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羰基化合物的气相色谱分析

来源:中国色谱网 作者:USEPA 2007-5-18
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摘要: 1 This is a gas chromatographic method optimized for the determination of selectedcarbonyl compounds in finished drinking water and raw source water。 The analytesapplicable to this method are derivatized to their corresponding pentafluorobenzyloximes。 The oxime derivatives are then extracted fr......


556-2 METHOD 556 DETERMINATION OF CARBONYL COMPOUNDS IN DRINKING WATER BY PENTAFLUORBENZYLHYROXYLAMINE DERIVATIZATION AND CAPILLARY GAS CHROMATOGRAPHY AND ELECTRON CAPTURE DETECTION 1. SCOPE AND APPLICATION 1.1 This is a gas chromatographic method optimized for the determination of selected carbonyl compounds in finished drinking water and raw source water. The analytes applicable to this method are derivatized to their corresponding pentafluorobenzyl oximes. The oxime derivatives are then extracted from the water with hexane. The hexane extracts are analyzed by capillary gas chromatography with electron capture detection (GC-ECD) and quantitated using procedural standard calibration. Accuracy, precision, and method detection limit (MDL) data have been generated for the following compounds: Chemical Abstract Services Analyte Registry Number Formaldehyde 50-00-0 Acetaldehyde 75-07-0 Propanal 123-38-6 Butanal 123-72-8 Pentanal 110-62-3 Hexanal 66-25-1 Heptanal 111-71-7 Octanal 124-13-0 Nonanal 124-19-6 Decanal 112-31-2 Cyclohexanone 108-94-1 Crotonaldehyde 123-73-9 Benzaldehyde 100-52-7 Glyoxal (ethanedial) 107-22-2 Methyl glyoxal (2-oxopropanal or pyruvic aldehyde) 78-98-8 1.2 This method applies to the determination of target analytes over the concentration ranges typically found in drinking water. Analyte retention times are in Section 17, Table 1. Other method performance data are presented in Section17, Tables 2-6. Experimentally determined method detection limits (MDLs) for the above listed analytes are provided in Section 17, Table 3. The MDL is defined as the statistically 556-3 calculated minimum amount that can be measured with 99% confidence that the reported value is greater than zero.(1) However, it should be noted that background levels of some method analytes (usually formaldehyde and acetaldehyde) are problematic. The minimum reporting level (MRL) for method analytes, for each analyst/ laboratory that uses this method, will depend on their ability to control background levels (Sect. 4). 1.3 This method is restricted to use by or under the supervision of analysts skilled in liquid-liquid extractions, derivatization procedures and the use of GC and interpretation of gas chromatograms. Each analyst must demonstrate the ability to generate acceptable results with this method, using the procedures described in Section 9. 2.0 SUMMARY OF METHOD 2.1 A 20 mL volume of water sample is adjusted to pH 4 with potassium hydrogen phthalate (KHP) and the analytes are derivatized at 35 oC for 2 hr with 15 mg of O- (2,3,4,5,6-Pentafluorobenzyl)-hydroxylamine (PFBHA) reagent. The oxime derivatives are extracted from the water with 4 mL hexane. The extract is processed through an acidic wash step, and then analyzed by GC-ECD. The target analytes are identified and quantitated by comparison to a procedural standard (Sect. 3.9). Two chromatographic peaks will be observed for many of the target analytes. Both (E) and (Z) isomers are formed for carbonyl compounds that are asymmetrical, and that are not sterically hindered. However, the (E) and (Z) isomers may not be chromatographically resolved in a few cases. Compounds where two carbonyl groups are derivatized, such as glyoxal and methyl glyoxal, have even more possible isomers. See Section17, Table 1 and Figure 1 for the chromatographic peaks used for analyte identification. NOTE: The absolute identity of the (E) and (Z) isomers was not determined during method development. Other researchers (2,3,4) have reported the first eluting peak as (E), and the second peak as (Z). For convenience, this method will follow this convention. Because more than 2 isomers are formed for glyoxal and methyl glyoxal, the peaks used for identification are referred to as “peak 1’ and “peak 2.’ 2.2 All results should be confirmed on a second, dissimilar capillary GC column. 3. DEFINITIONS 3.1 LABORATORY REAGENT BLANK (LRB) -- An aliquot of reagent water or other blank matrix that is treated exactly as a sample including exposure to all glassware, equipment, solvents and reagents, sample preservatives, internal standards, and surrogates that are used with other samples. The LRB is used to determine if method556-4 analytes or other interferences are present in the laboratory environment, the reagents, or the apparatus. 