Glossary of Environmental Terms


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  • Acceptance Limits – Limits established by a reference sample provider which are used to determine if a laboratory has acceptable accuracy.
  • Accuracy – A measure of the degree of conformity of a value generated by a specific procedure to the assumed or accepted true value.
  • Acidity – The quantitative capacity of aqueous media to react with hydroxyl (OH­) ions.
  • Acid rain – Air pollution produced when acid chemicals are incorporated into rain, snow, fog or mist. The “acid” in acid rain comes from sulfur oxides and nitrogen oxides, products of burning coal and other fuels, and from certain industrial processes. The sulfur oxides and nitrogen oxides are related to two strong acids: sulfuric acid and nitric acid. When sulfur dioxide and nitrogen oxides are released from power plants and other sources, winds blow them far from their source. If the acid chemicals in the air are blown into areas where the weather is wet, the acids can fall to Earth in the rain, snow, fog or mist. In areas where the weather is dry, the acid chemicals may become incorporated into dusts or smokes. Acid rain can damage the environment, human health and property.
  • Acute Toxicity – Any direct lethal action of one or more constituents of water to a living test organism that is demonstrable within 96 hours or less. Note: Death resulting from indirect action, such as the depletion of dissolved oxygen, is excluded.
  • Alkalinity – The quantitative capacity of aqueous media to react with hydrogen (H+) ions.
  • Alternative fuels – Fuels that can replace ordinary gasoline. Alternative fuels may have particularly desirable energy efficiency and pollution reduction features. Alternative fuels include compressed natural gas, alcohols, liquefied petroleum gas (LPG) and electricity. The 1990 Clean Air Act encourages development and sale of alternative fuels.
  • Analyte – The chemical substance or physical property being measured in a sample.
  • Aroclor – Monsanto trademark for polychlorinated biphenyls, designated by the percent total chlorination of the mixture.
  • Aquifer – Underground reservoir-like layer of water. Most of the water in aquifers is contained in beds of sand, gravel or other material, and can be pumped to the surface.
  • Audit – There is a variety of audits which are performed in the laboratory. The following is a breakdown of the various types:
    • Data audit – A systematic check of the data produced by an analytical test. Usually performed by a peer analyst or supervisor before the data are released to the client. The items checked vary depending on the testing performed.
    • External audit – A procedure conducted by an outside agency (e.g., WDNR or EPA) and usually associated with a laboratory’s certification. An external (or on-site) audit is a rigorous multi-day process in which the outside agency examines every aspect of the labortory’s processes which fall under their authority. These would include: analyst training and experience, management structure, quality assurance plans and quality control implementation, analytical methodology, data and sample handling, and documentation.
    • Internal audit – A procedure which mirrors an external certification audit. An internal audit is generally performed by the laboratory’s QA officer and is a systematic check of all documentation, quality control, sample handling, and analytical procedures associated with a particular analytical method.
  • Baseline – The level of minimum response as determined by detector background signal level and noise.
  • Batch – A group of samples which are carried through the preparatory and analysis procedures together. Generally it consists of a set number of samples and the associated quality control.
  • Bias – A systematic error inherent in a method or caused by some artifact or idiosyncrasy of the measurement system. Temperature effects and extraction inefficiencies are examples of this first kind of bias. Blanks, contamination, mechanical losses and calibration errors are examples of the latter kinds. Bias may be both positive and negative, and several kinds can exist concurrently, so net bias is all that can be evaluated.
  • Blank – There is a variety of blanks used in the laboratory. The following is a breakdown of the various types:
    • Reagent Blank – A sample of a carrier agent (generally a liquid such as Milli-Q water, organic solvent or an acid solution) that is normally used to capture a material or analyte of interest. A reagent blank is subjected to all of the usual analytical processes.
    • Method Blank – A blank made up of the same matrix as the sample, and subjected to the entire analytical process (including digestion, filtration, column chromatography, etc.). Such a blank may also be referred to as a digested blank (metals), wash (continuous flow), or a filter blank.
