Water: Monitoring & Assessment
Quality Assurance, Quality Control, and Quality Assessment Measures
Quality assurance/quality control measures are those activities you undertake to demonstrate the accuracy (how close to the real result you are) and precision (how reproducible your results are) of your monitoring. Quality Assurance (QA) generally refers to a broad plan for maintaining quality in all aspects of a program. This plan should describe how you will undertake your monitoring effort: proper documentation of all your procedures, training of volunteers, study design, data management and analysis, and specific quality control measures. Quality Control (QC) consists of the steps you will take to determine the validity of specific sampling and analytical procedures. Quality assessment is your assessment of the overall precision and accuracy of your data, after you've run the analyses.Quality Control and Assessment Measures: Internal Checks Internal checks are performed by the project field volunteers, staff, and lab.
- Field Blanks. A trip blank (also known as a field blank) is deionized water which is treated as a sample. It is used to identify errors or contamination in sample collection and analysis.
- Negative and Positive Plates (for bacteria). A negative plate results when the buffered rinse water (the water used to rinse down the sides of the filter funnel during filtration) has been filtered the same way as a sample. This is different from a field blank in that it contains reagents used in the rinse water. There should be no bacteria growth on the filter after incubation. It is used to detect laboratory bacteria contamination of the sample. Positive plates result when water known to contain bacteria (such as wastewater treatment plant influent) is filtered the same way as a sample. There should be plenty of bacteria growth on the filter after incubation. It is used to detect procedural errors or the presence of contaminants in the laboratory analysis that might inhibit bacteria growth.
- Field Duplicates. A field duplicate is a duplicate river sample collected by the same team or by another sampler or team at the same place, at the same time. It is used to estimate sampling and laboratory analysis precision.
- Lab Replicates. A lab replicate is a sample that is split into subsamples at the lab. Each subsample is then analyzed and the results compared. They are used to test the precision of the laboratory measurements. For bacteria, they are used to obtain an optimal number of bacteria colonies on filters for counting purposes.
- Spike Samples. A known concentration of the indicator being measured is added to the sample. This should increase the concentration in the sample by a predictable amount. It is used to test the accuracy of the method.
- Calibration Blank. A calibration blank is deionized water processed like any of the samples and used to "zero" the instrument. It is the first "sample" analyzed and used to set the meter to zero. This is different from the field blank in that it is "sampled" in the lab. It is used to check the measuring instrument periodically for "drift" (the instrument should always read "0" when this blank is measured). It can also be compared to the field blank to pinpoint where contamination might have occurred.
- Calibration Standards. Calibration standards are used to calibrate a meter. They consist of one or more "standard concentrations" (made up in the lab to specified concentrations) of the indicator being measured, one of which is the calibration blank. Calibration standards can be used to calibrate the meter before running the test, or they can be used to convert the units read on the meter to the reporting units (for example, absorbance to milligrams per liter).
- External Field Duplicates. An external field duplicate is a duplicate river sample collected and processed by an independent (e.g., professional) sampler or team at the same place at the same time as regular river samples. It is used to estimate sampling and laboratory analysis precision.
- Split Samples. A split sample is a sample that is divided into two subsamples at the lab. One subsample is analyzed at the project lab and the other is analyzed at an independent lab. The results are compared.
- Outside Lab Analysis of Duplicate Samples. Either internal or external field duplicates can be analyzed at an independent lab. The results should be comparable with those obtained by the project lab.
- Knowns. The quality control lab sends samples for selected indicators, labeled with the concentrations, to the project lab for analysis prior to the first sample run. These samples are analyzed and the results compared with the known concentrations. Problems are reported to the quality control lab.
- Unknowns. The quality control lab sends samples to the project lab for analysis for selected indicators, prior to the first sample run. The concentrations of these samples are unknown to the project lab. These samples are analyzed and the results reported to the quality control lab. Discrepancies are reported to the project lab and a problemidentification and solving process follows.
- Consult with your technical committee and/or program advisor to help you determine quality assurance/quality control measures you will use to answer your questions and meet your data quality requirements
- Locate a quality control lab--an independent lab that can run external checks for you.
- Determine which quality checks you have the resources and capabilities to carry out. Your human and financial resources and expertise might limit the water quality indicators your can monitor.
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|a - using an oxygen-saturated sample
b - using subsamples of different sizes