©EPA Method 1636: Determination of
Hexavalent Chromium by Ion
Chromatography
> Printed on Recycled Paper
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Method 1636
Acknowledgments
Method 1636 was prepared under the direction of William A. Telliard of the U.S. Environmental
Protection Agency's (EPA's) Office of Water (OW), Engineering and Analysis Division (BAD). lfce
method was prepared under EPA Contract 68-C3-0337 by the DynCorp Environmental Programs Division
with assistance from Interface, Inc.
The following researchers contributed to the philosophy behind this method. Their contribution is
gratefully acknowledged:
Shier Herman, National Research Council, Ottawa, Ontario, Canada
Nicholas Bloom, Frontier Geosciences Inc., Seattle, Washington
Paul Boothe and Gary Steinmetz, Texas A&M University, College Station, Texas
Eric CreceBus, Battelle Marine Sciences Laboratory, Sequim, Washington
Russell Flegal, University of California/Santa Cruz, California
Gary Gill, Texas A&M University at Galveston, Texas
Carlton Hunt and Dion Lewis, Battelle Ocean Sciences, Duxbuiy, Massachusetts
Carl Watras Wisconsin Department of Natural Resources, Boulder Junction, Wisconsin
Herb Windom and Ralph Smith, Skidaway Institute of Oceanography, Savannah, Georgia
In addition, the following personnel at the EPA Office of Research and Development's Environmental
Monitoring Systems Laboratory in Cincinnati, Ohio, are gratefuly acknowledged for the development of
the analytical procedures described in this method:
T.D. Martin
J.D.Pfaff
E J. Arar (DynCorp, formerly Technology Applications, Inc.)
S.E. Long (DynCorp, formerly Technology Applications, Inc.)
Disclaimer
This method has been reviewed and approved for publication by the Engineering and Analysis Division
of the U.S. Environmental Protection Agency. Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
i _
Questions concerning this method or its application should be addressed to:
W.A. Telliard
USEPA Office of Water
Analytical Methods Staff
Mail Code 4303
401 M Street, SW
Washington, DC 20460
Phone: 202/260-7120
Fax: 202/260-7185
Draft, January 1996
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Method1636
Introduction
This analytical method was designed to support water quality monitoring programs authorized under the
Clean Water Act. Section 304(a) of the Clean Water Act requires EPA to publish water quality criteria
that reflect the latest scientific knowledge about the physical fate (e.g., concentration and dispersal) of
pollutants, the effects of pollutants on ecological and human health, and the effect of pollutants on
biological community diversity, productivity, and stability.
Section 303 of the Clean Water Act requires states 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 waterbody
or a segment of a waterbody, the water quality criteria that are necessary to protect the designated use or
uses, and an antidegradation policy. These water quality standards serve two purposes: (1) they establish
the water quality goals for a specific waterbody, and (2) they are the basis for establishing water quality-
based treatment controls and strategies beyond the technology-based controls required by Sections 301(b)
and 306 of the Clean Water Act.
In defining water quality standards, the state may use narrative criteria, numeric criteria, or both.
However, the 1987 amendments to the Clean Water Act 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.
In some cases, these water quality criteria are as much as 280 times lower than those achievable using
existing EPA methods and required to support technology-based permits. Therefore, EPA developed new
sampling and analysis methods to specifically address state needs for measuring toxic metals at water
quality criteria levels, when such measurements are necessary to protect designated uses in state water
quality standards. The latest criteria published by EPA are those listed in the National Toxics Rule (57
FR 60848) and the Stay of Federal Water Quality Criteria for Metals (60 FR 22228). These rules include
water quality criteria for 13 metals, and it is these criteria on which the new sampling and analysis
methods are based. Method 1636 was specifically developed to provide reliable measurements of
hexavalent chromium at EPA WQC levels using ion chromatography techniques.
In developing these methods, EPA found that one of the greatest difficulties hi measuring pollutants at
these levels was precluding sample contamination during collection, transport, and analysis. The degree
of difficulty, however, is highly dependent on the metal and site-specific conditions. This analytical
method, therefore, is designed to provide the level of protection necessary to preclude contamination in
nearly all situations. It is also designed to provide the procedures necessary to produce reliable results
at the lowest possible water quality criteria published by EPA. In recognition of the variety of situations
to which this method may be applied, and in recognition of continuing technological advances, the method
is performance based. Alternative procedures may be used, so long as those procedures are demonstrated
to yield reliable results.
Requests for additional copies should be directed to:
US EPA NCEPI
11029 Kenwood Road
Cincinnati, OH 45242
513/489-8190
Draft, January 1996
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Method 1636
Note- This method is intended to be performance based, and. the laboratory is permitted to omit any
step or modify any procedure if all performance requirements set forth in this method are met The
laboratory is not allowed to omit any quality control analyses. The terms "must," may, and
"should" are included throughout this method and are intended to illustrate the importance of the
procedures in producing verifiable data at water quality criteria levels. The term "must" is used to
indicate that researchers in trace metals analysis have found certain procedures essential m
successfully analyzing samples and avoiding contamination; however, these procedures can be
modified or omitted if the laboratory can demonstrate that data quality is not affected.
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Method 1636
Method 1636
Determination of Hexavalent Chromium by Ion
Chromatography
1.0 Scope and Application
1.1 This method is for the determination of dissolved hexavalent chromium (as CrO42') in ambient
waters at EPA water quality criteria (WQC) levels using ion Chromatography (1C). This
method was developed by integrating the analytical procedures in EPA Method 218.6 with the
quality control (QC) and sample handling procedures necessary to avoid contamination and
ensure the validity of analytical results during sampling and analysis for metals at EPA WQC
levels. This method contains QC procedures that will ensure that contamination will be
detected when blanks accompanying samples are analyzed. This method is accompanied by
Method 1669: Sampling Ambient Water for Determination of Trace Metals at EPA Water
Quality Criteria Levels (the "Sampling Method"). The Sampling Method is necessary to
ensure that contamination will not compromise trace metals determinations during the
sampling process.
Analyte
Chemical Abstract Services
Registry Number (CASRN)
Hexavalent Chromium (as CrO/")
18540-29-9
1.2
1.3
1.4
Table 1 lists the EPA WQC level, the method detection limit (MDL), and the minimum level
(ML) for hexavalent chromium (Cr(VI)). Linear working ranges will be dependent on the
sample matrix, instrumentation, and selected operating conditions.
This method is not intended for determination of metals at concentrations normally found in
treated and untreated discharges from industrial facilities. Existing regulations (40 CFR Parts
400-500) typically limit concentrations in industrial discharges to the mid to high part-per-
billion (ppb) range, whereas ambient metals concentrations are normally in the low part-per-
trillion (ppt) to low ppb range.
The ease of contaminating ambient water samples with the metal(s) of interest and interfering
substances cannot be overemphasized. This method includes suggestions for improvements in
facilities and analytical techniques that should maximize the ability of the laboratory to make
reliable trace metals determinations and minimize contamination. These suggestions are given
in Section 4.0 and are based on findings of researchers performing trace metals analyses
(References 1-8). Additional suggestions for improvement of existing facilities may be found
in EPA's Guidance for Establishing Trace Metals Clean Rooms in Existing Facilities, which is
available from the National Center for Environmental Publications and Information (NCEPI) at
the address listed in the introduction to this document
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Method 1636
1 5 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 (Reference 9).
1.6 This method follows the EPA Environmental Methods Management Council's "Format for
Method Documentation" (Reference 10).
1 7 This method is "performance based"; i.e., an alternate procedure or technique may be used, as
long as the performance requirements in the method are met. Section 9.1.2 gives details of the
tests and documentation required to support and document equivalent performance.
1 8 For dissolved Cr(VI) determinations, samples must be filtered through a 0.45-um capsule filter
at the field site. The Sampling Method describes the filtering procedures. The filtered
samples should be preserved in the field; otherwise, samples must be analyzed within 24 h of
collection. The Sampling Method details procedures for field preservation.
1 9 Samples containing high levels of anionic species such as sulphate and chloride may cause
column overload. Samples containing high levels of organics or sulfides cause rapid reduction
of soluble Cr(VD to Cr(III). Samples must be stored at: 4°C and analyzed within 24 h of
collection unless preserved with sodium hydroxide.
110 This method should be used by analysts experienced hi the use of ion chromatography, and
should be used only by personnel thoroughly trained in the handling and analysis of samples
for determination of metals at EPA WQC levels. A minimum of six months expenence with
commercial instrumentation is recommended.