3.2 FIELD REAGENT BLANK (FRB) -- An aliquot of reagent water or other blank matrix that is placed in a sample container in the laboratory and treated as a sample in all respects, including shipment to the sampling site, storage, preservation, and all analytical procedures. The purpose of the FRB is to determine if method analytes or other interferences are introduced during sample shipping or storage. For this analysis the FRB should not be opened at the sampling site. 3.3 LABORATORY FORTIFIED BLANK (LFB) -- An aliquot of reagent water or other blank matrix to which known quantities of the method analytes are added in the laboratory. The LFB is analyzed exactly like a sample, and its purpose is to determine whether the methodology is in control, and whether the laboratory is capable of making accurate and precise measurements. 3.4 LABORATORY FORTIFIED SAMPLE MATRIX (LFM) -- An aliquot of an environmental sample to which known quantities of the method analytes are added in the laboratory. The LFM is analyzed exactly like a sample, and its purpose is to determine whether the sample matrix contributes bias to the analytical results. The background concentrations of the analytes in the sample matrix must be determined in a separate aliquot and the measured values in the LFM corrected for background concentrations. 3.5 STOCK STANDARD SOLUTION (SSS) -- A concentrated solution containing one or more method analytes prepared in the laboratory using assayed reference materials or purchased from a reputable commercial source. 3.6 PRIMARY DILUTION STANDARD SOLUTION (PDS) -- A solution of several analytes prepared in the laboratory from stock standard solutions and diluted as needed to prepare calibration solutions and other needed analyte solutions. 3.7 CALIBRATION STANDARD (CAL) -- A solution prepared from the primary dilution standard solution and stock standard solutions and the internal standards and surrogate analytes. The CAL solutions are used to calibrate the instrument response with respect to analyte concentration. 3.8 QUALITY CONTROL SAMPLE (QCS) -- A solution of method analytes of known concentrations which is used to fortify an aliquot of LRB or sample matrix. The QCS is obtained from a source external to the laboratory and different from the source of calibration standards. It is used to check laboratory performance with externally prepared test materials.556-5 3.9 PROCEDURAL STANDARD CALIBRATION -- A calibration method where aqueous calibration standards are prepared and processed (e.g. purged, extracted, and/or derivatized) in exactly the same manner as a sample. All steps in the process from addition of sampling preservatives through instrumental analyses are included in the calibration. Using procedural standard calibration compensates for any inefficiencies in the processing procedure. 3.10 INTERNAL STANDARD (IS) -- A pure analyte added to a sample, extract, or standard solution in known amount(s) and used to measure the relative responses of other method analytes and surrogates that are components of the same sample or solution. The internal standard must be an analyte that is not a sample component. 3.11 SURROGATE ANALYTE (SUR) -- A pure analyte, which is extremely unlikely to be found in any sample, and which is added to a sample aliquot in known amount(s) before extraction or other processing and is measured with the same procedures used to measure other sample components. The purpose of the SA is to monitor method performance with each sample. 3.12 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. 3.13 MATERIAL SAFETY DATA (MSDS) -- Written information provided by vendors concerning a chemical’s toxicity, health hazards, physical properties, fire, and reactivity data including storage, spill, and handling precautions. 3.14 CONTINUING CALIBRATION CHECK (CCC) -- A calibration standard containing one or more method analytes, which is analyzed periodically to verify the accuracy of the existing calibration for those analytes. 3.15 MINIMUM REPORTING LEVEL (MRL) -- The minimum concentration of an analyte that should be reported. This concentration is determined by the background level of the analyte in the LRBs and the sensitivity of the method to the analyte. Ideally, the MRL will be at or near the concentration of the lowest calibration standard. 