    • Calibration Blank – Also referred to as “0” (zero) standard when analyzed with calibration standards. It is made up of Milli-Q water and acidified with the same acid matrix as the calibration standards. The calibration blank is a zero standard and is used to calibrate the instrument. NOTE: This definition applies exclusively to the Inorganic Chemistry Unit.
    • Continuing Calibration Blank – A calibration blank that is measured at regular intervals throughout an analytical run to verify that the instrument calibration (baseline or zero concentration) has not changed. NOTE: This definition applies exclusively to the Inorganic Chemistry Unit.
    • System Blank – A sample of a carrier agent (generally a liquid such as Milli-Q water, organic solvent or an acid solution) that is normally used to capture a material or analyte of interest. This sample is simply “injected” onto the instrument (not taken through the entire method) to monitor instrument performance.
    • Field Reagent Blank – Reagent water placed in a sample container in the laboratory and treated as a sample in all respects, including exposure to sampling site conditions, storage, preservation and all analytical procedures. The purpose of the FRB is to determine if method analytes or other interferences are present in the field environment.
    • Trip Blank – Similar to the FRB above but the sample is not exposed to the sampling site conditions.

  • Blind Standard – A standard or a sample from an external source with a validated concentration of analytes. The concentration of the analytes is known to the laboratory manager or Quality Assurance Officer.
  • Bioassay – A standardized procedure for the determination of the effects of an environmental variable or substance on living organisms.
  • Calibration – A process used to determine a relationship between the response of the analytical equipment and a known amount of an analyte.
  • Calibration Standard – A solution prepared from the dilution of high purity standards (generally NIST traceable). The calibration standards are used to calibrate the instrument response to a known analyte concentration.
  • Certified Laboratory – A laboratory which performs tests for hire in connection with a program which requires data from a certified laboratory. Also called an Accredited Laboratory.
  • CFCs (chlorofluorocarbons) – These chemicals and some related chemicals have been used in great quantities in industry, for refrigeration and air conditioning, and in consumer products. CFCs and their relatives, when released into the air, rise into the stratosphere, a layer of the atmosphere high above the Earth. In the stratosphere, CFCs and their relatives take part in chemical reactions which result in reduction of the stratospheric ozone layer. (The ozone layer protects the Earth’s surface from harmful effects of radiation from the sun.) The 1990 Clean Air Act includes provisions for reducing releases (emissions) and eliminating production and use of these ozone-destroying chemicals.
  • CFR Code of Federal Regulations – A compilation of regulations which is revised each time a regulation is promulgated. It is published every year in July.
  • Check Sample – A solution having a known value and obtained from inside the laboratory. It may be analyzed as a blind sample or as a check on the instrumentation.
  • Check Standard – A standard with a known concentration of analytes. It is analyzed at a regular interval (every 10-20 samples depending on the method) to verify that the instrument is staying within calibration. Also known as a “Check” or “Gain.”
  • Clean Air Act – The original Clean Air Act was passed in 1963, but our national air pollution control program is actually based on the 1970 version of the law. The 1990 Clean Air Act Amendments are the most far-reaching revisions of the 1970 law. In this summary, we refer to the 1990 amendments as the 1990 Clean Air Act.
  • Coefficient of Determination – Denoted by “r-squared” it is the square of the correlation coefficient. Many software programs use this in place of “r”.
  • Coefficient of Variation – Relative standard deviation expressed as a percent, i.e., the standard deviation of a set of data expressed as a percentage of the mean.
  • Co-eluting Compounds – Compounds exhibiting the same retention times.
  • Common Sense Detection Limit – A limit of detection which is set using a common sense approach. The statistical LOD is used as a floor, but values may be adjusted higher depending on matrix effects, “real” instrument response and analyst experience.
  • Comparability – A measure of the confidence with which one data set can be compared to another.
  • Composite Samples – A non-discrete sample composed of more than one specific sample collected at various sampling sites and/or at different points in time. Composite samples may give an “average” concentration or may be an alternative to analyzing numerous grab samples when finding a statistical average value. If the constituents or concentrations are subject to significant changes due to collection, transport, or storage, composites cannot be taken.
  • Compound – A substance composed of atoms or ions of two or more elements in chemical combination.
