene (HDPE), half filled with 4 N HCl in reagent 

water. 

6.2.2 Panel immersion heater, 500-W, all-fluoropolymer coated, 120 vac (Cole-Parmer H03053-04, or equivalent) 

WARNING: Read instructions carefully!! The heater will maintain steady state, without 

temperature feedback control, of 60–75°C in a vat of the size described. However, the 

equilibrium temperature will be higher (up to boiling) in a smaller vat. Also, the heater plate 

MUST be maintained in a vertical position, completely submerged and away from the vat walls to 

avoid melting the vat or burning out! 

6.2.3 Laboratory sink—in Class-100 clean area, with high-flow reagent water (Section 7.1) 

for rinsing. 

6.2.4 Clean bench—Class-100, for drying rinsed bottles. 

6.2.5 Oven—stainless steel, in Class-100 clean area, capable of maintaining ± 5°C in the 

60–70°C temperature range. 

6.3 Cold vapor atomic fluorescence spectrometer (CVAFS): The CVAFS system used may either be 

purchased from a supplier, or built in the laboratory from commercially available components. 

Method 1631, Revision E - August 2002 9 
Method 1631, Revision E 

6.3.1 Commercially available CVAFS—Tekran (Toronto, ON) Series 2600 CVAFS, BrooksRand (Seattle, WA) Model III CVAFS, Leeman Labs Hydra AF Goldplus CVAFS, or 

equivalent 

6.3.2 Custom-built CVAFS (Reference 16.15). Figure 2 shows the schematic diagram. The 

system consists of the following: 

6.3.2.1 Low-pressure 4-W mercury vapor lamp 

6.3.2.2 Far UV quartz flow-through fluorescence cell—12 mm x 12 mm x 45 mm, with a 

10-mm path length (NSG Cells, or equivalent). 

6.3.2.3 UV-visible photomultiplier (PMT)—sensitive to < 230 nm. This PMT is isolated 

from outside light with a 253.7-nm interference filter (Oriel Corp., Stamford, CT, 

or equivalent). 

6.3.2.4 Photometer and PMT power supply (Oriel Corp. or equivalent), to convert PMT 

output (nanoamp) to millivolts 

6.3.2.5 Black anodized aluminum optical block—holds fluorescence cell, PMT, and light 

source at perpendicular angles, and provides collimation of incident and 

fluorescent beams (Frontier Geosciences Inc., Seattle, WA, or equivalent). 

6.3.2.6 Flowmeter—with needle valve capable of reproducibly keeping the carrier gas 

flow rate at 30 mL/min 

6.4 Hg purging system—Figure 2 shows the schematic diagram for the purging system. The system 

consists of the following: 

6.4.1 Flow meter/needle valve—capable of controlling and measuring gas flow rate to the 

purge vessel at 350 ± 50 mL/min. 

6.4.2 Fluoropolymer fittings—connections between components and columns are made using 

6.4-mm OD fluoropolymer tubing and fluoropolymer friction-fit or threaded tubing 

connectors. Connections between components requiring mobility are made with 3.2-

mm OD fluoropolymer tubing because of its greater flexibility. 

6.4.3 Acid fume pretrap—10-cm long x 0.9-cm ID fluoropolymer tube containing 2–3 g of 

reagent grade, nonindicating, 8–14 mesh soda lime chunks, packed between wads of 

silanized glass wool. This trap is cleaned of Hg by placing on the output of a clean 

cold vapor generator (bubbler) and purging for 1 h with N2 at 350 mL/min. 

6.4.4 Cold vapor generator (bubbler)—200-mL borosilicate glass (15 cm high x 5.0 cm 

diameter) with standard taper 24/40 neck, fitted with a sparging stopper having a coarse 

glass frit that extends to within 0.2 cm of the bubbler bottom (Frontier Geosciences, 

Inc. or equivalent). 

6.5 The dual-trap Hg(0) preconcentrating system 

6.5.1 Figures 2 and 3 show the dual-trap amalgamation system (Reference 16.5). 

10 Method 1631, Revision E - August 2002 
Method 1631, Revision E 

6.5.2 Gold-coated sand traps—10-cm long x 6.5-mm OD x 4-mm ID quartz tubing. The tube 

is filled with 3.4 cm of gold-coated 45/60 mesh quartz sand (Frontier Geosciences Inc., 

Seattle, WA, or equivalent). The ends are plugged with quartz wool. 

6.5.2.1 Traps are fitted with 6.5-mm ID fluoropolymer friction-fit sleeves for making 

connection to the system. When traps are not in use, fluoropolymer end plugs 

are inserted in trap ends to eliminate contamination. 

6.5.2.2 At least six traps are needed for efficient operation, one as the "analytical" trap, 

and the others to sequentially collect samples. 

6.5.3 Heating of gold-coated sand traps—To desorb Hg collected on a trap, heat for 3.0 min 

to 450–500 °C (a barely visible red glow when the room is darkened) with a coil 

consisting of 75 cm of 24-gauge Nichrome wire at a potential of 10-14 vac. Potential is 

applied and finely adjusted with an autotransformer. 

6.5.4 Timers—The heating interval is controlled by a timer-activated 120-V outlet (Gralab, 

or equivalent), into which the heating coil autotransformer is plugged. Two timers are 

required, one each for the "sample" trap and the "analytical" trap. 

6.5.5 Air blowers—After heating, traps are cooled by blowing air from a small squirrel-cage 

blower positioned immediately above the trap. Two blowers are required, one each for 

the "sample" trap and the "analytical" trap. 

6.6 Recorder—Any multi-range millivolt chart recorder or integrator with a range compatible with 

the CVAFS is acceptable. By using a two-pen recorder with pen sensitivity offset by a factor of 

10, the dynamic range of the system is extended to 103

. 

6.7 Pipettors—All-plastic pneumatic fixed-volume and variable pipettors in the range of 10 :L to 5.0 

mL. 

6.8 Analytical balance capable of weighing to the nearest 0.01 g 

7.0 Reagents and Standards 

Note: The quantities of reagents and the preparation procedures in this section are for illustrative 

purposes. Equivalent performance may be achievable using quantities of reagents and procedures 

other than those suggested here. The laboratory is responsible for demonstrating equivalent 

performance. 

7.1 Reagent water—18-MS minimum, ultrapure deionized water starting from a prepurified 

(distilled, reverse osmosis, etc.) source. Water should be monitored for Hg, especially after ion 

exchange beds are changed. 

7.2 Air—It is very important that the laboratory air be low in both particulate and gaseous mercury. 

Ideally, mercury work should be conducted in a new laboratory with mercury-free paint on the 

walls. A source of air that is very low in Hg should be brought directly into the Class-100 clean 

bench air intake. If this is not possible, air coming into the clean bench can be cleaned for 

mercury by placing a gold-coated cloth prefilter over the intake. Gold-coated cloth filter: Soak 2 

m2

 of cotton gauze in 500 mL of 2% gold chloride solution at pH 7. In a hood, add 100 mL of 

30% NH2OH@HCl solution, and homogenize into the cloth with gloved hands. The material will 

turn black as colloidal gold is precipitated. Allow the mixture to set for several hours, then rinse 

Method 1631, Revision E - August 2002 11 
Method 1631, Revision E 

with copious amounts of deionized water. Squeeze-dry the rinsed cloth, and spread flat on 

newspapers to air-dry. When dry, fold and place over the intake prefilter of the laminar flow 

hood. 

CAUTION: Great care should be taken to avoid contaminating the laboratory with gold dust. 

This could cause interferences with the analysis if gold becomes incorporated into the samples 

or equipment. The gilding procedure should be done in a remote laboratory if at all possible. 

7.3 Hydrochloric acid—trace-metal purified reagent-grade HCl containing less than 5 pg/mL Hg. 

The HCl should be analyzed for Hg before use. 

7.4 Hydroxylamine hydrochloride—Dissolve 300 g of NH2OH@HCl in reagent water and bring to 1.0 

L. This solution may be purified by the addition of 1.0 mL of SnCl2 solution and purging 

overnight at 500 mL/min with Hg-free N2. Flow injection systems may require the use of less 

SnCl2 for purification of this solution. 

7.5 Stannous chloride—Bring 200 g of SnCl2@2H2O and 100 mL concentrated HCl to 1.0 L with 

reagent water. Purge overnight with mercury-free N2 at 500 mL/min to remove all traces of Hg. 