Ill This method is accompanied by a data verification and validation guidance document titled
Guidance on the Documentation and Evaluation of Trace Metals Data Collected for CWA
Compliance Monitoring. Before using this method, data users should state the data quality
objectives (DQOs) required for a project.
2.0 Summary of Method
2.1 An aqueous sample is filtered through a 0.45-um filter and the filtrate is adjusted to a pH of 9
to 9.5 with a concentrated buffer solution. A measured volume of the sample (50-250 uL) is
introduced into the ion chromatograph. A guard column removes organics from the sample
before the Cr(VI), as CrO42% is separated on a high capacity anion exchange separator column.
Postcolumn derivatization of the Cr(VD with diphenylcarbazide is followed by detection of the
colored complex at 530 run.
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Method1636
3.0 Definitions
3.1 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 activities will be referred to collectively as the Apparatus.
3.2
Other definitions of terms are given in the glossary (Section 18) at the end of this method.
4.0 Contamination and Interferences
4.1 Preventing ambient water samples from becoming contaminated during the sampling and
analytical 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. More recently, historical trace metals data collected from
freshwater rivers and streams have been shown to be similarly biased because of contamination
during sampling and analysis (Reference 11). Therefore, it is imperative that extreme care be
taken to avoid contamination when collecting and analyzing ambient water 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
which contain high levels of zinc), containers, sampling equipment, reagents, and reagent
water; improperly cleaned and 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 that are directly exposed to exhalation
(Reference 3).
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 metals.
4.3.1.1 The integrity of the results produced cannot be compromised by contamination
of samples. Requirements and suggestions for control of sample contamination
are given in this method and the Sampling Method.
4.3.1.2 Substances in a sample cannot be allowed to contaminate the laboratory work
area or instrumentation used for trace metals measurements. Requirements and
suggestions for protecting the laboratory are given in this method.
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Method 1636
4.3.1.3 Although contamination control is essential, personnel health and safety remain
the highest priority. Requirements and suggestions for personnel safety are
given in the Sampling Method and Section 5 of this method.
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 hi 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.
433 Use a clean environment—The ideal environment for processing samples is a class 100
clean room (Section 6.1.1). 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
particle-free air or nitrogen. Digestions should be performed in a nonmetal fume hood
situated, ideally, in the clean room.
434 Minimize exposure—The Apparatus that will contact samples, blanks, or standard
solutions should be opened or exposed only hi 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 cle;m plastic wrap, stored in the clean
bench or in a plastic box or glove box, or bagg<5d in clean zip-type bags. Minimizing
the time between cleaning and use will also miiiimize contamination.
435 Clean work surfaces—Before a given batch of siamples is processed, all work surfaces
in the hood, clean bench, or glove box hi which the samples will be processed should
be cleaned by wiping with a lint-free cloth or wipe soaked with reagent water.
436 Wear gloves—Sampling personnel must wear clean, nontalc gloves (Section 6.6.8)
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 rninimal disruption to the
work activity.
437 Use metal-free Apparatus—All Apparatus used for determination of metals 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 the following materials should come hi contact
with samples: fluoropolymer (FEP, PTFE), conventional or linear
polyethylene, polycarbonate, polypropylene, polysulfone, or ultrapure quartz.
PTFE is less desirable than FEP because the sintered material hi FIFE may
contain contaminates and is susceptible to serious memory contamination
(Reference 6). Fluoropolymer or glass containers should be used for samples
that will be analyzed for mercury because mercury vapors can diffuse hi or out
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Method 2636
4.3.8
of the other materials, resulting either in contamination or low-biased results
(Reference 3). Stainless steel is a major source of chromium contamination.
All materials, regardless of construction, that will directly or indirectly contact
the sample must be cleaned using the procedures described in Section 11 and
must be known to be clean and metal free before proceeding.
4.3.7.2 The following materials have been found to contain trace metals and should
not contact the sample or be used to hold liquids that contact the sample,
unless these materials have been shown to be free of the metals of interest at
the desired level: Pyrex, Kimax, methacrylate, polyvinylchloride, nylon, and
Vycor (Reference 6). In addition, highly colored plastics, paper cap liners,
pigments used to mark increments on plastics, and rubber all contain trace
levels of metals and must be avoided (Reference 12).
4.3.7.3 Serialization—It is recommended that serial numbers be indelibly marked or
etched on each piece of Apparatus so that contamination can be traced, and
logbooks should be maintained to track the sample from the container through
the labware to injection 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.
4.3.7.4 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.
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 low concentrations of metals is processed immediately after a
sample containing relatively high concentrations of these metals. To reduce
carryover, the sample introduction system may be rinsed between samples with
dilute acid and reagent water. When an unusually concentrated sample is
encountered, it is followed by analysis of a laboratory blank to check for
carryover. For samples containing high levels of metals, it may be necessary
to acid-clean or replace the connecting tubing or inlet system to ensure that
contamination will not affect subsequent measurements. Samples known or
suspected to contain the lowest concentration of metals should be analyzed
first followed by samples containing higher levels. For instruments containing
autosamplers, the laboratory should keep track of which station is used for a
given sample. When an unusually high concentration of a metal is detected in
a sample, the station used for that sample should be cleaned more thoroughly
to prevent contamination of subsequent samples, and the results for subsequent
Draft, January 1996
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Method 1636
samples should be checked for evidence of the metal(s) that occurred in high
concentration.
4.3.8.2 Contamination by samples—Significant laboratory or instrument contamination
may result when untreated effluents, in-process waters, landfill leachates, and
other samples containing high concentrations of inorganic substances are
processed and analyzed. As stated hi Section 1.0, this method is not intended
for application to these samples, and samples containing high concentrations
should not be permitted into the clean room and laboratory dedicated for
processing trace metals samples. j
4.3.8.3 Contamination by indirect contact—Apparatus that may not directly come hi
contact with the samples may still be a source of contamination. For example,
clean tubing placed hi a duty plastic bag may pick up contamination from the
bag and then subsequently transfer the contamination to the sample.
Therefore, it is imperative that every piwe of the Apparatus that is directly or
indirectly used hi the collection, processing, and analysis of ambient water
samples be cleaned as specified hi Section 11.
4.3.8.4 Contamination by airborne paniculate matter—Less obvious substances capable
of contaminating samples include airborne particles. Samples may be
contaminated by airborne dust, dirt, panicles, or vapors from unfiltered air
supplies; nearby corroded or rusted pipes, wires, or other fixtures; or metal-
containing paint. Whenever possible, simple processing and analysis should
be done as far as possible from sources of airborne contamination.
I
4.4 Interferences which affect the accurate determination of Cr(VI) may come from several
sources. j
441 Reduction of Cr(VI) to Cr(ffl) can occur in the presence of reducing species hi an
acidic medium. At pH 6.5 or greater, however, CrO^, which is less reactive than
HCrO4", is the predominant species.
4.4.2 Overloading of the analytical column capacity with high concentrations of anionic
species, especially chloride and sulphate, will cause a loss of Cr(VI). The column
specified hi this method can handle samples containing up to 5% sodium sulphate or
2% sodium chloride (Reference 13). Poor recoveries from fortified samples and tailing
peaks are typical manifestations of column overload.
5.0 Safety
5 1 Hexavalent chromium is toxic and a suspected carcinogen and should be handled with
appropriate precautions. Extreme care should be exercised when weighing the salt for
preparation of the stock standard.
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Method 1636
5.2
5.3
Each laboratory is responsible for maintaining a current awareness file of OSHA regulations
for the safe handling of the chemicals specified in this method (References 14-17). A
reference file of material safety data sheets (MSDSs) should also be available to all personnel
involved in the chemical analysis. It is also 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. The references and bibliography at the end of
Reference 17 are particularly comprehensive in dealing with the general subject of laboratory
safety.
Concentrated nitric and hydrochloric acids present various hazards and are moderately toxic
and extremely irritating to skin and mucus membranes. Use these reagents in a fume hood
whenever possible and if eye or skin contact occurs, flush with large volumes of water.
Always wear protective clothing and safety glasses or a shield for eye protection, and observe
proper mixing when working with these reagents.
6.0 Apparatus, Equipment, and Supplies
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 and materials other than those suggested here. The
laboratory is responsible for demonstrating equivalent performance.
6.1 Facility
6.1.1 Clean room—Class 100, 200-ft2 minimum, with down-flow, positive-pressure
ventilation, air-lock entrances, and pass-through doors.
6.1.1.1 Construction materials—Nonmetallic, preferably plastic sheeting attached
without metal fasteners. If painted, paints that do not contain the metal(s) of
interest should be used.
6.1.1.2 Adhesive mats—for use at entry points to control dust and dirt from shoes.
6.1.2 Fume hoods—nonmetallic, two minimum, with one installed internal to the clean
room.