4. INTERFERENCES 4.1 Method interferences may be caused by contaminants in laboratory air, solvents, reagents (including reagent water), glassware, sample bottles and caps, and other sample processing hardware that lead to discrete artifacts and/or elevated baselines in the chromatograms. All of these materials must be routinely demonstrated to be free from interferences (less than 1/2 the MRL) under the conditions of the analysis by analyzing laboratory reagent blanks as described in Section 9.3. Subtracting blank values from sample results is not permitted.4.1.1 Before attempting analyses by this method, the analyst must obtain a source of reagent water free from carbonyl compounds and other interferences. The most likely interferences are the presence of formaldehyde and acetaldehyde in the reagent water. The most successful techniques for generating aldehyde free water are (1) exposure to UV light, or (2) distillation from permanganate. 4.1.2 Commercially available systems for generating reagent grade water have proved adequate, if a step involving exposure to UV light is included. For the data presented in this method, a Millipore Elix 3 reverse osmosis system followed by a Milli-Q TOC Plus polishing unit provided reagent water with background levels of l ug/L or less for each method analyte. Other researchers have reported typical blank values of 1-3 ug/L.(3,4) 4.1.3 Distillation of reagent water from acidified potassium permanganate has been reported as an effective method of eliminating background levels of aldehydes.(2) Distill 500 mL of reagent water to which 64 mg potassium permanganate and 1 mL conc. sulfuric acid have been added. In our laboratory, this procedure reduced formaldehyde levels to approximately 3 ug/L. 4.1.4 It may be necessary to purchase reagent grade water. If acceptably clean reagent grade water is purchased, care must also be taken to protect it from contamination caused by contact with laboratory air. 4.2 Formaldehyde is typically present in laboratory air and smaller amounts of other aldehydes may also be found. Care should be taken to minimize exposure of reagents and sample water with laboratory air. Because latex is a potential aldehyde contaminant source, protective gloves should not contain latex. Nitrile gloves, such as N-Dex Plus, are acceptable. Bottle caps should be made of polypropylene. Commonly used phenolic resin caps must be avoided because they can introduce formaldehyde contamination into samples. 4.3 Reagents must also be free from contamination. Many brands of solvents may contain trace amounts of carbonyl compounds. 4.4 Glassware must be scrupulously cleaned by detergent washing with hot water, and rinses with tap water and distilled water. Glassware should then be drained, dried, and heated in a laboratory oven at 130 oC for several hours before use. Solvent rinses with methanol or acetonitrile, followed by air drying, may be substituted for the oven heating. After cleaning, glassware should be stored in a clean environment to prevent any accumulation of dust or other contaminants.556-7 4.5 Matrix interferences may be caused by contaminants that are coextracted from the sample. The extent of matrix interferences will vary considerably from source to source, depending upon the nature and diversity of the matrix being sampled. 4.6 An interferant that elutes just prior to the acetaldehyde (E) isomer peak on the primary column is typically observed in chlorinated or chloraminated waters. If this peak interferes with the integration of the acetaldehyde (E) isomer peak, then acetaldehyde should be quantitated using only the acetaldehyde (Z) isomer, or from the confirmation column data. 5. SAFETY 5.1 The toxicity or carcinogenicity of each reagent used in this method has not been precisely defined; however, each chemical compound should be treated as a potential health hazard. From this viewpoint, exposure to these chemicals must be reduced to the lowest possible level by whatever means available. The laboratory is responsible for maintaining a current awareness file of OSHA regulations regarding the safe handling of the chemicals specified in this method. A reference file of material safety data sheets should also be made available to all personnel involved in the chemical analysis. Additional references to laboratory safety are available.(5-8) 5.2 Formaldehyde and acetaldehyde have been tentatively classified as known or suspected human or mammalian carcinogens. Glyoxal and methyl glyoxal have been shown to be mutagenic in in-vitro tests.(2) 6. EQUIPMENT AND SUPPLIES (All specifications are suggested. Brand names and/or catalog numbers are included for illustration only.) 6.1 SAMPLE CONTAINERS -- Grab Sample Bottle (aqueous samples) -- 30 mL amber glass, screw cap bottles and caps equipped with Teflon-faced silicone septa. Screw caps should be polypropylene. Typical phenolic resin caps should be avoided due to the possibility of sample contamination from formaldehyde. Prior to use, wash bottles and septa according to Section 4.4. 