Store tightly capped. 

7.6 Bromine monochloride (BrCl)—In a fume hood, dissolve 27 g of reagent grade KBr in 2.5 L of 

low-Hg HCl. Place a clean magnetic stir bar in the bottle and stir for approximately 1 h in the 

fume hood. Slowly add 38 g reagent grade KBrO3 to the acid while stirring. When all of the 

KBrO3 has been added, the solution color should change from yellow to red to orange. Loosely 

cap the bottle, and allow to stir another hour before tightening the lid. 

WARNING: This process generates copious quantities of free halogens (Cl2, Br2, BrCl), which 

are released from the bottle. Add the KBrO3 slowly in a fume hood! 

7.7 Stock mercury standard—NIST-certified 10,000-ppm aqueous Hg solution (NIST-3133). This 

solution is stable at least until the NIST expiration date. 

7.8 Secondary Hg standard—Add approx 0.5 L of reagent water and 5 mL of BrCl solution (Section 

7.6) to a 1.00-L Class A volumetric flask. Add 0.100 mL of the stock mercury standard (Section 

7.7) to the flask and dilute to 1.00 L with reagent water. This solution contains 1.00 μg/mL (1.00 

ppm) Hg. Transfer the solution to a fluoropolymer bottle and cap tightly. This solution is 

considered stable until the NIST expiration date. 

7.9 Working Hg Standard A—Dilute 1.00 mL of the secondary Hg standard (Section 7.8) to 100 mL 

in a Class A volumetric flask with reagent water containing 0.5% by volume BrCl solution 

(Section 7.6). This solution contains 10.0 ng/mL and should be replaced monthly, or longer if 

extended stability is demonstrated. 

7.10 Working Hg Standard B—Dilute 0.10 mL of the secondary Hg standard (Section 7.8) to 1000 mL 

in a Class A volumetric flask with reagent water containing 0.5% by volume BrCl solution 

(Section 7.6). This solution contains 0.10 ng/mL and should be replaced monthly, or longer if 

extended stability is demonstrated. 

7.11 Initial Precision and Recovery (IPR) and Ongoing Precision and Recovery (OPR) 

solutions—Using the working Hg standard A (Section 7.9), prepare IPR and OPR solutions at a 

12 Method 1631, Revision E - August 2002 
Method 1631, Revision E

Method 1631, Revision E - August 2002 13

concentration of 5 ng/L Hg in reagent water. IPR/OPR solutions are prepared using the same

amounts of reagents used for preparation of the calibration standards.

7.12 Nitrogen—Grade 4.5 (standard laboratory grade) nitrogen that has been further purified by the

removal of Hg using a gold-coated sand trap.

7.13 Argon—Grade 5.0 (ultra high-purity, GC grade) argon that has been further purified by the

removal of Hg using a gold-coated sand trap.

8.0 Sample Collection, Preservation, and Storage

8.1 Before samples are collected, consideration should be given to the type of data required (i.e.,

dissolved or total), so that appropriate preservation and pretreatment steps can be taken. An

excess of BrCl should be confirmed either visually (presence of a yellow color) or with starch

iodide indicating paper, using a separate sample aliquot, prior to sample processing or direct

analysis to ensure the sample has been properly preserved.

8.2 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

(Reference 16.4). Polyethylene sample bottles must not be used (Reference 16.15).

8.3 Collect samples using guidance provided in the Sampling Method (Reference 16.9). Procedures

in the Sampling Method are based on rigorous protocols for collection of samples for mercury

(References 16.4 and 16.15).

NOTE: Discrete samplers have been found to contaminate samples with Hg at the ng/L level. 

Therefore, great care should be exercised if this type of sampler is used. It may be necessary

for the sampling team to use other means of sample collection if samples are found to be

contaminated using the discrete sampler.

8.4 Sample filtration—For dissolved Hg, a sample is filtered through a 0.45-:m capsule filter

(Section 6.1.3.1) 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. The Sampling

Method describes sample filtration procedures.

8.5 Preservation—Samples are preserved by adding either 5 mL/L of pretested 12N HCl or 5 mL/L

BrCl solution to the sample bottle. If a sample will be used also for the determination of methyl

mercury, it should be collected and preserved according to procedures in the method that will be

used for determination of methyl mercury (e.g., HCl or H2SO4 solution). Preserved samples are

stable for up to 90 days of the date of collection.

8.5.1 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.

8.5.2 Samples that are acid-preserved may lose Hg to coagulated organic materials in the

water or condensed on the walls (Reference 16.16). 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
Method 1631, Revision E

14 Method 1631, Revision E - August 2002

cannot be added directly to the sample bottle, the bottle must be shaken vigorously

prior to sub-sampling.

8.5.3 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.

NOTE: Because of the potential for contamination, it is recommended that filtration and preservation of

samples be performed in the clean room in the laboratory. However, if circumstances prevent overnight

shipment of samples, samples should be filtered and preserved in a designated clean area in the field in

accordance with the procedures given in Method 1669 (Reference 16.9). If filtered in the field, samples

ideally should be filtered into the sample bottle.

8.6 Storage—Sample bottles should be stored in clean (new) polyethylene bags until sample analysis.

8.7 Sample preservation, storage, and holding time requirements also are given at 40 CFR part 136.3(e)

Table II.

9.0 Quality Control

9.1 Each laboratory that uses this Method is required to operate a formal quality assurance program

(Reference 16.17). 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.

9.1.1 The laboratory shall make an initial demonstration of the ability to generate acceptable

accuracy and precision. This ability is established as described in Section 9.2.

9.1.2 In recognition of advances that are occurring in analytical technology, the laboratory is

permitted certain options to improve results or lower the cost of measurements. These

options include automation of the dual-amalgamation system, single-trap amalgamation

(Reference 16.18), direct electronic data acquisition, calibration using gas-phase

elemental Hg standards, use of the bubbler or flow-injection systems, or changes in the

detector (i.e., CVAAS) when less sensitivity is acceptable or desired. Changes in the

determinative technique, such as the use of colorimetry, are not allowed. If an analytical

technique other than the CVAFS technique specified in this Method is used, that

technique must have a specificity for mercury equal to or better than the specificity of

the technique in this Method.

9.1.2.1 Each time this Method is modified, the laboratory is required to repeat the procedure in

Section 9.2 to demonstrate that an MDL (40 CFR part 136, Appendix B) less than or

equal to one-third the regulatory compliance limit or less than or equal to the MDL of

this Method (Table 1), whichever is greater, can be achieved. If the change will affect

calibration, the instrument must be recalibrated according to Section 10. 

Note: If the compliance limit is greater than the concentration of Hg in the OPR/OPR (5 ng/L), the

acceptance criteria for blanks and the concentrations of mercury spiked into quality control samples may

be increased to support measurements at the compliance limit. For example, if the compliance limit is 12
Method 1631, Revision E

Method 1631, Revision E - August 2002 15

ng/L (National Toxics Rule, 40 CFR 131.36), the MDL must be less than or equal to 4 ng/L;

concentrations of the calibration standards may be 5, 10, 20, 50 , and 100 ng/L; concentrations of the

IPR/OPR samples may be 10 ng/L; spike concentrations and acceptance criteria for MS/MSD samples

would remain as specified in Section 9.3; and an appropriate blank acceptance criterion would be 5 ng/L.

9.1.2.2 The laboratory is required to maintain records of modifications made to this Method. 

These records include the following, at a minimum:

9.1.2.2.1 The names, titles, addresses, and telephone numbers of the analyst(s)

who performed the analyses and modification, and the quality control

officer who witnessed and will verify the analyses and modification

9.1.2.2.2 A narrative stating the reason(s) for the modification(s)

9.1.2.2.3 Results from all quality control (QC) tests demonstrating the

performance of the modified method, including the following:

(a) Calibration (Section 10)

(b) Initial precision and recovery (Section 9.2.2)

(c) Analysis of blanks (Section 9.4)

(d) Matrix spike/matrix spike duplicate analysis (Section 9.3)

(e) Ongoing precision and recovery (Section 9.5)

(f) Quality control sample (Section 9.6)

(g) Method detection limit (Section 9.2.1)

9.1.2.2.4 Data that will allow an independent reviewer to validate each

determination by tracking the instrument output to the final result. These

data are to include the following:

(a) Sample numbers and other identifiers

(b) Processing dates

(c) Analysis dates

(d) Analysis sequence/run chronology

(e) Sample weight or volume

(f) Copies of logbooks, chart recorder, or other raw data output

(g) Calculations linking raw data to the results reported

9.1.3 Analyses of MS and MSD samples are required to demonstrate the accuracy and

precision and to monitor matrix interferences. Section 9.3 describes the procedure and

QC criteria for spiking.

