Page 53 - Modern Analytical Chemistry

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36 Modern Analytical Chemistry

3 A Analysis, Determination, and Measurement

The first important distinction we will make is among the terms “analysis,” “deter-
analysis mination,” and “measurement.” An analysis provides chemical or physical infor-
A process that provides chemical or mation about a sample. The components of interest in the sample are called ana-
physical information about the lytes, and the remainder of the sample is the matrix. In an analysis we determine
constituents in the sample or the sample
itself. the identity, concentration, or properties of the analytes. To make this determina-
tion we measure one or more of the analyte’s chemical or physical properties.
An example helps clarify the differences among an analysis, a determination,
analytes
The constituents of interest in a sample. and a measurement. In 1974, the federal government enacted the Safe Drinking
Water Act to ensure the safety of public drinking water supplies. To comply with
matrix this act municipalities regularly monitor their drinking water supply for potentially
All other constituents in a sample except harmful substances. One such substance is coliform bacteria. Municipal water de-
for the analytes. partments collect and analyze samples from their water supply. To determine the
concentration of coliform bacteria, a portion of water is passed through a mem-
determination brane filter. The filter is placed in a dish containing a nutrient broth and incu-
An analysis of a sample to find the bated. At the end of the incubation period the number of coliform bacterial
identity, concentration, or properties of
the analyte. colonies in the dish is measured by counting (Figure 3.1). Thus, municipal water
departments analyze samples of water to determine the concentration of coliform
bacteria by measuring the number of bacterial colonies that form during a speci-
measurement
An experimental determination of an fied period of incubation.
analyte’s chemical or physical properties.
3 B Techniques, Methods, Procedures, and Protocols

Suppose you are asked to develop a way to determine the concentration of lead in
drinking water. How would you approach this problem? To answer this question it
helps to distinguish among four levels of analytical methodology: techniques, meth-
ods, procedures, and protocols. 1
technique A technique is any chemical or physical principle that can be used to study an
A chemical or physical principle that can analyte. Many techniques have been used to determine lead levels. For example, in
2
be used to analyze a sample.
graphite furnace atomic absorption spectroscopy lead is atomized, and the ability of
the free atoms to absorb light is measured; thus, both a chemical principle (atom-
ization) and a physical principle (absorption of light) are used in this technique.
Chapters 8–13 of this text cover techniques commonly used to analyze samples.
method A method is the application of a technique for the determination of a specific
A means for analyzing a sample for a analyte in a specific matrix. As shown in Figure 3.2, the graphite furnace atomic ab-
specific analyte in a specific matrix.
sorption spectroscopic method for determining lead levels in water is different from
that for the determination of lead in soil or blood. Choosing a method for deter-
mining lead in water depends on how the information is to be used and the estab-
lished design criteria (Figure 3.3). For some analytical problems the best method
might use graphite furnace atomic absorption spectroscopy, whereas other prob-
lems might be more easily solved by using another technique, such as anodic strip-
ping voltammetry or potentiometry with a lead ion-selective electrode.
procedure A procedure is a set of written directions detailing how to apply a method to a
Written directions outlining how to particular sample, including information on proper sampling, handling of interfer-
analyze a sample.
ents, and validating results. A method does not necessarily lead to a single proce-
dure, as different analysts or agencies will adapt the method to their specific needs.
As shown in Figure 3.2, the American Public Health Agency and the American Soci-
ety for Testing Materials publish separate procedures for the determination of lead
levels in water.