United States
Environmental Protection
Agency
Office of Water
(4303)
EPA821-R-95-031
April 1995
Method 1638: Determination of Trace
Elements in Ambient Waters by
Inductively Coupled Plasma-Mass
Spectrometry
> Printed on Recycled Paper
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-S-EPA Method 1638: Determination of Trace
Elements in Ambient Waters by
Inductively Coupled Plasma-Mass
Spectrometry
> Printed on Recycled Paper
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Method 1638
Acknowledgements
Method 1638 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). The
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 Berman, 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 Crecelius, 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, Duxbury, Massachusetts;.
Carl Watras, Wisconsin Department of Natural Resources, Boulder Junction, Wisconsin; and
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 gratefully acknowledged for the development of
the analytical procedures described in this method:
C.A. Brockhoff
J.T. Creed
T.D. Martin
E.R. Martin
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.
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
202/260-7120
April 1995
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Method 1638
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 concerning 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). This rule includes water quality criteria for 13 metals, and it is these criteria on which the
new sampling and analysis methods are based. Method 1638 was specifically developed to provide
reliable measurements of nine of these metals at EPA WQC levels using inductively coupled plasma-mass
spectrometry techniques.
In developing these methods, EPA found that one of the greatest difficulties in 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:
U.S. EPA NCEPI
11029 Kenwood Road
Cincinnati, OH 45242
513/489-8190
April 1995 Hi
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Method 1638
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Note: This method is intended to be performance-based, and the laboratory is permitted to omit any
step or modify any procedure provided that 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 in 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.
IV
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Method 1638
Determination of Trace Elements in Ambient Waters by
Inductively Coupled Plasma — Mass Spectrometry
1.0 Scope and Application
1.1 This method is for the determination of dissolved elements in ambient waters at EPA water
quality criteria (WQC) levels using inductively coupled plasma-mass spectrometry (ICP-MS).
It may also be used for determination of total recoverable element concentrations in these
waters. This method was developed by integrating the analytical procedures in EPA Method
200.8 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 assure 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 ("Sampling Method"). The Sampling Method is
necessary to assure that trace metals determinations will not be compromised by contamination
during the sampling process.
1.2 This method is applicable to the following elements:
Analyte
Antimony
Cadmium
Copper
Lead
Nickel
Selenium
Silver
Thallium
Zinc
Symbol
(Sb)
(Cd)
(Cu)
(Pb)
(Ni)
(Se)
(Ag)
(Tl)
(Zn)
Chemical Abstract Services
Registry Number (CASRN)
7440-36-0
7440-43-9
7440-50-8
7439-92-1
7440-02-0
7782-49-2
7440-22-4
7440-28-0
7440-66-6
Table 1 lists the EPA WQC levels, the Method Detection Limit (MDL) for each metal, and the
minimum level for each metal in this method. Linear working ranges will be dependent on the
sample matrix, instrumentation, and selected operating conditions.
1.3 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-
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Method 1638
billion (ppb) range, whereas ambient metals concentrations are normally in the low part-per-
trillion (ppt) to low ppb range.
1.4 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, "Contamination and Interferences" 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.
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 of their lack of 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 hi 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 metal determinations, samples must be filtered through a 0.45-um capsule filter
at the field site. The filtering procedures are described in the Sampling Method. The filtered
samples may be preserved in the field or transported to the laboratory for preservation.
Procedures for field preservation are detailed in the Sampling Method; procedures for
laboratory preservation are provided in this method.
1.9 For the determination of total recoverable analytes in ambient water samples, a
digestion/extraction (see Section 12.2) is required before analysis when the elements are not in
solution (e.g., aqueous samples that may contain particulate and suspended solids).
1.10 The procedure given in this method for digestion of total recoverable metals is suitable for the
determination of silver in aqueous samples containing concentrations up to 0.1 mg/L. For the
analysis of samples containing higher concentrations of silver, succeedingly smaller volume,
well-mixed sample aliquots must be prepared until the analysis solution contains <0.1 mg/L
silver.
1.1.1 This method should be used by analysts experienced in the use of inductively coupled plasma
mass spectrometry (ICP-MS), including the interpretation of spectral and matrix interferences
and procedures for their correction, and this method 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 experience with commercial instrumentation is
recommended.
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Method 1638
1.12 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. Before using this metod, data users should state the data quality
objectives (DQOs) required for a project.
2.0 Summary of Method
2.1 An aliquot of a well-mixed, homogeneous aqueous sample is accurately measured for sample
processing. For total recoverable analysis of an aqueous sample containing undissolved
material, analytes are first solubilized by gentle refluxing with nitric: and hydrochloric acids.
After cooling, the sample is made to volume, mixed, and centrifuged or allowed to settle
overnight prior to analysis. For the determination of dissolved analytes in a filtered aqueous
sample aliquot, the sample is made ready for analysis by the appropriate addition of nitric acid,
and then diluted to a predetermined volume and mixed before analysis.
2.2 The digested sample is introduced into a radiofrequency plasma where energy transfer
processes cause desolvation, atomization, and ionization. The ions are extracted from the
plasma through a differentially pumped vacuum interface and separated on the basis of their
mass-to-charge ratio (m/z) by a mass spectrometer having a minimum resolution capability of
1 amu peak width at 5% peak height at m/z 300. Ions transmitted through the mass analyzer
are detected by an electron multiplier or Faraday detector and the resulting current is processed
by a data handling system (References 11-13).
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 Section 18.0 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 14). Therefore, it is imperative that extreme care be
taken to avoid contamination when collecting and analyzing ambient water samples for trace
metals.
4.2 There are numerous routes by which samples may become contaminated. 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
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Method 1638
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
4.3.2
4.3.3
4.3.4
4.3.5
4.3.6
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.
4.3.1.3 While contamination control is essential, personnel health and safety remain
the highest priority. Requirements and suggestions for personnel safety are
given in Section 5 of this method and the Sampling Method.
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.
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.
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.
Clean work surfaces—Before processing a given batch of samples, 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.
Wear gloves—Sampling personnel must wear clean, nontalc gloves (Section 6.9.7)
during all operations involving handling of the Apparatus, samples, and blanks. Only
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Method 1638
clean gloves may touch the Apparatus. If another object or substance is touched, the
glove(s) must be changed before handling the Apparatus again. 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 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 in 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 in PTFE 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 in or out
of the other materials resulting either in contamination or low-biased results
(Reference 3). 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 15).
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.
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Method 1638
4.4
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 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
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 in 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.
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 then subsequently transfer the contamination to the sample.
Therefore, it is imperative that every piece of the Apparatus that is directly or
indirectly used in the collection, processing, and analysis of ambient water
samples be cleaned as specified in Section 11.
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 metal-
containing paint. Whenever possible, sample processing and analysis should
occur as far as possible from sources of airborne contamination.
Interferences—Interference sources that may cause inaccuracies in the determination of trace
elements by ICP-MS are given below and must be recognized and corrected for. Instrumental
drift, as well as suppressions or enhancements of instrument response caused by the sample
matrix, should be corrected for by the use of internal standards.
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Method 1638
4.4.1 Isobaric elemental interferences—Are caused by isotopes of different elements that
form singly or doubly charged ions of the same nominal m/z and that cannot be
resolved by the mass spectrometer. All elements determined by this method have, at a
minimum, one isotope free of isobaric elemental interference*. Of the isotopes
recommended for use with this method (Table 5), only selenium-82 (krypton) has an
isobaric elemental interference. If an alternative isotope that has a higher natural
abundance is selected to achieve greater sensitivity, an isobaric interference may occur.
All data obtained under such conditions must be corrected by measuring the signal
from another isotope of the interfering element and subtracting the contribution the
isotope of interest based on the relative abundance of the alternate isotope and isotope
of interest. A record of this correction process should be included with the report of
the data. It should be noted that such corrections will only be as accurate as the
accuracy of the relative abundance used in the equation for data calculations. Relative
abundances should be established before applying any corrections.
4.4.2 Abundance sensitivity—Is a property defining the degree to which the wings of a mass
peak contribute to adjacent m/z's. The abundance sensitivity is affected by ion energy
and quadruple operating pressure. Wing overlap interferences may result when a small
m/z peak is being measured adjacent to a large one. The potential for these
interferences should be recognized and the spectrometer resolution adjusted to
minimize them.