9.1.4 Analyses of blanks are required to demonstrate acceptable levels of contamination. 

Section 9.4 describes the procedures and criteria for analyzing blanks.

9.1.5 The laboratory shall, on an ongoing basis, demonstrate through analysis of the ongoing

precision and recovery (OPR) sample and the quality control sample (QCS) that the

system is in control. Sections 9.5 and 9.6 describe these procedures, respectively.

9.1.6 The laboratory shall maintain records to define the quality of the data that are

generated. Sections 9.3.7 and 9.5.3 describe the development of accuracy statements.

9.1.7 Quality of the analyses is controlled by an analytical batch. An analytical batch is a set

of samples oxidized with the same batch of reagents, and analyzed during the same 12-

hour shift. A batch may be from 1 to as many as 20 samples. Each batch must be

accompanied by 3 system blanks (Section 9.4.2 for the flow-injection system), a
Method 1631, Revision E

16 Method 1631, Revision E - August 2002

minimum of 3 bubbler blanks (Section 9.4.1 for the bubbler system), 1 OPR sample at

the beginning and end of the batch (Section 9.5), a QCS (Section 9.6), and at least 3

method blanks (Section 9.4.4). In addition, there must be 1 MS and 1 MSD sample for

every 10 samples (a frequency of 10%). A typical analytical sequence would be:

(a) Three system blanks (Section 9.4.2) or a minimum of 3 bubbler blanks (Section

9.4.1)

(b) A minimum of five, non-zero calibration standards (Section 10.2.2.1)

(c) On-going precision and recovery (Section 9.5)

(d) Quality control sample (Section 9.6)

(e) Method blank (Section 9.4.4)

(f) Seven samples

(g) Method blank (Section 9.4.4)

(h) Three samples

(i) Matrix spike (Section 9.3)

(j) Matrix spike duplicate (Section 9.3)

(k) Four samples

(l) Method blank (Section 9.4.4)

(m) Six samples

(n) Matrix spike (Section 9.3) 

(o) Matrix spike duplicate (Section 9.3)

(p) Ongoing precision and recovery (Section 9.5)

The above sequence includes calibration. If system performance is verified at the end

of the sequence using the OPR, analysis of samples and blanks may proceed without

recalibration (i.e., the analytical sequence would be entered at Step (d) above), unless

more than 12 hours has elapsed since verification of system performance. If more than

12 hours has elapsed, the sequence would be initiated at Step (c) above. 

9.2 Initial demonstration of laboratory capability

9.2.1 Method detection limit—To establish the ability to detect Hg, the laboratory shall

achieve an MDL that is less than or equal to the MDL listed in Section 1.5 or one-third

the regulatory compliance limit, whichever is greater. The MDL shall be determined

according to the procedure at 40 CFR 136, Appendix B using the apparatus, reagents,

and standards that will be used in the practice of this Method. This MDL shall be used

for determination of laboratory capability only, and should be determined when a new

operator begins work or whenever, in the judgment of the laboratory, a change in

instrument hardware or operating conditions would dictate reevaluation of capability.

9.2.2 Initial precision and recovery (IPR)—To establish the ability to generate acceptable

precision and recovery, the laboratory shall perform the following operations:

9.2.2.1 Analyze four replicates of the IPR solution (5 ng/L, Section 7.11) according to

the procedure beginning in Section 11.

9.2.2.2 Using the results of the set of four analyses, compute the average percent

recovery (X), and the standard deviation of the percent recovery (s) for Hg.

9.2.2.3 Compare s and X with the corresponding limits for initial precision and recovery

in Table 2. If s and X meet the acceptance criteria, system performance is

acceptable and analysis of samples may begin. If, however, s exceeds the
Method 1631, Revision E

Method 1631, Revision E - August 2002 17

precision limit or X falls outside the acceptance range, system performance is

unacceptable. Correct the problem and repeat the test (Section 9.2.2.1).

9.3 Matrix spike (MS) and matrix spike duplicate (MSD)—To assess the performance of the Method

on a given sample matrix, the laboratory must spike, in duplicate, a minimum of 10% (1 sample

in 10) from a given sampling site or, if for compliance monitoring, from a given discharge. 

Therefore, an analytical batch of 20 samples would require two pairs of MS/MSD samples (four

spiked samples total).

9.3.1 The concentration of the spike in the sample shall be determined as follows:

 

9.3.1.1 If, as in compliance monitoring, the concentration of Hg in the sample is being

checked against a regulatory compliance limit, the spiking level shall be at that

limit or at 1–5 times the background concentration of the sample (as determined

in Section 9.3.2), whichever is greater.

9.3.1.2 If the concentration of Hg in a sample is not being checked against a limit, the

spike shall be at 1–5 times the background concentration or at 1-5 times the ML

in Table 1, whichever is greater.

9.3.2 To determine the background concentration (B), analyze one sample aliquot from each

set of 10 samples from each site or discharge according to the procedure in Section 11. 

If the expected background concentration is known from previous experience or other

knowledge, the spiking level may be established a priori.

9.3.2.1 If necessary, prepare a standard solution to produce an appropriate level in the

sample (Section 9.3.1).

9.3.2.2 Spike two additional sample aliquots with identical amounts of the spiking

solution and analyze these aliquots as described in Section 11.1.2 to determine

the concentration after spiking (A).

9.3.3 Calculate the percent recovery (R) in each aliquot using the following equation:

9.3.4 Compare percent recovery (R) with the QC acceptance criteria in Table 2.

9.3.4.1 If results of the MS/MSD are similar and fail the acceptance criteria, and

recovery for the OPR standard (Section 9.5) for the analytical batch is within the

acceptance criteria in Table 2, an interference is present and the results may not

be reported or otherwise used for permitting or regulatory compliance purposes. 

If the interference can be attributed to sampling, the site or discharge should be

resampled. If the interference can be attributed to a method deficiency, the

laboratory must modify the method, repeat the test required in Section 9.1.2, and

repeat analysis of the sample and MS/MSD. However, during the development
Method 1631, Revision E

18 Method 1631, Revision E - August 2002

of Method 1631, very few interferences have been noted in the determination of

Hg using this Method. (See Section 4.4 for information on interferences.)

9.3.4.2 If the results of both the spike and the OPR test fall outside the acceptance

criteria, the analytical system is judged to be not in control, and the results may

not be reported or used for permitting or regulatory compliance purposes. The

laboratory must identify and correct the problem and reanalyze all samples in the

sample batch.

9.3.5 Relative percent difference (RPD)—Compute the RPD between the MS and MSD

results according to the following equation using the concentrations found in the MS

and MSD. Do not use the recoveries calculated in Section 9.3.3 for this calculation

because the RPD is inflated when the background concentration is near the spike

concentration.

9.3.6 The RPD for the MS/MSD pair must not exceed the acceptance criterion in Table 2. If

the criterion is not met, the system is judged to be out of control. The problem must be

identified and corrected, and the MS/MSD and corresponding samples reanalyzed.

9.3.7 As part of the QC program for the laboratory, method precision and recovery for

samples should be assessed and records maintained. After analyzing five samples in

which the recovery passes the test in Section 9.3.4, compute the average percent

recovery (Ra) and the standard deviation of the percent recovery (sr

). Express the

accuracy assessment as a percent recovery interval from Ra - 2sr

 to Ra + 2sr. For

example, if Ra = 90% and sr

 = 10% for five analyses, the accuracy interval is expressed

as 70–110%. Update the accuracy assessment regularly (e.g., after every five to ten

new accuracy measurements).

9.4 Blanks—Blanks are critical to the reliable determination of Hg at low levels. The sections below

give the minimum requirements for analysis of blanks. Analysis of additional blanks is

recommended as necessary to pinpoint sources of contamination in, and external to, the

laboratory.

9.4.1 Bubbler blanks—Bubbler blanks are analyzed to demonstrate that bubbler systems are

free from contamination at levels that could affect data quality. At least three bubbler

blanks must be run during calibration and with each analytical batch. 

9.4.1.1 To analyze a bubbler blank, place a clean gold trap on the bubbler. Purge and

analyze previously purged water using the procedure in Section 11, and

determine the amount of Hg remaining in the system.

