Page 33 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

TYPES OF ANALYSIS 1.4

with the normal standard sampling procedures employed for different types of
materials.

1.4 TYPES OF ANALYSIS
With an appropriate sample available, attention must be given to the question
of the most suitable technique or techniques to be employed for the required
determinations. One of the major decisions to be made by an analyst is the
choice of the most effective procedure for a given analysis, and in order to arrive
at the correct decision, not only must he be familiar with the practical details
of the various techniques and of the theoretical principles upon which they are
based, he must also be conversant with the conditions under which each method
is reliable, aware of possible interferences which rnay arise, and capable of
devising means of circumventing such problems. He will also be concerned with
questions regarding the accuracy and the precision to be expected from given
methods and, in addition, he must not overlook such factors as time and costing.
The most accurate method for a certain determination rnay prove to be lengthy
or to involve the use of expensive reagents, and in the interests of economy it
rnay be necessary to choose a method which, although somewhat less exact,
yields results of sufficient accuracy in a reasonable time.
Important factors which must be taken into account when selecting an
appropriate method of analysis include (a) the nature of the information which
is sought, (b) the size of sample available and the proportion of the constituent
to be determined, and (c) the purpose for which the analytical data are required.
The nature of the information sought rnay involve requirement for very
detailed data, or alternatively, results of a general character rnay suffice. With
respect to the information which is furnished, different types of chemical analysis
rnay be classified as follows:
1. proximate analysis, in which the amount of each element in a sample is
determined with no concern as to the actual compounds present;
2. partial analysis, which deals with the determination of selected constituents
in the sample;
3. trace constituent analysis, a specialised instance of partial analysis in which
we are concerned with the determination of specified components present in
very minute quantity;
4. complete analysis, when the proportion of each component of the sample is
determined.
On the basis of sample size, analytical methods are often classified as:
1. macro, the analysis of quantities of 0.1 g or more;
2. meso (semimicro), dealing with quantities ranging from 10 - g to 10 - ' g;
3. micro, for quantities in the range g to 10-'g;
4. submicro, for samples in the range g to g;
5. ultramicro, for quantities below g.
The term 'semimicro' given as an alternative name for classification (2) is not
very apt, referring as it does to samples larger than micro.
A major constituent is one accounting for 1- 100 per cent of the sample under
investigation; a minor constituent is one present in the range 0.01-1 per cent;
a trace constituent is one present at a concentration of less than 0.01 per cent.