METHOD 3051A
MICROWAVE ASSISTED ACID DIGESTION OF
SEDIMENTS, SLUDGES, SOILS, AND OILS
SW-846 is not intended to be an analytical training manual. Therefore, method
procedures are written based on the assumption that they will be performed by analysts who are
formally trained in at least the basic principles of chemical analysis and in the use of the subject
technology.
In addition, SW-846 methods, with the exception of required method use for the analysis
of method-defined parameters, are intended to be guidance methods which contain general
information on how to perform an analytical procedure or technique which a laboratory can use
as a basic starting point for generating its own detailed Standard Operating Procedure (SOP),
either for its own general use or for a specific project application. The performance data
included in this method are for guidance purposes only, and are not intended to be and must
not be used as absolute QC acceptance criteria for purposes of laboratory accreditation.
1.0 SCOPE AND APPLICATION
1.1 This microwave extraction method is designed to mimic extraction using
conventional heating with nitric acid (HNO3), or alternatively, nitric acid and hydrochloric acid
(HCl), according to EPA Method 200.2 and Method 3050. Since this method is not intended to
accomplish total decomposition of the sample, the extracted analyte concentrations may not
reflect the total content in the sample. This method is applicable to the microwave-assisted acid
extraction/dissolution‡ of sediments, sludges, soils, and oils for the following elements:
Element CAS Registry No. a
*Aluminum (Al) 7429-90-5
*Antimony (Sb) 7440-36-0
Arsenic (As) 7440-38-2
*Barium (Ba) 7440-39-3
*Beryllium (Be) 7440-41-7
Boron (B) 7440-42-8
Cadmium (Cd) 7440-43-9
Calcium (Ca) 7440-70-2
*Chromium (Cr) 7440-47-3
Cobalt (Co) 7440-48-4
Copper (Cu) 7440-50-8
*Iron (Fe) 7439-89-6
Element CAS Registry No. a
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Lead (Pb) 7439-92-1
*Magnesium (Mg) 7439-95-4
Manganese (Mn) 7439-96-5
Mercury (Hg) 7439-97-6
Molybdenum (Mo) 7439-98-7
Nickel (Ni) 7440-02-0
Potassium (K) 7440-09-7
Selenium (Se) 7782-49-2
*Silver (Ag) 7440-22-4
Sodium (Na) 7440-23-5
Strontium (Sr) 7440-24-6
Thallium (Tl) 7440-28-0
*Vanadium (V) 7440-62-2
Zinc (Zn) 7440-66-6
a Chemical Abstract Service Registry Number
*Indicates elements which typically require the addition of HCl to achieve
equivalent results with Method 3050, as noted in Ref. 3.
‡Note: For matrices such as certain types of oils, this method may or may not
provide total sample dissolution. For other matrices, such as soils and sediments,
it should be considered an extraction method. Other elements and matrices may
be analyzed by this method if performance is demonstrated for the analyte of
interest, in the matrices of interest, at the concentration levels of interest (see
Sec. 9.0).
1.2 This method is provided as an alternative to EPA Method 200.2 and Method 3050.
This method provides options for improving the performance for certain analytes, such as
antimony, iron, aluminum, and silver by the addition of hydrochloric acid, when necessary. It is
intended to provide a rapid multi-element acid extraction or dissolution prior to analysis so that
decisions can be made about materials and site cleanup levels, the need for TCLP testing of a
waste (see Method 1311), and whether a BDAT process is providing acceptable performance.
Digests produced by the method are suitable for analysis by flame atomic absorption
spectrophotometry (FLAA), graphite furnace atomic absorption spectrophotometry (GFAA),
inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled
plasma mass spectrometry (ICP-MS). However, the addition of HCl may limit the quantitation
methods, or increase the difficulties of quantitation with some techniques.
Due to the rapid advances in microwave technology, consult your manufacturer's
recommended instructions for guidance on their microwave digestion system.
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1.3 Prior to employing this method, analysts are advised to consult the determinative
method that may be employed in the overall analysis for additional information on quality control
procedures, development of QC acceptance criteria, calculations, and general guidance.
