1.1 Overview
This quality assurance manual describes the quality control and quality assurance (QA/QC) procedures used by the U.S. Geological Survey (USGS) Wisconsin District Mercury Laboratory (WDML). These procedures include sample preparation, collection, and handling, as well as laboratory reagents, instrumentation, analysis, and documentation. Strict adherence to quality control and quality assurance procedures described in this manual is of critical importance to provide high-quality research and data.
1.2 Purpose
The purpose of this quality assurance manual is to provide a means to ensure high quality samples and data includingprecision, accuracy, and completeness from sample receipt through data entry and verification. This document is designed to inform WRD customers project chiefs, field and laboratory personnel, and others associated with the laboratory with regard to the QA/QC requirements.
1.3 Scope
The USGS Wisconsin District Mercury Laboratory was established in 1995 at the Wisconsin District Office of the Water Resources Division. The purpose of the laboratory is to provide assistance and support for USGS district and national projects in the collection and analysis of low level mercury samples using ultra trace level clean collection and analytical methods. The laboratory currently performs analyses for low-level total mercury, methyl mercury, gaseous mercury, and reactive mercury concentrations in filtered and unfiltered water samples. In addition, the laboratory performs analysis of total and methyl mercury in biota and sediment, total and dissolved organic carbon in water and total carbon in sediment. The procedures described in this manual include all activities currently being used in the Wisconsin District laboratory. Appendix B contains the standard operating procedures for the analytical methods used in the laboratory.
Use of trade names or commercial products is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.
1.4 Organization and Key Personnel
Dr. David P. Krabbenhoft, Research Hydrologist, is Project Chief of the USGS Mercury Laboratory. The Manager and Quality Assurance Officer for the laboratory is Mark L. Olson. Responsibilities of Mr. Olson include maintaining direction and coordination of the laboratory, assuring that QA/QC objectives are met, maintaining the quality assurance manual reviewing, daily activities, and approving changes in methodology. John F. DeWild is the Chemical Hygiene Officer, and his responsibilities include maintaining the chemical hygiene plan (Appendix C), chemical inventory, equipment, ordering, and coordinating daily laboratory efforts including assignment and review of technical tasks.
1.5 Facilities
The laboratory is located in the Wisconsin District office of the U.S. Geological Survey, Water Resources Division, in Middleton, Wisconsin. The laboratory currently consists of a main room where mercury analyses are conducted, a carbon lab, an acid wash prep room, and a storage room.
The main room where mercury analyses are done contains safeguards to minimize potential mercury contamination including: HEPA filters, mercury-free Class 100 laminar flow hoods, gold-coated cloth filters at the intakes of the laminar flow hoods, a vinyl curtain at the main doorway, and tacky mats. Water purification systems provide ultra-pure (> 18.2 Mohm) reagent grade water.
Sample integrity must be maintained throughout the sampling process, from field collection to delivery of samples to the laboratory. Contamination can occur from dust and particle matter, rain, or human breath, as well as from contact with contaminated surfaces such as skin and hair. The potential for sample contamination can be minimized through strict compliance with certain guidelines. Laboratory staff will instruct and assist field sampling personnel in the use of ultra clean trace metal collection techniques. These techniques include, for example, the use of Tyvek suits, gloves, and plastic bags to prevent sample contact with unclean surfaces. In hostile sampling conditions, sample preservation is delayed to minimize exposure of the sample. Prior to shipment to the laboratory for analysis, samples are stored in a cool place away from sunlight to limit the effect of photo degradation, biological activity and assure sample integrity.
2.1 Containers
All sample containers are Teflon and each is engraved with a unique identification number. Unique identification numbers and the size of the container are recorded in a laboratory notebook.
Following acid-bath cleaning and drying, two bar code labels are generated for each container, consisting of the unique identification number and a modifier representing the date that the container was removed from the acid bath. One bar code label is placed toward the bottom of the container and the container is sealed in a zip-type bag. The bagged container is sealed in another zip-type bag and the second label is placed in a plastic bag that is stored in the outer container bag. The outer bag also is labeled with the unique identification number and modifier using a permanent marker. Samples are tracked using the bar code to identify shipping and receiving dates, pre-analysis treatment (i.e., preservation, ultraviolet treatment, and bromine monochloride digestion), analysis date, and QA/QC data, data entry and verification.