  • Confidence interval – That range of values, calculated from an estimate of the mean and the standard deviation, which is expected to include the population mean with a stated level of confidence.
  • Confidence limit, 95 percent – The limits of the range of analytical values within which a single analysis will be included 95 percent of the time (note that in practice the constant ‘1.96’ will be rounded to ‘2’). 
     95% CL = x plus or minus 1.96S where: CL = confidence level S = standard deviation x = mean
  • Confirmation – Verification of the presence of a component through the use of an analytical technique based on a different scientific principle from the original method. (e.g. second column, alternate wavelength or detector, etc.)
  • Congener – One of 209 PCB homologs consisting of the general formula: 
     C12HnCl(10-n)
  • Continuing Calibration Verification (CCV) – See Check Standard
  • Control Sample (1) – A second source standard, independently prepared, with which the instrument performance and calibration standards can be verified. Also see Laboratory Control Sample and Second Source Standard.
  • Control Sample (2) – A previously analyzed sample run with consecutive batches to determine relative recovery of parameters.
  • Corrective Action – Actions intended to correct a quality control failure.
  • Correlation Coefficient – Denoted by the letter “r”, it is a statistically derived measure of the quality of a curve fit to a set of data points. Used to decide whether a given calibration curve is valid.
  • Detection – The ability to determine the presence of a chemical compound.
  • Dissolved Constituents – The constituents in a water sample which will pass through a 0.45 µm membrane filter.
  • Duplicate Sample – A sample which has been homogenized, split, and sent through the entire preparatory and analysis procedure to determine reproducibility (precision) of results.
  • Electron Capture Detector (ECD) – A selective detector for electro-negative compounds such as halogenated hydrocarbons, organophosphates, organosulfur, organometallic, and nitro organic compounds. An ECD ionizes the carrier gas with a low-energy beta emitter such as tritium or nickel 63. A voltage is applied and the ion flow is measured as a standing current. When an electronegative compound enters the detector, some electrons are captured and a signal is generated.
  • Error – The difference between an observed or measured value and the best obtainable estimate of its true value.
  • Flame Ionization Detector (FID) – An FID consists of a small hydrogen flame at the end of a length of capillary tubing. When hydrocarbons are introduced into the flame the compounds are ionized. Ion pairs are collected by applying a potential between the collector and the capillary jet. The collected electrons flow through a biasing circuit and create a voltage drop across an input load resistor. The signal then goes to the electrometer for amplification.
  • Florisil – A highly adsorbent material made of hard granular or powdered magnesia-silica gel (magnesium silicate).
  • Gas Chromatography (GC) – An analytical tool used to separate components in a complex mixture. Components migrate differentially when a mixture is applied to a stationary phase and put in the path of a liquid or gas mobile phase. A carrier gas drives the mixture through a column where the stationary phase is either a liquid-coated solid support (packed column) or a column coated with a thin film of a sorbent material (capillary column). The solutes partition between the inert carrier gas and the stationary phase with differing efficiencies. Resolution takes place due to varying factors including: the retarding action of the stationary phase, temperature of the GC oven, pressure of the carrier gas, etc.
  • Ghosting – A gas chromatographic interference, showing as a peak, which appears at the same elution time (co-eluter) as a known component.
  • Grab Samples – A discrete sample that is representative of one specific sample site at a specific point in time. The entire sample is collected at one particular location and all at one time. If the source is fairly consistent over a period of time and/or geographical area, the sample is considered to be representative.
  • Holding Time – The allowed time from when a sample was taken (or extracted) until it must be analyzed.
  • Homogeneity – The degree to which a property or substance (i.e., an analyte) is randomly distributed throughout a material.
  • High Performance Liquid Chromatography (HPLC) – A method of separating chemical mixtures into individual components. A sample extract is introduced to an HPLC column and is carried through the column by a continuous flow of mobile phase from the pump. Due to attraction to the columns solid phase, some components of the sample will travel through the column more slowly than others, thus separating by the time the elute from the columns end. HPLC is a separation technique only and is usually coupled with various analytical detectors.
