Glossary of Common Laboratory Terms

Accuracy

A measure of how close a measured value is to the true value. Assessed by means of percent recovery of spikes and standards.

Aerobic

Atmospheric or dissolved oxygen is available.

Aliquot

A measured portion of a sample.

Alkalinity

The capacity of the wastewater to neutralize acids. This is normally due to the presence of carbonate, bicarbonate, and/or hydroxide.

Ambient Temperature

The temperature of the surrounding area.

Anaerobic

Atmospheric or dissolved oxygen is not available.

Analyte

The compound or property that is to be determined or analyzed.

Anion

A negatively charged ion. This will normally try to combine or attach to a cation. Examples of anions are nitrate, nitrite, sulfate, phosphate, chloride, acetate, cyanide, carbonate, bicarbonate, and hydroxide.

Average

A way to represent a group of similar data points with a single number. The average can be reported in three different ways; mean, median, and mode. Of these, the mean is what is generally meant or intended.

Blank

Laboratory grade water and other reagents are set up and tested as though it was another sample. This checks for background interference from reagents and allows for correction.

Blind

QA/QC samples that are the same as unknowns with the exception that the person testing the samples does not know these are standards. Blind samples tend not to receive special treatment from the laboratory personnel.

Buffer

A solution or liquid mixture made up of an acid and its conjugate base. The chemical makeup allows it to neutralize small quantities of acids or bases or without great changes in pH when acids or bases are added. Buffers are generally used to calibrate pH meters.

Calibration

The process of preparing an instrument such that reliable operation will be ensured. The systematic determination of how the measurement system responds to the concentration of an analyte.

Cation

A positively charged dissolved particle which will normally try to combine or attach to an anion. Examples of cations may be ammonium, sodium, potassium, calcium, manganese, and magnesium.

Coefficient of variation

The measure of the ability to repeatedly obtain the same value for a single sample or method (i.e.; duplicate or replicate analyses). This may also be called the precision. [Note: Accuracy is not the point here, but rather how many times the same answer is obtained when all factors of the analysis remain constant same sample, same volume, same method, etc.] This value is calculated the same as a Relative Standard Deviation.

Composite Samples

A combination of grab samples collected at the same sampling point at different times. For most composite samples covering a specific time period, the grab samples should be obtained at regular intervals and placed in a single container. These grab samples can be collected automatically or manually. The sampling interval can be based on time or flow.

Desiccant

A drying agent or substance capable of absorbing moisture.

Desiccator

A sealed chamber in which samples can be dried in the presence of a desiccant.

Dissolved Solids

Solids that are in solution and cannot be filtered from the solution as suspended solids can.

Duplicate Testing

Two separate analyses are run on the same sample, using the same sample volume and testing technique. Usually the average result is reported. Example: running two TSS on the plant effluent. These are used to check laboratory procedure and precision. If more than two separate analyses are performed, it is referred to as replicate testing.

Electrode

A conductor through which a current of electricity enters or leaves. One or more may be contained in a probe. The term may be used interchangeably with the term probe when talking about a piece of laboratory equipment.

End Point

Where a titration is stopped. The point in a titration at which there is a sudden change in a physical property, such as indicator color, pH, conductivity or absorbance. The final value when titrating a solution.

Field Duplicate

Refers to two different samples collected at the same time, same location, and with the same sampling procedure. Used to check the sampling techniques.

Filtrate

The liquid that passes through a filter.

Grab Sample

A sample collected at a specific time and specific location, used to determine the nature of the water for that specific time and location only.

Gravimetric Analysis

An analytical method that relies on measuring the mass of solids, etc. in wastewater (and other liquids) by weighing precipitates or residues to complete the analysis.

Indicator

A chemical substance or compound having a physical property that changes abruptly, usually color, near the endpoint or equivalence point of a chemical reaction.

Instrument Blank

Laboratory pure water and other appropriate reagents are used to zero instrumentation.

Ion Selective Electrode

Also known by the acronym ISE. These electrodes respond to ions present in a sample. A potential develops across the membrane surface that is selectively dependent on the concentration of one particular ion in solution. The magnitude of the potential relates to the concentration of the ion concentration. The higher the potential, the higher the concentration.

Liter

Defined as exactly 1000 cm3.

Mean

A type of Average used by statisticians. A calculated value between the largest and smallest values in a series which is usually determined be adding all values and dividing by the total number of values. The mean is represented by a capital x with a line above it, ().

Median

A type of Average used by statisticians. The value above and below which there is an equal number of data points. The middle number in a series after all values have been arranged or sorted from highest to lowest. There are equal numbers of values above the median as below.

Meniscus

The curved surface of a liquid.

The picture shown on the left is a graphical representation of what a meniscus looks like. It is created by most liquids and is visible is glass columns. It is seen in burettes, pipettes, and volumetric flasks. The volume should be read from the bottom of the meniscus and perpendicular to the column. Accuracy and reproducibility will be maximized if the meniscus is viewed at eye level. Viewing the meniscus from above or below the liquid level will cause reading errors, called parallax errors.

Method Blank

A sample of laboratory pure water analyzed following the same step-by-step procedure used on the samples, including all reagents and solvents. The method blank will show any interference caused by a step in the procedure or by chemicals.

The result of the method blank should be zero or nearly zero. If it is not, the magnitude of the interference must be considered when calculating the concentration of the samples.

At least one method blank should be run a each time the analysis is performed.

Mode

A type of Average used by statisticians. The value that occurs most frequently in a group of values.