2.2 Sampling
Sample collection is done by personnel from, or in collaboration with, the U.S. Geological Survey, Wisconsin District Mercury Laboratory. The laboratory provides sampling equipment to support sample collection by said personnel upon receipt of a sampling equipment request form (See Appendix A for all forms). This request form must be received two weeks prior to the necessary shipping date. Laboratory personnel track all equipment from the point of shipping until receipt by the sampling personnel. Each container of equipment includes a copy of the sampling equipment request form with the items highlighted and a shipping inventory form. A set of request for analysis forms also is provided in the container of equipment. Field personnel will verify that all sampling equipment received from the laboratory is intact with no indications of shipping damage. Upon collection of a sample, the field sampling personnel places the second bar code (in bag stored in outer container bag) on a request for analysis form in the space adjacent to the type of mercury collection. If the sample was acidified in the field, the unique identification number of the bottle used for acid preservation must be recorded on the request for analysis form.
2.3 Sample Shipping
Sampling personnel will verify that all items listed on the shipping inventory form are included in the container of samples to be shipped and will note any information regarding known problems that may affect sample integrity. The completed shipping inventory form and request for analysis forms are then signed and dated by the person responsible for shipping the container of samples to the laboratory. The forms are sealed in a zip-type bag and included in the container of samples. The sampling personnel must inform laboratory personnel that a container was shipped and the method of shipment; all containers are shipped next day priority unless other arrangements are made with laboratory personnel. The sampling personnel are responsible for tracking the container until receipt by laboratory personnel.
2.4 Preservatives
Water samples for total mercury analysis are acidified within 24 hours to a concentration of 1% using a solution of high purity HCl that has been found to be low in mercury. Upon request, laboratory personnel provide acid preservative solutions to sampling personnel or acidify samples upon receipt at the laboratory. Acid preservative solutions are analyzed for total mercury after use to assess the contribution of the preservative volume to the concentration of total mercury in the sample. Samples collected for methyl mercury are frozen upon collection and shipped on dry ice. Carbon samples are shipped and stored at 4°C.
2.5 Field Duplicates and Blanks
Field duplicate and blank samples are collected in order to estimate sample site variability and potential for sample contamination at a sampling frequency determined by the field personnel coordinator or project chief. The coordinator must identify the need for additional equipment to collect duplicates and blanks on the sampling equipment request form. Upon request, laboratory personnel will provide the necessary equipment, including field blank water, to the sampling personnel. The field blank water will be of high quality (> 18 MW); sufficient quantity will be provided to allow for the rinsing of equipment and collection of a sample. The remaining field blank water is returned for analysis of the parameters desired.
3.1 Sample Receipt and Log-in
Immediately upon receipt at the laboratory, sample containers are checked against the shipping inventory form and request for analysis forms. The forms are dated and signed by the person responsible for log-in of the samples. A log-in number is assigned to each sample container that relates the bar code to the project and site, date, time, and depth of collection. This sample information is then entered into the database. Used or dirty sampling equipment is rinsed with tap water in the prep room and placed in bins to await cleaning. Samples requiring refrigeration or freezing upon receipt are handled appropriately and in a timely fashion to preserve their integrity.
3.2 Potential Problems
The laboratory personnel conducting sample log-in will note on the shipping inventory form any additional conditions that may affect sample integrity or identification including leaky bags or sample containers, and discrepancies between the sample label and request for analysis form. Laboratory personnel will notify the project chief of significant problems within one week of sample receipt.
3.3 Sample Preparation
Laboratory personnel will record information on sample preparation (e.g., UV treatment as appropriate, bromination, neutralization) on the mercury laboratory sample tracking form with the initials and dates of preparation.
3.4 Holding Times for Sample-Preparation and Analysis
Laboratory personnel will store a sample prior to preparation and analysis such that the elapsed time between collection and analysis results in no structural, physical, chemical, biological, or functional change. If samples are properly preserved and stored, carbon holding times are 14 days, total and methyl mercury are six months, reactive and dissolved gaseous mercury should be analyzed within 7 days.