6.1.3 Clean benches—Class 100, one installed hi the clean room; the other adjacent to the
analytical instrument(s) for preparation of samples and standards.
6.2 Ion Chromatograph
6.2.1 Instrument equipped with a pump capable of withstanding a minimum backpressure of
2000 psi and of delivering a constant flow in the range of 1-5 mL/min and containing
no metal parts in the sample, eluent, or reagent flow path
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Method 1636
i
6.2.2 Helium gas supply (high purity, 99.995%)
6.2.3 Pressurized eluent container, plastic, 1- or 2-L size
6.2.4 Sample loops of various sizes (50-250uL)
6.2.5 A pressurized reagent delivery module with a mixing tee and beaded mixing coil
i
6.2.6 Guard Column—A column placed before the separator column and containing a
sorbent capable of removing strongly absorbing organics and particles that would
otherwise damage the separator column (Dionex lonPac NG1 or equivalent).
6.2.7 Separator Column—A column packed with a high capacity anion exchange resin
capable of separating CrO42- from other sample constituents (Dionex lonPac AS7 or
equivalent).
6.2.8 A low-volume, flow-through cell, visible lamp detector containing no metal parts in
contact with the eluent flow path. Detection wavelength is at 530 nm.
6.2.9 Recorder, integrator, or computer for receiving analog or digital signals for recording
detector response (peak height or area) as a function of time
6.3 Alkaline detergent—Liquinox®, Alconox®, or equivalent.
6.4 pH meter or pH paper ;
6.5 Analytical balance—with capability to measure to 0.1 nig, for use in weighing solids and for
preparing standards
6.6 Labware—For determination of trace levels of elements, contamination and loss are of prime
consideration. Potential contamination sources include improperly cleaned laboratory
apparatus and general contamination within the laboratory environment from dust, etc. A
clean laboratory work area should be designated for trace element sample handling. Sample
containers can introduce positive and negative errors in the determination of trace elements by
(1) contributing contaminants through surface desorptkra or leaching, and (2) depleting element
concentrations through adsorption processes. All labware must be metal free. Suitable
construction materials are fluoropolymer (FEP, PTFE), conventional or linear polyethylene,
polycarbonate, and polypropylene. Huoropolymer shouild be used when samples are to be
analyzed for mercury. All labware should be cleaned according to the procedure in Section
11.4. Gloves, plastic wrap, storage bags, and filters maty all be used new without additional
cleaning unless results of the equipment blank pinpoint any of these materials as a source of
contamination. In this case, either an alternate supplier must be obtained or the materials must
be cleaned.
NOTE: Chrome acid must not be used for cleaning glassware.
6.6.1 Glassware—Class A volumetric flasks and a graduated cylinder.
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Method1636
6.6.2 Assorted Class A calibrated pipets
6.6.3 10-mL male luer-lock disposable syringes
6.6.4 0.45-pm syringe filters
6.6.5 Storage bottle—High density polypropylene, 1-L capacity.
6.6.6 Wash bottle—One-piece stem fluoropolymer, with screw closure, 125-mL capacity.
6.6.7 Tongs—For removal of Apparatus from acid baths. Coated metal tongs may not be
used.
6.6.8 Gloves—clean, nontalc polyethylene, latex, or vinyl; various lengths. Heavy gloves
should be worn when working in acid baths since baths will contain hot, strong acids.
6.6.9 Buckets or basins—5- to 50-L capacity, for acid soaking of the Apparatus.
6.6.10 Brushes—Nonmetallic, for scrubbing Apparatus.
6.6.11 Storage bags—Clean, zip-type, nonvented, colorless polyethylene (various sizes) to
store the Apparatus.
6.7
6.6.12 Plastic wrap—Clean, colorless polyethylene to store the Apparatus.
Sampling Equipment—The sampling team may contract with the laboratory or a cleaning
facility that is responsible for cleaning, storing, and shipping all sampling devices, sample
bottles, filtration equipment, and all other Apparatus used for the collection of ambient water
samples. Before the equipment is shipped to the field site, the laboratory or facility must
generate an acceptable equipment blank (Section 9.5.3) to demonstrate that the sampling
equipment is free from contamination.
6.7.1 Sampling Devices—Before ambient water samples are collected, consideration should
be given to the type of sample to be collected and the devices to be used (grab,
surface, or subsurface samplers). The laboratory or cleaning facility must clean all
devices used for sample collection. The Sampling Method describes various types of
samplers. Cleaned sampling devices should be stored in polyethylene bags or wrap.
6.7.2 Sample bottles—Fluoropolymer, conventional or linear polyethylene, polycarbonate, or
polypropylene; 500-mL with lids. Cleaned sample bottles should be filled with 0.1%
Hcl (v/v) until use.
NOTE: If mercury is a target analyte, fluoropolymer or glass bottles must be used.
6.7.3 Filtration Apparatus
6.7.3.1 Filter—Gehnan Supor 0.45-um, 15-mm diameter capsule filter (Gelman 12175,
or equivalent).
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Method 1636
6.7.3.2 Peristaltic pump—115-V a.c., 12-V d.c., internal battery, variable-speed,
single-head (Cole-Parmer, portable, "Masterfiex US," Catalog No. H-07570-10
drive with Quick Load pump head, Catalog No. H-07021-24, or equivalent).
6.7.3.3 Tubing for use with peristaltic pump—slyrene/ethylene/butylene/ silicone
(SEBS) resin, approx 3/8-in i.d. by approximately 3 ft (Cole-Parmer size 18,
Catalog No. G-06464-18, or approximately 1/4-in i.d., Cole-Parmer size 17,
Catalog No. G-06464-17, 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 hi 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.
7.0 Reagents and Standards
Reagents may contain elemental impurities that might affect the integrity of analytical data.
A trace amount of chromium is sometimes found in reagent grade salts. Since a concentrated
buffer solution is used hi this method to adjust the pH of samples, each reagent lot should be
tested for the metals of interest by diluting and analyzing an aliquot from the lot using the
techniques and instrumentation to be used for analysis of samples. The lot will be acceptable
if the concentration of the metal of interest is below the MDL listed in this method. All acids
used for this method must be of ultra high-purity grade. Suitable acids are available from a
number of manufacturers or may be prepared by sub-boiling distillation.
7.1 Reagents for cleaning Apparatus, sample bottle storage, and sample preservation and analysis
7.1.1 Nitric acid—concentrated (sp gr 1.41), Seastar or equivalent
7.1.2 Nitric acid (1+1)—Add 500 mL concentrated nitric acid to 400 mL of regent water
and dilute to 1 L.
7.1.3 Nitric acid (1+9)—Add 100 mL concentrated nitric acid to 400 mL of reagent water
and dilute to 1 L.
7.1.4 Hydrochloric acid—concentrated (sp gr 1.19).
7.1.5 Hydrochloric acid (1+1)—Add 500 mL concentrated hydrochloric acid to 400 mL of
reagent water and dilute to 1 L.
7.1.6 Hydrochloric acid (1+4)—Add 200 mL concenliated hydrochloric acid to 400 mL of
reagent water and dilute to 1 L.
7.1.7 Hydrochloric acid (HC1)—IN trace metal grade
7.1.8 Hydrochloric acid (HC1>—10% wt, trace metal grade
7.1.9 Hydrochloric acid (Hcl)—1% wt, trace metal grade
10
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Method 1636
7.2
7.3
7.4
7.5
7.6
7.7
7.1.10 Hydrochloric acid (Hcl)—0.5% (v/v), trace metal grade
7.1.11 Hydrochloric acid (Hcl)—0.1% (v/v) ultrapure grade
7.1.12 Ammonium hydroxide, NH4OH, (sp gr 0.902), (CASRN 1336-21-6)
7.1.13 Ammonium sulphate, (NH<)2SO4, (CASRN 7783-20-2)
7.1.14 1,5-Diphenylcarbazide, (CASRN 140-22-7)
7.1.15 Methanol, HPLC grade
7.1.16 Sulfuric acid, concentrated (sp gr 1.84)
Reagent water—Water demonstrated to be free from the metal(s) of interest and potentially
interfering substances at the MDL for that metal listed in Table 1. Prepared by distillation,
deionization, reverse osmosis, anodic/cathodic stripping voltammetry, or other technique that
removes the metal(s) and potential interferent(s).
Cr(VI) Stock Standard Solution—To prepare a 1000 mg/L solution, dissolve 4.501 g of
NaaCrO^HzO in reagent water and dilute to 1 L. Transfer to a polypropylene storage
container.