4.4.3 Isobaric polyatomic ion interferences—Are caused by ions consisting of more than one
atom which have the same nominal mass-to-charge ratio as the isotope of interest, and
which cannot be resolved by the mass spectrometer in use. These ions are commonly
formed in the plasma or interface system from support gases, or sample components.
Most of the common interferences have been identified (Reference 13), and these are
listed in Table 3 together with elements affected. Such interferences must be
recognized, and when they cannot be avoided by the selection of an alternative m/z,
appropriate corrections must be made to the data. Equations for the correction of data
should be established at the time of the analytical run sequence because the polyatomic
ion interferences will be highly dependent on the sample matrix and chosen instrument
conditions. In particular, the common 82Kr interference that affects the determination
of both arsenic and selenium can be greatly reduced with the use of high-purity
krypton-free argon.
4.4.4 Physical interferences—Are associated with the physical processes which govern the
transport of sample into the plasma, sample conversion processes in the plasma, and
the transmission of ions through the plasma-mass spectrometer interface. These
interferences may result in differences between instrument responses for the sample
and the calibration standards. Physical interferences may occur in the transfer of
solution to the nebulizer (e.g., viscosity effects), at the point of aerosol formation and
transport to the plasma (e.g., surface tension), or during excitation and ionization
processes within the plasma itself. High levels of dissolved solids in the sample may
contribute deposits of material on the extraction cone, skimmer cone, or both, reducing
the effective diameter of the orifices and therefore ion transmission. Dissolved solids
levels not exceeding 0.2% (w/v) have been recommended (Preference 13) to reduce
such effects. Internal standardization may be effectively used to compensate for many
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Method 1638
physical interference effects (Reference 16). Internal standards ideally should have
analytical behavior similar to the elements being determined.
4.4.5 Memory interferences—Result when isotopes of elements in a previous sample
contribute to the signals measured in a new sample. Memory effects can result from
sample deposition on the sampler and skimmer cones, and from the buildup of sample
material in the plasma torch and spray chamber. The site where these effects occur is
dependent on the element and can be minimized by flushing the system with a rinse
blank between samples (Section 7.6.3). The possibility of memory interferences
should be recognized within an analytical run and suitable rinse times should be used
to reduce them. The rinse times necessary for a particular element should be estimated
before analysis. This may be achieved by aspirating a standard containing elements
corresponding to ten times the upper end of the linear range for a normal sample
analysis period, followed by analysis of the rinse blank at designated intervals. The
length of time required to reduce analyte signals below the minimum level (ML)
should be noted. Memory interferences may also be assessed within an analytical run
by using a minimum of three replicate integrations for data acquisition. If the
integrated signal values drop consecutively, the analyst should be alerted to the
possibility of a memory effect, and should examine the analyte concentration in the
previous sample to identify if this was high. If a memory interference is suspected,
the sample should be reanalyzed after a long rinse period.
5.0 Safety
5.1 The toxicity or carcinogenicity of reagents used in this method have not been fully established.
Each chemical should be regarded as a potential health hazard and exposure to these
compounds should be as low as reasonably achievable.
5.1.1 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
17-20). 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 20 are particularly comprehensive
in dealing with the general subject of laboratory safety.
5.1.2 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.
5.2 The acidification of samples containing reactive materials may result in the release of toxic
gases, such as cyanides or sulfides. Acidification of samples should be done in a fume hood.
5.3 All personnel handling environmental samples known to contain or to have been in contact
with human waste should be immunized against known disease-causative agents.
8
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Method 1638
5.4 Analytical plasma sources emit radiofrequency radiation in addition to intense UV radiation
Suitable precautions should be taken to protect personnel from such hazards. The inductively
coupled plasma should only be viewed with proper eye protection from UV emissions.
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. Demonstration of equivalent
performance is the responsibility of the laboratory.
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 in the clean room; the other adjacent to the
analytical instrument(s) for preparation of samples and standards.
6.2 Inductively coupled plasma mass spectrometer:
6.2.1 Instrument capable of scanning the mass range 5-250 amu with a minimum resolution
capability of 1 amu peak width at 5% peak height. Instrument may be fitted with a
conventional or extended dynamic range detection system.
6.2.2 Radio-frequency generator compliant with FCC regulations.
6.2.3 Argon gas supply—High-purity grade (99.99%). When analyses are conducted
frequently, liquid argon is more economical and requires less frequent replacement of
tanks than compressed argon in conventional cylinders (Section 4.1.3).
6.2.4 A variable-speed peristaltic pump is required for solution delivery to the nebulizer.
6.2.5 A mass-flow controller on the nebulizer gas supply is required. A water-cooled spray
chamber may be of benefit in reducing some types of interferences (e.g., from
polyatomic oxide species).
6.2.6 If an electron multiplier detector is being used, precautions should be taken, where
necessary, to prevent exposure to high ion flux. Otherwise changes in instrument
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Method 1638
response or damage to the multiplier may result. Samples having high concentrations
of elements beyond the linear range of the instrument and with isotopes falling within
scanning windows should be diluted before analysis.
6.3 Analytical balance—with capability to measure to 0.1 mg, for use in weighing solids and for
preparing standards.
6.4 Temperature adjustable hot plate—capable of maintaining a temperature of 95°C.
6.5 Centrifuge with guard bowl, electric timer, and brake (optional).
6.6 Drying oven—gravity convection, with thermostatic control capable of maintaining 105°C (±
5°C).
6.7 Alkaline detergent—Liquinox®, Alconox®, or equivalent.
6.8 pH meter or pH paper.
6 9 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 desorption 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. Fluoropolymer should 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 may 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: Chromic acid must not be used for cleaning glassware. ,
6.9.1 Volumetric flasks, graduated cylinders, funnels and centrifuge tubes.
6.9.2 Assorted calibrated pipettes.
6.9.3 Beakers—fluoropolymer (or other suitable material), 250-mL with fluoropolymer
covers.
6.9.4 Storage bottles—Narrow-mouth, fluoropolymer with fluoropolymer screw closure, 125-
to 250-mL capacities.
6.9.5 Wash bottle—One-piece stem fluoropolymer, with screw closure, 125-mL capacity.
— April 1995
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Method 1638
6.9.6 Tongs—For removal of Apparatus from acid baths. Coated metal tongs may not be
used.
6.9.7 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.
6.9.8 Buckets or basins—5- to 50-L capacity, for acid soaking of the Apparatus.
6.9.9 Brushes—Nonmetallic, for scrubbing Apparatus.
6.9.10 Storage bags—Clean, zip-type, nonvented, colorless polyethylene (various sizes) for
storage of Apparatus.
6.9.11 Plastic wrap—Clean, colorless polyethylene for storage of Apparatus.
6.10 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 shipping the equipment to the field site, the laboratory or facility must
generate an acceptable equipment blank (Section 9.6.3) to demonstrate that the sampling
equipment is free from contamination.
6.10.1 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. Various types of samplers are described in the
Sampling Method. Cleaned sampling devices should be stored in polyethylene bags or
wrap.
6.10.2 Sample bottles—Fluoropolymer, conventional or linear polyethylene, polycarbonate, or
polypropylene; 500-mL with lids. Cleaned sample bottles should be filled with 0.1%
HC1 (v/v) until use.
NOTE: If mercury is a target analyte, fluoropolymer or glass bottles must be used.
6.10.3 Filtration Apparatus
6.10.3.1 Filter—Gelman Supor 0.45-pm, 15-mm diameter capsule filter
(Gelman 12175, or equivalent)
6.10.3.2 Peristaltic pump—115-V a.c., 12-V d.c., internal battery, variable-
speed, single-head (Cole-Parmer, portable, "Masterflex L/S," Catalog
No. H-07570-10 drive with Quick Load pump head, Catalog No. H-
07021-24, or equivalent).
6.10.3.3 Tubing for use with peristaltic pump—styrene/ethylene/butylene/
silicone (SEES) resin, approximately 3/8-in i.d. by approx 3 ft (Cole-
Parmer size 18, Catalog No. G-06464-18, or approximately 1/4-in i.d.,
April 1995
11
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Method 1638
Cole-Parmer size 17, Catalog. No. G-06464-17, or equivalent). Tubing
is cleaned by soaking in 5-10% HC1 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.
7.0 Reagents and Standards
Reagents may contain elemental impurities that might affect the integrity of analytical data.