Analysts also should consult the disclaimer statement at the front of the manual and the
information in Chapter Two for guidance on the intended flexibility in the choice of methods,
apparatus, materials, reagents, and supplies, and on the responsibilities of the analyst for
demonstrating that the techniques employed are appropriate for the analytes of interest, in the
matrix of interest, and at the levels of concern.
In addition, analysts and data users are advised that, except where explicitly specified in a
regulation, the use of SW-846 methods is not mandatory in response to Federal testing
requirements. The information contained in this method is provided by EPA as guidance to be
used by the analyst and the regulated community in making judgments necessary to generate
results that meet the data quality objectives for the intended application.
1.4 Use of this method is restricted to use by, or under supervision of, properly
personnel experienced and trained in the use of microwave digestion systems. Each analyst
must demonstrate the ability to generate acceptable results with this method.
2.0 SUMMARY OF METHOD
A representative sample is extracted and/or dissolved in concentrated nitric acid, or
alternatively, concentrated nitric acid and concentrated hydrochloric acid using microwave
heating with a suitable laboratory microwave unit. The sample and acid(s) are placed in a
fluorocarbon polymer (PFA or TFM) or quartz microwave vessel or vessel liner. The vessel is
sealed and heated in the microwave unit for a specified period of time. After cooling, the vessel
contents are filtered, centrifuged, or allowed to settle and then diluted to volume and analyzed
by the appropriate determinative method.
3.0 DEFINITIONS
Refer to Chapter One, Chapter Three and the manufacturer's instructions for definitions
that may be relevant to this procedure.
4.0 INTERFERENCES
4.1 Solvents, reagents, glassware, and other sample processing hardware may yield
artifacts and/or interferences to sample analysis. All of these materials must be demonstrated
to be free from interferences under the conditions of the analysis by analyzing method blanks.
Specific selection of reagents and purification of solvents by distillation in all-glass systems may
be necessary. Refer to each method to be used for specific guidance on quality control
procedures and to Chapter Three for general guidance on the cleaning of glassware. Also refer
to the determinative methods to be used for a discussion of interferences.
4.2 Very reactive samples or volatile materials may create high pressures due to the
evolution of gaseous digestion products. This may cause venting of the vessels with potential
loss of sample and/or analytes. The complete decomposition of either carbonates, or carbon
based samples, may produce enough pressure to vent the vessel if the sample size is greater
than 0.25 g (depending on the pressure capability of the vessel). Variations of the method to
accommodate very reactive materials are specifically addressed in Sec. 11.3.3.
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4.3 Many types of samples will be dissolved by this method. A few refractory sample
matrix compounds, such as quartz, silicates, titanium dioxide, alumina, and other oxides may
not be dissolved and in some cases may sequester target analyte elements. These bound
elements are considered non-mobile in the environment and are excluded from most aqueous
transport mechanisms of pollution.
4.4 Samples that are highly reactive or contaminated may require dilution before
analysis. If samples are diluted, then any dilutions must be accounted for in all subsequent
calculations. Highly reactive samples may also require pre-digestion in a hood to minimize the
danger of thermal runaway and excessively vigorous reactions.
5.0 SAFETY
5.1 This method does not address all safety issues associated with its use. The
laboratory is responsible for maintaining a safe work environment and a current awareness file
of OSHA regulations regarding the safe handling of the chemicals listed in this method. A
reference file of material safety data sheets (MSDSs) should be available to all personnel
involved in these analyses.
5.2 The microwave unit cavity must be corrosion resistant and well ventilated. All
electronics must be protected against corrosion for safe operation.
CAUTION: There are many safety and operational recommendations specific to the model and
manufacturer of the microwave equipment used in individual laboratories. A listing
of these specific suggestions is beyond the scope of this method. The analyst is
advised to consult the equipment manual, the equipment manufacturer, and other
appropriate literature for proper and safe operation of the microwave equipment
and vessels. For further details and a review of safety methods during microwave
sample preparation, see Ref. 3 and the document of Sec. 13.3.1.