7.3.1 Preparation of calibration standards—Fresh calibration standards should be prepared
every 2 weeks or as needed. Dilute the stock standard solution to levels appropriate to
the operating range of the instrument using reagent water. Before final dilution, the
standards should be adjusted to pH 9-9.5 with the buffer solution (Section 7.6).
Calibration standards should be prepared at a minimum of three concentrations, one of
which must be at the ML (Table 1), and another that must be near the upper end of
the linear dynamic range. Calibration standards should be verified initially using a
quality control sample (Section 7.8).
Eluent—Dissolve 33 g of ammonium sulphate in 500 mL of reagent water and add 6.5 mL of
ammonium hydroxide. Dilute to 1 L with reagent water.
Postcolumn Reagent—Dissolve 0.5 g of 1,5-diphenylcarbazide in 100 mL of HPLC grade
methanol. Add to about 500 mL of reagent water containing 28 mL of 98% sulfuric acid
while stirring. Dilute with reagent water to 1 L in a volumetric flask. Reagent is stable for 4
or 5 days but should be prepared only as needed.
Buffer Solution—Dissolve 33 g of ammonium sulphate hi 75 mL of reagent water and add 6.5
mL of ammonium hydroxide. Dilute to 100 mL with reagent water.
Blanks—The laboratory should prepare the following types of blanks. A calibration blank is
used to establish the analytical calibration curve; and the laboratory (method) blank is used to
assess possible contamination from the sample preparation procedure. In addition to these
blanks, the laboratory may be required to analyze field blanks (Section 9.5.2) and equipment
blanks (Section 9.5.3).
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Method 1636
7.7.1 Calibration blank—Consists of reagent water adjusted to pH 9-9.5 with the buffer
solution (Section 7.6).
772 Laboratory blank—Must contain all the reagents in the same volumes as used in
processing the samples. The laboratory blank must be carried through the same entire
preparation scheme as the samples.
7 8 Quality control sample (QCS)—The QCS should be obtained from a source outside the
laboratory. The concentration of the QCS solution analyzed will depend on the sensitivity of
the instrument. To prepare the QCS, dilute an appropriate aliquot of analytes to a concentration
^ 100 ug/L in reagent water and adjust the pH to 9-9.5 with the buffer solution (Section 7.6).
The QCS should be analyzed as needed to meet data quality needs, and a fresh solution should
be prepared quarterly or more frequently as needed.
7 9 Ongoing precision and recovery (OPR) Sample—To an aliquot of reagent water, add aliquots
from the stock standard (Section 7.3) to prepare the OPEL. The OPR must be earned through
the same entire preparation scheme as the samples. ;
8.0 Sample Collection, Filtration, Preservation, and Storage
8 1 Before samples are collected, consideration should be given to the type of data required so that
appropriate preservation and pretreatment steps can be taken. Filtration and pH adjustment
should be performed at the time of sample collection or as soon thereafter as practically
possible. i
8.2 Sample collection-samples are collected as described in the Sampling Method.
8 3 Sample filtration—For dissolved Cr(VI), samples and field blanks are filtered through a 0.45-
um capsule filter at the field site. The Sampling Method describes filtering procedures.
8 4 Field preservation is advised for hexavalent chromium to provide sample stability for up to 30
days (Reference 18). Samples are preserved with sodium hydroxide as described in the
Sampling Method.
8.5 If the samples are not preserved with sodium hydroxide, they must be analyzed within 24 h of
collection.
8.6 Samples should be stored in polyethylene bags at 0-4°C until analysis
9.0 Quality Assurance/Quality Control
i
9 1 Each laboratory that uses this method is required to operate a formal quality assurance
program (Reference 19). The minimum requirements of this program consist of an initial
demonstration of laboratory capability, analysis of samples spiked with metals of interest to
evaluate and document data quality, and analysis of standards and blanks as tests of continued
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Method 2636
performance. To determine that results of the analysis meet the performance characteristics of
the method, laboratory performance is compared to established performance criteria.
9.1.1
9.1.2
The analyst shall make an initial demonstration of the ability to generate acceptable
accuracy and precision with this method. This ability is established as described in
Section 9.2.
In recognition of advances that are occurring in analytical technology, the analyst is
permitted to exercise certain options to eliminate interferences or lower the costs of
measurements. These options include alternate digestion, concentration, and cleanup
procedures, and changes in instrumentation. Alternate determinative techniques, such
as the substitution of a colorimetric technique or changes that degrade method
performance, are not allowed. If an analytical technique other than the techniques
specified in the method is used, that technique must have a specificity equal to or
better than the specificity of the techniques in the method for the analytes of interest.
9.1.2.1 Each time the method is modified, the analyst is required to repeat the
procedure in Section 9.2. If the change will affect the detection limit of the
method, the laboratory is required to demonstrate that the MDL (40 CFR Part
136, Appendix B) is lower than the MDL for that analyte in this method, or
one-third the regulatory compliance level, whichever is higher. If the change
will affect calibration, the analyst must recalibrate the instrument according to
Section 10.
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
analyses) who performed the analyses and modification, and of
the quality control officer who witnessed and will verify the
analyses and modification
9.1.2.2.2 A listing of metals measured, by name and CAS Registry
number
9.1.2.2.3 A narrative stating reason(s) for the modification(s)
9.1.2.2A Results from all quality control (QC) tests comparing the
modified method to this method, including:
(a) Calibration
(b) Calibration verification
(c) Initial precision and recovery (Section 9.2)
(d) Analysis of blanks
(e) Accuracy assessment
9.1.2.2.5 Data that will allow an independent reviewer to validate each
determination by tracing the instrument output (peak height,
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Method 1636
area, or other signal) to the final result. These data are to
include, where possible:
(a) Sample numbers and other identifiers
(b) Digestion/preparation or extraction dates
(c) Analysis dates and times
(d) Analysis sequence/run chronology
(e) Sample weight or volume
(f) Volume before each extraction/concentration step
(g) Volume after each extraction/concentration step
(h) Final volume before analysis
(i) Injection volume
(j) Dilution data, differentiating between dilution of a
sample or extract
(k) Instrument and operating conditions (make, model,
revision, modifications)
(1) Columns (type, resin, etc.)
(m) Operating conditions (background corrections,
temperature program, flow rates, etc.)
(n) Detector (type, operating conditions, etc.)
(o) Printer tapes and. other recordings of raw data
(p) Quantitation reports, data system outputs, and other
data to link raw data to results reported
9.1.3 Analyses of blanks are required to demonstrate freedom from contamination. Section
9.5 describes the required types, procedures, and criteria for analysis of blanks.
9.1.4 The laboratory shall spike at least 10% of the samples with the metal of interest to
monitor method performance. This test is described in Section 9.3 of this method.
When results of these spikes indicate atypical method performance for samples, an
alternative extraction or cleanup technique must be used to bring method performance
within acceptable limits. If method performance for spikes cannot be brought within
the limits given in this method, the result may not be reported for regulatory
compliance purposes.
9.1.5 The laboratory shall, on an ongoing basis, demonstrate through calibration verification
and through analysis of the ongoing precision and recovery aliquot that the analytical
system is in control. Sections 10.4 and 9.6 describe these procedures.
9.1.6 The laboratory shall maintain records to define the quality of data that are generated.
Section 9.3.4 describes the development of accuracy statements.
i
9.2 Initial demonstration of laboratory capability
9.2.1 Method detection limit—To establish the ability to detect hexavalent chromium, the
analyst shall determine the MDL for Cr(VI) according to the procedure hi 40 CFR
136, Appendix B using the apparatus, reagents, and standards that will be used hi the
practice of this method. The laboratory must produce an MDL that is less than or
equal to the MDL listed hi Table 1, or one-third the regulatory compliance limit,
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Method 1636
whichever is greater. MDLs should be determined when a new operator begins work
or whenever, in the judgement of the analyst, a change in instrument hardware or
operating conditions would dictate that they be redetermined.
9.2.2 Initial precision and recovery (E?R)—To establish the ability to generate acceptable
precision and recovery, the analyst shall perform the following operations.
9.2.2.1 Analyze four aliquots of reagent water spiked with Cr(VI) at 2-3 times the ML
(Table 1), according to the procedures in Section 12. All digestion, extraction,
and concentration steps, and the containers, labware, and reagents that will be
used with samples must be used in this test.
9.2.2.2 Using results of the set of four analyses, compute the average percent recovery
(X) for the Cr(VI) in each aliquot and the standard deviation of the recovery
(s) for each metal.
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 blanks and samples may begin. If,
however, s exceeds the precision limit or X falls outside the range for
accuracy, system performance is unacceptable. Correct the problem and repeat
the test (Section 9.2.2.1).