9.4.1.2 If the bubbler blank is found to contain more than 50 pg Hg, the system is out of

control. The problem must be investigated and remedied, and the samples run on

that bubbler must be reanalyzed. If the blanks from other bubblers contain less

than 50 pg Hg, the data associated with those bubblers remain valid, provided

that all other criteria in Section 9 also are met.
Method 1631, Revision E

Method 1631, Revision E - August 2002 19

9.4.1.3 The mean result for all bubbler blanks (from bubblers passing the specification in

Section 9.4.1.2) must be < 25 pg (0.25 ng/L) Hg with a standard deviation (n-1)

of <10 pg (0.10 ng/L). If the mean is < 25 pg, the average peak area or height is

subtracted from all raw data before results are calculated (Section 12.2). 

9.4.1.4 If Hg in the bubbler blank exceeds the acceptance criteria in Section 9.4.1.3, the

system is out of control. The problem must be resolved and the system

recalibrated. Usually, the bubbler blank is too high for one of the following

reasons:

(a) Bubblers need rigorous cleaning;

(b) Soda-lime is contaminated; or

(c) Carrier gas is contaminated.

9.4.2 System blanks— System blanks are analyzed to demonstrate that flow injection

systems are free from contamination at levels that could affect data quality. Three

system blanks must be run during calibration and with each analytical batch. 

9.4.2.1 To analyze a system blank, analyze reagent water containing the same amount of

reagents used to prepare the calibration standards.

9.4.2.2 If a system blank is found to contain $ 0.50 ng/L Hg, the system is out of

control. The problem must be investigated and remedied, and the system

recalibrated. If the blanks contain < 0.50 ng/L Hg, the data associated with the

blanks remain valid, provided that all other criteria in Section 9 also are met.

9.4.2.3 The mean result for the three system blanks must be <0.5 ng/L Hg with a

standard deviation (n-1) <0.1 ng/L. If the mean exceeds these criteria, the system

is out of control, and the problem must be resolved and the system recalibrated. 

If the mean is <0.5 ng/L, the average peak height or area is subtracted from all

raw data before results are calculated (Section 12.3).

9.4.3 Reagent blanks—Reagent blanks are used to demonstrate that the reagents used to

prepare samples for Hg analyses are free from contamination. The Hg concentration in

reagent blanks is determined by analyzing the reagent solutions using either the bubbler

or flow-injection system. For the bubbler system, reagent may be added directly to

previously purged water in the bubbler.

9.4.3.1 Reagent blanks are required when the batch of reagents (bromine monochloride

plus hydroxylamine hydrochloride) are prepared. The amount of Hg in a reagent

blank containing 0.5% (v/v) BrCl solution (Section 7.6) and 0.2% (v/v)

hydroxylamine hydrochloride solution (Section 7.4) must be < 20 pg (0.2 ng/L).

9.4.3.2 The presence of more than 20 pg (0.2 ng/L) of Hg indicates a problem with the

reagent solution. The purging of certain reagent solutions, such as SnCl2 or

NH2OH, with mercury-free nitrogen or argon can reduce Hg to acceptable levels. 

Because BrCl cannot be purified, a new batch must be prepared and tested if the

BrCl is contaminated.

9.4.4 Method blanks— Method blanks are used to demonstrate that the analytical system is

free from contamination that could otherwise compromise sample results. Method

blanks are prepared and analyzed using sample containers, labware, reagents, and

analytical procedures identical to those used to prepare and analyze the samples.
Method 1631, Revision E

20 Method 1631, Revision E - August 2002

9.4.4.1 A minimum of three method blanks per analytical batch are required for both the

bubbler and flow-injection systems.

9.4.4.2 If the result for any method blank containing the nominal amount of reagent used

to prepare a sample (Section 11.1.1) is found to contain $ 0.50 ng/L (50 pg) Hg,

the system is out of control. Mercury in the analytical system must be reduced

until a method blank is free from contamination at the 0.50 ng/L level. Samples

associated with a contaminated method blank must be reanalyzed.

9.4.4.3 Because method blanks are analyzed using procedures identical to those used to

analyze samples, any sample requiring an increased amount of reagent must be

accompanied by at least one method blank that includes an identical amount of

reagent. 

9.4.5 Field blanks–Field blanks are used to demonstrate that samples have not been

contaminated by the sample collection and transport activities.

9.4.5.1 Analyze the field blank(s) shipped with each set of samples (samples collected

from the same site at the same time, to a maximum of 10 samples). Analyze the

blank immediately before analyzing the samples in the batch.

9.4.5.2 If Hg or any potentially interfering substance is found in the field blank at a

concentration equal to or greater than the ML (Table 1), or greater than one-fifth

the level in the associated sample, whichever is greater, results for associated

samples may be the result of contamination and may not be reported or otherwise

used for regulatory compliance purposes.

9.4.5.3 Alternatively, if sufficient multiple field blanks (a minimum of three) are

collected, and the average concentration (of the multiple field blanks) plus two

standard deviations is equal to or greater than the regulatory compliance limit or

equal to or greater than one-half of the level in the associated sample, results for

associated samples may be the result of contamination and may not be reported

or otherwise used for regulatory compliance purposes.

9.4.5.4 If contamination of the field blanks and associated samples is known or

suspected, the laboratory should communicate this to the sampling team so that

the source of contamination can be identified and corrective measures taken

before the next sampling event.

9.4.6 Equipment blanks—Before any sampling equipment is used at a given site, the

laboratory or cleaning facility is required to generate equipment blanks on all sampling

equipment that will be used to demonstrate that the sampling equipment is free from

contamination. 

9.4.6.1 Equipment blanks are generated in the laboratory or at the equipment cleaning

facility by processing reagent water through the sampling devices using the same

procedures that are used in the field (see Sampling Method). Therefore, the

"clean hands/dirty hands" technique used during field sampling should be

followed when preparing equipment blanks at the laboratory or cleaning facility

for low level mercury measurements. If grab samples are to be collected using

any ancillary equipment, e.g., an extension pole or a dipper, an equipment blank
Method 1631, Revision E

Method 1631, Revision E - August 2002 21

is generated by submersing this equipment into the reagent water and analyzing

the resulting reagent water collected.

9.4.6.2 The equipment blank must be analyzed using the procedures in this Method. If

mercury or any potentially interfering substance is detected in the blank at or

above the level specified for the field blank (Section 9.4.5), the source of

contamination or interference must be identified, and the problem corrected. The

equipment must be demonstrated to be free from mercury and interferences

before the equipment may be used in the field.

9.4.7 Bottle blanks— Bottles must be subjected to conditions of use to verify the

effectiveness of the cleaning procedures. A representative set of sample bottles

(Section 6.1.2) should be filled with reagent water acidified to pH <2 and allowed to

stand for a minimum of 24 h. At least 5% of the bottles from a given lot should be

tested, and the time that the bottles are allowed to stand should be as close as possible

to the actual time that the sample will be in contact with the bottle. After standing, the

water must be analyzed for any signs of contamination. If a bottle shows

contamination at or above the level specified for the field blank (Section 9.4.5), the

problem must be identified, the cleaning procedures corrected or cleaning solutions

changed, and all affected bottles re-cleaned.

9.5 Ongoing precision and recovery (OPR)—To demonstrate that the analytical system is within the

performance criteria of this Method and that acceptable precision and recovery is being

maintained within each analytical batch, the laboratory shall perform the following operations:

9.5.1 Analyze the OPR solution (5 ng/L, Section 7.11) prior to the analysis of each analytical

batch according to the procedure beginning in Section 11. An OPR also must be

analyzed at the end of an analytical sequence or at the end of each 12-hour shift. 

9.5.2 Compare the recovery with the limits for ongoing precision and recovery in Table 2. If

the recovery is in the range specified, the analytical system is in control and analysis of

samples and blanks may proceed. If, however, the concentration is not in the specified

range, the analytical process is not in control. Correct the problem and repeat the

ongoing precision and recovery test. All reported results must be associated with an

OPR that meets the Table 2 performance criteria at the beginning and end of each

batch.

9.5.3 The laboratory should add results that pass the specification in Section 9.5.2 to IPR and

previous OPR data and update QC charts to form a graphic representation of continued

laboratory performance. The laboratory also should develop a statement of laboratory

data quality by calculating the average percent recovery (Ra) and the standard deviation

of the percent recovery (sr

). Express the accuracy as a recovery interval from Ra – 2sr

to Ra + 2sr. For example, if Ra = 95% and sr = 5%, the accuracy is 85–105%.