5.3 This method requires microwave transparent and reagent resistant materials such
as fluorocarbon polymers (examples are PFA or TFM) or quartz to contain acids and samples.
For higher pressure capabilities the vessel may be contained within layers of different
microwave transparent materials for strength, durability, and safety. The internal volume of the
vessel should be at least 45 mL, and the vessel must be capable of withstanding pressures of at
least 30 atm (435 psi), and capable of controlled pressure relief. These specifications are to
provide an appropriate, safe, and durable reaction vessel of which there are many adequate
designs by many suppliers.
WARNING: The reagent combination (9 mL nitric acid to 3 mL hydrochloric acid) results in
greater pressures than those resulting from the use of only nitric acid. As
demonstrated in Figures 1 and 2, pressures of approximately 12 atm have been
reached during the heating of the acid mixture alone (no sample in the vessel).
Pressures reached during the actual decomposition of a sediment sample (SRM
2704, a matrix with low organic content) have more than doubled when using the 9
mL nitric and 3 mL hydrochloric acid mixture. These higher pressures necessitate
the use of vessels having higher pressure capabilities (30 atm or 435 psi).
Matrices having large organic content, such as oils, can produce approximately 25
atm of pressure inside the vessel (as described in Method 3052).
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WARNING: The outer layers of vessels are frequently not as acid or reagent resistant as the
liner material. In order to retain the specified performance and safety
requirements, these outer layers must not be chemically degraded or physically
damaged. Routine examination of the vessel materials is necessary to ensure
their safe use.
WARNING: Another safety concern relates to the use of sealed containers without pressure
relief devices. Temperature is the important variable controlling the reaction.
Pressure is needed to attain elevated temperatures, but must be safely contained.
Some digestion vessels constructed from certain fluorocarbons may crack, burst,
or explode in the unit under certain pressures. Only vessels approved by the
manufacturer of the microwave system being used are considered acceptable.
WARNING: When acids such as nitric and hydrochloric are used to effect sample digestion in
microwave units in open vessel(s), or sealed vessel(s), there is the potential for
any released acid vapors to corrode the safety devices that prevent the microwave
magnetron from shutting off when the door is opened. This can result in operator
exposure to microwave energy. Use of a laboratory-grade microwave equipment
system with isolated and corrosion resistant safety devices prevents this from
occurring. Use of laboratory-grade microwave equipment is needed to prevent
safety hazards. For further details, consult Ref. 3 and the document listed in Sec.
13.3.1.
Users are therefore advised not to use domestic (kitchen) type microwave ovens or
sealed containers which are not equipped with controlled pressure relief
mechanisms for microwave acid digestions by this method.
6.0 EQUIPMENT AND SUPPLIES
The mention of trade names or commercial products in this manual is for illustrative
purposes only, and does not constitute an EPA endorsement or exclusive recommendation for
use. The products and instrument settings cited in SW-846 methods represent those products
and settings used during method development or subsequently evaluated by the Agency.
Glassware, reagents, supplies, equipment, and settings other than those listed in this manual
may be employed provided that method performance appropriate for the intended application
has been demonstrated and documented.
This section does not list common laboratory glassware (e.g., beakers and flasks).
6.1 Microwave apparatus requirements
6.1.1 The temperature performance requirements necessitate the microwave
decomposition system to sense the temperature to within ± 2.5 EC and automatically
adjust the microwave field output power within 2 seconds of sensing. Temperature
sensors should be accurate to ± 2 EC (including the final reaction temperature of 175 ± 5
EC). Temperature feedback control provides the primary performance mechanism for the
method. Due to the variability in sample matrix types and microwave digestion equipment
(i.e., different vessel types and microwave oven designs), temperature feedback control is
preferred for reproducible microwave heating. For further details consult Ref. 3.
