Page 179 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

SAMPLINC STATlSTlCS 5.3

Bulk material

Increment

Gross sample

1

Sub-sample

1

Test or analysis sample

Fig. 5.1
of increments is taken in a random manner from points in the bulk material,
so that each part has an equal chance of being selected. The combination of
these increments then forms the gross sample. The gross sample is often too
large for direct analysis and must be divided further to produce a sub-sample.
The sub-sample may require treatment, for example reduction in particle size
or thorough mixing, before the analytical sample can be obtained. The analytical
sample should retain the same composition of the gross sample.
It must be stressed, however, that the whole object may be the analytical
sample, e.g. a specimen of moon-rock. Ideally this sample would be analysed
by non-destructive methods. Occasionally the bulk material may be homogeneous
(some water samples) and then only one increment may be needed to determine
the properties of the bulk. This increment should be of suitable size to provide
samples for replicate analyses.

5.3 SAMPLINC STATlSTlCS
The errors arising in sampling, particularly in the case of heterogeneous solids,
may be the most important source of uncertainty in the subsequent analysis of
the material. If we represent the standard deviation of the sampling operation
(the sampling error) by s, and the standard deviation of the analytical procedures
(the analytical error) by s,, then the overall standard deviation s, (the total
error) is given by