4.1 Reagent Specification
Reagent specifications are described in the standard operating procedures for each method (Appendix B). Reagents are labeled with the receipt and opening dates; reagent preparations are labeled with the contents, concentration, preparation date, and initialed. Light and temperature sensitive reagents and preparations are stored in the refrigerator in black Teflon bottles or dark zip-type bags. New reagent preparations are analyzed for total mercury content. Material Safety Data Sheets (MSDS) for all purchased reagents are stored in a notebook in the laboratory, and laboratory personnel are instructed as to the content and location of the MSDS. Laboratory personnel use exhaust hoods, protective clothing, gloves, and glasses when working with reagents that are hazardous.
4.2 Container Specification
Teflon containers are used for sample collection. Before each use, containers are soaked for a minimum of 24 hours in a 4 N Hydrochloric acid (HCl) bath at 65 degrees Centigrade. New Teflon containers are soaked for at least 48 hours. Containers are then immersed in a fresh deionized water bath, rinsed a minimum of three times and filled 25% full with a 1% trace pure HCl solution. Containers are then dried in a Class 100 laminar flow hood. A minimum of 5 percent of all containers from each bath are filled with reagent water (18.2 MW) and analyzed for total mercury concentration within 2 weeks to assess potential contamination from the cleaning procedure. The increase in total mercury concentration must be less than 0.1 ng/liter or all containers and equipment from that bath are cleaned again and rechecked. Acid baths are analyzed monthly for total mercury concentration.
4.3 Laboratory Services
Laboratory personnel maintain a logbook to record the status and maintenance of critical laboratory services. The water-purification systems provide reagent water that is MW resistance from a primary reverse-osmosis water-polishing system. Laboratory personnel record water system parameters daily during use. To meet criteria, the output must be > 18.2 MW and the pressure drop across the RO filters must be less than 5 psi or laboratory personnel must replace the appropriate filter cartridges. Compressed gases include argon (grade 5) and nitrogen (grade 5 and reagent grade). The quantity of gas and the regulator pressure are recorded daily; installation of new tanks also is recorded. Laboratory personnel monitor the temperature of all freezers and refrigerators daily during laboratory operation. Laboratory personnel will verify that temperatures are 4oC + 2o for refrigerators and less than 10o C for freezers or temperature adjustments must be made and recorded. Exhaust hoods are used where hazardous or toxic fumes are generated. Exhaust hoods are rated at > 100 linear feet per minute air flow and checked quarterly. If velocities are less than required rating, use of the hood is suspended and a service technician is called to correct the problem.
4.4 Laboratory Waste Disposal
Laboratory personnel will make every effort to make efficient use of all laboratory chemicals and thereby minimize generation of hazardous waste. Mercury waste from standards and diluted standard preparations is properly disposed of in coordination with the University of Wisconsin - Madison Water Chemistry Department. Other chemicals constituting hazardous waste also are disposed of in this manner. Non-hazardous chemical wastes are neutralized prior to disposal down the sanitary sewer. Environmental samples that do not exceed criteria for total mercury concentration are disposed of in the sanitary sewer.
5.1 Analytical Balances
Analytical balances in the laboratory are manufactured by OHAUS. The scale used for weighing reagents and calibration standards is capable of measuring to the nearest 0.01 mg, the scales used for weighing of samples is capable of measuring to the nearest 0.1g.
5.2 Spectrophotometers
Mercury analyses utilize Cold Vapor Atomic Fluorescence Spectrometers (CVAFS) model 2500 manufactured by Tekran (Toronto, Ontario) equipped with mass flow controllers. These are connected to Hewlett Packard HP3395 Integrators.
5.3 Carbon Analyzers
Carbon analyses utilize an OI Analytical Total Organic Carbon Analyzer model 1010 equipped with a OI model 1051 autosampler.
5.4 Hot plates, furnaces, and ovens
Corning hot plates, Thermolyne muffle furnaces, and a Labline oven.
5.5 Pipettes
Wheaton and VWR Scientific pipettes used in analyses and preparation range from 5 µl to 10 ml.