  • Initial Demonstration of Capability (IDC) – Procedure to establish the ability to generate acceptable accuracy and precision. The procedure consists of analyzing four to seven replicate samples prepared in the laboratory and taken through the entire method. The standard deviation, mean, mean recovery, and percent relative standard deviation are determined and compared to the required values in the determinative method.
  • Interference – A biological or chemical attribute of a test sample that positively or negatively offsets the measured result from the true value. If interference that is not segregated and identified is present, it enlarges or reduces the method bias.
  • Internal Audit – An internal process conducted by the laboratory QA Officer and used to determine continuing compliance with established method SOPs and with external regulatory requirements.
  • Internal Standard – A standard added to a sample extract just before GC analysis to determine relative retention time and quantitation for parameters of interest.
  • Laboratory Control Sample – A check standard obtained from an independent source (other than the calibration standard) and made up in a laboratory matrix (e.g., Ottawa sand to simulate sediment). It is taken through the method just like a sample and used to determine the amount of matrix effect by comparing a “clean” matrix with a true sample matrix.
  • Laboratory Fortified Blank – An aliquot of “blank” sample matrix (Milli-Q water, Ottawa Sand), to which known quantities of the method analytes are added. The LFB is analyzed exactly like a sample, and its purpose is to determine whether the methodology is in control, and whether the laboratory is capable of making accurate and precise measurements at the required MDL.
  • Limit of Detection (LOD) – The minimum concentration of a substance which can be measured and reported with 99% confidence that the analyte concentration is greater than zero. The LOD is typically determined from replicate analyses (minimum of seven) of natural or synthetic samples. The LOD is calculated using the following equation: 
    LOD = s(t-value; 99%CL n-1 DF)
  • Limit of Quantitation (LOQ) – The concentration of a substance above which quantitative results may be obtained with a specific degree of confidence (99% CL). Confidences in the apparent analyte concentration increase as the analyte signal increases above the LOD. The LOQ equals 10 times the standard deviation of the LOD replicate analysis.
  • Linearity – The range of concentration over which the detector maintains a constant sensitivity.
  • Lower Control Limit (LCL) – The maximum allowable limit for a given QC result. Usually defined as the statistical mean minus three standard deviations (99% confidence limit). It applies to accuracy measurements. See also Upper Control Limit and Lower Warning Limit
  • Lower Warning Limit (LWL) – The point at which the analyst needs to be more concerned with the integrity of the analytical process. Usually defined as the statistical mean minus two standard deviations (95% confidence limit). It applies to accuracy measurements. See also Upper Warning Limit and Lower Control Limit
  • Mass Spectrometry – An analytical identification technique in which a compound is placed in an evacuated chamber and exposed to a stream of electrons (EI) or ionized gas (CI). The compounds are ionized (either as whole molecules or fragments), and the resulting ions are accelerated by an electric impulse and then passed through a magnetic field, where they can be selectively (due to mass) allowed to pass to an electron multiplier.
  • Material safety data sheets (MSDS) – Product safety information sheets prepared by manufacturers and marketers of products containing toxic chemicals. These sheets can be obtained by requesting them from the manufacturer or marketer. Some stores, such as hardware stores, may have material safety data sheets on hand for products they sell.
  • Matrix Spike – A sample (preferably a real sample but may be a laboratory matrix) which is spiked with some or all of the method parameters and treated just as a sample would be. It is an accuracy measurement whose purpose is to determine whether the methodology is in control.
  • Matrix Spike Duplicate – A second Matrix spike. The pair are used in lieu of a real sample duplicate (generally when analytes are not expected to be found in the real world) to gauge the precision of the method.
  • Mean (arithmetic) – The mean (or average) of a set of “n” values is the sum of the values divided by “n”.
  • Metabolite – An intermediate substance produced by the processes of a living cell or organism. Environmental analysis often looks at the real world concentration of a contaminants metabolites to gather information about the parent compound.
  • Method – An assemblage of extraction and measurement techniques or processes and the order in which they are performed.
  • Minimum Sensitivity – The lowest amount (mass) that produces a sample peak of 10% full-scale deflection.
  • Monitoring – A type of sample or sampling program designed to determine baseline conditions and/or time trends.
