by CAS Element · 2007 · Cited by 24 — MICROWAVE ASSISTED ACID DIGESTION OF. SEDIMENTS, SLUDGES, SOILS, AND OILS. SW-846 is not intended to be an analytical training manual. Therefore, method.
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3051A – 1Revision 1 February 2007METHOD 3051AMICROWAVE ASSISTED ACID DIGESTION OF SEDIMENTS, SLUDGES, SOILS, AND OILSSW-846 is not intended to be an analytical training manual. Therefore, methodprocedures 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 subjecttechnology.In addition, SW-846 methods, with the exception of required method use for the analysisof 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.1.0SCOPE AND APPLICATION 1.1This 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:ElementCAS 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
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ElementCAS Registry No. a3051A – 2Revision 1 February 2007Lead(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 notprovide 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 ofinterest, in the matrices of interest, at the concentration levels of interest (see Sec. 9.0).1.2This 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 isintended 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 coupledplasma 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’srecommended instructions for guidance on their microwave digestion system.
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3051A – 3Revision 1 February 20071.3 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 aregulation, the use of SW-846 methods is not mandatory in response to Federal testingrequirements. 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.1.4Use 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 analystmust demonstrate the ability to generate acceptable results with this method. 2.0SUMMARY OF METHOD A representative sample is extracted and/or dissolved in concentrated nitric acid, oralternatively, concentrated nitric acid and concentrated hydrochloric acid using microwaveheating 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.3.0DEFINITIONS Refer to Chapter One, Chapter Three and the manufacturer’s instructions for definitionsthat may be relevant to this procedure.4.0INTERFERENCES 4.1Solvents, 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 maybe 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.4.2Very 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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3051A – 4Revision 1 February 20074.3Many types of samples will be dissolved by this method. A few refractory samplematrix compounds, such as quartz, silicates, titanium dioxide, alumina, and other oxides maynot 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 aqueoustransport mechanisms of pollution.4.4Samples 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. 5.0SAFETY 5.1This 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.5.2The 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 re commendations specific to the model andmanufacturer 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. 13.3.1.5.3This 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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3051A – 5Revision 1 February 2007WARNING: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 microwavemagnetron from shutting off when the door is opened. This can result in operator exposure to microwave energy. Use of a laboratory-grade microwave equipmentsystem with isolated and corrosion resistant safety devices prevents this fromoccurring. 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 orsealed containers which are not equipped with controlled pressure relief mechanisms for microwave acid digestions by this method.6.0EQUIPMENT AND SUPPLIES The mention of trade names or commercial products in this manual is for illustrativepurposes 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). 6.1Microwave apparatus requirements 6.1.1The temperature performance requirements necessitate the microwave decomposition system to sense the temperature to within ± 2.5 C and automaticallyadjust the microwave field output power within 2 seconds of sensing. Temperature sensors should be accurate to ± 2 C (including the final reaction temperature of 175 ± 5C). Temperature feedback control provides the primary performance mechanism for themethod. 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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3051A – 6Revision 1 February 2007power for a specific number and type of vessels, vessel load, and heat loss characteristicsof 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 describedin 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 controlprovides backward compatibility with older laboratory microwave systems which may notbe 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).6.1.2The accuracy of the temperat ure measurement system should beperiodically validated at an elevated temperature. This can be done using a container of silicon oil (a high temperature oil) and an external, calibrated temperature measurementsystem. The oil should be adequately stirred to ensure a homogeneous temperature, and both the microwave temperature sensor and the external temperature sensor placed intothe oil. After heating the oil to a constant temperature of 180 ± 5 C, the temperatureshould be measured using both sensors. If the measured temperatures vary by more than1 to 2 C, the microwave temperature measurement system should be calibrated. Consultthe microwave manufacturer™s instructions about the specific temperature sensor calibration procedure.6.1.3A 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.6.2Filter paper, qualitative or equivalent. 6.3Filter funnel, glass, polypropylene, or other appropriate material. 6.4Analytical balance, of appropriate capacity and resolution meeting data quality objectives.7.0REAGENTS AND STANDARDS 7.1Reagent-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. Othergrades 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.7.2All 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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3051A – 8Revision 1 February 20079.2.2The 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 tomore 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.9.2.3To 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.9.2.4Preparation of reference samples for specific determinative methods The following sections provide guidance on the QC reference sample concentratesfor 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.9.2.4.1Method 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 6010.9.2.4.2Method 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 6020.9.2.4.2Method 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 7000.9.2.4.3Method 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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3051A – 9Revision 1 February 2007in reagent water with appropriat type(s) and volume(s) of acid(s). See Method7010.9.2.4.4Method 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 7061.9.2.4.5Method 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 7062.9.2.4.5Method 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 7063.9.2.4.6Method 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 7470.9.2.4.7Method 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 7471.9.2.4.8Method 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 7472.9.2.4.9Method 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. 9.2.4.10Method 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 7474.9.2.4.11Method 7741, Selenium by AA, Gaseous Reduction — The QC reference sample concentrate should contain selenium at 1,000 mg/L in reagent water. See Method 7741.9.2.4.12Method 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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3051A – 10Revision 1 February 20079.2.5Analyze 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. 9.3Sample quality control for preparation and analysis 9.3.1Documenting 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 thespecific 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 degreeto 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. 9.3.2A 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.9.3.3The concentration of the matrix spike sample and/or the LCS should be determined as described in the following sections.
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3051A – 11Revision 1 February 20079.3.3.1If, 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.9.3.3.2If 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.9.3.3.3If 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.9.3.4Analyze 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.9.3.5Blanks — 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.9.4The 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. 9.5Periodically, the accuracy of the temper ature measurement system used to controlthe 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 C) in a covered, microwave-transparent vessel for 2 min at thesetting 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. 10.0.9.7The 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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