Calibration and maintenance of equipment is performed on a regular basis to assure proper functioning of equipment and minimize time lost due to interruption of laboratory functions. An "out of calibration" sign is placed on any equipment that is does not meet quality control objectives. Instruments requiring repair or replacement are not used for data generating purposes and are posted with an "out of service" sign until the situation is remedied. Preventive maintenance functions for major equipment are described in detail in the manuals of each individual instrument. Maintenance and calibration information is recorded in a logbook at the time the procedures are performed. Calibration and maintenance of equipment is done as specified in the analytical method or as indicated below:
6.1 Analytical Balances
Analytical balances are checked monthly using Class S weights and are cleaned and certified annually by a contract service technician. Balances are checked daily for zero point adjustment and cleaned after each use. Balances are mounted on surfaces that minimize vibration and are located away from drafts.
6.2 Cold Vapor Atomic Fluorescence Spectrometers
Spectrophotometers are calibrated before each use using the external standard technique (refer to Section 7). Lamps are replaced as required based on performance and according to manufacture guidelines.
6.3 Total Organic Carbon Analyzer
The carbon analyzer is calibrated before use using the external standard technique (refer to Section 7).
6.4 Hot Plates, Furnaces, and Ovens
The temperature calibration of hot plates, furnaces and ovens is checked quarterly with a NIST certified non-mercury thermometer.
6.5 Pipettes
Pipettes are calibrated monthly using three calibration points.
7.1 Preparation of Standards
Primary Standards for total mercury are obtained commercially and are standardized and certified against a NIST standard reference material. Standards for methyl mercury and carbon are prepared from dry chemicals according to the specific method. Lot number and opening date are recorded in the logbook. Diluted standard solutions are checked against the previous sets of standards before they are used to standardize the instrument. These solutions are clearly marked as to concentration, date of preparation, and preparer's initials. Standards are kept a period not to exceed that recommended by the manufacturer or the method.
7.2 Frequency of Standardization
Standardization is performed at least at the beginning of a daily sample run. For all analyses, a standard curve is used to calculate sample concentrations measured from an instrument response. The curve is generated by measuring instrument responses for a series of standard solutions of the analyte. Sample concentrations are then calculated by interpolating between the standard points.
7.3 Number of Standards
A set of at least three standards that bracket the expected sample concentrations is used for standardization.
7.4 Verification of Standardization
Instrument responses used to generate the standard curve must be linear according to criteria established for the specific method or a second series of standard solutions are analyzed prior to analysis of any samples.
7.5 Reference Material Check
An NIST certified reference material is prepared and analyzed for total mercury at the beginning of a days run, after approximately every 10 samples and at the end of the run. No reference materials are currently available for methyl mercury. Carbon standards are checked by an independent standard prepared from a stock of a different lot number than the stock used for standardization.
Standard Operating Procedures for all methods are described in detail in Appendix B. Precision and accuracy objectives vary with the parameter being determined.
8.1 Precision - Duplicates
The precision of an analytical procedure is determined by performing replicate analysis of a sample and must meet the criteria established for the specific method. The indexes of precision used are relative percent difference (RPD) and relative standard deviation (RSD):
RPD (%) = ((|X1-X2|)/mean) x 100
RSD (%) = (standard deviation/mean) x 100where X1 and X2 are the measured values for the first and second replicates, respectively. The Limit of Detection (LOD) is the concentration that is three standard deviations of multiple blank analysis (IUPAC definition for a 99% confidence level). Below this concentration, the analyte is considered to be undetectable. The region from three to five times the standard deviation of the blanks is the region of detection but not quantification. A concentration greater that five times the standard deviation of the blanks is the region of quantification. The RPD and RSD are applicable only in the region of quantification. If the RPD or RSD exceeds 10 percent for total mercury the sample must be reanalyzed.