9.2.3 Linear dynamic range (LDR)—The LDR should be determined by analyzing a
minimum of 7 calibration standards ranging in concentration from 1 pg/L to 5,000
ug/L across all sensitivity settings of the spectrophotometer. To normalize responses,
divide the response by the sensitivity setting multiplier. Perform the linear regression
of normalized response vs. concentration and obtain the constants m and b, where m is
the slope of the line and b is the y-intercept. Incrementally analyze standards of
higher concentration until the measured absorbance response, R, of a standard no
longer yields a calculated concentration, C±, that is ± 10% of the known concentration,
C, where Cc = (R - b)lm. That concentration defines the upper limit of the LDR for
that instrument and analytical operating conditions. Samples having a concentration
that is > 90% of the upper limit of the LDR must be diluted to fall within the bounds
of the current calibration curve concentration range and reanalyzed.
9.2.4 Quality control sample (QCS)—When beginning the use of this method, quarterly or
as required to meet data quality needs, verify the calibration standards and acceptable
instrument performance with the preparation and analyses of a QCS (Section 7.8). To
verify the calibration standards the determined mean concentration from 3 analyses of
the QCS must be within ± 10% of the stated QCS value. If the QCS is not within the
required limits, an immediate second analysis of the QCS is recommended to confirm
unacceptable performance. If the calibration standards, acceptable instrument
performance, or both cannot be verified, the source of the problem must be identified
and corrected before proceeding with further analyses.
9.3 Method accuracy—To assess the performance of the method on a given sample matrix, the
laboratory must perform matrix spike (MS) and matrix spike duplicate (MSD) sample analyses
on 10% of the samples from each site being monitored, or at least one MS sample analysis
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Method 1636
and one MSD sample analysis must be performed for each sample batch (samples collected
from the same site at the same time, to a maximum of 10 samples), whichever is more
frequent Blanks (e.g., field blanks) may not be used for MS/MSD analysis.
9.3.1 The concentration of the MS and MSD is determined as follows:
9.3.1.1 If, as in compliance monitoring, the concentration of Cr(VI) in the sample is
being checked against a regulatory concentration limit, the spike must be at
that limit or at 1-5 times the background concentration, whichever is greater.
9.3.1.2 If the concentration is not being checked against a regulatory limit, the
concentration must be at 1-5 times the background concentration or at 1-5
times the ML in Table 1, whichever is greater.
I
9.3.2 Assessing spike recovery
9.3.2.1 Determine the background concentration (B) of Cr(VI) by analyzing one
sample aliquot according to the procedure in Section 12.
9.3.2.2 If necessary, prepare a QC check sample concentrate that will produce the
appropriate level (Section 9.3.1) in the sample when the concentrate is added.
9.3.2.3 Spike a second sample aliquot with the QC check sample concentrate and
analyze it to determine the concentration after spiking (A) of Cr(VI).
I
9.3.2.4 Calculate each percent recovery (P) as 100(A - B)/T, where T is the known
true value of the spike. |
9.3.3 Compare the percent recovery (P) for Cr(VI) with the corresponding QC acceptance
criteria found in Table 2. If P falls outside the designated range for recovery, the
acceptance criteria have not been met.
9.3.3.1 If the acceptance criteria were not met, analyze the ongoing precision and
recovery standard (Section 9.6). If the OPR is within limits for Cr(VI) (Table
2), the analytical system is in control and the problem can be attributed to the
sample matrix.
9.3.3.2 For samples that exhibit matrix problems, further isolate the metal(s) from the
sample matrix using dilution, chelation, extraction, concentration, hydride
generation, or other means and repeat the accuracy test (Section 9.3.2).
9.3.3.3 If the recovery for Cr(VI) remains outside the acceptance criteria, the
analytical result for Cr(VI) in the unspiked sample is suspect and may not be
reported for regulatory compliance purposes.
9.3.4 Recovery for samples should be assessed and records maintained.
I
9.3.4.1 After the analysis of five samples of a given matrix type (river water, lake
water, etc.) for which Cr(VI) passes the tests in Section 9.3.3, compute the
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Method 2636
average percent recovery (R) and the standard deviation of the percent
recovery (SR). Express the accuracy assessment as a percent recovery interval
from R - 2SR to R + 2SR for each matrix. For example, if R - 90% and SR
= 10% for five analyses of river water, the accuracy interval is expressed as
70-110%.
9.3.4.2 Update the accuracy assessment for Cr(VI) hi each matrix on a regular basis
(e.g., after each five to ten new measurements).
9.4 Precision of matrix spike and duplicate
9.4.1 Calculate the relative percent difference (RPD) between the MS and MSD per the
equation below using the concentrations found in the MS and MSD. Do not use the
recoveries calculated in Section 9.3.2.4 for this calculation because the RPD is inflated
when the background concentration is near the spike concentration.
RPD =
(Dl+D2)/2
Where:
Dl = concentration of the analyte in the MS sample
D2 = concentration of the analyte in the MSD sample
9.4.2 The relative percent difference between the matrix spike and the matrix spike duplicate
must be less than 20%. If this criterion is not met, the analytical system is be judged
to be out of control. Correct the problem and reanalyze all samples in the sample
batch associated with the MS/MSD that failed the RPD test.
9.5 Blanks—Blanks are analyzed to demonstrate freedom from contamination.
9.5.1 Laboratory (method) blank
9.5.1.1 Prepare a method blank with each sample batch (samples of the same matrix
started through the sample preparation process (Section 12) on the same 12-
hour shift, to a maximum of 10 samples). To demonstrate freedom from
contamination, analyze the blank immediately after analysis of the OPR
(Section 9.6).
9.5.1.2 If Cr(VI) or any potentially interfering substance is found hi the blank at a
concentration equal to or greater than the MDL (Table 1), sample analysis
must be halted, the source of the contamination determined, the samples and a
new method blank prepared, and the sample batch and fresh method blank
reanalyzed.
9.5.1.3 Alternatively, if a sufficient number of blanks (three minimum) are analyzed to
characterize the nature of a blank, the average concentration plus two standard
deviations must be less than the regulatory compliance level.
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Method 1636
9.5.1.4 If the result for a single blank remains above the MDL or if the result for the
average concentration plus two standard deviations of three or more blanks
exceeds the regulatory compliance level, results for samples associated with
those blanks may not be reported for regulatory compliance purposes. Stated
another way, results for all initial precision and recovery tests (Section 9.2)
and all samples must be associated with an uncontaminated method blank
before these results may be reported for regulatory compliance purposes.
9.5.2 Field blank
.
9.5.2.1 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.
9.5.2.2 If Cr(VD or any potentially interfering substance is found in the field blank at
a concentration equal to or greater than ithe 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
for regulatory compliance purposes.
9.5.2.3 Alternatively, if a sufficient number of field blanks (three minimum) are
analyzed to characterize the nature of the field blank, the average concentration
plus two standard deviations must be lesis than the regulatory compliance level
or less than one-half the level in the associated sample, whichever is greater.
9.5.2.4 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.
9.5.3 Equipment Blanks—Before any sampling equipment is used at a given site, the
laboratory or cleaning facility is required to generate equipment blanks to demonstrate
that the sampling equipment is free from contamination. Two types of equipment
blanks are required: bottle blanks and sampler check blanks.
9.5.3.1 Bottle blanks—After undergoing appropriate cleaning procedures (Section
11.4), bottles should be subjected to conditions of use to verify the
effectiveness of the cleaning procedures,, A representative set of sample bottles
should be filled with reagent water adjusted to a pH 9-9.5 with the buffer
solution (Section 7.6) and allowed to sfaind for a minimum of 24 h. Ideally,
the time that the bottles are allowed to stand should be as close as possible to
the actual time that sample will be in contact with the bottle. After standing,
the water should be analyzed for any signs of contamination. If any bottle
shows signs of contamination, the problem must be identified, the cleaning
procedures corrected or cleaning solutions changed, and all affected bottles
recleaned.
9.5.3.2 Sampler check blanks—Sampler check blanks are generated in the laboratory
or at the equipment cleaning contractor's facility by processing reagent water
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Method 1636
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
sampler check blanks at the laboratory or cleaning facility.
9.5.3.2.1 Sampler check blanks are generated by filling a large carboy or
other container with reagent water (Section 7.2) and processing
the reagent water through the equipment using the same
procedures used in the field (see Sampling Method). For
example, manual grab sampler check blanks are collected by
directly submerging a sample bottle into the water, filling the
bottle, and capping. Subsurface sampler check blanks are
collected by immersing the sampler into the water and
pumping water into a sarhplelcontainer.
