Page 178 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

CHAPTER 5

SAMPLING

5.1 THE BASlS OF SAMPLING
The purpose of analysis is to determine the quality or composition of a material;
and for the analytical results obtained to have any validity or meaning it is
essential that adequate sampling procedures be adopted. Sampling is the process
of extracting from a large quantity of material a small portion which is truly
representative of the composition of the whole material.
Sampling methods fa11 into three main groups:
1. those in which al1 the material is examined;
2. casual sampling on an ad hoc basis;
3. methods in which portions of the material are selected based upon statistical
probabilities.
Procedure ( 1) is normally impracticable, as the majority of methods employed
are destructive, and in any case the amount of material to be examined is
frequently excessive. Even for a sample of manageable size the analysis would
be very time-consuming, it would require large quantities of reagents, and would
monopolise instruments for long periods.
Sampling according to (2) is totally unscientific and can lead to decisions
being taken on inadequate information. In this case, as the taking of samples
is entirely casual, any true form of analytical control or supervision is impossible.
For these reasons the only reliable basis for sampling must be a mathematical
one using statistical probabilities. This means that although not every item or
every part of the sample is analysed, the limitations of the selection are carefully
calculated and known in advance. Having calculated the degree of acceptable
risk or margin of variation, the sampling plan is then chosen that will give the
maximum information and control that is compatible with a rapid turnover of
samples. For this reason, in the case of sampling from batches the selection of
individual samples is carried out according to special random tables6 which
ensure that persona1 factors do not influence the choice.

5.2 SAMPLING PROCEDURE
The sampling procedure may involve a number of stages prior to the analysis
of the material. The sampling stages are outlined in Fig. 5.1.
For the most part, bulk materials are non-homogeneous, e.g. minerals,
sediments, and foodstuffs. They may contain particles of different composition
which are not uniformly distributed within the material. In this case, a number