Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
materials and tools coming into contact with the samples.
Samples must be handled and stored in such a way that the level
of the substance to be analysed remains stable. Processes that are
most likely to decrease sample stability include precipitation,
chemical deterioration (e.g., photolysis), surface absorption and
evaporation of the analyte. Another factor that could be important is
changes in the matrix, which may affect recovery of the analyte.
11.4.5 Equipment maintenance and calibration
Adequate equipment should be available for the receipt, handling,
storage and analysis of samples. Its maintenance and calibration is
essential to QA practise. Equipment used for analysis must be tested,
calibrated and standardized.
11.4.6 Internal audit and corrective action
Internal audit and corrective action are critical activities in
QA management. Audit procedures are to verify that the procedures are
conducted the way they were planned and described in the study plan.
Non-conformance must to be corrected to attain the study objectives.
11.5 Quality control/quality assurance for sample measurement
11.5.1 Method selection and validation
A chemical compound can usually be analysed by a variety of
different methods. Some methods emphasize quality of analysis, i.e.,
accuracy and precision, whereas others are directed mainly towards
practicality and low cost. However, even the best method may give
incorrect results if improperly used. Methods are customarily
classified according to their main use in the analytical field into
definitive, reference and field routine methods. The best
approximation of the true value obtained by analysis is the
definitive value, i.e., the result obtained by the definitive
method. The definitive method is, however, generally not considered
practicable for daily laboratory use. Therefore, the purpose of a
reference method is often to serve as a basis for the determination
of the accuracy of the field methods. Field methods (or routine
methods) are those in everyday use in the laboratory. They are not
necessarily as precise and accurate as reference and definitive
methods. Their use is justified, however, because the definitive and
reference methods are often too cumbersome and expensive and may not
be available. The precision and bias relating to the method should be
known for every routine method used.
The following data quality specifications have to be validated in
method selection to meet the requirements and objectives of the
exposure assessment project.
11.5.1.1 Accuracy
Accuracy is defined as the closeness of agreement between a test
result and the accepted true value. Accuracy or validity of an
analysis is primarily determined by the specificity of the method and
the analytical recovery.
Fig. 31A illustrates the ideal case where the reported values
correspond exactly to the expected values. However, all analytical
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