9.5.3.2.2 The sampler check blank must be analyzed using the
procedures given in this method. If Cr(VI) or any potentially
interfering substance is detected in the blank, the source of
contamination or interference must be identified, and the
problem corrected. The equipment must be demonstrated to be
free from Cr(VT) before the equipment may be used in the
field.
9.5.3.2.3 Sampler check blanks must be run on all equipment that will
be used in the field. If, for example, samples are to be
collected using both a grab sampling device and a subsurface
sampling device, a sampler check blank must be run on both
pieces of equipment.
9.6 Ongoing precision and recovery
9.6.1 Prepare an ongoing precision and recovery sample (laboratory fortified method blank)
identical to the initial precision and recovery aliquots (Section 9.2) with each sample
batch (samples of the same matrix started through the sample preparation process
(Section 12) on the same 12-hour shift, to a maximum of 10 samples) by spiking an
aliquot of reagent water with the metal(s) of interest
9.6.2 Analyze the OPR sample before analysis of the method blank and samples from the
same batch.
9.6.3 Compute the percent recovery of Cr(VI) in the OPR sample.
9.6.4 Compare the concentration to the limits for ongoing recovery in Table 2. If the
acceptance criteria are met, system performance is acceptable and analysis of blanks
and samples may proceed. If, however, the recovery falls outside of the range given,
the analytical processes are not being performed properly. Correct the problem,
reprepare the sample batch, and repeat the ongoing precision and recovery test (Section
9.6).
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Method 1636
i
9.6.5 Add results that pass the specifications in Section 9.6.4 to initial and previous ongoing
data for Cr(VI) in each matrix. Update QC charts to form a graphic representation of
continued laboratory performance. Develop a statement of laboratory accuracy for
each matrix type by calculating the average percent recovery (R) and the standard
deviation of percent recovery (SR). Express the accuracy as a recovery interval from
R - 2SR to R + 2SR. For example, if R = 95% and SR = 5%, the accuracy is 85-
105%.
9.7 The specifications contained hi this method can be met if the instrument used is calibrated
properly and then maintained in a calibrated state. A given instrument will provide the most
reproducible results if dedicated to the settings and conditions required for the analyses of
metals by this method.
9.8 Depending on specific program requirements, the laboratory may be required to analyze field
duplicates collected to determine the precision of the sampling technique. 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
10.0 Calibration and Standardization
10.1 Operating conditions—Because of the diversity of instrument hardware, no detailed instrument
operating conditions are provided. The analyst is advisesd to follow the recommended
operating conditions provided by the manufacturer. It is the responsibility of the analyst to
verify that the instrument configuration and operating conditions satisfy the quality control
requirements in this method. Table 3 lists instrument operating conditions that may be used as
a guide for analysts in determining instrument configuration and operating conditions. The
flow rate of the eluent pump is set at 1.5 mL/min and the pressure of the reagent delivery
module adjusted so that the final flow rate of the postcolumn reagent (Section 7.5) from the
detector is 2.0 mL/min. This requires manual adjustment and measurement of the final flow
rate using a graduated cylinder and a stop watch. A warm-up period of approximately 30 min
after the flow rate has been adjusted is recommended, and the flow rate should be checked
prior to calibration and sample analysis.
10.2 Injection sample loop size should be chosen based on anticipated sample concentrations and
the selected sensitivity setting of the spectrophotometer. The sample volume used to load the
sample loop should be at least 10 times the loop size so that all tubing in contact with sample
is thoroughly flushed with the new sample to minimize cross-contamination.
10.3 For initial and daily operation, calibrate the instrument according to the instrument
manufacturer's recommended procedures using the calibration blank (Section 7.7.1) and
calibration standards (Section 7.3.1) prepared at three oir more concentrations, one of which
must be at the ML (Table 1), and another that must be near the upper end of the linear
dynamic range.
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Method 1636
10.4 Calibration verification—Immediately following calibration, an initial calibration verification
should be performed. Adjustment of the instrument is performed until verification criteria are
met. Only after these criteria are met may blanks and samples be analyzed.
10.4.1 Analyze the mid-point calibration standard (Section 10.3).
10.4.2 Compute the percent recovery of Cr(VT) using the calibration curve obtained hi the
initial calibration.
10.4.3 Compare the recovery with the corresponding limit for calibration verification hi Table
2. If all metals meet the acceptance criteria, system performance is acceptable and
analysis of blanks and samples may continue using the response from the initial
calibration. If the value falls outside the range given, system performance is
unacceptable. Locate and correct the problem and/or prepare a new calibration check
standard and repeat the test (Sections 10.4.1-10.4.3), or recalibrate the system
according to Section 10.3. I
10.4.4 Calibration must be verified following every ten samples by analyzing the mid-point
calibration standard. If the recovery does not meet the acceptance criteria specified in
Table 2, analysis must be halted, the problem corrected, and the instrument
recalibrated. All samples after the last acceptable calibration verification must be
reanalyzed.
10.5 A calibration blank must be analyzed following every calibration verification to demonstrate
that there is no carryover of Cr(VT) and that the analytical system is free from contamination.
If the concentration of an analyte hi the blank result exceeds the MDL, correct the problem,
verify the calibration (Section 10.4), and repeat the analysis of the calibration blank.
11.0 Procedures for Cleaning the Apparatus
11.1 All sampling equipment, sample containers, and labware should be cleaned hi a designated
cleaning area that has been demonstrated to be free of trace element contaminants. Such areas
may include class 100 clean rooms as described by Moody (Reference 20), labware cleaning
areas as described by Patterson and Settle (Reference 6), or clean benches.
11.2 Materials such as gloves (Section 6.6.8), storage bags (Section 6.6.11), and plastic wrap
(Section 6.6.12) may be used new without additional cleaning unless the results of the
equipment blank pinpoint any of these materials as a source of contamination. In this case,
either an alternate supplier must be obtained or the materials must be cleaned.
11.3 Cleaning procedures—Proper cleaning of the Apparatus is extremely important, because the
Apparatus may not only contaminate the samples but may also remove the analytes of interest
by adsorption onto the container surface.
NOTE: If laboratory, field, and equipment blanks (Section 9.5) from Apparatus
cleaned with fewer cleaning steps than those detailed below show no levels of analytes
above the MDL, those cleaning steps that do not eliminate these artifacts may be
omitted if all performance criteria outlined in Section 9 are met.
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Method 1636
11.3.1 Bottles, labware, and sampling equipment
11.3.1.1 Fill a precleaned basin (Section 6.6.9) with a sufficient quantity of a
0.5% solution of liquid detergent (Section 6.3), and completely
immerse each piece of ware. Allow to soak in the detergent for at
least 30 min.
11.3.1.2 Using a pair of clean gloves (Section 6.6.8) and clean nonmetallic
brushes (Section 6.6.10), thoroughly scrub down all materials with the
detergent.
11.3.1.3 Place the scrubbed materials in a precleaned basin. Change gloves.
11.3.1.4 Thoroughly rinse the inside and outside of each piece with reagent
water until there is no sign of detergent residue (e.g., until all soap
bubbles disappear).
11.3.1.5 Change gloves, immerse the rinsed equipment in a hot (50-60°C) bath
of concentrated reagent grade HNO3 (Section 7.1.1) and allow to soak
for at least 2 h. j
11.3.1.6 After soaking, use clean gloves and tongs to remove the Apparatus and
thoroughly rinse with distilled, deionized water (Section 7.2).
11.3.1.7 Change gloves and immerse the Apparatus hi a hot (50-60°C) bath of
IN trace metal grade Hcl (Section 7.1.7), and allow to soak for at least
48 h.
11.3.1.8 Thoroughly rinse all equipment and bottles with reagent water.
Proceed with Section 11.3.2 for labware and sampling equipment.
Proceed with Section 11.3.3 for sample bottles.
11.3.2 Labware and sampling equipment j
11.3.2.1 After cleaning, air-dry in a class 100 clean air bench.
11.3.2.2 After drying, wrap each piece of ware or equipment hi two layers of
polyethylene film.
11.3.3 Fluoropolymer sample bottles—These bottles should be used if mercury is a target
analyte.
11.3.3.1 After cleaning, fill sample bottles with 0.1% (v/v) ultrapure Hcl
(Section 7.1.11) and cap tightly. To ensure a tight seal, it may be
necessary to use a strap wrench,
11.3.3.2 After capping, double-bag each botfle hi polyethylene zip-type bags.
Store at room temperature until sample collection.