Because of the high sensitivity of ICP-MS, high-purity reagents should be used. 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. Nitric acid is
preferred for ICP-MS to minimize polyatomic ion interferences. Several polyatomic ion
interferences result when hydrochloric acid is used (Table 3); however, hydrochloric acid is
required to maintain stability in solutions containing antimony and silver. When hydrochloric
acid is used, corrections for the chloride polyatomic ion interferences must be applied to all
data.
7.1 Reagents for cleaning Apparatus, sample bottle storage, and sample preservation.
7.1.1 Nitric acid—concentrated (sp gr 1.41), Seastar or equivalent
7.1.2 Nitric acid (1+1)—Add 500 mL cone, nitric acid to 400 mL of regent water and dilute
to 1 L.
7.1.3 Nitric acid (1+9)—Add 100 mL cone, 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 concentrated 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 (HC1)—1% wt, trace metal grade.
7.1.10 Hydrochloric acid (HC1)—0.5% (v/v), trace metal grade.
12 April 1995
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Method 1638
7.1.11 Hydrochloric acid (HC1)—0.1% (v/v) ultrapure grade.
7.1.12 Tartaric acid (CASRN 87-69-4).
7.2 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).
7.3 Stock standard solutions—Stock standards may be purchased from a reputable commercial
source or prepared from ultra high-purity grade chemicals or metals (99.99-99.999% pure).
All salts should be dried for 1 h at 105°C, unless otherwise specified. Stock solutions should
be stored in FEP bottles. Replace stock standards when succeeding dilutions for preparation of
the multielement stock standards can not be verified.
CAUTION: Many metal salts are extremely toxic if inhaled or swallowed. Wash
hands thoroughly after handling.
The following procedures may be used for preparing standard stock solutions:
NOTE: Some metals, particularly those which form surface oxides, require cleaning
prior to being weighed. This may be achieved by pickling the surface of the metal in
acid. An amount in excess of the desired weight should be pickled repeatedly, rinsed
with water, dried, and weighed until the desired weight is achieved.
7.3.1 Antimony solution, stock 1 mL = 1000 ug Sb—Dissolve 0.100 g antimony powder in
2 mL (1+1) nitric acid and 0.5 mL concentrated hydrochloric acid, heating to effect
solution. Cool, add 20 mL reagent water and 0.15 g tartaric acid. Warm the solution
to dissolve the white precipitate. Cool and dilute to 100 mL with reagent water.
7.3.2 Beryllium solution, stock 1 mL = 1000 ug Be—Dissolve 1.965 g BeSO4.4H?O (DO
NOT DRY) in 50 mL reagent water. Add 1 mL concentrated nitric acid. Dilute to
100 mL with reagent water.
7.3.3 Bismuth solution, stock 1 mL = 1000 ug Bi—Dissolve 0.1115 g Bi2O3 in 5 mL
concentrated nitric acid. Heat to effect solution. Cool and dilute to 100 mL with
reagent water.
7.3.4 Cadmium solution, stock 1 mL = 1000 ug Cd—Pickle cadmium metal in (1+9) nitric
acid to an exact weight of 0.100 g. Dissolve in 5 mL (1+1) nitric acid, heating to
effect solution. Cool and dilute to 100 mL with reagent water.
7.3.5 Cobalt solution, stock 1 mL = 1000 pg Co—Pickle cobalt rnetal in (1+9) nitric acid to
an exact weight of 0.100 g. Dissolve in 5 mL (1+1) nitric acid, heating to effect
solution. Cool and dilute to 100 mL with reagent water.
April 1995 13
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Method 1638
7.4
7.3.6 Copper solution, stock 1 mL - 1000 pg Cu—Pickle copper metal in (1+9) nitric acid
to an exact weight of 0.100 g. Dissolve in 5 mL (1+1) nitric acid, heating to effect
solution. Cool and dilute to 100 mL with reagent water.
7.3.7 Indium solution, stock 1 mL = 1000 ug In—Pickle indium metal in (1+1) nitric acid to
an exact weight of 0.100 g. Dissolve in 10 mL (1+1) nitric acid, heating to effect
solution. Cool and dilute to 100 mL with reagent water.
7.3.8 Lead solution, stock 1 mL = 1000 pg Pb—Dissolve 0.1599 g PbNO3 in 5 mL (1+1)
nitric acid. Dilute to 100 mL with reagent water.
7.3.9 Magnesium solution, stock 1 mL = 1000 pg Mg—Dissolve 0.1658 g MgO in 10 mL
(1+1) nitric acid, heating to effect solution. Cool and dilute to 100 mL with reagent
water.
7.3.10 Nickel solution, stock 1 mL = 1000 pg Ni—Dissolve 0.100 g nickel powder in 5 mL
concentrated nitric acid, heating to effect solution. Cool and dilute to 100 mL with
reagent water.
7.3.11 Scandium solution, stock 1 mL = 1000 pg Sc—Dissolve 0.1534 g Sc2O3 in 5 mL
(1+1) nitric acid, heating to effect solution. Cool and dilute to 100 mL with reagent
water.
7.3.12 Selenium solution, stock 1 mL = 1000 pg Se—Dissolve 0.1405 g SeO2 in 20 mL
reagent water. Dilute to 100 mL with reagent water.
7.3.13 Silver solution, stock 1 mL = 1000 pg Ag—Dissolve 0.100 g silver metal in 5 mL
(1+1) nitric acid, heating to effect solution. Cool and dilute to 100 mL with reagent
water. Store in dark container.
7.3.14 Terbium solution, stock 1 mL - 1000 pg Tb—Dissolve 0.1176 g Tb4O7 in 5 mL
concentrated nitric acid, heating to effect solution. Cool and dilute to 100 mL with
reagent water.
7.3.15 Thallium solution, stock 1 mL - 1000 pg Tl—Dissolve 0.1303 g T1NO3 in a solution
mixture of 10 mL reagent water and 1 mL concentrated nitric acid. Dilute to 100 mL
with reagent water.
7.3.16 Yttrium solution, stock 1 mL - 1000 pg Y—Dissolve 0.1270 g Y2O3 in 5 mL (1+1)
nitric acid, heating to effect solution. Cool and dilute to 100 mL with reagent water.
7.3.17 Zinc solution, stock 1 mL = 1000 pg Zn—Pickle zinc metal in (1+9) nitric acid to an
exact weight of 0.100 g. Dissolve in 5 mL (1+1) nitric acid, heating to effect
solution. Cool and dilute to 100 mL with reagent water.
Multielement stock standard solutions—Care must be taken in the preparation of multielement
stock standards so that the elements are compatible and stable. Originating element stocks
should be checked for the presence of impurities which might influence the accuracy of the
standard. Freshly prepared standards should be transferred to acid-cleaned, not previously
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April 1995
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Method 1638
used, FEP fluorocarbon bottles for storage and monitored periodically for stability. The
following combinations of elements are suggested:
Standard Solution A Standard Solution B
Antimony Nickel Silver
Cadmium Selenium
Copper Thallium
Lead Zinc
Except for selenium, multielement stock standard solutions A and B (1 mL = 10 ug) may be
prepared by diluting 1.0 mL of each single element stock standard in the combination list to
100 mL with reagent water containing 1% (v/v) nitric acid. For selenium in solution A, an
aliquot of 5.0 mL of the stock standard should be diluted to the specified 100 mL (1 ml = 50
ug Se). Replace the multielement stock standards when succeeding dilutions for preparation of
the calibration standards cannot be verified with the quality control sample.
7.4.1 Preparation of calibration standards—Fresh multielement calibration standards should
be prepared every two weeks or as needed. Dilute each of the stock multielement
standard solutions A and B to levels appropriate to the operating range of the
instrument using reagent water containing 1% (v/v) nitric acid. Calibration standards
should be prepared at a minimum of three concentrations, one of which must be at the
minimum level (Table 1), and another which must be near the upper end of the linear
dynamic range. It should be noted the selenium concentration is always a factor of 5
> the other analytes. If the direct addition procedure is being used (Method A, Section
10.3), add internal standards (Section 7.5) to the calibration, standards and store in
fluoropolymer bottles. Calibration standards should be verified initially using a quality
control sample (Section 7.8).
7.5 Internal standard stock solution—1 mL = 100 ug. Dilute 10 mL of scandium, yttrium, indium,
terbium, and bismuth stock standards (Section 7.3) to 100 mL with reagent water, and store in
a FEP bottle. Use this solution concentrate for addition to blanks, calibration standards and
samples, or dilute by an appropriate amount using 1% (v/v) nitric acid, if the internal standards
are being added by peristaltic pump (Method B, Section 10.3).
