174
10 Interlaboratory Comparisons
the laboratory, especially those that are doubtful and unsatisfactory. This explains
the question as to what results should be qualified as doubtful or unsatisfactory and
whether the criteria used by the organizers always meets the requirements regarding
quality that are valid for the laboratory?
10.1 Assigning the Reference Value
The reference value, which is attributed to the quantity being measured in the samples
distributed within the given ILC/PT round, can be determined in various ways. In
practice, five scenarios are used by the organizers of ILC/PT.
10.2 By Formulation of Synthetic Test Samples
In this case, samples are used that are synthetically prepared, which means that the
chemical substances of high purity are dosed by weighing the exact portion on the
analytical balance. The known quantity of a pure substance can be mixed with other
components by weight, which allows production of samples with a known content
of the substance to be measured. A known quantity of a pure substance can be
added to a natural sample (matrix enrichment), which allows production of samples
mimicking the characteristics of test objects. The advantage of the production of
synthetic samples by weight is the known quantity of the added substance with
a great precision. It allows the measurement traceability to be determined and to
assign an uncertainty to the reference value. Despite these advantages, in practice,
it is not always possible to produce a synthetic material with expected metrological
characteristics. Often the problem is the mixing of various components so as to obtain
a satisfactory homogeneity of the sample. Moreover, it is not always possible to find
a natural sample that does not contain the analyte. In addition, attention should be
paid to the chemical form of a given substance as well as its affinity to the matrix.
10.3 The Use of Certified Reference Material
Although there is a limited number of available CRMs, whenever their matrix and the
content of analyte correspond to that of test samples analyzed in the laboratory, they
are considered to be the best option. The certified value for a given quantity, along
with its uncertainty, is used as a reference (assigned) value. In that case, besides
checking its competencies, the laboratory can also use the given CRM to ensure
measurement traceability. From the metrological perspective, the use of a CRM is an
ideal way to test the laboratory’s performance. However, the high price of the CRM
and their limited diversity means that they are seldom used by ILC/PT providers.
10 Interlaboratory Comparisons
the laboratory, especially those that are doubtful and unsatisfactory. This explains
the question as to what results should be qualified as doubtful or unsatisfactory and
whether the criteria used by the organizers always meets the requirements regarding
quality that are valid for the laboratory?
10.1 Assigning the Reference Value
The reference value, which is attributed to the quantity being measured in the samples
distributed within the given ILC/PT round, can be determined in various ways. In
practice, five scenarios are used by the organizers of ILC/PT.
10.2 By Formulation of Synthetic Test Samples
In this case, samples are used that are synthetically prepared, which means that the
chemical substances of high purity are dosed by weighing the exact portion on the
analytical balance. The known quantity of a pure substance can be mixed with other
components by weight, which allows production of samples with a known content
of the substance to be measured. A known quantity of a pure substance can be
added to a natural sample (matrix enrichment), which allows production of samples
mimicking the characteristics of test objects. The advantage of the production of
synthetic samples by weight is the known quantity of the added substance with
a great precision. It allows the measurement traceability to be determined and to
assign an uncertainty to the reference value. Despite these advantages, in practice,
it is not always possible to produce a synthetic material with expected metrological
characteristics. Often the problem is the mixing of various components so as to obtain
a satisfactory homogeneity of the sample. Moreover, it is not always possible to find
a natural sample that does not contain the analyte. In addition, attention should be
paid to the chemical form of a given substance as well as its affinity to the matrix.
10.3 The Use of Certified Reference Material
Although there is a limited number of available CRMs, whenever their matrix and the
content of analyte correspond to that of test samples analyzed in the laboratory, they
are considered to be the best option. The certified value for a given quantity, along
with its uncertainty, is used as a reference (assigned) value. In that case, besides
checking its competencies, the laboratory can also use the given CRM to ensure
measurement traceability. From the metrological perspective, the use of a CRM is an
ideal way to test the laboratory’s performance. However, the high price of the CRM
and their limited diversity means that they are seldom used by ILC/PT providers.