Alternatively, for a specific vessel type, specific set of reagent(s), and sample type,
a calibration control mechanism can be developed. Through calibration of the microwave
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power for a specific number and type of vessels, vessel load, and heat loss characteristics
of a specific vessel series, the reaction temperature profile described in Sec. 11.3.5 can
be reproduced. The calibration settings are specific for the number and type of vessels
and microwave system being used, in addition to the specific reagent combination being
used. Therefore, no specific calibration settings are provided in this method. These
settings may be developed by using temperature monitoring equipment for each specific
set of microwave equipment and vessel type. They may be used as previously described
in such methods as Methods 3015 and 3052. In this circumstance, the microwave system
provides programmable power, which can be programmed to within ± 12 W of the required
power. Typical systems provide a nominal 600 W to 1200 W of power. Calibration control
provides backward compatibility with older laboratory microwave systems which may not
be equipped for temperature monitoring or feedback control and with lower cost
microwave systems for some repetitive analyses. Older vessels with lower pressure
capabilities may not be compatible (see Refs. 1, 2, and 3 and the documents listed in
Secs. 13.3.3 and 13.3.5).
6.1.2 The accuracy of the temperature measurement system should be
periodically validated at an elevated temperature. This can be done using a container of
silicon oil (a high temperature oil) and an external, calibrated temperature measurement
system. The oil should be adequately stirred to ensure a homogeneous temperature, and
both the microwave temperature sensor and the external temperature sensor placed into
the oil. After heating the oil to a constant temperature of 180 ± 5 EC, the temperature
should be measured using both sensors. If the measured temperatures vary by more than
1 to 2 EC, the microwave temperature measurement system should be calibrated. Consult
the microwave manufacturer's instructions about the specific temperature sensor
calibration procedure.
6.1.3 A rotating turntable is employed to ensure the homogeneous distribution
of microwave radiation within the unit. The speed of the turntable should be a minimum of
3 rpm. Other types of equipment that are used to assist in achieving uniformity of the
microwave field may also be appropriate.
6.2 Filter paper, qualitative or equivalent.
6.3 Filter funnel, glass, polypropylene, or other appropriate material.
6.4 Analytical balance, of appropriate capacity and resolution meeting data quality
objectives.
7.0 REAGENTS AND STANDARDS
7.1 Reagent-grade chemicals must be used in all tests. Unless otherwise indicated, it
is intended that all reagents conform to the specifications of the Committee on Analytical
Reagents of the American Chemical Society, where such specifications are available. Other
grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity
to permit its use without lessening the accuracy of the determination.
7.2 All acids should be sub-boiling distilled where possible to minimize the blank levels
due to metallic contamination. Other grades may be used, provided it is first ascertained that
the reagent is of sufficient purity to permit its use without decreasing the accuracy of the
determination. If the purity of a reagent is questionable, the reagent should be analyzed to
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determine the level of impurities. The reagent blank must be less than the lower level of
quantitation in order to be used.
7.2.1 Concentrated nitric acid (HNO3) -- The acid should be analyzed to
determine levels of impurity. If the method blank is less than the lower level of
quantitation, the acid can be used.
7.2.2 Concentrated hydrochloric acid (HCl) -- The acid should be analyzed to
determine levels of impurity. If the method blank is less than the lower level of
quantitation, the acid can be used.
7.3 Reagent water -- Reagent water must be interference free. All references to water
in this method refer to reagent water unless otherwise specified. For further details, consult the
document listed in Sec. 13.3.2.
8.0 SAMPLE COLLECTION, PRESERVATION, AND STORAGE
8.1 See the introductory material to Chapter Three, "Inorganic Analytes."
8.2 All sample containers must be prewashed with acids and water, and metal-free
detergents, if necessary, depending on the history of use of the container (Ref. 3). Plastic and
glass containers are both suitable. For further information, see Chapter Three.
9.0 QUALITY CONTROL
9.1 Refer to Chapter One for additional guidance on quality assurance (QA) and
quality control (QC) protocols. When inconsistencies exist between QC guidelines, methodspecific
QC criteria take precedence over both technique-specific criteria and those criteria
given in Chapter One, and technique-specific QC criteria take precedence over the criteria in
Chapter One. Any effort involving the collection of analytical data should include development
of a structured and systematic planning document, such as a Quality Assurance Project Plan
(QAPP) or a Sampling and Analysis Plan (SAP), which translates project objectives and
specifications into directions for those that will implement the project and assess the results.
