The selection of a valid method of analysis

In order to be able to choose a suitable analytical method it is essential to know something about the chemical and physical properties of the test substance (Table 1.1). Because the relationship between the property and the amount of substance is not always a simple one, some methods are only suitable for the detection of the substance (qualitative) while others may be quantitative. For any method it is important to appreciate the nature of the relationship between

Table 1.1 Physical basis of analytical methods

Physical properties that Examples of properties used in can be measured with some the quantitation of degree of precision Protein Lead Oxygen

Extensive Mass +

Volume

Mechanical

Specific gravity +

Viscosity +

Surface tension +

Spectral

Absorption + +

Emission

Fluorescence

Turbidity +

Rotation

Electrical Conductivity

Current/voltage +

Half-cell potential +

Nuclear Radioactivity

Proteins are the major components by bulk in many biological samples and hence the weighing of a dried sample should give an estimate of the amount of protein present. Similarly, solutions that contain protein show values for specific gravity and surface tension which are in some way related to protein content. Measurements of the turbid ity resulting from the precipitation of protein and the absorption of radiation at specif ic wavelengths have all been used quantitatively

The lead content of biological samples is usually very small, rendering gravimetric methods impracticable, and methods have often relied upon the formation of coloured complexes with a variety of dyes. More recently, the development of absorption spectroscopy using vaporized samples has provided a sensitive quantitative method. Oxygen measurements using specific electrodes offer a level of sensitivity which is unobtainable using volumetric gas analysis.

the measurement obtained and the amount of substance in the sample. Most analytical methods involve several preparative steps before the final measurement can be made and it is possible to produce a flow diagram representing a generalized method of analysis (Table 1.2). Not all the steps may be necessary in any particular method and it may be possible to combine two or more by careful choice of instrumentation. It is important when selecting a particular method to consider not only its analytical validity but also the cost of the analysis in terms of the instrumentation and reagents required and the time taken.

Table 1.2 Generalized method of analysis

The major manipulative steps in a generalized method of analysis

Purification of the test substance

Development of a physical characteristic by the formation of a derivative

Detection of an inherent or induced physical characteristic

Signal amplification

Signal measurement

Computation V

Presentation of result

>■ Atomic spectroscopy - see Sections 2.2 and 2.5.

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