78
6 Reference Materials in Chemical Measurements
The appropriate reference material should contain the analyte at a concentration
similar to its contents in the test samples, and in the same chemical form.
It is possible that natural materials are enriched by the addition of the analyte, so
that its content is similar to that expected in the test samples. In this case, the form of
the chemical substance added is extremely important. Enrichment of the material can
be carried out in various ways. The simplest method is to add a known quantity of a
substance to a prepared material (e.g., the addition of a standard solution of a mercury
salt to serum; the addition of a standard solution of a pesticide to comminuted meat).
In some cases, after the addition of a substance to the material, it is left for a certain
time, assuming that it is possible to restore the chemical processes in the matrix (e.g.,
changes in a degree of oxidation; binding with protein).
For example, for the determination of total mercury content in blood serum, an
additive of standard solution of HgCl 2 was used. It has been found that recovery
is different depending on whether the measurements were carried out directly after
the addition of the standard solution (almost 100% recovery was obtained) or if the
sample was mixed under the incubation conditions before measurement (at the typical
temperature of living organisms). In the latter case, recovery was only 80%. This
suggests that incubation of the samples at around 40 °C has allowed the occurrence of
biochemical processes that cause the incorporation of mercury with organic matrixes,
therefore decreasing its ability for extraction.
An interesting example is the production of RM for testing the content of pesticides
in foods of plant origin. In the case of tomatoes, it is possible to grow them in soil
enriched with pesticides of interest or by adding to tomato pulp, dissolved in acetone
mixtures of pesticides. One should therefore be aware of the method of preparation
of the material, because it allows for the assessment of the possible impact of nonhomogeneity of the material on the uncertainty of measurements.
It is worth noting that there are cases where the production of a RM is not possible;
such an example may be where there is a need to determine volatile organic compounds in water. It is not possible to prepare sufficiently stable RM containing volatile
organic compounds. In this case, the recommended solution is to produce mixtures
of these compounds and add them directly to the test sample prior to measurement.
6.10 The Uncertainty of the Certified Value
and the Expanded Uncertainty of the Measurement
Result
In order to select the adequate CRM, it is also essential to consider the uncertainty
associated to the reference values since this has an impact on the uncertainty of final
result obtained for the test samples. The uncertainty attributed to the reference value
depends on the type of material and the method used for the determination of the
6 Reference Materials in Chemical Measurements
The appropriate reference material should contain the analyte at a concentration
similar to its contents in the test samples, and in the same chemical form.
It is possible that natural materials are enriched by the addition of the analyte, so
that its content is similar to that expected in the test samples. In this case, the form of
the chemical substance added is extremely important. Enrichment of the material can
be carried out in various ways. The simplest method is to add a known quantity of a
substance to a prepared material (e.g., the addition of a standard solution of a mercury
salt to serum; the addition of a standard solution of a pesticide to comminuted meat).
In some cases, after the addition of a substance to the material, it is left for a certain
time, assuming that it is possible to restore the chemical processes in the matrix (e.g.,
changes in a degree of oxidation; binding with protein).
For example, for the determination of total mercury content in blood serum, an
additive of standard solution of HgCl 2 was used. It has been found that recovery
is different depending on whether the measurements were carried out directly after
the addition of the standard solution (almost 100% recovery was obtained) or if the
sample was mixed under the incubation conditions before measurement (at the typical
temperature of living organisms). In the latter case, recovery was only 80%. This
suggests that incubation of the samples at around 40 °C has allowed the occurrence of
biochemical processes that cause the incorporation of mercury with organic matrixes,
therefore decreasing its ability for extraction.
An interesting example is the production of RM for testing the content of pesticides
in foods of plant origin. In the case of tomatoes, it is possible to grow them in soil
enriched with pesticides of interest or by adding to tomato pulp, dissolved in acetone
mixtures of pesticides. One should therefore be aware of the method of preparation
of the material, because it allows for the assessment of the possible impact of nonhomogeneity of the material on the uncertainty of measurements.
It is worth noting that there are cases where the production of a RM is not possible;
such an example may be where there is a need to determine volatile organic compounds in water. It is not possible to prepare sufficiently stable RM containing volatile
organic compounds. In this case, the recommended solution is to produce mixtures
of these compounds and add them directly to the test sample prior to measurement.
6.10 The Uncertainty of the Certified Value
and the Expanded Uncertainty of the Measurement
Result
In order to select the adequate CRM, it is also essential to consider the uncertainty
associated to the reference values since this has an impact on the uncertainty of final
result obtained for the test samples. The uncertainty attributed to the reference value
depends on the type of material and the method used for the determination of the