12 —
Overall Analytical Procedures and Their Automation
In Chapter 1 a general pattern for analytical procedures was introduced and the various stages of an
analysis identified as: sampling; pretreatment; separation or masking; measurement; interpretation of
results. Subsequent chapters have dealt with separation methods, measurement techniques and the
interpretation of results in more detail. It remains to examine sampling more closely and to consider, by
way of example, some overall analytical schemes.
12.1—
Sampling and Sample Pretreatment
Representative Samples and Sample Storage
It is axiomatic that unless the sample is correctly selected, taken and handled then there is no point in
carrying out an analysis. A sample must provide a meaningful measure of the analyte, and only careful
consideration of the overall problem by the analyst and his technical colleagues will ensure correct
sampling. For a completely homogeneous system any part of the whole is suitable for analysis. With a
non-homogeneous system a more difficult sampling problem exists. For example, a truck load of an ore
will contain particles of vastly different sizes and of variable composition and to obtain a sample
representing the average composition of the load, a systematic selection of material from various parts
and levels must be made. The selected material must then be crushed, ground and mixed to provide a
single homogeneous sample. Thus the analyst may be presented with several kilograms of crushed
material within which there is still some variation in particle size. The final selection of the material on
which the analysis is to be carried out may be made by coning and quartering. In this process the
powdered material is poured into a cone shaped heap, and subsequently divided into four equal parts.
Two opposite quarters are combined into a second cone, the remaining two being discarded. The
process is repeated until the amount of sample is reduced to that required for analysis. Coning and
quartering should ensure that a representative selection of particle sizes is taken at each division.