Environmental Chemical Determinations

Abstract

Environmental chemical determinations are identifications and measurements of the concentrations of elements, compounds, or ions in environmental media. In a chemical determination, equal importance is given to the correct identification of the substance and to its accurate and precise measurement. There has been a tendency in some environmental work to place more emphasis on making accurate and precise measurements and to give less attention to ascertaining the correctness of the identification of the substance being measured.

Glossary

Accuracy

The degree of agreement between the measured concentration of a substance in a sample and the true value of the concentration in the sample.

Analysis

The process of investigation of a sample of the physical world to learn about its chemical components, composition, structure, or other physical or chemical characteristics. Generally only samples are analyzed, and individual elements, compounds, and ions are separated from one another, identified, measured, or determined. A pure compound or multielement ion is analyzed only when it is investigated to determine its components, composition, structure, or other physical or chemical characteristics.

Analyte

A general term for any element, compound, or ion that is present in a sample or is targeted for determination in a sample.

Analytical method

The complete process used to determine an analyte or analytes in a sample. The analytical method documents all the individual steps in the process from sampling to reporting the results.

Analytical method attributes

Measures of the quality, reliability, and uncertainty of the determinations obtained with an analytical method. Typical analytical method attributes are selectivity, sensitivity, detection limits, signal/noise, recovery, accuracy, bias, precision, and validation. Analytical method attributes are sometimes called figures of merit.

Bias

The systematic error in a measurement of the amount or concentration of an analyte in a sample.

Congeners

Compounds or ions that are members of a series of related substances that differ only by the number of hydrogens that have been substituted by the same atom.

Derivatives

Compounds or ions that are produced by chemical reactions of analytes. An analytically useful derivative has physical or chemical properties that are not possessed by the analyte but that can be employed to determine the analyte.

Detection limit

The minimum quantity or concentration of an analyte that can be detected with an analytical method or technique. There are no generally accepted standard criteria for detection, and detection limits often depend on the sample matrix. Therefore detection limits must include the criteria for detection and the nature of the sample matrix. A technique with a higher analyte sensitivity does not always provide a lower detection limit because interfering sample matrix components may also be observed with higher sensitivities.

Determination

The identification and the measurement of the concentration of an analyte in a sample.

Isomers

Two or more compounds or multielement ions that have the same elemental composition but different structures.

Precision

The degree of random variation in repetitive measurements of the concentration of an analyte in a sample. Precision is usually measured by the standard deviation or the relative standard deviation of the measurements.

Procedure

A specific part of an analytical method that is concerned with one aspect of the method, for example, the liquid–liquid extraction of groups of similar analytes from a water sample.

Qualitative analysis

The process of only identifying the analytes in a sample.

Quantitative analysis

The process of both identifying and measuring the concentrations of the analytes in a sample.

Recovery

The amount of analyte measured in a sample matrix as a fraction of the amount of the same analyte that was added to the sample. If the analyte is present in the sample before the addition, the native amount is subtracted from the measured quantity before calculating the recovery.

Sample matrix

The general nature of the sample and its components that can have a significant impact on the performance of an analytical method. For example, sea water and freshwater sample matrices are significantly different, and this difference can affect the performance of an analytical method.

Selectivity

Is a qualitative estimate of how well the analyte identification procedure is able to distinguish an analyte in a sample from one or many similar analytes with similar, or even some of the same, physical or chemical properties.

Sensitivity

Is the electronic or other measurable signal produced by the analytical method or measurement technique per unit amount of analyte.

Signal/Noise

Is the ratio of analyte electronic or other measurable signal to the mean background matrix signal.

Technique

Is a specific way of manipulating a sample or substance or measuring a substance. One or more techniques may be used within each procedure, and several procedures may be used within an analytical method.

Validation

Is the determination of the attributes, or figures of merit, of an analytical method for one or more analytes in one or more sample matrices by one or more analysts in one or more analytical laboratories and the acceptance of the attributes as reasonable and useful by the users of the data. There are many levels of analytical method validation ranging from the validation of a method for a single analyte in a single matrix by a single analyst in a single laboratory to a multi-analyte, multi-matrix, multi-analyst, and multi-laboratory validation.