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Method 1636
11.3.4 Bottles, labware, and sampling equipment (polyethylene or material other than
fluoropolymer)
11.3.4.1 Apply the steps outlined in Sections 11.3.1.1-11.3.1.8 to all bottles,
labware, and sampling equipment. Proceed with Section 11.3.4.2 for
bottles or Section 11.3.4.3 for labware and sampling equipment.
11.3.4.2 After cleaning, fill each bottle with 0.1% (v/v) ultrapure Hcl (Section
7.1.11). Double-bag each bottle in a polyethylene bag to prevent
contamination of the surfaces with dust and dirt. Store at room
temperature until sample collection.
11.3.4.3 After rinsing labware and sampling equipment, air-dry in a class 100
clean ah- bench. After drying, wrap each piece of ware or equipment
hi two layers of polyethylene film.
NOTE: Polyethylene bottles cannot be used to collect samples that will be analyzed
for mercury at trace (e.g., 0.012 ug/L) levels because of the potential for vapors to
diffuse through the polyethylene.
IIA
11.5
11.3.4.4 Polyethylene bags—If polyethylene bags need to be cleaned, clean
according to the following procedure:
11.3.4.4.1 Partially fill with cold, (1+1) HNO3 (Section 7.1.2) and rinse
with distilled deioruzed water (Section 7.2).
11.3.4.4.2 Dry by hanging upside down from a plastic line with a plastic
clip.
11.3.5 Silicone tubing, fluoropolymer tubing, and other sampling apparatus—Clean any
silicone, fluoropolymer, or other tubing usedito collect samples by rinsing with 10%
Hcl (Section 7.1.8) and flushing with water from the site before sample collection.
11.3.6 Extension pole—Because of its length, it is impractical to submerse the 2-m
polyethylene extension pole (used in with the optional grab sampling device) in acid
solutions as described above. If such an extension pole is used, a nonmetallic brush
(Section 6.6.10) should be used to scrub the pole with reagent water and the pole
wiped down with acids described hi Section 11.3.4. After cleaning, the pole should be
wrapped in polyethylene film. ;
Storage—Store each piece or assembly of the Apparatus in a clean, single polyethylene zip-
type bag. If shipment is required, place the bagged apparatus hi a second polyethylene zip-
type bag. ;>
All cleaning solutions and acid baths should be periodically monitored for accumulation of
metals that could lead to contamination. When levels of'metals hi the solutions become too
Draft, January 1996
23
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Method 1636
high, the solutions and baths should be changed and the old solutions neutralized and
discarded in compliance with state and federal regulations.
12.0 Procedures for Sample Preparation and Analysis
12.1 Filtered, pH-adjusted samples at 4°C should be brought to ambient temperature before
analysis. |
12.2 Initiate instrument operating configuration and calibrate the instrument as described hi Section
10.
I
12.3 Construct a calibration curve of analyte response (peak height or area) vs. analyte
concentration over a concentration range of one or two orders of magnitude. The calibration
range should bracket the anticipated concentration range of samples. The coefficient of
correlation (r) for the curve should be 0.999 or greater.
12.4 Draw into a new, unused syringe (Section 6.6.3) approximately 3 mL of sample. Inject 10X
the volume of the sample loop into the injection valve of the 1C. Sample concentrations that
exceed the calibration range must be diluted and reanalyzed.
12.5 During analysis of samples, the laboratory must comply with the required quality control
described in Sections 9 and 10.
13.0 Data Analysis and Calculations
13.1 The sample concentration can be calculated from the calibration curve. Report values in ug/L.
Report results at or above the ML for metals found hi ssunples and determined in standards.
Report all results for metals found hi blanks, regardless of level.
13.2 For data values less than the ML, two significant figures should be used for reporting element
concentrations. For data values greater than or equal to the ML, three significant figures
should be used.
13.3 The QC data obtained during the analyses provide an indication of the quality of the sample
data and should be provided with the sample results.
14.0 Method Performance
i
14.1 The method detection limit (MDL) listed hi Table 1 and! the quality control acceptance criteria
listed in Table 2 were validated hi a single laboratory (Reference 21) for dissolved hexavalent
chromium.
15.0 Pollution Prevention
i
15.1 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. The EPA has established a preferred hierarchy of environmental
24
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Method I636
15.2
management techniques that places pollution prevention as the management option of first
choice. Whenever feasible, laboratory personnel should use pollution prevention techniques to
address their waste generation. When wastes cannot be feasibly reduced 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. To minimize the volume of expked standards to be disposed, standards should be
prepared in volumes consistent with laboratory use.
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 Government Relations and
Science Policy, 1155 16th Street NW, Washington DC 20036, 202/872-4477.
16.0 Waste Management
16.1 The Environmental Protection Agency requires that laboratory waste management practices be
conducted consistent with all applicable rules and regulations. The Agency urges laboratories
to protect the air, water, and land by minimizing and controlling all releases from hoods and
bench operations, complying with the letter and spirit of any sewer discharge permits and
regulations, and by complying with all solid and hazardous waste regulations, particularly the
hazardous waste identification rules and land disposal restrictions. For further information on
waste management, consult The Waste Management Manual for Laboratory Personnel,
available from the American Chemical Society at the address listed in Section 15.2.
17.0 References
1 Adeloju, S.B.; Bond, A.M. "Influence of Laboratory Environment on the Precision and
Accuracy of Trace Element Analysis," Anal. Chem. 1985, 57, 1728.
2 Berman, S.S.; Yeats, P.A. "Sampling of Seawater for Trace Metals," CRC Reviews in
Analytical Chemistry 1985,16, 1.
3 Bloom, N.S. "Ultra-Clean Sampling, Storage, and Analytical Strategies for the Accurate
Determination of Trace Metals in Natural Waters"; Presented at the 16th Annual EPA
Conference on the Analysis of Pollutants in the Environment, Norfolk, VA, May 5, 1993.
4 Bruland, K.W. 'Trace Elements in Seawater," Chemical Oceanography 1983, 8, 157.
5 Nriagu, J.O.; Larson, G.; Wong, H.K.T.; Azcue, J.M. "A Protocol for Minimizing
Contamination in the Analysis of Trace Metals in Great Lakes Waters," /. Great Lakes
Research 1993,19, 175.
6 Patterson, C.C.; Settle, D.M. "Accuracy in Trace Analysis," In National Bureau of Standards
Special Publication 422; LaFleur, P.D., Ed., U.S. Government Printing Office: Washington,
DC, 1976.
Draft, January 1996
25
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Method 1636
7 Fitzgerald, W.F.; Watras, CJ. Science of the Total Environment 1989, 87188, 223.
8 Gill, G.A.; Fitzgerald, W.F. Deep Sea Res. 1985, 32, 287.
9 Prothro, M.G., "Office of Water Policy and Technical Gruidance on Interpretation and
Implementation of Aquatic Life Metals Criteria"; EPA Memorandum to Regional Water
Management and Environmental Services Division Directors, Oct. 1, 1993.
10 "Format for Method Documentation," Distributed by the EPA Environmental Monitoring
Management Council, Washington, DC, Nov. 18, 1993.
11 Windom, H.L; Byrd, J.T.; Smith, R.G., Jr.; Huan, F. "Inadequacy of NASQAN Data for
Assessing Metal Trends in the Nation's Rivers," Environ. Sci. Technol. 1991, 25, 1137.
12 Zief, M.; Mitchell, J.W. "Contamination Control in Trace Metals Analysis"; In Chemical
Analysis, 1976, Vol. 47, Chapter 6.
13 Dionex Technical Note No. 26, May 1990.
14 "Carcinogens - Working With Carcinogens," Department of Health, Education, and Welfare.
Public Health Service. Centers for Disease Control. National Institute for Occupational Safety
and Health, Publication No. 77-206, Aug. 1977. Available from the National Technical
Information Service (NTIS) as PB-277256.
15 "OSHA Safety and Health Standards, General Industry"; 29 CFR 1910, Occupational Safety
and Health Administration, OSHA 2206 (revised January 1976).
16 "Safety in Academic Chemistry Laboratories," American Chemical Society Committee on
Chemical Safety, 3rd ed., 1979.
17 "Proposed OSHA Safety and Health Standards, Laboratories," Occupational Safety and Health
Administration, Fed. Regist., July 24, 1986.
18 Grohse, P. Research Triangle Institute, Institute Drive, Building 6, Research Triangle Park, NC
27709.
I
19 Handbook of Analytical Quality Control in Water and Wastewater Laboratories; U.S.
Environmental Protection Agency, EMSL. Cincinnati, OH: March 1979. EPA-600/4-79-019.
20 Moody, J.R. "NBS Clean Laboratories for Trace Element Analysis," Anal Chem. 1982, 54,
1358A.
