A similar situation arises with a large metallurgical specimen although crushing and grinding is not
possible. Sawing or drilling is used to obtain a systematic set of samples from different parts of the
specimen. These may then be analysed individually to provide a profile of the analyte distribution, or
mixed, coned and quartered to provide a sample representative of the overall composition. A rather
different and more complex sampling situation is discussed in Example 12.3 (p. 511). Having obtained
a properly representative sample, care and thought must be given to its storage prior to analysis. Many
factors may need to be taken into account, but the prime objects, as in the sample treatment discussed
below, must be to avoid losses or contamination and to prevent undesirable changes in chemical form.
Use of the right container is important. For instance, solutions can leach trace metals from the wall of a
glass container or lose them by adsorption onto it. Biological samples present special problems as they
decompose rapidly unless refrigerated or stored in a sterile environment.
In a limited number of cases, direct analysis of a sample is possible and is generally preferable;
normally, however, some degree of pretreatment or 'conditioning' is required. There are three important
pitfalls to guard against in pretreatment. Firstly, additional analyte(s) may contaminate the sample
during chemical or mechanical conditioning. This is a particular problem where trace constituents are
concerned. Secondly, losses of a volatile analyte may easily occur and, thirdly, the analyte may undergo
undesirable changes of chemical form. Hence, it is necessary to consider carefully what pretreatment is
suitable in the light of the required analysis. Generally, moisture content should be determined before
any pretreatment. Dissolution is by far the most important method of preparing a sample for analysis.
Where a solution cannot easily be obtained, chemical attack is necessary. Aqueous acids or alkalis are
effective in many cases. Organic samples are generally 'dry oxidized' by heating in an open crucible or
sealed 'bomb' or 'wet oxidized' with concentrated HNO 3 /H 2 SO 4 mixtures. Inorganic samples which resist
attack by acids or alkalis are best fused with a suitable flux, e.g. LiBO 2 , Na 2 CO 3 , Na 2 O 2 , KOH.
Alternatively, hydrofluoric acid may be used to effect dissolution and simultaneous volatilization of
SiO 2 if it is not an analyte. However, this acid attacks glass and is hazardous to use. Acid leaching of the
analyte(s) from an insoluble matrix is sometimes preferred so as to simplify the preparation procedure
and minimize subsequent interference from matrix elements. Typical sequences for the analysis of
organic and inorganic samples are illustrated in Figures 12.1 and 12.2.
Sample Concentration and Clean-up:
Solid Phase Extraction
Concentration of an analyte prior to measurement may be necessary where the level is likely to be close
to or below the practical detection limit of the technique to be used. Solvent extraction, solid phase
extraction and ion-exchange may be used for this purpose. Where a complex