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Terms
Definitions
Accuracy
Accuracy is a measurement of how close an analytical result is to the true or known value. Accuracy data are generated by performing laboratory control samples and/or matrix spikes and calculating the percent recovery for the analyte(s) of interest. NCL establishes its own acceptance limits for accuracy by calculating the standard deviation “in-house” spike recovery values. Upper and lower warning limits are set at + 2 standard deviations from the mean, and upper and lower control limits are set at + 3 standard deviations from the mean. Prior to NCL’s calculation and establishment of in-house limits for an analytical method, NCL may elect to establish temporary limits of 70-120% recovery as a guideline. In order to see trends in accuracy, percent recovery values can be graphed on Shewhart type charts with appropriate limits noted.
Action Level
The level of chemical which, if exceeded, triggers treatment or other requirements that a water system must follow.
Analytical Batch
An analytical batch is a group of 20 or fewer samples of the same matrix, which are prepared and analyzed together. If there are more than 20 samples to be analyzed, each group of 20 or fewer will be handled as a separate batch.
Calibration Standards
Calibration standards are used to calibrate instrumentation and to ensure adequate instrument performance. Calibration procedures are as stated in the individual methods.
CAM
California Assessment Manual.
Coliform
A group of related bacteria whose presence in drinking water may indicate contamination by disease-causing microorganisms.
Community Water System
A water system which supplies drinking water to 25 or more of the same people year-round in their residences.
Continuing Calibration Verification Standard (CCV)
A continuing calibration verification standard is a mid-level standard analyzed during and/or at the end of an analytical batch. A continuing calibration verifies that the instrument response has not drifted from the initial calibration response. The continuing calibration standard should be within the method specified ranges to be acceptable.
Cooler Blanks
Cooler blanks are small containers of water that are placed in the coolers with the samples. The temperature of the cooler blank is taken upon receipt at the laboratory to determine the sample temperature.
EPA
US Environmental Protection Agency.
External Reference Standards
External reference standards are used to validate the accuracy of working standards and second lot standards. These standards are usually obtained from Environmental Resource Associates (ERA), or equivalent. They are prepared twice yearly or more frequently if necessary.
Finished Water
Water that has been treated and is ready to be delivered to customers.
GLP
Good Laboratory Practice Standards of the EPA (40 CFR Part 160)
Initial Calibration Blank (ICB)
Primarily used for metals analysis. A blank that is analyzed following the calibration curve to verify that the instrument is “clean.”
Initial Calibration Verification (ICV)
Primarily used for metals analysis. A standard that is analyzed following the calibration curve to verify the calibration.
Internal Standard
Internal standards are added to the samples and their associated QC after extraction and prior to analysis. The same amount of internal standard is added to each standard, QC sample, and client sample. A calibration curve, that incorporates an internal standard, will “correct” the results for changes in instrument response, injection variability, etc.
Laboratory Control Sample (LCS)
Laboratory Control Samples are analyzed to verify the acceptable performance of an analytical method. Laboratory control samples are prepared by fortifying a matrix blank with a known amount of analytes(s) using a primary standard, or equivalent. LCS’s are spiked with all the analytes in each method, except for multi-peak analytes. These are prepared and analyzed at a frequency of one per batch. The results are reported as percent recovery, which is used as a measure of the accuracy of the analysis.
Matrix
The component or substrate (e.g., surface water, drinking water, soil) that contains the analyte(s) of interest.
Matrix Spike (MS)
A matrix spike is performed on a specific matrix to assess the accuracy of an analytical result. A matrix spike is prepared by fortifying a known amount of analyte(s) on an actual sample. Matrix spikes are fortified with all the analytes in the method, except for the multi-peak analytes. Matrix spikes are prepared using second lot standards, if available. Matrix spikes are analyzed for every 10 drinking water samples, and matrix spikes and matrix spike duplicates are analyzed for every 20 non-drinking water samples, or at least once per quarter, whichever is more frequent.