21 "Results of the Validation Study for Determination of Trace Metals at EPA Water Quality
Criteria Levels," April 1995. Available from the Sample Control Center (operated by
DynCorp), 300 N. Lee Street, Alexandria, VA 22314, 703/519-1140.
26
Draft, January 1996
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Method 1636
18.0 Glossary
Many of the terms and definitions listed below are used in the EPA 1600-series methods, but
terms have been cross-referenced to terms commonly used in other methods where possible.
18.1 Ambient Water—Waters in the natural environment (e.g., rivers, lakes, streams, and other
receiving waters), as opposed to effluent discharges.
18.2 Analyte—A metal tested for by the methods referenced in this method. The analytes are
listed in Table 1.
18.3 Apparatus—The sample container and other containers, filters, filter holders, labware, tubing,
pipets, and other materials and devices used for sample collection or sample preparation, and
that will contact samples, blanks, or analytical standards.
18.4 Calibration Blank—A volume of reagent water acidified with the same acid matrix as in the
calibration standards. The calibration blank is a zero standard and is used to calibrate the ICP
instrument (Section 7.7.1).
18.5 Calibration Standard (CAL)—A solution prepared from a dilute mixed standard and/or stock
solutions and used to calibrate the response of the instrument with respect to analyte
concentration.
18.6 Dissolved Analyte—The concentration of analyte in an aqueous sample that will pass through
a 0.45-um membrane filter assembly before sample acidification (Section 8.3).
18.7 Equipment Blank—An aliquot of reagent water that is subjected in the laboratory to all
aspects of sample collection and analysis, including contact with all sampling devices and
apparatus. The purpose of the equipment blank is to determine if the sampling devices and
apparatus for sample collection have been adequately cleaned before shipment to the field site.
An acceptable equipment blank must be achieved before the sampling devices and apparatus
are used for sample collection. In addition, equipment blanks should be run on random,
representative sets of gloves, storage bags, and plastic wrap for each lot to determine if these
materials are free from contamination before use.
18.8 Field Blank—An aliquot of reagent water that is placed in a sample container in the
laboratory, shipped to the field, and treated as a sample in all respects, including contact with
the sampling devices and exposure to sampling site conditions, storage, preservation, and all
analytical procedures, which may include filtration. The purpose of the field blank is to
determine if the field or sample transporting procedures and environments have contaminated
the sample.
18.9 Field Duplicates (FD1 and FD2)—Two separate samples collected in separate sample bottles
at the same time and place under identical circumstances and treated exactly the same
throughout field and laboratory procedures. Analyses of FD1 and FD2 give a measure of the
precision associated with sample collection, preservation, and storage, as well as with
laboratory procedures.
Draft, January 1996
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Method 1636
18.10 Initial Precision and Recovery (EPR)—Four aliquots of the OPR standard analyzed to
establish the ability to generate acceptable precision and accuracy. IPRs are performed before
a method is used for the first time and any time the meithod or instrumentation is modified.
18.11 Instrument Detection Limit (IDL)—The concentration equivalent to the analyte signal which
is equal to three tunes the standard deviation of a series of ten replicate measurements of the
calibration blank signal at the selected analytical wavelength.
18.12 Laboratory Blank—An aliquot of reagent water that is treated exactly as a sample including
exposure to all glassware, equipment, solvents, reagents, internal standards, and surrogates that
are used with samples. The laboratory blank is used to determine if method analytes or
interferences are present hi 'the laboratory environment, the reagents, or the apparatus (Sections
7.7.2 and 9.5.1).
18.13 Laboratory Control Sample (LCS)—See Ongoing Precision and Recovery (OPR) Standard.
18.14 Laboratory Duplicates (LD1 and LD2)—Two aliquots of the same sample taken hi the
laboratory and analyzed separately with identical procedures. Analyses of LD1 and LD2
indicates precision associated with laboratory procedures, but not with sample collection,
preservation, or storage procedures.
18.15 Laboratory Fortified Blank (LFB)—See Ongoing Precision and Recovery (OPR) Standard.
18.16 Laboratory Fortified Sample Matrix (LFM)—See Matrix Spike (MS) and Matrix Spike
Duplicate (MSD).
18.17 Laboratory Reagent Blank (LRB)—See Laboratory Blank.
18.18 Linear Dynamic Range (LDR)—The concentration range over which the instrument response
to an analyte is linear (Section 9.2.3).
18.19 Matrix Spike (MS) and Matrix Spike Duplicate (MSD)—Aliquots of an environmental
sample to which known quantities of the method analytes are added hi 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 hi the
sample matrix must be determined hi a separate aliquot and the measured values hi the MS
and MSD corrected for background concentrations (Section 9.3).
18.20 May—This action, activity, or procedural step is optional.
18.21 May Not—This action, activity, or procedural step is prohibited.
18.22 Method Blank—See Laboratory Blank.
18.23 Method Detection Limit (MDL)—The minimum concentration of an analyte that can be
identified, measured, and reported with 99% confidences that the analyte concentration is
greater than zero (Section 9.2.1 and Table 1).
28
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Method 1636
18.24 Minimum Level (ML)—The lowest level at which the entire analytical system gives a
recognizable signal and acceptable calibration point (Reference 9).
18.25 Must—This action, activity, or procedural step is required.
18.26 Ongoing Precision and Recovery (OPR) Standard—A laboratory blank spiked with known
quantities of the method analytes. The OPR is analyzed exactly like a sample. Its purpose is
to determine whether the methodology is in control and to assure that the results produced by
the laboratory remain within the method-specified limits for precision and accuracy (Sections
7.9 and 9.6).
18.27 Preparation Blank—See Laboratory Blank.
18.28 Primary Dilution Standard—A solution containing the analytes that is purchased or prepared
from stock solutions and diluted as needed to prepare calibration solutions and other solutions.
18.29 Qualify Control Sample (QCS)—A sample containing all or a subset of the method analytes
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 to check laboratory performance with test materials prepared external to the normal
preparation process.
18.30 Reagent Water—Water demonstrated to be free from the method analytes and potentially
interfering substances at the MDL for that metal in the method.
18.31
18.32
Should—This action, activity, or procedural step is suggested but not required.
Stock Standard Solution—A solution containing one or more method analytes that is
prepared using a reference material traceable to EPA, the National Institute of Science and
Technology (NIST), or a source that will attest to the purity and authenticity of the reference
material.
Draft, January 1996
29
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Method 1636
Table 1
I
Hexavalent Chromium Analysis Using Method 1636: Lowest Water Quality Criterion, Method
Detection Limit, and Minimum Level
Metal
Hexavalent Chromium
Lowest Ambient Water
Quality Criterion (pgflL)1
10
Method Detection Limit (MDL)
and Minimum Level (ML); ug/L
MDL1
0.23
ML3
0.5
Notes:
1.
2.
3.
Lowest of the freshwater, marine, or human health WQC at 40 CFR Part 131 (57 FR 60848 for human health criteria and 60 FR 22228
for aquatic criteria). Hardness-dependent freshwater aquatic life criteria also calculated to reflect a hardness of 25 mg/L CaCO3, and
all aquatic life criteria have been adjusted to reflect dissolved levels in accordance with the equations provided in 60 FR 22228.
Method Detection Limit as determined by 40 CFR Part 136, Appendix B.
Minimum level (ML) calculated by multiplying laboratory-determined MDL by 3.18 and rounding result to nearest multiple of 1, 2,
5, 10, etc. in accordance with procedures used by HAD and described in the EPA Draft National Guidance for the Permitting,
Monitoring, and Enforcement of Water Quality-Based Effluent Limitations Set Below Analytical Detection/Quantitation Levels, March
22,1994.
Table 2
Quality Control Acceptance Criteria for Performance Tests in EPA Method 16361
Metal
Hexavalent Chromium
initial Precision and
Recovery (Section 9.2)
s X
20 80-120
Calibration Verification
(Section 10.4)
90-110
Ongoing Precision and
Recovery (Section 9.6)
79-122
Spike Recovery
(Section 9.3)
79-122
1. All specifications expressed as percent.
30
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Method 1636
Table 3: Recommended Ion Chromatographic Conditions
Columns:
Eluent:
Postcolumn Reagent:
Detector:
Retention Time:
Guard Column—Dionex lonPac NG1
Separator Column—Dionex lonPac AS7
250 mM (NH^SO^
100 mM NH4OH
Flow rate =1.5 mL/min
2mM Diphenylcarbohydrazide
10% v/v CH3OH
IN H2SO4
Flow rate = 0.5 mL/min
Visible 530 nm
3.8 min
Draft, January 1996
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