Instant Notes: Analytical Chemistry

Section A – The nature and scope of analytical chemistry

A4 SAMPLING AND SAMPLE

HANDLING

The importance of obtaining a representative sample for analysis cannot be

overemphasized. Without it, results may be meaningless or even grossly

misleading. Sampling is particularly crucial where a heterogeneousmaterial is to

be analyzed. It is vital that the aims of the analysis are understood and an appro-

priate sampling procedure adopted. In some situations, a sampling planor

strategy may need to be devised so as to optimize the value of the analytical

information collected. This is necessary particularly where environmental

samples of soil, water or the atmosphere are to be collected or a complex indus-

trial process is to be monitored. Legal requirements may also determine a

sampling strategy, particularly in the food and drug industries. A small sample

taken for analysis is described as a laboratory sample. Where duplicate analyses

or several different analyses are required, the laboratory sample will be divided

into sub-samples which should have identical compositions.

Homogeneous materials (e.g., single or mixed solvents or solutions and most

gases) generally present no particular sampling problem as the composition of

any small laboratory sample taken from a larger volume will be representative of

the bulk solution. Heterogeneous materialshave to be homogenized prior to

obtaining a laboratory sample if an average or bulk composition is required.

Conversely, where analyte levels in different parts of the material are to be

Representative
sample

Key Notes

A representative sample is one that truly reflects the composition of the

material to be analyzed within the context of a defined analytical

problem.

Due to varying periods of time that may elapse between sample

collection and analysis, storage conditions must be such as to avoid

undesirable losses, contamination or other changes that could affect the

results of the analysis.

Preliminary treatment of a sample is sometimes necessary before it is in a

suitable form for analysis by the chosen technique and method. This may

involve a separation or concentration of the analytes or the removal of

matrix components that would otherwise interfere with the analysis.

Samples generally need to be brought into a form suitable for

measurements to be made under controlled conditions. This may involve

dissolution, grinding, fabricating into a specific size and shape,

pelletizing or mounting in a sample holder.

Related topic Analytical problems and procedures (A2)

Representative

sample

Sample storage

Sample

pre-treatment

Sample preparation