  • Mullin Mix – A stock solution of Aroclors 1232, 1248, and 1262 at 183 µg/mL. The Aroclor mixture serves as the stock standard for calibration mixtures. Reference: Mullin MD. PCB Workshop; U.S. EPA Large Lakes Research Station; June 1985; Grosse Ile, MI.
  • NELAC – National Environmental Laboratory Accreditation Conference – A voluntary organization of state, federal, and other groups formed to establish mutually acceptable standards for accrediting environmental laboratories.
  • Noise – An extraneous electronic signal that affects baseline stability.
  • Nonpoint source pollution – Water pollution that cannot be traced to a specific source.
  • Nitrogen Phosphorous Detector (NPD) – A selective detector which is highly sensitive to compounds containing nitrogen or phosphorous. The detector contains a jet and a collector (similar to an FID); however, the collector contains a small alumina cylinder coated with a rubidium salt and heated electrically. In the presence of this thermionic source, N- or P-containing organic molecules are efficiently ionized. The ions are collected and the current is measured.
  • Ozone – A gas which is a variety of oxygen. The oxygen gas found in the air consists of two oxygen atoms stuck together; this is diatomic molecular oxygen. Ozone consists of three oxygen atoms stuck together into an ozone molecule. Ozone occurs in nature; it produces the sharp smell you notice near a lightning strike. High concentrations of ozone gas are found in a layer of the atmosphere–the stratosphere–high above the Earth. Stratospheric ozone shields the Earth against harmful rays from the sun, particularly ultraviolet B. Smog’s main component is ozone; this ground-level ozone is a product of reactions among chemicals produced by burning coal, gasoline and other fuels, and chemicals found in products including solvents, paints, hairsprays, etc.
  • Parts per billion (ppb) – A generic measure of concentration indicating one part of contaminant for every one billion parts of matrix (i.e., one microgram of chloroform in one liter of water).
  • Parts per million (ppm) – A generic measure of concentration indicating one part of contaminant for every one million parts of matrix (i.e., one milligram of chloroform in one liter of water).
  • Parts per trillion (ppt) – A generic measure of concentration indicating one part of contaminant for every one trillion parts of matrix (i.e., one nanogram of chloroform in one liter of water).
  • Performance Audit – The use of a performance standard (an external sample obtained from the EPA, or another approved supplier) or an independent standard (typically prepared internally) to check the analytical accuracy of a method. See Blind Standard
  • Performance Sample – A solution obtained from an outside source having a known value and analyzed as a blind sample or as a check on the instrumentation.
  • Petroleum Volatile Organic Chemicals (PVOCs) – A specific list of 10 gasoline-related VOCs.
  • pH – The negative logarithm of the hydrogen ion activity in an aqueous solution, or, the logarithm of the reciprocal of the hydrogen ion activity. Used as a measure of alkalinity or acidity.
  • Point source pollution – Pollution entering a stream at specific, detectable points.
  • Polychlorinated Biphenyls (PCBs) – A class of chlorinated, aromatic compounds which found widespread applications because of their general stability and inertness. These compounds were used for a variety of commercial purposes and were marketed under trade-names such as Aroclor, Clophen, Kanechlor and others.
  • Polycyclic Aromatic Hydrocarbons (PAHs) – A class of organic compounds consisting of two or more aromatic rings bonded together (e.g., anthracene, pyrene, etc).
  • Precision – The degree of agreement of repeated measurements of the same parameter expressed quantitatively as the standard deviation computed from the results of a series of controlled determinations. (The measured values may differ from the true value and still be precise.)
  • Purge and Trap – An extraction technique used in the analysis of volatile organic compounds (VOCs). An inert gas (generally helium) is bubbled through an aliquot of liquid. The gas is passed through a trap which is filled with any number of different sorbents. The compounds of interest are retained and then driven off the trap and onto a GC column by rapid heating.
  • Quality Assessment – The activities which assure that the quality control techniques employed have worked. An example of such an activity is a field blank to assess the field cleaning procedures.
  • Quality Assurance – The sum of all policies and activities which the laboratory utilizes to ensure that quality control is carried out.
  • Quality Assurance Plan – A document stating the quality policy, quality system, and quality practices of the organization. It may also be referred to as the Quality Assurance Manual.