8.2 Accuracy - Spikes
Sample accuracy is determined by adding a known amount of the analyte (spike) to the sample and measuring the change in concentration. The percent recovery is used as the index for measuring accuracy and is calculated as follows:
Percent Recovery = ((C2-C1)/C2) x 100
Where C2 is the spiked sample concentration and C1 is the sample concentration. Percent recoveries must meet criteria established for the specific method or a second spiked sample must be analyzed. If the second spike does not meet criteria then all sample data for that run are suspect and need to be reanalyzed or a flag is assigned to draw the project chiefs attention to that data.
8.3 Blanks
Method blanks will be analyzed to verify that the analytical system is free of contamination and sample carryover. The mean of the instrument responses from the blanks is used as the zero value in the calibration curve and in the calculation of the LOD. The LOD/volume of sample in liters, as calculated from the first three blanks, must be less than the expected sample concentration.
8.4 Completeness
To achieve the quality control objectives for completeness, every effort is made to avoid sample loss through accident or inadvertence. Collection of sufficient sample generally allows reanalysis in the event of partial sample loss involving a sample aliquot. The laboratory analyst determines the completeness of each set of samples by dividing the total number of analyses completed by the number that should have been performed on that set. The criterion for completeness is 95 percent for all parameters.
9.1 Logbooks
The laboratory analyst enters the bench-generated data from sample analyses in a bound logbook using permanent ink and initials and dates each page of entries. Laboratory personnel will maintain the logbook in such as way as to preclude its discreditation, and will make any corrections by crossing out the original entry with a single line and noting the corrected entry. The data recorded will consist of the instrument response, standard, spike, and instrument blank concentrations, sample identification information, and any other pertinent information. Analyte concentrations of new reagents and standard solutions are recorded in a separate logbook as are maintenance of equipment and facilities. The Quality Assurance Officer is responsible for the security of all logbooks.
9.2 Quality Control Checks per Analytical Run
The laboratory analyst performs quality control checks immediately after calibrating the instrument and periodically throughout the day. Quality control checks include analysis of a Quality Control Sample (QCS) at the beginning and end of each daily run and approximately every ten samples to monitor instrument changes in sensitivity. A QCS is a sample of known concentration with established warning and control limits specific to each parameter. If the measurement of a QCS is outside the warning limits, but within the control limits, a second QCS is analyzed. If the second control samples is within the warning limits, the analysis is assumed to be in a state of statistical control, and all routine sample data since the last acceptable QCS measurement are accepted, and routine sample analysis are continued. If the second QCS is outside the warning limits, it is assumed the analysis is no longer in a state of statistical control. All routine data analyzed since the last acceptable QCS measurement are suspect. Routine analysis are suspended until corrective action is taken. After corrective action, statistical control must be reestablished and demonstrated before analysis continue. The reestablishment of control is demonstrated by the result of three consecutive QCS measurements that are in control. Once statistical control has been demonstrated, all routine samples since the last acceptable QCS measurement and reanalyzed.
9.3 Development of Control Charts
Precision control charts will be kept for bath blanks and relative percent difference or relative standard deviation between duplicate sample analysis. Accuracy control charts will be kept for quality control check sample analysis and percent recovery of spike analysis. Control charts should be updated by the analyst as soon as the sample measurement is completed. If a measurement is our of control corrective actions must be taken.
10.1 External Performance-audit Samples
The laboratory participates in interlaboratory standard sample analysis programs including USGS Standard Reference Sample (SRS) audit and the Florida Department of Environmental Protection Mercury Intercomparison Program (FMIP) on a biannual basis. Results of the performance-audit samples are compared with results from other participating laboratories. If review of the data indicates that the analyses do not meet criteria, then sample analysis must immediately cease and the methods and instrumentation investigated to determine the source of the problem. The U.S. Geological Survey, Office of Water Quality, Branch of Quality Systems and the National Water Quality Laboratory, Methods Research and Development Group also perform technical reviews of the Mercury Laboratory at intervals not to exceed three years. Corrective actions required as a result of such technical reviews will be implemented by the Quality Assurance Officer.
10.2 Training
In-house training of new employees is the responsibility of the laboratory manager or designee. New employees will be trained in the content of the Quality Assurance Manual, Chemical Safety Hygiene Plan, standard operating procedures for the analytical method(s), and any other pertinent manuals. The laboratory manager will verify that an trainee is qualified for an analytical method after a set of quality control samples is analyzed within established criteria by the trainee.