7.6 Blanks—The laboratory should prepare the following types of blanks. A calibration blank is
used to establish the analytical calibration curve; the laboratory (method) blank is used to
assess possible contamination from the sample preparation procedure and to assess spectral
background; and the rinse blank is used to flush the instrument between samples to reduce
memory interferences. In addition to these blanks, the laboratory may be required to analyze
field blanks (Section 9.6.2) and equipment blanks (Section 9.6.3).
7.6.1 Calibration blank—Consists of 1% (v/v) nitric acid in reagent water. If the direct
addition procedure (Method A, Section 10.3) is being used,, add internal standards.
7.6.2 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
April 1995 15
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Method 1638
preparation scheme as the samples including digestion, when applicable (Section
9.6.1). If the direct addition procedure (Method A, Section 10.3) is being used, add
internal standards to the solution after preparation is complete.
7.6.3 Rinse blank—Consists of 2% (v/v) nitric acid in reagent water.
7.7 Tuning solution—This solution is used for instrument tuning and mass calibration prior to
analysis. The solution is prepared by mixing beryllium, magnesium, cobalt, indium, and lead
stock solutions (Section 7.3) in 1% (v/v) nitric acid to produce a concentration of 100 ug/L of
each element. Internal standards are not added to this solution. (Depending on the sensitivity
of the instrument, this solution may need to be diluted 10-fold.)
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 pg/L in 1% (v/v) nitric acid. Because of lower sensitivity, selenium may be diluted to a
concentration of < 500 ug/L. If the direct addition procedure (Method A, Section 10.3) is
being used, add internal standards after dilution, mix, and store in a FEP bottle. 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 multielement stock standards A and B (Section 7.4) to prepare the OPR. The OPR must
be carried through the same entire preparation scheme as the samples including sample
digestion, when applicable (Section 9.7). If the direct addition procedure (Method A, Section
10.3) is being used, add internal standards to this solution after preparation has been
completed.
8.0 Sample Collection, Filtration, Preservation, and Storage
8.1 Before an aqueous sample is collected, consideration should be given to the type of data
required, (i.e., dissolved or total recoverable), so that appropriate preservation and pretreatment
steps can be taken. The pH of all aqueous samples must be tested immediately before
aliquotting for processing or direct analysis to ensure the sample has been properly preserved.
If properly acid-preserved, the sample can be held up to six months before analysis.
8.2 Sample collection—Samples are collected as described in the Sampling Method.
8.3 Sample filtration—For dissolved metals, samples and field blanks are filtered through a 0.45-
um capsule filter at the field site. Filtering procedures are described in the Sampling Method.
For the determination of total recoverable elements, samples are not filtered but should be
preserved according to the procedures in Section 8.4.
8.4 Sample preservation—Preservation of samples and field blanks for both dissolved and total
recoverable elements may be performed in the field at time of collection or in the laboratory.
However, to avoid the hazards of strong acids in the field and transport restrictions, to
minimize the potential for sample contamination, and to expedite field operations, the sampling
team may prefer to ship the samples to the laboratory within two weeks of collection.
Samples and field blanks should be preserved at the laboratory immediately upon receipt. For
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April 1995
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Method 1638
all metals, preservation involves the addition of 10% HNO3 (Section 7.1.3) to bring the sample
to pH <2. For samples received at neutral pH, approx 5 mL of 10% HNO3 per liter will be
required.
8 4 1 Wearing clean gloves, remove the cap from the sample bottle, add the volume of
reagent grade acid that will bring the pH to <2, and recap the bottle immediately. If
the bottle is full, withdraw the necessary volume using a p-recleaned pipet and then add
the acid. Record the volume withdrawn and the amount of acid used.
NOTE: Do not dip pH paper or a pH meter into the sample; remove a small aliquot
with a clean pipet and test the aliquot. When the nature of the sample is either
unknown or known to be hazardous, acidification should be done in a fume hood. See
Section 5.2.
8 4.2 Store the preserved sample for a minimum of 48 h at 0-4°C to allow the acid to
completely dissolve the metal(s) adsorbed on the container walls. The sample pH
should be verified as <2 immediately before withdrawing an aliquot for processing or
direct analysis. If, for some reason such as high alkalinity, the sample pH is verified
to be >2, more acid must be added and the sample held for sixteen hours until verified
to be pH <2. See Section 8.1.
8.4.3 With each sample batch, preserve a method blank and an OPR sample in the same way
as the sample(s).
8.4.4 Sample bottles should be stored in polyethylene bags at 0-4 °C until analysis.
9.0 Quality Assurance/Quality Control
9 1 Each laboratory that uses this method is required to operate a formal quality assurance
program (Reference 21). 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
performance. Laboratory performance is compared to established performance criteria to
determine that results of the analysis meet the performance characteristics of the method.
9.1.1 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.
9.1.2 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
April 1995 1?
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Method 1638
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 hi Section 9.2. If the detection limit of the method will be affected
by the change, 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
calibration will be affected by the change, 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
9.1.2.2.2
9.1.2.2.3
9.1.2.2.4
9.1.2.2.5
The names, titles, addresses, and telephone numbers of the
analyst(s) who performed the analyses and modification, and of
the quality control officer who witnessed and will verify the
analyses and modification.
A listing of metals measured, by name and CAS Registry
number.
A narrative stating reason(s) for the modification(s).
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.
Data that will allow an independent reviewer to validate each
determination by tracing the instrument output (peak height,
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 prior to extraction/concentration step.
(g) Volume after each extraction/concentration step.
(h) Final volume prior to analysis.
(i) Injection volume.
(j) Dilution data, differentiating between dilution of a
sample or extract.
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April 1995
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Method 1638
(k) Instalment and operating conditions (make, model,
revision, modifications).
(1) Sample introduction system (ultrasonic nebulizer, flow
injection system, etc).
(m) Operating conditions (background corrections,
temperature program, flow rates, etc).
(n) Detector (type, operating conditions, etc).
(o) Mass spectra, 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. The
required types, procedures, and criteria for analysis of blanks are described in Section
9.6.
9.1.4 The laboratory shall spike at least 10% of the samples with the metal(s) 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. These procedures are described in Sections 10.2 and 9.7 of this
method.
9.1.6 The laboratory shall maintain records to define the quality of data that are generated.
Development of accuracy statements is described in Section 9.3.4.
9.2 Initial demonstration of laboratory capability
9.2.1 Method detection limit—To establish the ability to detect the: trace metals of interest,
the analyst shall determine the MDL for each analyte according to the procedure in 40
CFR 136, Appendix B using the apparatus, reagents, and standards that will be used in
the practice of this method. The laboratory must produce an MDL that is less than or
equal to the MDL listed in Table 1, or one-third the regulatory compliance limit,
whichever is greater. MDLs should be determined when a new operator begins work
or whenever, in the judgment of the analyst, a change hi instrument hardware or
operating conditions would dictate that they be redetermined,
9.2.2 Initial precision and recovery (IPR)—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 the metal(s) of interest at
2-3 tunes the ML (Table 1), according to the procedures in Section 12. All
April 1995
19
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Method 1638
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 metal(s) in each aliquot and the standard deviation of the
recovery(ies) for each metal.
9.2.2.3 For each metal, compare s and X with the corresponding limits for initial
precision and recovery in Table 2. If s and X for all metal(s) meet the
acceptance criteria, system performance is acceptable and analysis of blanks
and samples may begin. If, however, any individual s exceeds the precision
limit or any individual X falls outside the range for accuracy, system
performance is unacceptable for that metal. Correct the problem and repeat the
test (Section 9.2.2.1).
9.2.3 Linear calibration ranges—Linear calibration ranges are primarily detector limited.
The upper limit of the linear calibration range should be established for each analyte
by determining the signal responses from a minimum of three different concentration
standards, one of which is close to the upper limit of the linear range. Care should be
taken to avoid potential damage to the detector during this process. The linear
calibration range that may be used for the analysis of samples should be judged by the
analyst from the resulting data. The upper limit should be an observed signal no more
than 10% below the level extrapolated from lower standards. Determined sample
analyte concentrations that are greater than 90% of the determined upper limit must be
diluted and reanalyzed. The upper limits should be verified whenever, hi the judgment
of the analyst, a change in analytical performance caused by either a change in
instrument hardware or operating conditions would dictate they be redetermined.