9.6 Quality control sample (QCS) – The laboratory must obtain a QCS from a source different from

the Hg used to produce the standards used routinely in this Method (Sections 7.7–7.10). The

QCS should be analyzed as an independent check of system performance.

9.7 Depending on specific program requirements, the laboratory may be required to analyze field

duplicates and field spikes collected to assess the precision and accuracy of the sampling, sample

transportation, and storage techniques. The relative percent difference (RPD) between field

duplicates should be less than 20%. If the RPD of the field duplicates exceeds 20%, the

laboratory should communicate this to the sampling team so that the source of error can be

identified and corrective measures taken before the next sampling event.
Method 1631, Revision E

22 Method 1631, Revision E - August 2002

10.0 Calibration and Standardization

10.1 Calibration and standardization— Separate calibration procedures are provided for a bubbler

system (Section 10.2) and flow-injection system (Section 10.3). Both systems are calibrated

using standards traceable to NIST Standard Reference Materials. If system performance is

verified at the end of an analytical batch using the OPR, analysis of samples and blanks may

proceed without recalibration, unless more than 12 hours has elapsed since verification of system

performance.

10.2 Bubbler system calibration

10.2.1 Establish the operating conditions necessary to purge Hg from the bubbler and to

desorb Hg from the traps in a sharp peak. Further details for operation of the purgeand-trap, desorption, and analysis systems are given in Sections 11.2.1 and 11.2.2. 

10.2.2 The calibration must contain a minimum of five non-zero points and the results of

analysis of three bubbler blanks. The lowest calibration point must be at the Minimum

Level (ML).

NOTE: The purge efficiency of the bubbler system is 100% and is independent of volume at the

volumes used in this Method. Calibration of this system is typically performed using units of

mass. For purposes of working in concentration, the volume is assumed to be 100 mL.

10.2.2.1 Standards are analyzed by the addition of aliquots of Hg working standard A

(Section 7.9) and Hg working standard B (Section 7.10) directly into the

bubblers. Add 0.50 mL of working standard B and 0.5 mL SnCl2 to the bubbler. 

Swirl to produce a standard containing 50 pg of Hg (0.5 ng/L). Purge under the

optimum operating conditions (Section 10.2.1). Sequentially follow with the

addition of aliquots of 0.05, 0.25, 0.50 and 1.0 mL of working standard A to

produce standards of 500, 2500, 5000, and 10,000 pg Hg (5.0, 25.0, 50.0 and

100.0 ng/L). 

NOTE: If calibration to the higher levels results in carryover (Section 4.3.8.1), calibrate the

system across a narrower range (Section 10.4) 

10.2.2.2 Analyze the standards beginning with the lowest concentration and proceeding to

the highest. Tabulate the height or area for each peak.

10.2.2.3 Prepare and analyze a minimum of 3 bubbler blanks. If multiple bubblers are

used, there must be 1 bubbler blank per bubbler (to a maximum of 4 bubblers). 

Calculate the mean peak area or height for the bubbler blanks. 

10.2.2.4 For each calibration point, subtract the mean peak height or area of the bubbler

blanks from the peak height or area for each standard. Calculate the calibration

factor (CFx) for Hg in each of the five standards using the mean bubbler-blanksubtracted peak height or area and the following equation:
Method 1631, Revision E

Method 1631, Revision E - August 2002 23

10.2.2.5 Calculate the mean calibration factor (CFm), the standard deviation of the

calibration factor (SD; n-1), and the relative standard deviation (RSD) of the

calibration factor, where RSD = 100 x SD/CFm. 

10.2.2.6 If RSD # 15%, calculate the recovery for the lowest standard using CFm. If the

RSD # 15% and the recovery of the lowest standard is in the range of 75-125%,

the calibration is acceptable and CFm may be used to calculate the concentration

of Hg in samples. If RSD > 15% or if the recovery of the lowest standard is not

in the range of 75-125%, recalibrate the analytical system and repeat the test.

10.2.2.7 Calculate the concentration of Hg in the bubbler blanks (Section 10.2.2.1) using

CFm. The bubbler blanks must meet the criteria in Section 9.4.1; otherwise,

mercury in the system must be reduced and the calibration repeated until the

bubbler blanks meet the criteria.

10.3 Flow-injection system calibration

10.3.1 Establish the operating conditions necessary to purge Hg from the gas-liquid separator

and dryer tube and desorb Hg from the traps in a sharp peak. Further details for

operating the analytical system are given in Section 11.2.1. 

10.3.2 The calibration must contain a minimum of 5 non-zero points and the results of

analysis of 3 system blanks. The lowest calibration point must be at the minimum level

(ML).

10.3.2.1 Place 25-30 mL of reagent water and 250 μL of concentrated BrCl solution

(Section 7.6) in each of 5 calibrated 50-mL autosampler vials. Prepare the 0.5

ng/L calibration standard by adding 250 μL of working standard B (Section 7.10)

to the vial. Dilute to the mark with reagent water. Sequentially follow with the

addition of aliquots of 25, 125, 250 and 500 μL of working standard A (Section

7.9) to produce standards of 5.0, 25.0, 50.0 and 100.0 ng/L, respectively. Cap

the vials and invert once to mix.

10.3.2.2 Immediately prior to analysis, remove the caps and add 125 μL of NH2OH

solution (Section 7.4). Re-cap, invert once to mix, and allow to stand until the

yellow color disappears. Remove all caps and place vials into the analysis rack.

10.3.2.3 Analyze the standards beginning with the lowest concentration and proceeding to

the highest. Tabulate the height or area for the Hg peak.

10.3.2.4 Prepare and analyze a minimum of 3 system blanks and tabulate the peak heights

or areas. Calculate the mean peak area or height for the system blanks.

10.3.2.5 For each calibration point, subtract the mean peak height or area of the system

blanks (Section 9.4.2) from the peak height or area for each standard. Calculate
Method 1631, Revision E

24 Method 1631, Revision E - August 2002

the calibration factor (CFx) for Hg in each of the five standards using the mean

reagent-blank-subtracted peak height or area and the following equation:

10.3.2.6 Calculate the mean calibration factor (CFm), the standard deviation of the

calibration factor (SD; n-1), and the relative standard deviation (RSD) of the

calibration factor, where RSD = 100 x SD/CFm. 

10.3.2.7 If RSD # 15%, calculate the recovery for the lowest standard (0.5 ng/L) using

CFm. If the RSD # 15% and the recovery of the lowest standard is in the range of

75-125%, the calibration is acceptable and CFm may be used to calculate the

concentration of Hg in samples, blanks, and OPRs. If RSD > 15% or if the

recovery of the lowest standard is not in the range of 75-125%, recalibrate the

analytical system and repeat the test.

10.3.2.8 Calculate the concentration of Hg in the system blanks (Section 9.4.2) using CFm. 

The system blanks must meet the criteria in Section 9.4.2; otherwise, mercury in

the system must be reduced and the calibration repeated until the system blanks

meet the criteria.

10.4 Calibration to a range other than 0.5 to 100 ng/L—This Method may be calibrated to a range

other than 0.5 to 100 ng/L, provided that the following requirements are met:

(a) There must be a minimum of five non-zero calibration points.

(b) The difference between successive calibration points must be no greater than a factor of

10 and no less than a factor of 2 and should be approximately evenly spaced on a

logarithmic scale over the calibration range.

(c) The relative standard deviation (RSD) of the calibration factors for all calibration

points must be less than 15%.

(d) The calibration factor for any calibration point at a concentration greater than 100 ng/L

must be within ±15% of the average calibration factor for the points at or below 100

ng/L.

(e) The calibration factor for any point <0.5 ng/L must be within 25% of the average

calibration factor for all points.

(f) If calibration is to a higher range and this Method is used for regulatory compliance,

the ML must be less than one-third the regulatory compliance limit

11.0 Procedure

NOTE: The following procedures for analysis of samples are provided as guidelines.

