Laboratory QA/QC procedures ensure correct and representative analyses through instrument calibration and the processing of control samples. The precision of laboratory findings refers to the reproducibility of results. In a laboratory QC program, a sample is analyzed independently (more than once) using the same methods and set of conditions. Precision is estimated by comparing the measurements. Accuracy refers to the degree of difference between observed values and known or actual values. The accuracy of a method may be determined by analyses of samples to which known amounts of a reference standard have been added.
Laboratory QA/QC procedures can be quite complex. Often, the analytical methods specify QA/QC requirements for calibration, interference checks (e.g. inductively coupled plasma mass spectrometry analyses), control samples, spiking (including the method of standard additions), blank contaminant level, and instrument tuning. Accuracy is normally determined through the analysis of blanks, standards, blank spikes, laboratory control samples, and spiked samples. Precision is determined through the comparison of duplicate results or duplicate spiked results for organic analysis. For more information on laboratory QA/QC, the POTW should contact their QA manager or approval authority.
The methods used by in‐house or contract laboratories to analyze industrial user samples must be methods that are EPA‐approved under 40 CFR part 136 and thus are acceptable to a court of law as the most reliable and accurate methods of analyzing water and wastewater. Although some field test kits are useful as indicators of current conditions (and, thus, may be used for process control considerations), they are not appropriate for sampling that is conducted to verify or determine compliance. If non EPA‐approved methods are used for analysis, the data will not be considered admissible as evidence of noncompliance. When choosing a contract laboratory, POTWs should consult their state’s accredited laboratory standards, when applicable, or obtain and review a copy of the laboratory’s QA/QC plan. For more information on laboratory considerations, the POTW should see the EPA’s Guidance Manual for POTW Pretreatment Program Development (USEPA 1983, 1985a).
The laboratories performing the sample analyses should use the following control measures as part of their QA/QC plan:
- Prep/reagent blank – A sample consisting of reagent(s), without the target analyte or sample matrix, introduced into the analytical procedure at the appropriate point and carried through all subsequent steps to determine the contribution of the reagents and/or the contribution of the analytical steps to any error in the observed value.
- Quality control sample – This is an uncontaminated sample matrix spiked with known amounts of analytes from a source independent from the calibration standards. Use this sample to establish intra‐laboratory or analyst‐specific precision and bias or to assess the performance of all or a portion of the measurements’ system.
- Matrix spike (MS) and matrix spike duplicate (MSD) – Two aliquots of an environmental sample to which known quantities of the analytes of interest and surrogates (if applicable) are added in the laboratory. The MS/MSD are prepared and analyzed exactly like a field sample. Their purpose is to quantify any additional bias and imprecision caused by the sample matrix. The background concentrations of the analytes in the sample matrix must be determined in a separate aliquot and the measured values in the MS/MSD corrected for background concentrations.
- Use an MS to verify accuracy of the analytical procedures.
- An MSD is a duplicate of an MS sample. It measures the precision of the analysis in terms of relative percent difference.
- Laboratory duplicate – This is a sample that is received by the laboratory and then subsequently divided into two or more portions. Each portion is separately and identically prepared and analyzed. The results from laboratory duplicate samples check the laboratory precision.
Sampling Location
The first step in preparing to sample is to verify that the sample location is appropriate. The industrial user permit must specify the sampling location for compliance sampling [40 CFR 403.8(f)(1)(iii)(B)(4)] (CFR, §52.21(aa)(2)(v) 1980). Sampling points should be located where there is no discharge other than the discharge from the industrial user process effluent being monitored. This sampling location must be representative of the actual discharge from the facility. A representative sampling location is where a sample can be obtained that is as close as possible to the same composition as that in the larger volume of wastewater being discharged and is typical of the discharge from the facility on a normal operating day. Convenience, accessibility, and safety are important considerations, but they are secondary to the representativeness of the sample. The most representative samples will be drawn from a wastewater depth where the flow is turbulent and well mixed and the chance of solids settling is minimal. The depth of sample collection should be 40–60% of the wastestreams depth. To avoid contamination, the inspector should take care to collect samples from the center of the flow with the opening of the sampling device or container facing upstream. Wide channels or paths of flow may require dye testing to determine the most representative sampling site. If dye testing is inconclusive, multiple samples may need to be collected by cross‐sectional sampling.
The inspector should use the sampling location specified in the industrial user’s permit. If the sample location specified in the permit is not adequate to collect a representative sample, the inspector should determine an alternative location. This determination should be based on the inspector’s knowledge of the plant itself, the production processes, and the outfalls. If there is a conflict between the sample location described in the permit and the location the inspector believes is most representative, samples should be collected at both sites. The reason for the conflict should be thoroughly documented for later resolution by the POTW. If necessary, the permit or other control mechanism must be amended to reflect the more representative sampling location(s). The CWA federal categorical standards apply at the end‐of‐process (or at the end of treatment, if treatment exists) unless the standard specifies a different location to collect the sample (e.g. in 40 CFR part 433, metal finishing, the sample location for cyanide is after the cyanide destruct system prior to dilution with other streams). If process effluent is mixed prior to treatment with unregulated wastestreams or dilution water, the CWA (USEPA 1985b) must be used (40 CFR 403.6). In this case, the POTW will designate, in the industrial user’s permit or other control mechanism, where the sampling point(s) will be (end‐of‐pipe, end‐of‐process, or both). The POTW must ensure that any change in the industrial user’s sampling point(s) will not allow the user to substitute dilution for adequate treatment to achieve compliance with applicable standards. Where a treated, regulated, process, wastestream is combined prior to treatment with wastewaters other than those generated by the regulated process, the industrial user may monitor either the segregated process wastestream or the combined wastestream for the purpose of determining compliance with applicable pretreatment standards.
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