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
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 a specific metal in the
sample is being checked against a regulatory concentration limit, the spike
20
April 1995
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Method 1638
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.
9.3.2 Assessing spike recovery
9.3.2.1 Determine the background concentration (B) of each metal 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 each metal.
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 each metal with the corresponding QC
acceptance criteria found in Table 2. If any individual P falls outside the designated
range for recovery, that metal has failed the acceptance criteria.
9.3.3.1 For a metal that has failed the acceptance criteria, analyze the ongoing
precision and recovery standard (Section 9.7). If the OPR is within its
respective limit for the metal(s) that failed (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 the metal remains outside the acceptance criteria, the
analytical result for that metal 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.
9.3.4.1 After the analysis of five samples of a given matrix type (river water, lake
water, etc.) for which the metal(s) pass the tests in Section 9.3.3, compute the
average percent recovery (R) and the standard deviation of the percent
/?.'•. recovery (SR) for the metal(s)... Express the accuracy assessment as a percent
recovery interval from R - 2SS. 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%.
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Method 1638
9.3.4.2 Update the accuracy assessment for each metal in each matrix regularly (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 =
(D1+D2)I2
Where:
Dl = concentration of the analyte in the MS sample
D2 = concentration of the analyte in the MSD sample
9.5
9.6
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 judged to
be out of control. In this case, correct the problem and reanalyze all samples in the
sample batch associated with the MS/MSD which failed the RPD test.
Internal standards responses—The analyst is expected to monitor the responses from the
internal standards throughout the sample batch being analyzed. Ratios of the internal standards
responses against each other should also be monitored routinely. This information may be
used to detect potential problems caused by mass dependent drift, errors incurred in adding the
internal standards, or increases in the concentrations of individual internal standards caused by
background contributions from the sample. The absolute response of any one internal standard
must not deviate more than 60-125% of the original response in the calibration blank. If
deviations greater than these are observed, flush the instrument with the rinse blank and
monitor the responses in the calibration blank. If the responses of the internal standards are
now within the limit, take a fresh aliquot of the sample, dilute by a further factor of two, add
the internal standards, and reanalyze. If, after flushing, the responses of the internal standards
in the calibration blank are out of limits, terminate the analysis and determine the cause of the
drift. Possible causes of drift may be a partially blocked sampling cone or a change in the
tuning condition of the instrument.
Blanks—Blanks are analyzed to demonstrate freedom from contamination.
9.6.1 Laboratory (method) blank
9.6.1.1 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). Analyze the blank immediately after
analysis of the OPR (Section 9.7) to demonstrate freedom from contamination.
9.6.1.2 If the metal of interest or any potentially interfering substance is found in the
blank at a concentration equal to or greater than the MDL (Table 1), sample
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Method 1638
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.
9.6.1.3 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.
9.6.1.4 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.
9.6.2 Field blank
9.6.2.1 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.
9.6.2.2 If the metal of interest 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,
then results for associated samples may be the result of contamination and may
not be reported for regulatory compliance purposes.
9.6.2.3 Alternatively, if a sufficient number of field blanks (3 minimum) are analyzed
to characterize the nature of the field blank, the average concentration plus two
standard deviations must be less than the regulatory compliance level or less
than one-half the level in the associated sample, whichever is greater.
9.6.2.4 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
prior to the next sampling event.
9.6.3 Equipment Blanks—Before any sampling equipment is used al: 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.
9.6.3.1 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 acidified to pH<2 and allowed to stand 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
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Method 1638
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.
9.6.3.2 Sampler check blanks—Sampler check blanks are generated in the laboratory
or at the equipment cleaning contractor's facility by processing reagent water
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.
9.6.3.2.1 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 that are 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 sample container.
9.6.3.2.2 The sampler check blank must be analyzed using the procedures given
in this method. If any metal of interest 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 the metal(s) of
interest before the equipment may be used in the field.
9.6.3.2.3 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.
9.7 Ongoing precision and recovery
9.7.1 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.
9.7.2 Analyze the OPR sample before analyzing the method blank and samples from the
same batch.
9.7.3 Compute the percent recovery of each metal in the OPR sample.
9.7.4 For each metal, compare the concentration to the limits for ongoing recovery in Table
2. If all metals meet the acceptance criteria, system performance is acceptable and
analysis of blanks and samples may proceed. If, however, any individual recovery falls
24 April 1995
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Method 1638
9.8
9.9
outside of the range given, the analytical processes are not being performed properly
for that metal. In this event, correct the problem, reprepare the sample batch, and
repeat the ongoing precision and recovery test (Section 9.7).
9.7.5 Add results that pass the specifications in Section 9.7.4 to initial and previous ongoing
data for each metal in each matrix. Update QC charts to form a graphic representation
of continued laboratory performance. Develop a statement of laboratory accuracy for
each metal in 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%.
The specifications contained in 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.
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 advised to follow the recommended
operating conditions provided by the manufacturer. The analyst is reaponsible for verifying
that the instrument configuration and operating conditions satisfy the quality control
requirements in this method. Table 7 lists instrument operating conditions that may be used as
a guide for analysts in determining instrument configuration and operating conditions.
10.2 Precalibration routine—The following precalibration routine should be completed before
calibrating the instrument until it can be documented with periodic performance data that the
instrument meets the criteria listed below without daily tuning.
10.2.1 Initiate proper operating configuration of instrument and data system. Allow a period
of not less than 30 minutes for the instrument to warm up. During this period,
conduct mass calibration and resolution checks using the tuning solution. Resolution
at low mass is indicated by magnesium isotopes 24, 25, 26. Resolution at high mass
is indicated by lead isotopes 206, 207, 208. For good performance adjust spectrometer
resolution to produce a peak width of approximately 0.75 amu at 5% peak height.
Adjust mass calibration if it has shifted by more than 0.1 amu from unit mass.
10.2.2 Instrument stability must be demonstrated by running the tuning solution (Section 7.7)
a minimum of five times with resulting relative standard deviations of absolute signals
for all analytes of less than 10%.
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Method 1638
10.3 Internal Standardization—Internal standardization must be used in all analyses to correct for
instrument drift and physical interferences.
10.3.1 A list of acceptable internal standards is provided in Table 4. For full mass range
scans, a minimum of three internal standards must be used. Procedures described in
this method for general application detail the use of five internal standards: scandium,
yttrium, indium, terbium, and bismuth.
10.3.2 Internal standards must be present in all samples, standards, and blanks at identical
levels. This may be achieved by directly adding an aliquot of the internal standards to
the CAL standard, blank, or sample solution (Method A), or alternatively by mixing
with the solution before nebulization using a second channel of the peristaltic pump
and a mixing coil (Method B).
10.3.3 The concentration of the internal standard should be sufficiently high to obtain good
precision in the measurement of the isotope used for data correction and to minimize
the possibility of correction errors if the internal standard is naturally present in the
sample. Depending on the sensitivity of the instrument, a concentration range of 1
pg/L to 200 pg/L of each internal standard is recommended. Internal standards should
be added to blanks, samples, and standards in a like manner, so that dilution effects
resulting from the addition may be disregarded.
10.4 Calibration—Before initial calibration, set up proper instrument software routines for
quantitative analysis. The instrument must be calibrated at a minimum of three points for each
analyte to be determined.
10.4.1 Inject the calibration blank (Section 7.6.1) and calibration standards A and B (Section
7.4.1) prepared at three or more concentrations, one of which must be at the Minimum
Level (Table 1), and another that must be near the upper end of the linear dynamic
range. A minimum of three replicate integrations is required for data acquisition. Use
the average of the integrations for instrument calibration and data reporting.
10.4.2 Compute the response factor at each concentration, as follows:
RF =
Where:
Cs - concentration of the analyte in the standard or blank solution
Cfr - concentration of the internal standard in the solution
As = height or area of the response at the mlz for the analyte
Ais = height or area of the mlz for the internal standard
10.4.3 Using the individual response factors at each concentration, compute the mean RF for
each analyte.
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Method 1638
10.4.4 Linearity—If the RF over the calibration range is constant (<20% RSD), the RF can be
assumed to be invariant and the mean RF can be used for calculations. Alternatively,
the results can be used to plot a calibration curve of response ratios, A/Ais, vs. RF.
10.5 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.
10.5.1 Analyze the mid-point calibration standard (Section 10.4).
10.5.2 Compute the percent recovery of each metal using the mean RF or calibration curve
obtained in the initial calibration.