Maximum Contaminant Level (MCL)
The highest level of a contaminant that EPA allows in drinking water. MCLs ensure that drinking water does not pose either a short-term or long-term health risk. EPA sets MCLs at levels that are economically and technologically feasible. Some states set MCLs which are more strict than EPA's.
Method Blank (MB)
A method blank is a blank matrix (such as de-ionized water or Ottawa sand) to which all reagents are added in the same volumes or proportions as specified in the method processing. A method blank is analyzed with each analytical batch. Method blanks are analyzed to monitor the introduction of contaminants into the analytical process.
Method Detection Limit (MDL)
The method detection limit, as outlined in 40 CFR Part 136, Appendix B, is the minimum concentration of a substance that can be measured and reported with 99% confidence that the analyte concentration is greater than zero. This limit is determined by analysis of a sample in a given matrix containing the analytes. Seven replicates fortified at one to five times the expected detection limit are analyzed. The standard deviation times the Student’s t value is calculated to determine the MDL.
Precision
Precision is the degree to which analytical results are reproducible. Precision data are generated by analyzing laboratory control sample, matrix spike, or samples in duplicate. Duplicate samples are analyzed every batch or every 20 samples. Duplicate pairs are compared to one another by calculating the relative percent difference (RPD) for each pair. In general, the RPD must be within + 20% (the limit specified by the EPA in its Contract Lab Program). However, NCL establishes its own precision warning and control limits for each method. These limits are determined by calculating the standard deviation for the RPDs on in-house duplicate pairs. The warning limits for a method are set at + 2 standard deviations from the mean and the control limits are set at + 3 standard deviation from the mean. In order to clearly see trends in precision data, RPDs can be graphed on Shewhart type charts with the appropriate limits noted.
% Recovery
The % recovery is a measure of the accuracy of a given analysis. It is the spiked result minus the unspiked result divided by the amount spiked and is expressed as a percent.
RPD
The Relative Percent Difference is a measure of the precision of a given analysis. It is the difference between duplicate results divided by the arithmetic mean of the duplicates.
Raw Water
Water in its natural state, prior to any treatment for drinking.
Reporting Limit (RL)
The reporting limit is the lowest concentration routinely determined during laboratory operations.
STLC
Soluble Threshold Limit Concentration.
Second Source Standards
Second source (lot) standards are made using a stock standard from a different lot # or manufacturer than the primary stock standard that is used to prepare working standards. Second lot standards, when available, are analyzed to verify the response of the primary stock standard.
Secondary Drinking Water Standards
Non-enforceable federal guidelines regarding cosmetic effects (such as tooth or skin discoloration) or aesthetic effects (such as taste, odor, or color) of drinking water.
Source Water
Water in its natural state, prior to any treatment for drinking.
Surface Water
The water that water systems pump and treat from sources open to the atmosphere, such as rivers, lakes, and reservoirs.
Surrogate
Surrogates are compounds which are similar to the analytes of interest in chemical behavior, but which are not normally found in environmental samples. Surrogates are added to the samples and their associated QC prior to sample preparation to monitor the effect of the sample matrix on the accuracy of the analysis. Percent recoveries are calculated for each surrogate. Acceptance limits are established for surrogate percent recoveries.
TCLP
Toxicity Characteristic Leaching Procedure.
TTLC
Total Threshold Limit Concentration.
Travel Blanks
Travel blanks are prepared for the analysis of volatile compounds. Travel blanks are VOAs containing analyte-free water, which are prepared by the laboratory and sent with the sample. They accompany the coolers and sample containers from the laboratory to the site and back to the laboratory again. If any of the samples show levels of analytes above the reporting limit, the travel blanks are analyzed. Travel blanks are useful for determining contamination attributable to shipping and field handling procedures.
WET
Waste Extraction Test
Calibration Standards
Calibration standards are used to calibrate instrumentation and to ensure adequate instrument performance. Calibration procedures are as stated in the individual methods.