6.2 VIALS -- 8 mL or 12 mL vials for the acid wash step (Sect. 11.1.10), and GC autosampler vials, both types must be glass with Teflon-lined polypropylene caps. 6.3 VOLUMETRIC FLASKS -- various sizes used for preparation of standards. 6.4 BALANCE -- Analytical, capable of accurately weighing to the nearest 0.0001 g. 6.5 WATER BATH or HEATING BLOCK -- Capable of maintaining 35 ± 2 oC6.6 GAS CHROMATOGRAPH -- Capillary Gas Chromatograph equipped with a split/splitless injector, or other injector suitable for trace analysis, and an electron capture detector. 6.6.1 Primary Column -- 30 m x 0.25mm J&W DB-5ms, 0.25 um film thickness (or equivalent). Note: The J&W DB-5 was not found to be equivalent for this application. The surrogate analyte is not resolved from octanal with the DB-5 column. 6.6.2 Confirmation Column -- 30 m x 0.25 mm Restek Rtx- 1701, 0.25 um film thickness (or equivalent) 7. REAGENTS AND STANDARDS 7.1 Reagent grade or better chemicals should be used in all tests. Unless otherwise indicated, it is intended that all reagents shall 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 REAGENT WATER --. Reagent water as free as possible from interferences and contamination is critical to the success of this method. See Section 4.1. 7.3 ACETONITRILE -- High purity, demonstrated to be free of analytes and interferences. 7.4 HEXANE -- High purity, demonstrated to be free of analytes and interferences: B&J Brand, GC2 grade or equivalent. 7.5 POTASSIUM HYDROGEN PHTHALATE (KHP) -- ACS Grade or better. 7.6 0-(2,3,5,6-PENTAFLUOROBENZYL)-HYDROXYLAMINE HYDROCHLORIDE (PFBHA) -- 98+%, Aldrich cat# 19,448-4. (Store in a desiccator - Do not refrigerate). 7.7 SULFURIC ACID -- ACS Grade or better. 7.8 COPPER SULFATE PENTAHYDRATE -- ACS Grade or better. 7.9 AMMONIUM CHLORIDE, NH4Cl or AMMONIUM SULFATE, (NH4)2SO4. 7.10 SOLUTIONS556-9 7.10.1 PFBHA REAGENT -- Prepare a fresh 15 mg/mL solution in reagent water daily. Prepare an amount appropriate to the number of samples to be derivatized. One mL of solution is added per sample. For example, if 14 sample vials are being extracted, prepare 15 mL of solution. For a 15 mL volume of solution, weigh 0.225 grams of PFBHA into a dry 40 mL vial, add 15 mL water and shake to dissolve. 7.10.2 0.2 N SULFURIC ACID -- Add 5 mL of concentrated sulfuric acid to 900 mL of reagent water. 7.11 STOCK STANDARD SOLUTIONS -- When a compound purity is assayed to be 96% or greater, the weight can be used without correction to calculate the concentration of the stock standard. 7.11.1 INTERNAL STANDARD (IS) -- 1,2-DIBROMOPROPANE, 98+% purity. An alternate compound may be used as the IS at the discretion of the analyst. If an alternate is selected, an appropriate concentration will need to be determined. 7.11.1.1 INTERNAL STANDARD STOCK SOLUTION, (10,000 ug/mL) -- Accurately weigh approximately 0.1 gram to the nearest 0.0001g, into a tared 10 mL volumetric flask containing hexane up to the neck. After determining weight difference, fill to mark with hexane. Stock solutions can be used for up to 6 months when stored at -10 oC. 7.11.1.2 INTERNAL STANDARD FORTIFIED EXTRACTION SOLVENT, 400 ug/L in hexane -- This is the solvent used to extract the derivatized samples. The internal standard is added to the solvent prior to performing the extraction. The volume of this solvent to be prepared should be determined by the sample workload. The following example illustrates preparation of 1 L of fortified solvent. If fewer samples are to be analyzed each month, prepare smaller batches of working solvent. Add 40 uL of internal standard stock solution directly to 1 L of hexane in a volumetric flask. Cap flask and invert three times to ensure thorough mixing. Transfer to l L storage bottle with Teflon lined cap. This solution can be used up to 4 weeks. As a check, run a sample of this working solvent on the GC before the first extraction of aqueous samples. Have enough working solvent available to extract all calibration and aqueous samples in each extraction set. Never use two different batches of working solvent for one set of extractions.556-10 7.11.2 SURROGATE (SUR) -- 2’,4’,5’ -TRIFLUOROACETOPHENONE This compound was found to be an appropriate surrogate analyte for these analyses. However, the chromatograms for this analysis are very crowded, and all possible matrix interferences cannot be anticipated. An alternate carbonyl compound may be selected as the surrogate analyte if matrix interferences or chromatographic problems are encountered. Any surrogate analyte selected must form an oxime derivative, because its purpose is to monitor the derivatization process. If an alternate surrogate is selected, its concentration may also be adjusted to meet the needs of the laboratory. 