Laboratories may find it necessary to optimize the procedures, such as drying time or potential

applied to the Nichrome wires, for the laboratory's specific instrument set-up.
Method 1631, Revision E

Method 1631, Revision E - August 2002 25

11.1 Sample Preparation

11.1.1 Pour a 100-mL aliquot from a thoroughly shaken, acidified sample, into a 125-mL

fluoropolymer bottle. If BrCl was not added as a preservative (Section 8.5), add the

amount of BrCl solution (Section 7.6) given below, cap the bottle, and digest at room

temperature for a 12 h minimum. 

11.1.1.1 For clear water and filtered samples, add 0.5 mL of BrCl; for brown water and

turbid samples, add 1.0 mL of BrCl. If the yellow color disappears because of

consumption by organic matter or sulfides, more BrCl should be added until a

permanent (12-h) yellow color is obtained.

11.1.1.2 Some highly organic matrices, such as sewage effluent, will require high levels

of BrCl (e.g., 5 mL/100 mL of sample) and longer oxidation times, or elevated

temperatures (e.g., place sealed bottles in oven at 50 °C for 6 h). The oxidation

must be continued until it is complete. Complete oxidation can be determined by

either observation of a permanent yellow color remaining in the sample or the

use of starch iodide indicating paper to test for residual free oxidizer. The

sample also may be diluted to reduce the amount of BrCl required, provided that

the resulting level of mercury is sufficient for reliable determination.

11.1.2 Matrix spikes and matrix spike duplicates—For every 10 or fewer samples, pour 2

additional 100-mL aliquots from a selected sample (see Section 9.3), spike at the level

specified in Section 9.3, and process in the same manner as the samples. There must be

a minimum of 2 MS/MSD pairs for each analytical batch of 20 samples.

11.2 Hg reduction and purging—Separate procedures are provided for the bubbler system (Section

11.2.1) and flow-injection (Section 11.2.2). 

11.2.1 Hg reduction and purging for the bubbler system

11.2.1.1 Add 0.2-0.25 mL of NH2OH solution to the BrCl-oxidized sample in the 125-mL

sample bottle. Cap the bottle and swirl the sample. The yellow color will

disappear, indicating the destruction of the BrCl. Allow the sample to react for 5

min with periodic swirling to be sure that no traces of halogens remain.

NOTE: Purging of free halogens onto the gold trap will result in damage to the trap and low or

irreproducible results.

11.2.1.2 Connect a fresh trap to the bubbler, pour the reduced sample into the bubbler,

add 0.5 mL of SnCl2 solution, and purge the sample onto a gold trap with N2 at

350 ± 50 mL/min for 20 min.

11.2.1.3 When analyzing Hg samples, the recovery is quantitative, and organic

interferents are destroyed. Thus, standards, bubbler blanks, and small amounts of

high-level samples may be run directly in previously purged water. After very

high samples (Section 4.3.8.1), a small degree of carryover (<0.01%) may occur. 

Bubblers that contain such samples must be demonstrated to be clean prior to

proceeding with low level samples. Samples run immediately following a

sample that has been determined to result in carryover must be reanalyzed using a

bubbler that is demonstrated to be clean as per Section 4.3.8.1.

11.2.2 Hg reduction and purging for the flow-injection system
Method 1631, Revision E

26 Method 1631, Revision E - August 2002

11.2.2.1 Add 0.2-0.25 mL of NH2OH solution (Section 7.4) to the BrCl-oxidized sample

in the 125-mL sample bottle or in the autosampler tube (the amount of NH2OH

required will be approximately 30 percent of the BrCl volume). Cap the bottle

and swirl the sample. The yellow color will disappear, indicating the destruction

of the BrCl. Allow the sample to react for 5 minutes with periodic swirling to be

sure that no traces of halogens remain. 

NOTE: Purging of free halogens onto the gold trap will result in damage to the trap and low or

irreproducible results.

11.2.2.2 Pour the sample solution into an autosampler vial and place the vial in the rack.

11.2.2.3 Carryover may occur after analysis of a sample containing a high level of

mercury. Samples run immediately following a sample that has been determined

to result in carryover (Section 4.3.8.1) must be reanalyzed using a system

demonstrated to be clean as per Section 4.3.8.1. 

11.3 Desorption of Hg from the gold trap

11.3.1 Remove the sample trap from the bubbler, place the Nichrome wire coil around the trap

and connect the trap into the analyzer train between the incoming Hg-free argon and

the second gold-coated (analytical) sand trap (Figure 2).

11.3.2 Pass argon through the sample and analytical traps at a flow rate of approximately 30

mL/min for approximately 2 min to drive off condensed water vapor.

11.3.3 Apply power to the coil around the sample trap for 3 minutes to thermally desorb the

Hg (as Hg(0)) from the sample trap onto the analytical trap.

11.3.4 After the 3-min desorption time, turn off the power to the Nichrome coil, and cool the

sample trap using the cooling fan.

11.3.5 Turn on the chart recorder or other data acquisition device to start data collection, and

apply power to the Nichrome wire coil around the analytical trap. Heat the analytical

trap for 3 min (1 min beyond the point at which the peak returns to baseline).

11.3.6 Stop data collection, turn off the power to the Nichrome coil, and cool the analytical

trap to room temperature using the cooling fan.

11.3.7 Place the next sample trap in line and proceed with analysis of the next sample.

NOTE: Do not heat a sample trap while the analytical trap is still warm; otherwise, the analyte

may be lost by passing through the analytical trap.

11.4 Peaks generated using this technique should be very sharp and almost symmetrical. Mercury

elutes at approximately 1 minute and has a width at half-height of about 5 seconds.

11.4.1 Broad or asymmetrical peaks indicate a problem with the desorption train, such as

improper gas flow rate, water vapor on the trap(s), or an analytical trap damaged by

chemical fumes or overheating.
Method 1631, Revision E

Method 1631, Revision E - August 2002 27

11.4.2 Damage to an analytical trap is also indicated by a sharp peak, followed by a small,

broad peak.

11.4.3 If the analytical trap has been damaged, the trap and the fluoropolymer tubing

downstream from it should be discarded because of the possibility of gold migration

onto downstream surfaces.

11.4.4 Gold-coated sand traps should be tracked by unique identifiers so that any trap

producing poor results can be quickly recognized and discarded.

12.0 Data Analysis and Calculations

12.1 Separate procedures are provided for calculation of sample results using the bubbler system

(Section 12.2) and the flow-injection system (Section 12.3), and for method blanks (Section

12.4). 

12.2 Calculations for the bubbler system

12.2.1 Calculate the mean peak height or area for Hg in the bubbler blanks measured during

system calibration or with the analytical batch (ABB; n = 3 minimum).

12.2.2 Calculate the concentration of Hg in ng/L (parts-per-trillion; ppt) in each sample

according to the following equation:

12.3 Calculations for the flow-injection system

12.3.1 Calculate the mean peak height or area for Hg in the system blanks measured during

system calibration or with each analytical batch (ASB; n = 3)

12.3.2 Calculate the concentration of Hg in ng/L in each sample according to the following

equation:

12.4 Calculations for concentration of Hg in method blanks, field blanks, and reagent blanks. 
Method 1631, Revision E

28 Method 1631, Revision E - August 2002

12.4.1 Calculate the concentration of Hg in the method blanks (CMB), field blanks (CFB), or

reagent blanks (CRB) in ng/L, using the equation in Section 12.2.2 (if bubbler system is

used) or Section 12.3.2 (if flow injection system is used) and substituting the peak

height or area resulting from the method blank, field blank, or reagent blank for As. 

12.4.2 Determine the mean concentration of Hg in the method blanks associated with the

analytical batch (a minimum of three). If a sample requires additional reagent(s) (e.g.,

BrCl), a corresponding method blank containing an identical amount of reagent must

be analyzed (Section 9.4.4.3). The concentration of Hg in the corresponding method

blank may be subtracted from the concentration of Hg in the sample per Section 12.5.2.

12.5 Reporting

12.5.1 Report results for Hg at or above the ML, in ng/L, to three significant figures. Report

results for Hg in samples below the ML as <0.5 ng/L, or as required by the regulatory

authority or in the permit. Report results for Hg in reagent blanks and field blanks at or

above the ML, in ng/L, to three significant figures. Report results for Hg in reagent

blanks, method blanks, or field blanks below the ML but at or above the MDL to two

significant figures. Report results for Hg not detected in reagent blanks, method

blanks, or field blanks as <0.2 ng/L, or as required by the regulatory authority or in the

permit.

