The distribution of toxic heavy metals or organic compounds in a land rede-
velopment site presents a different problem. Here, to economize on the number
of analyses, a grid is superimposed on the site dividing it up into approximately
one- to five-metre squares. From each of these, samples of soil will be taken at
several specified depths. A three-dimensional representation of the distribution
of each analyte over the whole site can then be produced, and any localized high
concentrations, or hot spots, can be investigated by taking further, more closely-
spaced, samples. Individual samples may need to be ground, coned and
quartered as part of the sampling strategy.
Repeated sampling over a period of time is a common requirement. Examples
include the continuous monitoring of a process stream in a manufacturing plant
and the frequent sampling of patients’ body fluids for changes in the levels of
drugs, metabolites, sugars or enzymes, etc., during hospital treatment. Studies of
seasonal variations in the levels of pesticide, herbicide and fertilizer residues in
soils and surface waters, or the continuous monitoring of drinking water supplies
are two further examples.
Having obtained a representative sample, it must be labeledand stored under
appropriate conditions. Sample identification through proper labeling, increas-
ingly done by using bar codes and optical readers under computer control, is an
essential feature of sample handling.Sample storage Samples often have to be collected from places remote from the analytical labora-
tory and several days or weeks may elapse before they are received by the labo-
ratory and analyzed. Furthermore, the workload of many laboratories is such that
incoming samples are stored for a period of time prior to analysis. In both
instances, sample containers and storage conditions (e.g., temperature, humidity,
light levels and exposure to the atmosphere) must be controlled such that no
significant changes occur that could affect the validity of the analytical data. The
following effects during storage should be considered:
● increases in temperature leading to the loss of volatile analytes, thermal or
biological degradation, or increased chemical reactivity;
● decreases in temperature that lead to the formation of deposits or the precipi-
tation of analytes with low solubilities;
● changes in humidity that affect the moisture content of hygroscopic solids and
liquids or induce hydrolysis reactions;
● UV radiation, particularly from direct sunlight, that induces photochemical
reactions, photodecomposition or polymerization;
● air-induced oxidation;
● physical separation of the sample into layers of different density or changes in
crystallinity.In addition, containers may leak or allow contaminants to enter.
A particular problem associated with samples having very low (traceand
ultra-trace) levels of analytes in solution is the possibility of losses by adsorp-
tion onto the walls of the container or contamination by substances being
leached from the container by the sample solvent. Trace metals may be depleted
by adsorption or ion-exchange processes if stored in glass containers, whilst
sodium, potassium, boron and silicates can be leached from the glass into the
sample solution. Plastic containers should always be used for such samples.12 Section A – The nature and scope of analytical chemistry