Each laboratory should maintain a formal quality assurance program. The laboratory should
also maintain records to document the quality of the data generated. All data sheets and quality
control data should be maintained for reference or inspection.
9.2 Initial demonstration of proficiency
Each laboratory must demonstrate initial proficiency with each sample preparation and
determinative method combination it utilizes by generating data of acceptable accuracy and
precision for target analytes in a clean reference matrix. This will include a combination of the
sample extraction method and the determinative method (a 6000 or 7000 series method). The
laboratory must also repeat the demonstration of proficiency whenever new staff are trained or
significant changes in instrumentation are made.
9.2.1 Prepare the reference samples from a spiking solution containing each
analyte of interest. The reference sample concentrate (spiking solution) may be prepared
from pure standard materials, or purchased as certified solutions. If prepared by the
laboratory, the reference sample concentrate should be made using stock standards
prepared independently from those used for calibration.
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9.2.2 The procedure for preparation of the reference sample concentrate is
dependent upon the method being evaluated. Guidance for reference sample
concentrations for certain methods is listed in Sec. 9.2.4. In other cases, the
determinative methods may contain guidance on preparing the reference sample
concentrate and the reference sample. In the absence of any other guidance, consult
Sec. 9.3.3 and prepare the spiking solution accordingly.
The concentration of target analytes in the reference sample may be adjusted to
more accurately reflect the concentrations that will be analyzed by the laboratory. If the
concentration of an analyte is being evaluated relative to a regulatory limit or action level,
see Sec. 9.3.3 for information on selecting an appropriate spiking level.
9.2.3 To evaluate the performance of the total analytical process, the reference
samples must be handled in exactly the same manner as actual samples. See the note in
Sec. 9.3.1 for important information regarding spiking samples.
9.2.4 Preparation of reference samples for specific determinative methods
The following sections provide guidance on the QC reference sample concentrates
for many determinative methods. The concentration of the target analytes in the QC
reference sample for the methods listed below may need to be adjusted to more
accurately reflect the concentrations of interest in different samples or projects. If the
concentration of an analyte is being evaluated relative to a regulatory limit or action level,
see Sec. 9.3.3 for information on selecting an appropriate spiking level. In addition, the
analyst may vary the concentration of the spiking solution and the volume of solution
spiked into the sample. However, because of concerns about the effects of the spiking
solution solvent on the sample, the total volume spiked into a sample should generally be
held to no more than 1 mL. For any determinative method not listed below, the analyst
should consult Sec. 9.3.3 and is free to choose analytes and spiking concentrations
appropriate for the intended application. See the note in Sec. 9.3.1 for important
information regarding spiking samples.
NOTE: All of the concentrations listed below refer to the concentration of the spiking
solution itself, not the concentration of the spiked sample.
9.2.4.1 Method 6010, Inorganic Elements by ICP-AES -- The QC
reference sample concentrate should contain each analyte at 1,000 mg/L in
reagent water with appropriate type(s) and volume(s) of acid(s). See Method
6010.
9.2.4.2 Method 6020, Inorganic Elements by ICP-MS -- The QC
reference sample concentrate should contain each analyte at 1,000 mg/L in
reagent water with appropriate type(s) and volume(s) of acid(s). See Method
6020.
9.2.4.2 Method 7000, Inorganic Elements by Flame AAS -- The QC
reference sample concentrate should contain each analyte at 1,000 mg/L in
reagent water with appropriate type(s) and volume(s) of acid(s). See Method
7000.
9.2.4.3 Method 7010, Inorganic Elements by Graphite Furnace AAS --
The QC reference sample concentrate should contain each analyte at 1,000 mg/L
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in reagent water with appropriat type(s) and volume(s) of acid(s). See Method
7010.
9.2.4.4 Method 7061, Arsenic by AA, Gaseous Hydride -- The QC
reference sample concentrate should contain arsenic at 1,000 mg/L in reagent
water with appropriate volume of concentrated nitric acid. See Method 7061.