12.5.2 Report results for Hg in samples, method blanks and field blanks separately. In addition

to reporting results for the samples and blank(s) separately, the concentration of Hg in

the method blanks or field blanks associated with the sample may be subtracted from

the results for that sample, or must be subtracted if requested or required by a

regulatory authority or in a permit. 

12.5.3 Results from tests performed with an analytical system that is not in control must not be

reported or otherwise used for permitting or regulatory compliance purposes, but do not

relieve a discharger or permittee of reporting timely results.

13.0 Method Performance

13.1 This Method was tested in 12 laboratories using reagent water, freshwater, marine water and

effluent (Reference 16.19). The quality control acceptance criteria listed in Table 2 were verified

by data gathered in the interlaboratory study, and the method detection limit (MDL) given in

Section 1.5 was verified in all 12 laboratories. In addition, the techniques in this Method have

been compared with other techniques for low-level mercury determination in water in a variety of

studies, including ICES-5 (Reference 16.20) and the International Mercury Speciation

Intercomparison Exercise (Reference 16.21).

13.2 Precision and recovery data for reagent water, freshwater, marine water, and secondary effluent

are given in Table 3.
Method 1631, Revision E

Method 1631, Revision E - August 2002 29

14.0 Pollution Prevention

14.1 Pollution prevention encompasses any technique that reduces or eliminates the quantity or

toxicity of waste at the point of generation. Many opportunities for pollution prevention exist in

laboratory operation. EPA has established a preferred hierarchy of environmental management

techniques that places pollution prevention as the management option of first choice. Whenever

feasible, laboratory personnel should use pollution prevention techniques to address waste

generation. When it is not feasible to reduce wastes at the source, the Agency recommends

recycling as the next best option. The acids used in this Method should be reused as practicable

by purifying by electrochemical techniques. The only other chemicals used in this Method are

the neat materials used in preparing standards. These standards are used in extremely small

amounts and pose little threat to the environment when managed properly. Standards should be

prepared in volumes consistent with laboratory use to minimize the disposal of excess volumes of

expired standards.

14.2 For information about pollution prevention that may be applied to laboratories and research

institutions, consult Less is Better: Laboratory Chemical Management for Waste Reduction,

available from the American Chemical Society's Department of Governmental Relations and

Science Policy, 1155 16th Street NW, Washington DC 20036, 202/872–4477.

15.0 Waste Management

15.1 The laboratory is responsible for complying with all Federal, State, and local regulations

governing waste management, particularly hazardous waste identification rules and land disposal

restrictions, and for protecting the air, water, and land by minimizing and controlling all releases

from fume hoods and bench operations. Compliance with all sewage discharge permits and

regulations is also required. An overview of requirements can be found in Environmental

Management Guide for Small Laboratories (EPA 233-B-98-001).

15.2 Acids, samples at pH <2, and BrCl solutions must be neutralized before being disposed of, or

must be handled as hazardous waste.

15.3 For further information on waste management, consult The Waste Management Manual for

Laboratory Personnel and Less is Better: Laboratory Chemical Management for Waste

Reduction, both available from the American Chemical Society's Department of Government

Relations and Science Policy, 1155 16th Street NW, Washington, DC 20036.

16.0 References

16.1 Bloom, Nicolas, Draft “Total Mercury in Aqueous Media,” Frontier Geosciences, Inc., September

7, 1994.

16.2 Fitzgerald, W.F.; Gill, G.A. "Sub-Nanogram Determination of Mercury by Two-Stage Gold

Amalgamation and Gas Phase Detection Applied to Atmospheric Analysis," Anal. Chem. 1979,

15, 1714.

16.3 Bloom, N.S; Crecelius, E.A. "Determination of Mercury in Sea water at Subnanogram per Liter

Levels," Mar. Chem. 1983, 14, 49.

16.4 Gill, G.A.; Fitzgerald, W.F. "Mercury Sampling of Open Ocean Waters at the Picogram Level,"

Deep Sea Res 1985, 32, 287.
Method 1631, Revision E

30 Method 1631, Revision E - August 2002

16.5 Bloom, N.S.; Fitzgerald, W.F. "Determination of Volatile Mercury Species at the Picogram Level

by Low-Temperature Gas Chromatography with Cold-Vapor Atomic Fluorescence Detection,"

Anal. Chim. Acta. 1988, 208, 151.

16.6 Guidance on Establishing Trace Metal Clean Rooms in Existing Facilities, U.S. EPA, Office of

Water, Office of Science and Technology, Engineering and Analysis Division (4303), 401 M

Street SW, Washington, DC 20460, January 1996, EPA 821-B-96-001.

16.7 Trace Metal Cleanroom, prepared by Research Triangle Institute for U.S. Environmental

Protection Agency, 26 W. Martin Luther King Dr., Cincinnati, OH 45268, RTI/6302/04-02 F.

16.8 Guidance on the Documentation and Evaluation of Trace Metals Data Collected for Clean Water

Act Compliance Monitoring, U.S. Environmental Protection Agency, Office of Water, Office of

Science and Technology, Engineering and Analysis Division (4303), 401 M Street SW,

Washington, DC 20460, July 1996, EPA 821-B-96-004.

16.9 Method 1669, "Method for Sampling Ambient Water for Determination of Metals at EPA

Ambient Criteria Levels," U.S. Environmental Protection Agency, Office of Water, Office of

Science and Technology, Engineering and Analysis Division (4303), 401 M Street SW,

Washington, DC 20460, April 1995 with January 1996 revisions.

16.10 Correspondence from Nicolas Bloom, Frontier Geosciences, Inc. to Dale Rushneck, Interface,

Inc., December 31, 1998.

16.11 "Working with Carcinogens," Department of Health, Education, and Welfare, Public Health

Service. Centers for Disease Control. NIOSH Publication 77-206, Aug. 1977, NTIS PB-277256.

16.12 "OSHA Safety and Health Standards, General Industry," OSHA 2206, 29 CFR 1910.

16.13 "Safety in Academic Chemistry Laboratories," ACS Committee on Chemical Safety, 1979.

16.14 "Standard Methods for the Examination of Water and Wastewater," 18th ed. and later revisions,

American Public Health Association, 1015 15th Street NW, Washington, DC 20005. 1-35:

Section 1090 (Safety), 1992.

16.15 Bloom, N.S. "Trace Metals & Ultra-Clean Sample Handling," Environ. Lab. 1995, 7, 20.

16.16 Bloom, N.S. "Influence of Analytical Conditions on the Observed 'Reactive Mercury,'

Concentrations in Natural Fresh Waters," In Mercury as a Global Pollutant; Huckabee, J. and

Watras, C.J., Eds.; Lewis Publishers, Ann Arbor, MI: 1994.

16.17 "Handbook of Analytical Quality Control in Water and Wastewater Laboratories," U.S.

Environmental Protection Agency. Environmental Monitoring Systems Laboratory, Cincinnati,

OH 45268, EPA-600/4-79-019, March 1979.

16.18 Liang, L.; Bloom, N.S. "Determination of Total Mercury by Single-Stage Gold Amalgamation

with Cold Vapor Atom Spectrometric Detection," J. Anal. Atomic Spectrom. 1993, 8, 591.

16.19 "Results of the EPA Method 1631 Validation Study," February, 1998. Available from the EPA

Sample Control Center, 6101 Stevenson Avenue, Alexandria, VA, 22304; 703/461-2100.
Method 1631, Revision E

Method 1631, Revision E - August 2002 31

16.20 Cossa, D.; Couran, P. "An International Intercomparison Exercise for Total Mercury in Sea

Water," App. Organomet. Chem. 1990, 4, 49.

16.21 Bloom, N.S.; Horvat, M.; Watras, C.J. "Results of the International Mercury Speciation

Intercomparison Exercise," Wat. Air. Soil Pollut., 1995, 80, 1257.

17.0 Glossary

The definitions and purposes below are specific to this Method, but have been conformed to common

usage as much as possible.

17.1 Ambient Water—Waters in the natural environment (e.g., rivers, lakes, streams, and other

receiving waters), as opposed to effluent discharges.

17.2 Analytical Batch—A batch of up to 20 samples that are oxidized with the same batch of reagents

and analyzed during the same 12-hour shift. Each analytical batch must also include at least three

bubbler blanks, an OPR, and a QCS. In addition, MS/MSD samples must be prepared at a

frequency of 10% per analytical batch (one MS/MSD for every 10 samples).