11.1 Data Entry
Laboratory personnel manually enter the results for sampleanalyses into the computer spreadsheet using the logbook and then check the preliminary spreadsheet entries for correctness and completeness. The laboratory is currently developing a laboratory information management system (LIMS) and anticipates full operation of the system during 1998.
11.2 Data Validation
After initial review of the computer entries, laboratory personnel verify the spreadsheet using the original instrument response printout and make any necessary corrections. The Quality Assurance Officer or designee will then review the verified spreadsheet.
11.3 Data Reporting
A final written data report is generated from the verified and reviewed analytical data and is provided to the project chiefs. The data report also will include a statement of the uncertainty of the data. Laboratory personnel will resolve any significant problems in the analyses prior to the release of the data report; otherwise the data must be qualified and the reason for the qualification clearly stated. Data reports are forwarded to the project chief within 10 working days after final review by the Quality Assurance Officer.
11.4 Data Archiving
All laboratory logbooks, instrument response printouts, completed analytical services request forms, and sample tracking forms are retained and easily located according to project, date, and method of analysis. The spreadsheets and laboratory database are maintained on PC's.
11.5 Computer and Database Security
All computer files for the laboratory are safeguarded by incremental backups performed daily (Monday through Friday) and total/complete backups performed monthly (first or last weekend of the month) by the data base owner. The database owner archives all backup tapes for two years in a temperature-controlled room with restricted access. All computer files are subject to restricted access for laboratory personnel only.
Quality assurance reports will be prepared by the Quality Assurance Officer and submitted to the project chiefs to ensure that QA/QC objectives are met. The reports will include (1) an assessment of precision, accuracy, and completeness for each project, (2) significant quality control problems and the status of corrective action, (3) the results of QA performance audits,and (4) any changes to the QA Manual.
12.1 Corrective Action
Corrective procedures will be taken as necessary to define and resolve any measurement problems or questionable findings. Significant nonconformance will be reported to the Quality Assurance Officer, who will initiate immediate corrective action. The laboratory analyst will resolve problems with errors and out-of-control situations in the method as possible. When problems cannot be resolved at the bench level, the analyst will promptly notify the Quality Assurance Officer so that the appropriate corrective action may be taken. When QA objectives are not met, the analysis is no longer in a state of control. All routine data analyzed since the last acceptable QA sample measurement are suspect. Routine sample analyses are suspended until corrective action is taken. After corrective action, statistical control must be reestablished and demonstrated before sample analyses continue.
12.2 Problems and Changes
Laboratory personnel will verbally report any significant problems to the laboratory manager with recommendations for corrective action. Laboratory personnel will submit any proposed changes in QA or analytical procedures to the Quality Assurance Officer for review and approval prior to implementation. The analyst will record all approved modifications to methods in the appropriate
modification, and the Quality Assurance Officer who witnessed and will verify the modification. All complaints are to be brought up with either the Quality Assurance Officer or the Laboratory Manager, if warranted, changes will be made, if not, the suggestion will be filed in the appropriate location.
Forms used by the Wisconsin District Mercury Laboratory
This appendix presents the equipment and data tracking forms used by the Wisconsin District Mercury Laboratory.
WDML F01 Wisconsin District Mercury Laboratory Sampling Equipment Request Form.
WDML F02 Wisconsin District Mercury Laboratory Shipping Inventory Form.
WDML F03 Wisconsin District Mercury Laboratory Request for Analysis Form.
WDML F04 Wisconsin District Mercury Laboratory Total Mercury Sample Tracking Form.
Standard Operating Procedure used by the Wisconsin District Mercury Laboratory
This appendix presents the standard operating procedures used for analysis by the Wisconsin District Mercury Laboratory.
WDML SOP001 Determination of Total Mercury in Water by Oxidation, Purge and Trap, and Cold Vapor Atomic Fluorescence Spectrometry.
WDML SOP002 Determination of Reactive Mercury in Water by Purge and Trap, and Cold Vapor Atomic Fluorescence Spectrometry.