9.2.4.5 Method 7062, Antimony and Arsenic by AA, Borohydride
Reduction -- The QC reference sample concentrate should contain each analyte at
1,000 mg/L in reagent water with appropriate volume of concentrated nitric acid.
See Method 7062.
9.2.4.5 Method 7063, Arsenic by ASV -- The QC reference sample
concentrate should contain mercury at 1,000 mg/L in reagent water with
appropriate volume of concentrated nitric acid. Stock solutions are commercially
available as spectrophotometric standards. See Method 7063.
9.2.4.6 Method 7470, Mercury in Liquid Waste by Manual Cold-Vapor
Technique -- The QC reference sample concentrate should contain mercury at
1,000 mg/L in reagent water with appropriate volume of concentrated nitric acid.
Stock solutions are also commercially available as spectrophotometric standards.
See Method 7470.
9.2.4.7 Method 7471, Mercury in Solid or Semisolid Waste by Manual
Cold-Vapor Technique -- The QC reference sample concentrate should contain
mercury at 1,000 mg/L in reagent water with appropriate volume of concentrated
nitric acid. Stock solutions are also commercially available as spectrophotometric
standards. See Method 7471.
9.2.4.8 Method 7472, Mercury by ASV -- The QC reference sample
concentrate should contain mercury at 1,000 mg/L in reagent water with
appropriate volume of concentrated nitric acid. Stock solutions are also
commercially available as spectrophotometric standards. See Method 7472.
9.2.4.9 Method 7473, Mercury by Thermal, Decomposition,
Amalgamation, and AA -- The QC reference sample concentrate should contain
mercury at 1,000 mg/L in reagent water with appropriate volume of concentrated
nitric acid. Stock solutions are also commercially available as spectrophotometric
standards. See Method 7473.
9.2.4.10 Method 7474, Mercury by Atomic Fluorescence -- The QC
reference sample concentrate should contain mercury at 1,000 mg/L in reagent
water with appropriate volume of concentrated nitric acid. Stock solutions are also
commercially available as spectrophotometric standards. See Method 7474.
9.2.4.11 Method 7741, Selenium by AA, Gaseous Reduction -- The QC
reference sample concentrate should contain selenium at 1,000 mg/L in reagent
water. See Method 7741.
9.2.4.12 Method 7742, Selenium by AA, Borohydride Reduction -- The
QC reference sample concentrate should contain selenium at 1,000 mg/L in
reagent water. See Method 7742.
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9.2.5 Analyze at least four replicate aliquots of the well-mixed reference
samples by the same procedures used to analyze actual samples. This will include a
combination of the sample preparation method and the determinative method (a 6000 or
7000 series method). Follow the guidance on data calculation and interpretation
presented in the determinative method.
9.3 Sample quality control for preparation and analysis
9.3.1 Documenting the effect of the matrix should include the analysis of at
least one matrix spike and one duplicate unspiked sample or one matrix spike/matrix spike
duplicate pair per analytical batch. The decision on whether to prepare and analyze
duplicate samples or a matrix spike/matrix spike duplicate must be based on a knowledge
of the samples in the sample batch. If samples are expected to contain target analytes,
laboratories may use one matrix spike and a duplicate analysis of an unspiked field
sample. If samples are not expected to contain target analytes, then laboratories should
use a matrix spike and matrix spike duplicate pair. The consideration as to which sample
for a given batch is selected for QC analyses should be decided during the project
planning process and documented in an approved sampling and analysis plan. The actual
sample selected for QC analyses should be representative of the entire matrix composition
for a given extraction batch, since data quality assumptions will likely be applied to all
batch samples based on compliance to the stated data quality objectives and meeting the
recommended precision and accuracy criteria. Therefore, it is inappropriate to combine
dissimilar matrices in a single sample preparatory batch and expect to use a single set of
QC samples. Sec. 9.3.3 provides guidance on establishing the concentration of the matrix
spike compounds in the sample chosen for spiking.
