Quality assurance can be defined as a system of activities whose purpose is to provide to the producer or user of a product the assurance that it meets defined standards of quality with a stated level of confidence. The Water-Quality Data & Analysis Section ensures the representativeness and quality of its data by submitting various blank, replicate, and spike samples along with its environmental samples to the laboratory for chemical analysis. Results of these quality assurance (QA) samples helps the Water-Quality Specialist to identify needed areas of improvement in our sample collection and processing protocols. Below are descriptions of each QA sample used to ensure the quality of USGS data.
The purpose of a filtration blank is to periodically check the cleanliness of the filtration equipment. Filtration blanks are prepared with certified inorganic blank water. To prepare a filtration blank sample, select a laboratory schedule (represents a list of constituents to be analyzed from a sample) typically used that employs filtration as part of its sample processing. Fill each shipping container with blank water that has passed through the filtration mechanism (membrane included) and add the required preservatives. Submit to the laboratory for regular analysis. The resulting analysis should represent a list of constituents with concentrations below the analytical detection level.
The purpose of a replicate sample is to assess the adequacy of the sampling process in obtaining a representative sample. Replicate samples are essentially two complete samples of stream water collected at the same time and point within the stream. Both samples are prepared, packed for shipping, and analyzed identically. Results are compared on the basis of Relative Percent Difference (RPD). If the RPD is greater than 5%, then it is determined that error exists in the sample collection technique and another sample is scheduled to be collected as a replacement.
The purpose of an equipment blank is to assess the cleanliness of sampling equipment and the churn. Equipment blanks are prepared with certified inorganic blank water. Select a laboratory schedule that includes a filtered sample. Start with either a DH-81 (hand-held sampler) or bag sampler and process one set of samples from the sampler. Pour blank water in the churn and process another set of samples. Finally, process all filtered samples from the churn and sampler. Equipment blanks are typically done once per year. Results should indicate that the collection and processing equipment is not compromising the quality of the environmental samples.
This is typically used for pesticide samples and other organic compound samples. The purpose of a matrix spike is to provide the means to determine precision and accuracy or recoveries for sampling conditions of specific sample sites. Field spikes can be used to determine the extent of matrix bias or interferences on analyte recovery and degradation during shipping. Three to four matrix spikes are performed on an annual basis.
U.S. Department of the Interior, U.S. Geological Survey
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Updated: August 27, 2003
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URL: http://missouri.usgs.gov/wtrqual/quality_assuraance.htm