WDML SOP003 Determination of Dissolved Gaseous Mercury in Water by Purge and Trap, and Cold Vapor Atomic Fluorescence Spectrometry.
WDML SOP004 Standard Operating Procedure for the Distillation of Water Samples for Subsequent Determination of Methyl mercury by Aqueous Phase Ethylation, Followed by Gas Chromatography Separation with Cold Vapor Atomic Fluorescence Detection.
WDML SOP005 Standard Operating Procedure for the Determination of Methyl mercury by Aqueous Phase Ethylation, Followed by Gas Chromatography Separation with Cold Vapor Atomic Fluorescence Detection.
WDML SOP006 Standard Operating Procedure for the Determination of Dissolved Organic Carbon in Water.
Chemical Hygiene Plan used by the Wisconsin District Mercury Laboratroy
This appendix presents the chemical hygiene plan used for by the Wisconsin District Mercury Laboratory.
Glossary
The definitions below are required to better understand the meaning of terms frequently used throughout the text. Definitions to terms are as cited in EPA 1631 and the Quality Assurance/Control Manual of the USGS National Water Quality Laboratory.
- Accuracy
- A measure of the degree of conformance of the values generated by a specific mathod or procedure with the true value. The concept of accuracy includes both bias and precision.
- Bias
- Systematic error that is manifested as a consistent positive or negative deviation from the known or true value. It differs from random error which shows no such deviation.
- Blank
- Solution that is free of the analyte(s) of interest. Such a solution would be used to develop specific types of blank samples.
- Certified Reference Material
- A reference material, for which one or more property values are certified by a technically valid procedure, accompanied by or traceable to a certificate or other documentation which is issued by a certifying body.
- Duplicate Analysis
- The analysis or measurement of the variable of interest performed as identically as possible on two subsequent subsamples of a sample.
- Filtered
- Pertains to the constituents in a representative water sample that pass through a 0.45 micrometer membrane filter.
- Intercomparison Study
- An exercise in which samples are prepared and split by a reference laboratory, then analyzed by one or more testing laboratories and the reference laboratory. The intercomparison, with a reputable laboratory as the reference laboratory, serves as the best test of the precision and accuracy of the analyses at natural environmental levels.
- Limit of Detection
- The minimum concentration of a substance that can be identified, measured, and reported with 99-percent confidence that the analyte concentration is greater than zero; determined from analysis of a sample in a given matrix containing analyte.
- Matrix Spike
- An aliquot of an environmental sample to which known quantities of the analyte(s) of interest is added in the laboratory. The matrix spike is analyzed exactly like a sample. The purpose is to quantify the bias and precision 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 matrix spike corrected for those background concentrations.
- Precision
- The degree of similarity among independent measurements of the same quantity, without reference to the known or true value.
- Quality Assurance (QA)
- Those planned or systematic actions necessary to provide adequate confidence that a product or service will satisfy given requirements for quality.
- Quality Control (QC)
- The operational techniques and activities used to fulfill requirements of quality.
- Quality Control Sample (QCS)
- A sample containing known concentration of a given analyte. The QCS is obtained from a source external to the laboratory, or is prepared from a source of standards different from the source of calibration standards. It is used to check laboratory performance with test materials prepared external to the usual preparation process.
- Reagent Water
- Water known not to contain the analyte(s) of interest at the detection limit of the method. For these methods, water is first purified by reverse osmosis then passes through a deionizing system. Output from the deionizing system must exceed 18 MW and pass through a 0.2 µM filter.
- Sample
- A representative part of a larger whole; a finite part or subset of a statistical population.
- Standard Operating Procedure (SOP)
- A written document which details the method of operation, analysis, or action whose techniques and procedures are thoroughly prescribed and which is accepted as the method for performing certain routine or repetitive tasks. It may be a standard method or one developed by the user.
- Stock Solution
- A solution containing an analyte that is prepared from a reference material traceable to EPA, NIST, or a source that will attest to the purity and authenticity of the reference material.
- Total
- Pertains to the constituent in a representative water sample. This term is used only when the analytical procedures ensures measurement of at least 95 percent of the constituent present in both the dissolved and suspended phases of the sample.