MSDS

Material Safety Data Sheet A document that provides information about a given chemical product. It includes the name, composition (chemicals in the product), hazards, first aid measures, fire fighting measures, information regarding the proper steps to take with spills, handling and storage, personal protection to be used, physical and chemical properties, and information about stability & reactivity, toxicology, disposal, transporting, and regulatory requirements.

Nephelometry

A technique in which the intensity of light scattered by a suspension is measured to determine the concentration of suspended particles.

NPDES

National Pollutant Discharge Elimination System. A provision of the Clean Water Act that prohibits discharge of pollutants into waters within the United States unless the state or EPA has issued a special permit.

Percent Recovery

A means for expressing the accuracy of a test. Percent recovery expresses what proportion of a known quantity can be measured by a given analytical technique. This QA/QC measurement is applicable to standards, spiked blanks, and spiked samples. It is calculated by dividing the result of the analysis (less any blank or sample contribution) by the known quantity of the analyte, expressed as a percentage. An example of the calculation is shown below.

Precision

The measure of the ability to repeatedly obtain the same value for a single sample or method (i.e. duplicate or replicate analyses). This may also be called the coefficient of variation. [Note: Precision is a measure of reproducibility. Precision is not synonymous with accuracy.]

Probe

An instrument or device used to get information about an environment. The use of a probe in a wastewater treatment laboratory generally has electrodes arranged in a given manner within the probe to allow attachment to a meter or other reading device to obtain desired information.

Quality

Reliably and repeatedly attaining a standard of excellence.

Quality Assurance

A program monitoring and controlling laboratory procedures and results, to insure the reliability of results. The paperwork involved in monitoring the program, such as control charts and graphs.

Quality Control

Use of scientific methods to maintain the most accurate data possible. Procedures performed to check against a standard, such as blanks, duplicates, and spikes.

Range

The difference between the highest and lowest values of a set.

Reagents

Reagents come in a variety of different qualities. For the work that is done in environmental testing, the use of chemicals meeting American Chemical Society (A.C.S.) requirements is recommended.

Reagent Blank

Used to determine the background or "blank" of all the reagents used in a given method. A complete reagent blank, made up of all reagents used for a specific method is sometimes used to zero instruments for spectrophotometric analysis. Measures the response to of the analytical method to impurities in the reagents and other components other than the analyte.

Relative Standard Deviation (%RSD)

A measure of the reproducibility of an analysis. This is determined by dividing the standard deviation (of a sample rather than the population) by the mean for the same set and then multiplying by 100%. An example of the calculation is shown below.

Replicates

More that two separate analyses are run on the same sample using the same sample volume and testing technique. Usually the average result is reported. Example: running more than two TSS on the plant effluent. These are used to check laboratory procedure and precision.

Spiked Sample

A precise volume of a sample to which a precise volume of standard reference material is added. The spiked sample concentration equals the original sample plus the added standard. Spikes should be run 1 in every 20 samples. The concentration of the standard added should be approximately equal to the original sample value.

Split Samples

Two or more parts of a homogeneous sample divided for analysis by different labs or by different analysts. Split samples are used as a QC check of the analytical procedures.

Standard

Standards are samples for which the analyst knows the true value before running the test. Standards can be made in-house or purchased from laboratory supply companies. Standards are often used to calibrate instruments and to evaluate the accuracy of an analysis.

Standard Deviation

A measurement of the extent to which individual values in a sample set differ from each other (or how closely they are clustered about the mean value). This manual will use a lowercase s to indicate the standard deviation for a finite set of data. Some calculators have a dedicated function key for the standard deviation of a sample labeled σn-1.

Standardization

The process by which the concentration of a reagent is determined by reaction with a known quantity of a second reagent.

Tare or Tare Weight

The weight of the vessel or filter before the item of interest is added. The tare weight subtracted from the gross weight of the item of interest and accompanying vessel gives the net weight of the item of interest. Many balances can be tared. That is, with the empty vessel in place, the balance can be set to read zero grams.

Titration

The process by which the quantity of analyte in a solution is determined from the amount of a standard reagent it consumes. Ordinarily, a titration is performed by the careful addition of a known amount of a chemical solution of known strength (titrant) to the completion of the reaction indicated by the end point. The volume of standard reagent is then measured.

Turbidity

The light-scattering property associated with suspended particles in a liquid. A turbid solution appears cloudy.

Unknown

USEPA Performance Evaluation (PE) study samples are an example of unknown standards. The test person is aware that a standard is being run, but does not know the true value of the standard. Only after the standard has been run and the test result calculated, will the true value be revealed. There are several supply houses where similar unknowns can be purchased throughout the year.

Water Quality

The quality of water to be used depends greatly on the purpose for which it is intended. The water to be used when performing analytical procedures should be free from the analyte of interest as well as any interfering substances.

Type 1

This water is used in procedures requiring maximum accuracy and precision. This type of water is used in HPLC, ICP/MS, and atomic absorption. The methods in this manual do not require water of this purity.

Type 2

This water is used for most analytical or general laboratory testing.

Water Treatment

Deionization

Also called demineralization or ion exchange. Synthetic resins are used to remove ions and minerals from feed water. This is very effective at removing ionized impurities, but does not remove organic, bacterial, pathogenic, or particulate matter efficiently.

Distillation

Feed water is placed in a vessel, heated to a gaseous state (boiled), and the vapor is then condensed in a separate vessel. It is the most effective method for removing a broad range of impurities, but does not remove gases very well.