10.5.3 For each metal, compare the recovery with the corresponding limit for calibration
verification in 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 any individual value falls outside the range given, system
performance is unacceptable for that compound. In this event, locate and correct the
problem and/or prepare a new calibration check standard and repeat the test (Sections
10.5.1-10.5.3), or recalibrate the system according to Section 10.4.
10.5.5 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.
10.6 A calibration blank must be analyzed following every calibration verification to demonstrate
that there is no carryover of the analytes of interest and that the analytical system is free from
contamination. If the concentration of an analyte in the blank result exceeds the MDL, correct
the problem, verify the calibration (Section 10.5), 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 in 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 22), labware cleaning
areas as described by Patterson and Settle (Reference 6), or clean benches.
11.2 Materials, such as gloves (Section 6.9.7), storage bags (Section 6.9.10), and plastic wrap
(Section 6.9.11), 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.
April 1995 27
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Method 1638
NOTE: If laboratory, field, and equipment blanks (Section 9.6) from the Apparatus
cleaned with fewer cleaning steps than those detailed below show no levels ofanalytes
above the MDL, those cleaning steps that do not eliminate these artifacts may be
omitted provided all performance criteria outlined in Section 9 are met.
11.3.1 Bottles, labware, and sampling equipment
11.3.1.1 Fill a precleaned basin (Section 6.9.8) with a sufficient quantity of a
0.5% solution of liquid detergent (Section 6.7), and completely
immerse each piece of ware. Allow to soak in the detergent for at
least 30 minutes.
11.3.1.2 Using a pair of clean gloves (Section 6.9.7) and clean nonmetallic
brushes (Section 6.9.9), 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 hours.
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 in a hot (50-60°C) bath of
IN trace metal grade HC1 (Section 7.1.7), and allow to soak for at
least 48 hours.
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
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 and equipment in two layers of
polyethylene film.
11.3.3 Fluoropolymer sample bottles—These bottles should be used if mercury is a target
analyte.
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Method 1638
11.3.3.1 After cleaning, fill sample bottles with 0.1% (v/v) ultrapure HC1
(Section 7.1.11) and cap tightly. It may be necessary to use a strap
wrench to assure a tight seal.
11.3.3.2 After capping, double-bag each bottle in polyethylene zip-type bags.
Store at room temperature until sample collection.
11.3.4 Bottles, labware, and sampling equipment (polyethylene or material other than,
fluoropolymer)
11.3.4.1 Apply the steps outlined above 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 HC1 (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 air bench. After drying, wrap each piece of ware and equipment
in 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 of vapors
diffusing through the polyethylene.
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 deionized 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 used to collect samples by rinsing with 10%
HC1 (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 6.9.9) should be used to scrub the pole with reagent water and the pole wiped
down with acids described in Section 11.3.4 above. After cleaning, the pole should be
wrapped in polyethylene film.
April 1995 29
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Method 1638
11.4 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 in a second polyethylene zip-
type bag.
11.5 All cleaning solutions and acid baths should be periodically monitored for accumulation of
metals that could lead to contamination. When levels of metals in the solutions become too
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 Aqueous sample preparation—dissolved analytes
12.1.1 For determination of dissolved analytes in ground and surface waters, pipet an aliquot
(> 20 mL) of the filtered, acid-preserved sample into a clean 50-mL polypropylene
centrifuge tube. Add an appropriate volume of (1+1) nitric acid to adjust the acid
concentration of the aliquot to approximate a 1% (v/v) nitric acid solution (e.g., add
0.4 mL (1+1) HNO3 to a 20-mL aliquot of sample). Add the internal standards, cap
the tube, and mix. The sample is now ready for analysis. Allowance for sample
dilution should be made in the calculations.
12.2 Aqueous sample preparation—total recoverable analytes
NOTE: To preclude contamination during sample digestion, it may be necessary to
perform the open beaker, total-recoverable digestion procedure described in Sections
12.2.1—12.2.7 in a fume hood that is located in a clean room. An alternate digestion
procedure is provided in Section 12.2.8; however, this procedure has not undergone
interlaboratory testing.
12.2.1 For the determination of total recoverable analytes in ambient water samples, transfer a
100-mL (±1 mL) aliquot from a well-mixed, acid-preserved sample to a 250-mL
Griffin beaker (Section 6.9.3). If appropriate, a smaller sample volume may be used.
12.2.2 Add 2 mL (1+1) nitric acid and 1.0 mL of (1+1) hydrochloric acid to the beaker and
place the beaker on the hot plate for digestion. The hot plate should be located in a
fume hood and previously adjusted to provide evaporation at a temperature of
approximately but no higher than 85°C. (See the following note.) The beaker should
be covered or other necessary steps should be taken to prevent sample contamination
from the fume hood environment.
NOTE: For proper heating, adjust the temperature control of the hot plate such that
an uncovered Griffin beaker containing 50 mL of water placed in the center of the hot
plate can be maintained at a temperature approximately but no higher than 85°C.
(Once the beaker is covered with a watch glass, the temperature of the water will rise
to approximately 95°C.)
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Method 1638
12.2.3 Reduce the volume of the sample aliquot to about 20 mL by gentle heating at 85°C.
Do not boil. This step takes about 2 hours for a 100-mL aliquot with the rate of
evaporation rapidly increasing as the sample volume approaches 20 mL. (A spare
beaker containing 20 mL of water can be used as a gauge.)
12.2.4 Cover the lip of the beaker with a watch glass to reduce additional evaporation and
gently reflux the sample for 30 minutes. (Slight boiling may occur, but vigorous
boiling must be avoided to prevent loss of the HC1-H2O azeotrope.)
12.2.5 Allow the beaker to cool. Quantitatively transfer the sample solution to a 50-mL
volumetric flask or 50-mL class A stoppered graduated cylinder, make to volume with
reagent water, stopper, and mix.
12.2.6 Allow any undissolved material to settle overnight, or centrifuge a portion of the
prepared sample until clear. (If, after centrifuging or standing overnight, the sample
contains suspended solids that would clog the nebulizer, a portion.of the sample may
be filtered to remove the solids before analysis. However, care should be exercised to
avoid potential contamination from filtration.)
12.2.7 Prior to analysis, adjust the chloride concentration by pipetting 20 mL of the prepared
solution into a 50-mL volumetric flask, dilute to volume with reagent water and mix.
(If the dissolved solids in this solution are >0.2%, additional dilution may be required
to prevent clogging of the extraction and/or skimmer cones.)' Add the internal
standards and mix. The sample is now ready for analysis. Because the effects of
various matrices on the stability of diluted samples cannot be characterized, all
analyses should be performed as soon as possible after the completed preparation.
12.2.8 Alternate total recoverable digestion procedure
12.2.8.1 Open the preserved sample under clean conditions. Add ultrapure
nitric and hydrochloric acid at the rate of 10 mL/L and 5 mL/L,
respectively. Remove the cap from the original container only long
enough to add each aliquot of acid. The sample container should not
be rilled to the lip by the addition of the acids. However, only
minimal headspace is needed to avoid leakage during heating.
12.2.8.2 Tightly recap the container and shake thoroughly. Place the container
in an oven preheated to 85°C. The container should be placed on an
insulating piece of material such as wood rather than directly on the
typical metal grating. After the samples have reached 85°C, heat for 2
hours. (Total time will be 2.5-3 hours depending on the sample size).
Temperature can be monitored using an identical sample container with
distilled water and a thermocouple to standardize heating time.
12.2.8.3 Allow the sample to cool. Add the internal standards and mix. The
sample is now ready for analysis. Remove aliquots for analysis under
clean conditions.
12.3 Sample Analysis
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Method 1638
12.3.1 For every new or unusual matrix, it is highly recommended that a semiquantitative
analysis be carried out to screen the sample for elements that may be present at high
concentration. Information gained from this screening may be used to prevent
potential damage to the detector during sample analysis and to identify elements that
may exceed the linear range. Matrix screening may be carried out using intelligent
software, if available, or by diluting the sample by a factor of 500 and analyzing in a
semiquantitative mode. The sample should also be screened for background levels of
all elements chosen for use as internal standards to prevent bias in the calculation of.
the analytical data.
12.3.2 Initiate instrument operating configuration. Tune and calibrate the instrument for the
analytes of interest (Section 10.0).
