Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
of statistical power is described more fully in Chapter 4.
On the basis of the power analysis, acceptance intervals (AI) can
be calculated for a laboratory. The AI will be narrower than the
interval between the MAD lines and the AI lines will be closer to the
MAD lines with increasing number of QC samples. Also, methods with
inherently smaller errors will decrease the difference between the MAD
lines and the AI. Fig. 35 shows the regression line for the reported
results of analysis of six QC samples versus the reference values. The
MAD interval is indicated by solid lines and the acceptance interval
by broken lines (Vahter, 1982). Fig. 36 shows the results of lead in
blood from one laboratory during the training phase in the global
UNEP/WHO Project on Assessment of Exposure to Lead and Cadmium through
Biological Monitoring. The results of QC 2 and QC 3 were rejected,
i.e., the regression lines fall outside the acceptance lines when
evaluated between the reference values 100 and 400 µg/litre (Lind et
al., 1987).
The experience from the QA programme of the WHO/UNEP HEAL
Programme shows that detailed protocols, instructions, and control
activities covering the whole process, from sampling to analysis and
data reporting, are essential if reliable data are to be obtained. The
results clearly demonstrated that good analytical performance for a
pollutant in one medium (e.g, lead in blood) is no guarantee of good
performance for the same pollutant in another medium (e.g., lead in
air filters). Similarly, good analytical performance at one
concentration is no guarantee of good performance at higher or lower
concentrations. It is, therefore, important that the QC samples have a
matrix similar to that of the monitoring samples and that the
concentrations of the QC samples cover the expected range of
concentrations. One, or a few, reference samples are not sufficient
for the evaluation of the evaluation of analytical performance using
rigorous MAD criteria (Vahter & Slorach, 1990).
11.5.4 Reference materials
Reference materials are important means of ensuring comparability
of analytical results across laboratories, over time, among
populations and among studies. Reference samples for internal as well
as external QA have been used in the analysis of environmental media,
such as air, water and food, as well as for biological tissues and
body fluids, such as internal organs, blood and urine. It is important
that reference materials have a matrix which is the same or very
similar to the matrix the "real" monitoring samples are comprised of.
Matrix effects may seriously invalidate analytical results. The
concentrations of a substance to be measured should also cover the
same range as is expected in the monitoring samples. For several
substances, the chemical forms in which they may exist must be taken
into consideration. For example, arsenic, mercury, tin and several
other metals occur in a number of different valence states and in
organic and inorganic forms which vary in stability.
Various types of reference samples are commercially available.
The International Atomic Energy Agency (IAEA) conducted a survey on
analytical reference materials of biological and environmental origin.
The IAEA database presently contains over 10 000 analyte values for
455 substances in 650 reference materials produced by 30 different
suppliers (IAEA, 1995, 1996). It is expected that this survey will
help analysts to select reference materials for QA purposes that match
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