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6 Reference Materials in Chemical Measurements
reference value assigned to the CRM and the value obtained in the laboratory using
a given measurement procedure. A well-selected RM should reflect the behavior of
the real samples, indicating that the content of determined substances, the content of
the matrix, and the physical form of the reference should be the same as or close to
the test sample.
The similarity (or the lack of it) of the RM to the test sample is an extremely
important criterion for selection. For this, the term ‘commutability’ is used, defining
the degree of conformity (similarity) of properties and behavior of the RM and sample
in the measurement procedure.
Commutability: the property of a reference material, demonstrated by the
equivalence of the mathematical relationships among the results of different
measurement procedures for reference materials and for representative samples
of the type intended to be measured.
Clause 2.1.20, ISO Guide 30:2015
In fact, the lack of full compliance of characteristics of the RM and test sample
should be considered as a source of uncertainty, which should be then included in the
uncertainty budget. It is not easy selecting the ideal RM that meets the requirements
of full compliance. This is due to a number of constraints, which are explained below.
6.8 The Type of Reference Material Versus the Type
of Sample
The RMPs offer RMs produced with the use of different kind of matrices; for example,
water, soil or food products. At first glance, it seems that for the laboratory dealing
with the examination of soil, an available reference material prepared from the soil
would be straightforward. By examining this in more detail, it is essential to consider
the kind of soil; for example, the content of organic matter or the content of silica
in the soil from which the RM was prepared. Extraction processes work differently
in the case of sandy soils, and otherwise in the case of soils containing substantial
amounts of humic substances. Similarly, when purchasing a RM of milk powder,
we must consider the content of fat; and, for geological materials, the presence of
various minerals. Where the composition of the matrix of the sample and the RM
is different, it can have a significant effect on the extraction process, and hence the
effectiveness of transferring the substance to be examined into solution.
Another problem is the physical form of the RM. In many cases, manufacturers
prepare materials in the form of a particulate powder of a high homogeneity. Such
material is certainly very valuable from the point of view of its metrological quality,
but, soil raw samples are after all not finely powdered and as well homogeneous as
RMs. The question then arises whether the extraction process of the analyte from the
6 Reference Materials in Chemical Measurements
reference value assigned to the CRM and the value obtained in the laboratory using
a given measurement procedure. A well-selected RM should reflect the behavior of
the real samples, indicating that the content of determined substances, the content of
the matrix, and the physical form of the reference should be the same as or close to
the test sample.
The similarity (or the lack of it) of the RM to the test sample is an extremely
important criterion for selection. For this, the term ‘commutability’ is used, defining
the degree of conformity (similarity) of properties and behavior of the RM and sample
in the measurement procedure.
Commutability: the property of a reference material, demonstrated by the
equivalence of the mathematical relationships among the results of different
measurement procedures for reference materials and for representative samples
of the type intended to be measured.
Clause 2.1.20, ISO Guide 30:2015
In fact, the lack of full compliance of characteristics of the RM and test sample
should be considered as a source of uncertainty, which should be then included in the
uncertainty budget. It is not easy selecting the ideal RM that meets the requirements
of full compliance. This is due to a number of constraints, which are explained below.
6.8 The Type of Reference Material Versus the Type
of Sample
The RMPs offer RMs produced with the use of different kind of matrices; for example,
water, soil or food products. At first glance, it seems that for the laboratory dealing
with the examination of soil, an available reference material prepared from the soil
would be straightforward. By examining this in more detail, it is essential to consider
the kind of soil; for example, the content of organic matter or the content of silica
in the soil from which the RM was prepared. Extraction processes work differently
in the case of sandy soils, and otherwise in the case of soils containing substantial
amounts of humic substances. Similarly, when purchasing a RM of milk powder,
we must consider the content of fat; and, for geological materials, the presence of
various minerals. Where the composition of the matrix of the sample and the RM
is different, it can have a significant effect on the extraction process, and hence the
effectiveness of transferring the substance to be examined into solution.
Another problem is the physical form of the RM. In many cases, manufacturers
prepare materials in the form of a particulate powder of a high homogeneity. Such
material is certainly very valuable from the point of view of its metrological quality,
but, soil raw samples are after all not finely powdered and as well homogeneous as
RMs. The question then arises whether the extraction process of the analyte from the