7.11.2.1 SURROGATE STOCK SOLUTION, 10,000 ug/mL -- Accurately weigh approximately 0.1 gram SUR to the nearest 0.0001g, into a 10 mL tared volumetric flask containing acetonitrile up to the neck. After determining weight difference, fill to mark with acetronitrile. Stock solutions can be used for up to 6 months when stored at -10 oC or less. 7.11.2.2 SURROGATE ADDITIVE SOLUTION, 20 ug/mL -- Dilute the surrogate stock solution to 20 ug/mL in acetonitrile. This solution can be used up to 3 months when stored at 4 oC or less. 7.11.3 STOCK STANDARD SOLUTION (SSS) Prepare stock standard solutions for each analyte of interest at a concentration of 1 to 10 mg/mL in acetonitrile, or purchase SSSs or primary dilution standards (PDSs) from a reputable supplier. Method analytes may be obtained as neat materials or as ampulized solutions from commercial suppliers. The stock standard solutions should be stored at -10 oC or less and protected from light. Standards prepared in this manner were stable for at least 60 days. Standards may be used for longer periods of time if adequate records of stability are kept. Laboratories should use standard QC practices to determine when their standards need to be replaced. 7.11.3.1 For analytes which are solids in their pure form, prepare stock standard solutions by accurately weighing approximately 0.1 gram of pure material to the nearest 0.0001g in a 10 mL volumetric flask. Dilute to volume with acetonitrile. 7.11.3.2 Stock standard solutions for analytes which are liquid in their pure form at room temperature can be accurately prepared in the following manner.7.11.3.2.1 Place about 9.8 mL of acetonitrile into a 10- mL volumetric flask. Allow the flask to stand, unstoppered, for about 10 min. to allow solvent film to evaporate from the inner walls of the volumetric, and weigh to the nearest 0.0001 gram. 7.11.3.2.2 Use a 100-uL syringe and immediately add 100 uL of standard material to the flask by keeping the syringe needle just above the surface of the acetonitrile. Be sure the standard material falls dropwise directly into the acetonitrile without contacting the inner wall of the volumetric. 7.11.3.2.3 Reweigh, dilute to volume, stopper, then mix by inverting several times. Calculate the concentration in milligrams per milliliter from the net gain in weight. 7.11.4 PRIMARY DILUTION STANDARD (PDS) -- The PDS for this method should include all method analytes of interest to the analyst. The PDS is prepared by combining and diluting stock standard solutions with acetonitrile to a concentration of 100 ug/mL. Store at -10 oC or less and protect from light. Standards prepared in this manner were stable for at least 60 days. Standards may be used for longer periods of time if adequate records of stability are kept. Laboratories should use standard QC practices to determine when their standards need to be replaced. This primary dilution standard is used to prepare calibration spiking solutions, which are prepared at 5 concentration levels for each analyte, and are used to spike reagent water to prepare the aqueous calibration standards. 7.11.5 CALIBRATION SPIKING SOLUTIONS -- Five calibration spiking solutions are prepared, each at a different concentration, and are used to spike reagent water to prepare the calibration standards. The calibration spiking solutions are prepared from the PDS. Store the calibration spiking solutions at -10 oC or less and protect from light. Solutions prepared in this manner were stable for at least 60 days. Solutions may be used for longer periods of time if adequate records of stability are kept. Laboratories should use standard QC practices to determine when solutions need to be replaced. An example of how the calibration spiking solutions are prepared is given in the following table. Modifications of this preparation scheme may be made to meet the needs of the laboratory.556-12 PREPARATION OF CALIBRATION SPIKING SOLUTIONS Cal. Level PDS Conc., ug/mL Vol. PDS Std., uL Final Vol., Cal Spike Sol’n, mLs Final Conc., Cal Spike Sol’n, ug/mL 1 100 250 5 5 2 100 500 5 10 3 100 1000 5 20 4 100 1500 5 30 5 100 2000 5 40 7.11.6 PROCEDURAL CALIBRATION STANDARDS -- A designated amount of each calibration spiking solution is spiked into five separate 20 mL aliquots of reagent water in a 30 mL sample container, to produce aqueous calibration standards. The reagent water used to make the calibration