  • Quality Assurance Project Plan (QAPP) – An outline for a particular project, including the project goals, schedule, data quality objectives, etc.
  • Quality Control – The actual implementation of Quality Assurance policies. The Quality Control program monitors the reliability (precision and accuracy) of results through a specific series of measurements. The QC program is designed to test the analyst, the analytical process, and the instrument performance by the use of quantifiable measurements.
  • Quality Control Audit – A systematic review of analytical data at the bench level, conducted by an authorized chemist. The chemist will verify that proper Quality Control procedures were followed before any data may be reported.
  • Quality Control Matrix – A group of samples with similar matrices which are grouped together for quality control purposes (e.g., Sample Tap, Municipal Well, Private Well are all Drinking Water for QC purposes).
  • Range(1) – The difference between the highest and lowest values reported for a sample.
  • Range(2) – A classification used to differentiate different concentrations from each other. Analyte results may be in a low concentration (range 1), mid-level concentration (range 2), or high level (range 3). Each level may have different quality control criteria associated with it.
  • Recovery (percent) – A measure of the ratio, expressed as a percentage, of the amount of the determinant found to the true amount known to be present in the sample. This gives an indication of the presence or absence of interfering substances in a sample.
  • Reference Sample – A sample used to determine accuracy, which is acquired from a source other than the laboratory conducting the analysis, in which the true value and acceptance limits are unknown to laboratory at the time of analysis.
  • Reformulated gasoline – Specially refined gasoline with low levels of smog-forming volatile organic compounds (VOCs) and low levels of hazardous air pollutants. The 1990 Clean Air Act requires sale of reformulated gasoline in the nine smoggiest areas. Reformulated gasolines were sold in several smoggy areas even before the 1990 Clean Air Act was passed.
  • Relative Percent Difference – The difference between measurements divided by the average, expressed as a percent. RPD = (Difference/Average) x 100
  • Relative Response – The ratio of responses of a given parameter to a reference parameter.
  • Relative Retention Time – The ratio of retention times of a given parameter to a reference parameter.
  • Relative Standard Deviation – The standard deviation of a set of data expressed as a percentage of the mean.
  • Report Limit – The lowest level for a particular parameter determined to be significant. This is a subjective judgement based on analyst experience and instrument performance. The report limit is used as a minimum reportable concentration of an analyte when a statistically determined LOD has not been developed.
  • Resolved Peak – A peak which begins and ends at the baseline.
  • Response – The signal amplitude-to-mass ratio for a given parameter.
  • Retention Time – The time that elapses from the introduction of the sample until the components peak maximum is reached.
  • Runoff – Water, including rain and snow, which is not absorbed into the ground. Instead, it flows across the land and eventually runs into streams and rivers. Runoff can pick up pollutants from the air and land, carrying them into the stream.
  • Safe Drinking Water Act (SDWA) – The SDWA is the principal law governing drinking water safety in the United States. Enacted initially in 1974 (SDWA 1974), the amended SDWA authorizes the U.S. Environmental Protection Agency (EPA) to establish comprehensive national drinking water regulations to ensure drinking water safety.
  • Sample Matrix – The mechanical, physical and chemical properties of the sample which contains the parameters of interest. See QC Matrix
  • Sample Weathering – The effects of the environment on individual components of a complex mixture (i.e., Aroclors, toxaphene, or technical chlordane).
  • Sediment – Fragmental material, both mineral and organic, that is in suspension or is being transported by the water mass or has been deposited on the bottom of the aquatic environment.
  • Silica Gel – A regenerative adsorbent consisting of amorphous silica. It is used in the laboratory to remove interferences and/or fractionate different analytes of interest.
  • Sludge – A water-­formed sedimentary deposit.
  • Split Sample – A subsample or aliquot of a collected sample, to be analyzed by another accepted analytical technique or another qualified laboratory in order to verify the original data.
  • Standard Addition – The method of standard addition is an analytical technique used to quantify samples whose matrices differ significantly form those of the standards. This technique is generally utilized in Atomic Absorption furnace analysis. It is accomplished by analyzing the sample mixed one-to-one with standards and blank, plotting the response vs. added concentration, and extrapolating the x-intercept for the unknown concentration.