12.3.3 Establish instrument software run procedures for quantitative analysis. For all sample
analyses, a minimum of three replicate integrations is required for data acquisition.
Use the average of the integrations for data reporting.
12.3.4 All m/z's that may affect data quality must be monitored during the analytical run. As
a minimum, those m/z's prescribed in Table 5 must be monitored in the same scan as
is used for the collection of the data. This information should be used to correct the
data for identified interferences.
12.3.5 The rinse blank should be used to flush the system between samples. Allow sufficient
time to remove traces of the previous sample or a minimum of 1 minute. Samples
should be aspirated for 30 seconds before data is collected.
12.3.6 Samples having concentrations higher than the established linear dynamic range should
be diluted into range and reanalyzed. The sample should first be analyzed for the
trace elements in the sample, protecting the detector from the high concentration
elements if necessary, by the selection of appropriate scanning windows. The sample
should then be diluted for the determination of the remaining elements. Alternatively,
the dynamic range may be adjusted by selecting an alternative isotope of lower natural
abundance, if quality control data for that isotope have been established. The dynamic
range must not be adjusted by altering instrument conditions to an uncharacterized
state.
13.0 Data Analysis and Calculations
13.1 Table 6 lists elemental equations recommended for sample data calculations. Sample data
should be reported in units of ug/L (parts-per-billion; ppb). Report results at or above the ML
for metals found in samples and determined in standards. Report all results for metals found
in 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 For aqueous samples prepared by total recoverable procedure (Sections 12.2.1-12.2.7),
multiply solution concentrations by the dilution factor 1.25. If additional dilutions were made
32 April 1995
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Method 1638
to any samples, the appropriate factor should be applied to the calculated sample
concentrations.
13.4 Compute the concentration of each analyte in the sample using the response factor determined
from calibration data (Section 10.4) and the following equation:
A x C.
Where the terms are as defined in Section 10.4,2.
13.5 Corrections for characterized spectral interferences should be applied to the data. Chloride
interference corrections should be made on all samples, regardless of the addition of
hydrochloric acid, because the chloride ion is a common constituent of environmental samples.
13.6 If an element has more than one monitored m/z, examination of the concentration
calculated for each m/z, or the relative abundances, will provide useful information for
the analyst in detecting a possible spectral interference. Consideration should therefore
be given to both primary and secondary m/z's in the evaluation of the element
concentration. In some cases, the secondary m/z may be less sensitive or more prone
to interferences than the primary recommended m/z; therefore, differences between the
results do not necessarily indicate a problem with data calculated for the primary m/z.
13.7 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.
13.8 Do not perform blank subtraction on the sample results. Report results for samples
and accompanying blanks.
14.0 Method Performance
14.1 The method detection limits (MDLs) listed in Table 1 and the quality control acceptance
criteria listed in Table 2 were validated in two laboratories (Reference 23) for dissolved
analytes.
15.0 Pollution Prevention
15.1 Pollution prevention encompasses any technique that reduces or eliminates the quantity or
toxicity of waste at the point of generation. Numerous opportunities for pollution prevention
exist in laboratory operation. The 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 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
April 1995 33
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Method 1638
properly. Standards should be prepared in volumes consistent with laboratory use to minimize
the volume of expired standards to be disposed.
15.2 For information about pollution prevention that may be applicable 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 Herman, 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, Virginia, 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," J. Great Lakes
Research 1993,19, 175.
6 Patterson, C.C.; Settle, D.M. In National Bureau of Standards Special Publication 422;
LaFleur, P.D., Ed., U.S. Government Printing Office, Washington, DC, 1976. "Accuracy in
Trace Analysis."
7 Fitzgerald, W.F.; Watras, CJ. Science of the Total Environment 1989, 87/88, 223.
8 Gill, G.A.; Fitzgerald, W.F. Deep Sea Res. 1985, 32, 287.
9 Prothro, M.G. "Office of Water Policy and Technical Guidance on Interpretation and
Implementation of Aquatic Life Metals Criteria," EPA Memorandum to Regional Water
Management and Environmental Services Division Directors, October 1, 1993.
34 April 1995
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Method 1638
10 "Format for Method Documentation," Distributed by the EPA Environmental Monitoring
Management Council, Washington, DC, November 18, 1993.
11 Gray, A.L.; Date, A.R. Analyst 1983, 108, 1033.
12 Houk, R.S. et al. Anal Chem. 1980, 52, 2283.
13 Houk, R.S. Anal. Chem. 1986, 58, 91 A.
14 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.
15 Zief, M.; Mitchell, J.W. "Contamination Control in Trace Metals Analysis"; In Chemical
Analysis 1976, Vol. 47, Chapter 6.
16 Thompson, J.J.; Houk, R.S. Appl. Spec. 1987, 41, 801.
17 Carcinogens - Working With Carcinogens, Department of Health, Education, and Welfare,
Public Health Service, Center 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.
18 "OSHA Safety and Health Standards, General Industry," Occupational Safety and Health
Administration, OSHA 2206, 29 CFR 1910 (Revised, January 1976).
19 "Safety in Academic Chemistry Laboratories," American Chemical Society Publication,
Committee on Chemical Safety, 3rd Edition, 1979.
20 "Proposed OSHA Safety and Health Standards, Laboratories, Occupational Safety and Health
Administration," Fed. Regist. July 24, 1986.
21 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.
22 Moody, J.R. "NBS Clean Laboratories for Trace Element Analysis," Anal. Chem. 1982, 54
1358A.
23 "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.
24 Hinners, T.A. "Interferences in ICP-MS by Bromine Species"; Presented at Winter Conference
on Plasma Spectrochemistry, San Diego, CA, January 10-15, 1994.
April 1995 35
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Method 1638
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 7.6.1).
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 prior to 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 they are shipped 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.
3
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Method 1638
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 method or instrumentation is modified.
18.11 Instrument Detection Limit (IDL)—The concentration equivalent to the analyte signal which
is equal to three times the standard deviation of a series of ten replicate measurements of the
calibration blank signal at the selected analytical mass(es).
18.12 Internal Standard—Pure analyte(s) added to a sample, extract, or standard solution in known'
amount(s) and used to measure the relative responses of other method analytes that are
components of the same sample or solution. The internal standard must be an analyte that is
not a sample component (Sections 7.5 and 9.5).
18.13 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 in the laboratory environment, the reagents, or the apparatus (Sections
7.6.2 and 9.6.1).
18.14 Laboratory Control Sample (LCS)—See Ongoing Precision and Recovery (OPR) Standard.
18.15 Laboratory Duplicates (LD1 and LD2)—Two aliquots of the same sample taken in 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.
18.16 Laboratory Fortified Blank (LFB)—See Ongoing Precision and Recovery (OPR) Standard.
18.17 Laboratory Fortified Sample Matrix (LFM)—See Matrix Spike (MS) and Matrix Spike
duplicate (MSD).
18.18 Laboratory Reagent Blank (LRB)—See Laboratory Blank.
18.19 Linear Dynamic Range (LDR)—The concentration range over which the instrument response
to an analyte is linear (Section 9.2.3).
18.20 Matrix Spike (MS) and Matrix Spike Duplicate (MSD)—Aliquots of an environmental
sample to which known quantities of the method analytes are 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 background concentrations (Section 9.3).
18.21 m/z—mass-to-charge ratio
18.22 May—This action, activity, or procedural step is optional.
18.23 May Not—This action, activity, or procedural step is prohibited.
April 1995 37
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Method 1638
18.24 Method Blank—See Laboratory Blank.
18.25 Method Detection Limit (MDL)—The minimum concentration of an analyte that can be
identified, measured, and reported with 99% confidence that the analyte concentration is
greater than zero (Section 9.2.1 and Table 1).
18.26 Minimum Level (ML)—The lowest level at which the entire analytical system gives a
recognizable signal and acceptable calibration point (Reference 9).
18.27 Must—This action, activity, or procedural step is required.
18.28 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 9.7).
18.29 Preparation Blank—See Laboratory Blank.
18.30 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.
18.31 Quality 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.
18.32 Reagent Water—Water demonstrated to be free from the method analytes and potentially
interfering substances at the MDL for that metal in the method.
18.33 Should—This action, activity, or procedural step is suggested but not required.
18.34 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.
18.35 Total Recoverable Analyte—The concentration of analyte determined by analysis of the
solution extract of an unfiltered aqueous sample following digestion by refluxing with hot
dilute mineral acid(s) as specified in the method (Section 12.2).