The choice of analytes to be spiked should reflect the analytes of interest for the
specific project. Thus, if only a subset of the list of target analytes provided in a
determinative method are of interest, then these would be the analytes of interest for the
project. In the absence of project-specific analytes of interest, it is suggested that the
laboratory periodically change the analytes that are spiked with the goal of obtaining
matrix spike data for most, if not all, of the analytes in a given determinative method. If
these compounds are not target analytes for a specific project, or if other compounds are
known to be of greater concern at a given site, then other matrix spike compounds should
be employed.
CAUTION: The utility of the data for the matrix spike compounds depends on the degree
to which the spiked compounds mimic the compounds already present in a
field sample. Therefore, it is CRITICAL that any compounds added to a
sample are added to the sample aliquot PRIOR TO any additional processing
steps. It is also CRITICAL that the spiked compounds be in the same
chemical form as the target compounds.
9.3.2 A laboratory control sample (LCS) should be included with each analytical
batch. The LCS consists of an aliquot of a clean (control) matrix similar to the sample
matrix and of the same weight or volume: e.g., reagent water for the water matrix or sand
or soil for the solid matrix. The LCS is spiked with the same analytes at the same
concentrations as the matrix spike, when appropriate. When the results of the matrix
spike analysis indicate a potential problem due to the sample matrix itself, the LCS results
are used to verify that the laboratory can perform the analysis in a clean matrix.
9.3.3 The concentration of the matrix spike sample and/or the LCS should be
determined as described in the following sections.
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9.3.3.1 If, as in compliance monitoring, the concentration of a specific
analyte in the sample is being checked against a regulatory limit or action level, the
spike should be at or below the regulatory limit or action level, or 1 - 5 times the
background concentration (if historical data are available), whichever concentration
is higher.
9.3.3.2 If historical data are not available, it is suggested that an
uncontaminated sample of the same matrix from the site be submitted for matrix
spiking purposes to ensure that high concentrations of target analytes and/or
interferences will not prevent calculation of recoveries.
9.3.3.3 If the concentration of a specific analyte in a sample is not
being checked against a limit specific to that analyte, then the concentration of the
matrix spike should be at the same concentration as the reference sample (Sec.
9.2.4), near the middle of calibration range, or approximately 10 times the
quantitation limit in the matrix of interest. It is again suggested that a background
sample of the same matrix from the site be submitted as a sample for matrix
spiking purposes.
9.3.4 Analyze these QC samples (the LCS and the matrix spikes or the optional
matrix duplicates) following the procedures in the determinative method. Calculate and
evaluate the QC data as outlined in the determinative method.
9.3.5 Blanks -- The preparation and analysis of method blanks and other blanks
are necessary to track potential contamination of samples during the extraction and
analysis processes. Refer to Chapter One for specific quality control procedures.
9.4 The laboratory must also have procedures for documenting the effect of the matrix
on method performance. Refer to Chapter One and each determinative method for specific
guidance on developing method performance data.
9.5 Periodically, the accuracy of the temperature measurement system used to control
the microwave equipment should be validated per Sec. 6.1.2.
9.6 (This step is not necessary if using temperature feedback control.) Each day that
samples are extracted, the microwave-power calibration should be verified by heating 1 kg of
ASTM Type II water (at 22 ± 3 EC) in a covered, microwave-transparent vessel for 2 min at the
setting for 490 W and measuring the water temperature after heating per Sec. 10.5. If the
power calculated (according to Sec. 12.0) differs from 490 W by more than ± 10 W, the
microwave settings should be recalibrated according to Sec. 10.0.
9.7 The choice of an acid or acid mixture for digestion will depend on the analytes of
interest and no single acid or acid mixture is universally applicable to all analyte groups.
Whatever acid or acid mixture is employed, including those specifically listed in this method, the
analyst must demonstrate adequate performance for the analytes of interest, at the levels of
interest. At a minimum, such a demonstration will encompass the initial demonstration of
proficiency described in Method 3500, using a clean reference matrix. Method 8000 describes
procedures that may be used to develop performance criteria for such demonstrations as well
as for matrix spike and laboratory control sample results.