  • Standard Deviation – The most widely used measure to describe the dispersion of a set of data about the mean. Normally x + s will include 68%; x + 2s will include about 95%; and x + 3s will include 99% of the data from a study.
  • Standard Operating Procedure (SOP) – A written document which details the method of an operation, analysis or action whose techniques and processes are thoroughly prescribed, and which is accepted as the method for performing certain routine or repetitive tasks.
  • Standard Sample – A previously analyzed sample run with consecutive batches to determine relative recovery of parameters. Also called a control sample.
  • Subsample – A portion taken from a sample.
  • SUMMA – SUMMA is a trademarked electropolishing technique for passivating metal surfaces. For ambient air sampling it is done internally to the stainless steel canister after welding the two halves together, resulting in 100% passivation of all internal surfaces with no active sites on welds or seams.
  • Surface water – Water found in rivers, streams, creeks, lakes, ponds, or oceans.
  • Surrogate Spike – A standard–not found in samples and chemically similar to the parameters of interest–that is added to each sample at the beginning of the analytical procedure to determine extraction/recovery efficiency for each sample.
  • Toxic Congener – A small group of polychlorinated biphenyls which exhibit a similar structure to the tetra-chlorodioxins. These congeners exhibit an increased toxicity when compared to the other PCBs.
  • Tributary – A stream or river flowing into a larger stream or river.
  • Trihalomethanes (THMs) – A class of four tri-halogenated compounds often formed as by-products of chlorination. The four compounds are: bromoform, bromodichloromethane, chlorodibromomethane and chloroform.
  • Uncertainty – The range of values within which the true value is estimated to lie. It is a best estimate of possible inaccuracy due to both random and systematic error.
  • Upper Control Limit (UCL) – The maximum allowable limit for a given QC result. Usually defined as the statistical mean + three standard deviations (99% confidence limit). It applies to both precision and accuracy measurements. See Lower Control Limit and Upper Warning Limit
  • Upper Warning Limit (UWL) – The point at which the analyst needs to be more concerned with the integrity of the analytical process. Usually defined as the statistical mean + two standard deviations (95% confidence limit). It applies to both precision and accuracy measurements. See Lower Warning Limit and Upper Control Limit
  • Unresolved Peak – A peak which begins or ends on the shoulder of a neighboring peak.
  • Vapor Pressure – The pressure (usually expressed in mmHg) characteristic at any given temperature of a vapor in equilibrium with its liquid or solid form.
  • Volatile organic compounds (VOCs) – Organic chemicals all contain the element carbon (C); organic chemicals are the basic chemicals found in living things and in products derived from living things, such as coal, petroleum and refined petroleum products. Many of the organic chemicals we use do not occur in nature, but were synthesized by chemists in laboratories. Volatile chemicals produce vapors readily; at room temperature and normal atmospheric pressure, vapors escape easily from volatile liquid chemicals. Volatile organic chemicals include gasoline, industrial chemicals such as benzene, solvents such as toluene and xylene, and tetrachloroethylene (perchloroethylene, the principal dry cleaning solvent). Many volatile organic chemicals are also hazardous air pollutants; for example, benzene causes cancer.
  • Water Quality – The kinds and amounts of matter dissolved and suspended in natural waters, the physical characteristics of the waters, and the ecological relationships between aquatic organisms and the environment.
  • Watershed – All the land that serves as a drainage for a specific stream or river. Imagine a maple leaf. The stalk of the leaf is the river. The veins threading into the stalk are the tributaries flowing into the river. The complete leaf represents a river drainage system, or watershed.
  • Water table – The position of the underground water or the depth to which one must drill to reach it. The water table may be a few feet down or hundreds of feet. The depth depends in part on the amount of water that has been removed.
  • Working Standard – Standard solutions which may be directly injected for GC calibration.
  • XAD-2 Resin – A hydrophobic styrene-divinyl benzene co-polymer, manufactured as beads of controlled mesh-size and porosity. It is widely used in water and air sampling procedures as an adsorbent for non-polar organics such as pesticides, PAHs and PCBs.

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