18.36 Tuning Solution—A solution which is used to determine acceptable instrument performance
before calibration and sample analyses (Section 7.7).
38
April 1995
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Method 1638
Table 1
List of Analytes Amenable to Analysis Using Method 1638: Lowest Water Quality Criterion
for Each Metal Species, Method Detection Limits, Minimum Levels,
and Recommended Analytical M/Z's
Metal
Antimony
Cadmium
Copper
Lead
Nickel
Selenium
Silver
Thallium
Zinc
Lowest
Ambient
Water Quality
Criterion
tog/L)1
14
0.32
2.5
0.14
7.1
5
0.31
1.7
28
Method Detection
Limit (MDL) and
Minimum Level (ML);
V9/L
MDL1
0.0097
0.025
0.087
0.015
0.33
1.2
0.029
0.0079
0.14
ML3
0.02
0.1
0.2
0.05
1
5
0.1
0.02
0.5
Recommended
Analytical m/z
123
111
63
206, 207, 208
60
82
107
205
66
Notes:
1. Lowest of the freshwater, marine, and human health WQC promulgated by EPA for 14 states at 40 CFR Part 131 (57 FR 60848) with
hardness-dependent freshwater aquatic life criteria adjusted in accordance with 57 FR 60848 to reflect the worst case hardness of 25 'mg/L
CaC03 and all aquatic life criteria adjusted in accordance with the Oct. 1,1993 Office of Water guidance to reflect dissolved metals criteria.
2. Method Detection Limit as determined by 40 CFR Part 136, Appendix B.
3. 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 BAD and described in the EPA Draft National Guidance for the Permitting, Monitoring', and
Enforcement of Water Quality-Based Effluent Limitations Set Below Analytical DetectionlQuantitation Levels, March 22, 1994.
April 1995
39
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Method 1638
Table 2
Quality Control Acceptance Criteria for Performance Tests in EPA Method 16381
Metal
Antimony
Cadmium
Copper
Lead
Nickel
Selenium
Silver
Thallium
Zinc
Initial Precision and
Recovery (Section
9.2)
s X
20 81-120
13 85-112
43 55-141
30 75-140
30 71-131
41 63-145
19 82-120
30 66-134
43 55-142
Calibration
Verification
(Section 10.5)
90-111
91-105
76-120
91-120
86-116
69-127
81-107
82-118
76-121
Ongoing Precision
and
Recovery (Section
9.7)
79-122
84-113
51-145
72-143
68-134
59-149
74-119
64-137
46-146
Spike
Recovery
(Section 9.3)
79-122
84-113
51-145
72-143
68-134
59-149
74-119
64-137
46-146
1 All specifications expressed as percent.
40
April 1995
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Method 1638
TABLE 3:
COMMON MOLECULAR ION INTERFERENCES IN ICP-MS
BACKGROUND MOLECULAR IONS
Molecular Ion
NH*
OPT
OH2*
C2+
CN*
CO*
N2*
N2H*
NO*
NOH*
02*
O2H*
3SArH*
38ArH*
^ArH*
CO2*
CO2H*
ArC*,ArO*
ArN*
ArNH*
ArO*
ArOH*
*Ai*Ai*
"OAr^Ar*
^Atj*
m/z Element Interference*
15
17
18
24
26
28
28
29
30
31
32
33
37
39
41
44
45 Sc
52 Cr
54 Cr
55 Mn
56
57
76 Se
78 Se
80 Se
b Elements or Internal standards affected by the molecular Ions.
April 1995
41
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Method 1638
TABLE 3 (continued)
MATRIX MOLECULAR IONS
BROMIDE (Reference 24)
Molecular Ion
"BrH*
wBrO*
"BrO*
"BrOH*
Ar^'Br*
CHLORIDE
Molecular Ion
"CIO*
MCIOH*
"CIO*
"C10H*
Ar^Cl*
Ar"Cl*
SULFATE
Molecular Ion
"SO*
"SOH*
J'SO*
"SOH*
SO,*, S2*
AP2S*
Ai^'S*
PHOSPHATE
Molecular Ion
PO*
POH*
POj*
ArP*
GROUP I, II METALS
Molecular Ion
ArNa*
ArK*
ArCa*
42
m/z Element Interference
82 Se
95 ' Mo
97 Mo
98 Mo
121 Sb
m/z Element Interference
51 V
52 Cr
53 Cr
54 Cr
75 As
77 Se
m/z Element Interference
48
49
50 V,Cr
51 V
64 Zn
72
74
m/z Element Interference
47
48
63 Cu
71
m/z Element Interference
63 Cu
79
80
April 1995
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Method 1638
TABLE 3 (continued)
MATRIX MOLECULAR IONS
MATRIX OXIDES* m/z's Element Interference
Molecular Ion
TiO 62-66 Ni,Cu,Zn
ZrO 106-112 Ag.Cd
MoO 108-116 Cd
* Oxide interferences will normally be very small and will only impact the method elements when present at relatively high
concentrations. Some examples of matrix oxides of which the analyst should be aware arc listed.
April 1995 43
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Method 1638
TABLE 4: INTERNAL STANDARDS AND LIMITATIONS OF USE
Internal Standard
6Lithium
Scandium
Yttrium
Rhodium
Indium
Terbium
Holmium
Lutetium
Bismuth
m/z
6
45
89
103
115
159
165
175
209
Possible Limitation
a
polyatomic ion interference
a,b
isobaric interference by
a
Sn
(a) May be present in environmental samples.
(b) In some instruments, yttrium may form measurable amounts of YO+ (105 amu) and YOH+ (106 amu). If this
is the case, care should be taken in the use of the cadmium elemental correction equation.
Note:
Internal standards recommended for use with this method are shown in boldface. Preparation
procedures for these are included in Section 7.3.
44
April 1995
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Method 1638
TABLE 5: RECOMMENDED ISOTOPES AND ADDITIONAL
M/Z'S THAT MUST BE MONITORED
Isotope
27
121.123
75
135.137
9
106.108.111.114
52,53
59
63,65
206,207,208
55
95,97,98
60,62
77,82
107,109
203.205
232
238
51
66,67,68
83
99
105
118
Element of Interest
Aluminum
Antimony
Arsenic
Barium
Beryllium
Cadmium
Chromium
Cobalt
Copper
Lead
Manganese
Molybdenum
Nickel
Selenium
Silver
Thallium
Thorium
Uranium
Vanadium
Zinc
Krypton
Ruthenium
Palladium
Tin
Note: Isotopes recommended for analytical determination are underlined.
April 1995 45
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Method 1638
TABLE 6: RECOMMENDED ELEMENTAL EQUATIONS FOR DATA CALCULATIONS
Element
Sb
Cd
Cu
Pb
NI
Se
Ag
Tl
Zn
Elemental Equation
(1.000)(123C)
(I.OOO)(I11C)-(1.073)[(108C)-(0.712)(I06C)]
(1.000)(MC)
(1.000)(206C)+(1.000)(207C)+(1.000)(208C)
(1.000)(WC)
(i.ooox^c)
(1.000)(1OTC)
(1.000)(205C)
(1.000X*SC)
Note
(1)
(2)
(3)
INTERNAL STANDARDS
Element Elemental Equation
Bi
In
Sc
Tb
Y
(1.000)("5C)-(0.016)("8C)
(1.000X4SQ
(1.000)('59C)
(1.000)(89C)
Note
(4)
C—counts at specified m/z
(1)—Correction for MoO interference. M/z 106 must be from Cd only, not ZrO+. An additional correction should be made if palladium is
present.
(2)—allowance for variability of lead isotopes
(3)—Some argon supplies contain krypton as an impurity. Selenium is corrected for 82Kr by background subtraction.
(4)—correction for tin
46
April 1995
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Method 1638
TABLE 7: RECOMMENDED INSTRUMENT OPERATING CONDITIONS
Instrument
Plasma forward power
Coolant flow rate
Auxiliary flow rate
Nebulizer flow rate
Solution uptake rate
Spray chamber temperature
Data Acquisition
Detector mode
Replicate integrations
Mass range
Dwell time
Number of MCA channels
Number of scan sweeps
Total acquisition time
VG PlasmaQuad Type I
1.35 kW
13.5 IVmin
0.6 IVmin
0.78 L/min
0.6 mL/min
15°C
Pulse counting
3
8-240 amu
320 us
2048
85
3 minutes per sample
April 1995
47
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