17.3 Bottle Blank—The bottle blank is used to demonstrate that the bottle is free from contamination

prior to use. Reagent water known to be free of mercury at the MDL of this Method is added to a

bottle, acidified to pH <2 with BrCl or HCl, and allowed to stand for a minimum of 24 hours. 

The time that the bottle is allowed to stand should be as close as possible to the actual time that

the sample will be in contact with the bottle. After standing, the water is analyzed. 

17.4 Bubbler Blank—For this Method, the bubbler blank is specific to the bubbler system and is used

to determine that the analytical system is free from contamination. After analysis of a standard,

blank, or sample, the solution in the bubbler is purged and analyzed. A minimum of three

bubbler blanks is required for system calibration.

17.5 Equipment Blank—Reagent water that has been processed through the sampling device

at a laboratory or other equipment cleaning facility prior to shipment of the sampling

equipment to the sampling site. The equipment blank is used to demonstrate that the

sampling equipment is free from contamination prior to use. Where appropriate, the

"clean hands/dirty hands" technique used during field sampling should be followed

when preparing equipment blanks at the laboratory or cleaning facility.

17.6 Field Blank—Reagent water that has been transported to the sampling site and exposed to the

same equipment and operations as a sample at the sampling site. The field blank is used to

demonstrate that the sample has not been contaminated by the sampling and sample transport

systems.

17.7 Intercomparison Study—An exercise in which samples are prepared and split by a reference

laboratory, then analyzed by one or more testing laboratories and the reference laboratory. The

intercomparison, with a reputable laboratory as the reference laboratory, serves as the best test of

the precision and accuracy of the analyses at natural environmental levels.
Method 1631, Revision E

32 Method 1631, Revision E - August 2002

17.8 Matrix Spike (MS) and Matrix Spike Duplicate (MSD)—Aliquots of an environmental sample

to which known quantities of the analyte(s) of interest is added in the laboratory. The MS and

MSD are analyzed exactly like a sample. Their purpose is to quantify the bias and precision

caused by the sample matrix. The background concentrations of the analytes in the sample matrix

must be determined in a separate aliquot and the measured values in the MS and MSD corrected

for these background concentrations.

17.9 May—This action, activity, or procedural step is allowed but not required.

17.10 May not—This action, activity, or procedural step is prohibited.

17.11 Method blank— Method blanks are used to determine the concentration of mercury in

the analytical system during sample preparation and analysis, and consist of a volume of

reagent water that is carried through the entire sample preparation and analysis. Method

blanks are prepared by placing reagent water in a sample bottle and analyzing the water

using reagents and procedures identical to those used to prepare and analyze the

corresponding samples. A minimum of three method blanks is required with each

analytical batch.

17.12 Minimum Level (ML)—The lowest level at which the entire analytical system must give a

recognizable signal and acceptable calibration point for the analyte. It is equivalent to the

concentration of the lowest calibration standard, assuming that all method-specified sample

weights, volumes, and cleanup procedures have been employed. The ML is calculated by

multiplying the MDL by 3.18 and rounding the result to the number nearest to (1, 2, or 5) x 10n

,

where n is an integer (See Section 1.5).

17.13 Must—This action, activity, or procedural step is required.

17.14 Quality Control Sample (QCS)—A sample containing Hg at known concentrations. The QCS is

obtained from a source external to the laboratory, or is prepared from a source of standards

different from the source of calibration standards. It is used as an independent check of

instrument calibration.

17.15 Reagent blank—Reagent blanks are used to determine the concentration of mercury in the

reagents (BrCl, NH2OH@HCl, and SnCl2) that are used to prepare and analyze the samples. In this

Method, reagent blanks are required when each new batch of reagents is prepared.

17.16 Reagent Water—Water demonstrated to be free of mercury at the MDL of this Method. It is

prepared from 18 MS ultrapure deionized water starting from a prepurified source. Reagent

water is used to wash bottles, as trip and field blanks, and in the preparation of standards and

reagents.

17.17 Regulatory Compliance Limit—A limit on the concentration or amount of a pollutant or

contaminant specified in a nationwide standard, in a permit, or otherwise established by a

regulatory authority.

17.18 Shall—This action, activity, or procedure is required.

17.19 Should—This action, activity, or procedure is suggested, but not required. 
Method 1631, Revision E

Method 1631, Revision E - August 2002 33

17.20 Stock Solution— A solution containing an analyte that is prepared from a reference material

traceable to NIST, or a source that will attest to the purity and authenticity of the reference

material.

17.21 System Blank— For this Method, the system blank is specific for the flow-injection system and

is used to determine contamination in the analytical system and in the reagents used to prepare the

calibration standards. A minimum of three system blanks is required during system calibration.

17.22 Ultraclean Handling— A series of established procedures designed to ensure that samples are

not contaminated during sample collection, storage, or analysis.
Method 1631, Revision E

34 Method 1631, Revision E - August 2002

18.0 Tables and Figures

Table 1

Lowest Ambient Water Quality Criterion for Mercury and the Method Detection Limit and

Minimum Level of Quantitation for EPA Method 1631

Metal

Lowest Ambient Water

Quality Criterion(1)

Method Detection Limit (MDL)

and Minimum Level (ML)

MDL(2) ML(3)

Mercury (Hg) 1.3 ng/L 0.2 ng/L 0.5 ng/L

1. Lowest water quality criterion for the Great Lakes System (Table 4, 40 CFR 132.6). 

The lowest Nationwide criterion is 12 ng/L (40 CFR 131.36).

2. Method detection limit (40 CFR 136, Appendix B)

3. Minimum level of quantitation (see Glossary)

Table 2

Quality Control Acceptance Criteria for Performance Tests in EPA Method 1631

Acceptance Criteria Section Limit (%)

Initial Precision and Recovery (IPR) 9.2.2

Precision (RSD) 9.2.2.3 21

Recovery (X) 9.2.2.3 79-121

Ongoing Precision and Recovery (OPR) 9.5.2 77-123

Matrix Spike/Matrix Spike Duplicate (MS/MSD) 9.3

Recovery 9.3.4 71-125

Relative Percent Difference (RPD) 9.3.5 24
Method 1631, Revision E

Method 1631, Revision E - August 2002 35

Table 3

Precision and Recovery for Reagent Water, Fresh Water, Marine Water, and Effluent Water 

Using Method 1631

Matrix

*Mean Recovery

(%)

*Precision 

(% RSD)

Reagent Water 98.0 5.6

Fresh Water (Filtered) 90.4 8.3

Marine Water (Filtered) 92.3 4.7

Marine Water (Unfiltered) 88.9 5.0

Secondary Effluent (Filtered) 90.7 3.0

Secondary Effluent (Unfiltered) 92.8 4.5

*Mean percent recoveries and RSDs are based on expected Hg concentrations.
Method 1631, Revision E

36 Method 1631, Revision E - August 2002

Figure 1. Schematic Diagram of Bubbler Setup
Method 1631, Revision E

Method 1631, Revision E - August 2002 37

Figure 2. Schematic Diagram of the Bubbler, Purge and Trap, Cold Vapor Atomic

Fluorescence Spectrometer (CVAFS) System
Method 1631, Revision E

38 Method 1631, Revision E - August 2002 Waste Reducing Liquid Sample Handling Agent Gas / Liquid Separator System Controller (PC) Analytical Cartridge V24-way V13-way Detector Carrier (Argon) Detector Carrier Flow (Mass Flow Controller) Separator Flow (Ball Flowmeter) HEATER-A AF DETECTOR Data Acquisition/Control UnitFAN-A Dual Stage Gold Preconcentrator Key: Main Pump Speed Control DI water



















Autosampler

Auto-Sampler Port

FAN-B

Soda LimeScrubber 







HEATER-B

SamplingCartridge

Wash Station Pump Control

V1

VENT

V2

LOAD

Separator

Flow

(Argon or

Nitrogen) Hi-Resolution

A/D Input

DI WaterLiquid SampleReducing AgentLiquid WasteArgon Carrier

Phase SeparatorCarrier Gas

Control Line

P

P

P3

2

P

Wash Station

Vent Vent

Sample

Sample

Rack

Probe

WashPump

(2 channel)

Main Peristaltic

Pump 

MFC

450 ml/m

80

ml/m Carrier Gas In

Liquid

Liquid

Waste

Sample

In

LoadedCarrierOut

P1

Figure 3. Schematic Diagram of the Flow-Injection, Cold Vapor Atomic

Fluorescence Spectrometer (CVAFS) System