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10.0 CALIBRATION AND STANDARDIZATION
The following sections provide information regarding the calibration of microwave
equipment.
NOTE: If the microwave unit uses temperature feedback control to control the performance
specifications of the method, then performing the calibration procedure is not
necessary.
10.1 Calibration is the normalization and reproduction of a microwave field strength to
permit reagent and energy coupling in a predictable and reproducible manner. It balances
reagent heating and heat loss from the vessels and is equipment dependent due to the heat
retention and loss characteristics of the specific vessel. Available power is evaluated to permit
the microwave field output in watts to be transferred from one microwave system to another.
Use of calibration to control this reaction requires balancing output power, coupled energy,
and heat loss to reproduce the temperature heating profile given in Sec. 11.3.5. The conditions
for each acid mixture and each batch containing the same specified number of vessels must be
determined individually. Only identical acid mixtures and vessel models and specified numbers
of vessels may be used in a given batch.
10.2 For cavity type microwave equipment, calibration is accomplished by measuring
the temperature rise in 1 kg of water exposed to microwave radiation for a fixed period of time.
The analyst can relate power in watts to the partial power setting of the system. The calibration
format needed for laboratory microwave systems depends on the type of electronic system used
by the manufacturer to provide partial microwave power. Few systems have an accurate and
precise linear relationship between percent power settings and absorbed power. Where linear
circuits have been utilized, the calibration curve can be determined by a three-point calibration
method (see Sec. 10.4). Otherwise, the analyst must use the multiple point calibration method
(see Sec. 10.3). Assistance in calibration and software guidance of calibration are available in
Ref. 3 and the document listed in Sec. 13.3.5.
10.3 The multiple point calibration involves the measurement of absorbed power over a
large range of power settings. Typically, for a 600 W unit, the following power settings are
measured: 100, 99, 98, 97, 95, 90, 80, 70, 60, 50, and 40% using the procedure described in
Sec. 10.5. These data are clustered about the customary working power ranges. Non-linearity
has been encountered at the upper end of the calibration. Non-linearity is primarily encountered
when using older instrumentation, however, multi-point calibration is recommended for use with
all instrumentation when accurate and precise temperature feedback control is not available. If
the system's electronics are known to have nonlinear deviations in any region of proportional
power control, it will be necessary to make a set of measurements that bracket the power to be
used. The final calibration point should be at the partial power setting that will be used in the
test. This setting should be checked periodically to evaluate the integrity of the calibration. If a
significant change is detected (± 10 W), then the entire calibration should be re-evaluated.
10.4 The three-point calibration involves the measurement of absorbed power at three
different power settings. Measure the power at 100% and 50% using the procedure described
in Sec. 10.5. From this 2-point line, determine the partial power setting that corresponds to the
power, in watts, specified in the procedure to reproduce the heating profile specified in Sec.
11.3.5. Measure the absorbed power at that partial power setting. If the measured absorbed
power does not correspond to the specified power within ± 10 W, use the multiple point
calibration in Sec. 10.3. This point should also be used to periodically verify the integrity of the
calibration.
3051A - 13 Revision 1
February 2007
10.5 Equilibrate a large volume of water to room temperature (22 ± 3 EC). One kg of
reagent water is weighed (1,000.0 ± 0.1 g) into a fluorocarbon beaker or a beaker made of
some other material that does not significantly absorb microwave energy (glass absorbs
microwave energy and is not recommended). The initial temperature of the water should be 22
± 3 EC measured to ± 0.05 EC. The covered beaker is circulated continuously (in the normal
sample path) through the microwave field for 2 min at the desired partial power setting with the
system's exhaust fan on maximum (as it will be during normal operation). The beaker is
removed and the water vigorously stirred. Use a magnetic stirring bar inserted immediately
after microwave irradiation (irradiating with the stir bar in the vessel could cause electrical
arcing) and record the maximum temperature within the first 30 seconds to ± 0.05 EC. Use a
new sample for each additional measurement. If the water is reused (after making adjustments
for any loss of weight due to heating), both the water and the beaker must have returned to 22 ±
3 EC. Three measurements at each power setting should be made.