standards should contain the preservation reagents described in Section 8.1.2 (ammonium chloride or ammonium sulfate at 500 mg/L and copper sulfate pentahydrate at 500 mg/L). Aqueous calibration standards are processed and analyzed according to the procedures in Section 11. Resulting data are used to generate a calibration curve. An example of the preparation of aqueous calibration standards is given below. The lowest concentration calibration standard should be at or near (within 25% of) the MRL. Modifications of this preparation scheme may be made to meet the needs of the laboratory. Preparing aqueous calibration standards using varying volumes of one calibration spiking solution is an acceptable alternative to the example below. PREPARATION OF PROCEDURAL (AQUEOUS) CALIBRATION STANDARDS Cal. Level Cal. Spike Sol’n Conc., ug/mL Vol. Cal. Spike Sol’n., uL Final Vol., Cal Std mL Final Conc., Cal Std ug/L 1 5 20 20 5 2 10 20 20 10 3 20 20 20 20 4 30 20 20 30 5 40 20 20 40556-13 8. SAMPLE COLLECTION, PRESERVATION, AND STORAGE 8.1 SAMPLE VIAL PREPARATION 8.1.1 Grab samples must be collected in accordance with conventional sampling practices (6) using amber glass 30 mL containers with PTFE-lined screwcaps, or caps with PTFE-faced silicon septa. 8.1.2 Prior to shipment to the field, 15 mg of copper sulfate pentahydrate must be added to each bottle. This material acts as a biocide to inhibit bacteriological decay of method analytes. If samples to be collected contain free chlorine, then 15mg of ammonium chloride or ammonium sulfate must also be added to the bottle prior to sample collection. The ammonium compound will react with the free chlorine to form monochloramine, and retard the formation of additional carbonyl compounds. Add these materials as dry solids to the sample bottle. The stability of these materials in concentrated aqueous solution has not been verified. NOTE: Aldehydes have been demonstrated to be extremely susceptible to microbiological decay. The use of other chlorine reducing agents such as sodium thiosulfate or ascorbic acid, has also been shown to produce invalid data. Proper sample collection and preservation is important to obtaining valid data. The data in Section17, Table 6 illustrates the importance of proper sample preservation. 8.2 SAMPLE COLLECTION 8.2.1 Fill sample bottles to just overflowing but take care not to flush out the sample preservation reagents. The capped sample should be head-space free. 8.2.2 When sampling from a water tap, remove the aerator so that no air bubbles will be trapped in the sample. Open the tap, and allow the system to flush until the water temperature has stabilized (usually about 3-5 min). Collect samples from the flowing system. 8.2.3 When sampling from an open body of water, fill a 1 quart wide-mouth bottle or 1L beaker with sample from a representative area, and carefully fill sample bottles from the container. 8.2.4 After collecting the sample, cap carefully to avoid spillage, and agitate by hand for 1 min.8.3 SAMPLE STORAGE/HOLDING TIMES 8.3.1 Samples must be iced or refrigerated at 4 oC and maintained at these conditions away from light until extraction. Samples must be extracted within 7 days of sampling. However, since aldehydes are subject to decay in stored samples, all samples should be derivatized and extracted as soon as possible. NOTE: A white or blue precipitate is likely to occur. This is normal and does not indicate any problem with sample collection or storage. 8.3.2 Extracts (Sect. 11.1.11) must be stored at 4 oC or less away from light in glass vials with Teflon-lined caps. Extracts must be analyzed within 14 days of extraction. 8.4 FIELD REAGENT BLANKS -- Processing of a field reagent blank (FRB) is required along with each sample set. A sample set is composed of the samples collected from the same general sampling site at approximately the same time. Field reagent blanks are prepared at the laboratory before sample vials are sent to the field. At the laboratory, fill a sample container with reagent water (Sect. 7.2), add sample preservatives as described in Section 8.1.2, seal and ship to the sampling site along with the empty sample containers. FRBs should be confirmed to be free (less than 1/2 the MRL) of all method analytes prior to shipping them to the field. Return the FRB to the laboratory with filled sample bottles. DO NOT OPEN THE FRB AT THE SAMPLING SITE. If any of the analytes are detected at concentrations equal to or greater than 1/2 the MRL , then all data for the problem analyte(s) should be considered invalid for all samples in the shipping batch.

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