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6 Reference Materials in Chemical Measurements
6.15 Summary
There is no doubt that the use of RMs by laboratories is a crucial demand to ensure
the quality of the results of chemical measurements. It is worth noting that RMs
are often very expensive, so it often happens that laboratory management looks for
opportunities to reduce costs by limiting the expenses relating to CRMs. Thus, the
proper selection of the standards is essential to as to ensure the metrologically sound
reference values with the acceptable uncertainty within the economically reasonable
cost of the CRMs.
As previously emphasized several times, when choosing the appropriate RM one
should primarily take into consideration its similarity to the test material in respect to
the matrix, and in the content of the analyte. The uncertainty assigned to the reference
value should be evaluated in respect to the expected uncertainty budget.
Each reference material should be accompanied by the appropriate document,
often called a certificate or a certificate of reference material. According to ISO
17034, this document should include relevant information—namely, it must contain
a reference value and its assigned uncertainty and information on the measurement
traceability for the reference value (in the case of certified material). It is also expected
that the manufacturer should give information on the aim of the use of the material
and the storage conditions. In the case of chemical measurements, it is important
to correctly determine the measured quantity. It is important to know whether the
reference value is independent of the measurement procedure (e.g., determination of
the element after the total digestion of soil samples), or is operationally defined (e.g.,
determination of the content of an element after the extraction of its water-soluble
compounds). The RMP should clearly specify what the reference value refers to, and
the laboratory should be thoroughly familiar with this information. Otherwise, you
can expect that the laboratory will not be able to reproduce the expected reference
value. There have been cases when the manufacturer gives the reference value in
relation to the dried material at a suitable temperature, which can result from a
hygroscopic nature of the substance. This means that the drying of the material
under specified conditions should be included in the measurement procedure.
It is important to also have in mind the physical form of the RM; for example, in
the case of powdered solids, it is the size of grains. The certificate should contain
information about the minimum weight, which guarantees acceptable homogeneity
of the substance. In this case, misplaced saving as well as retrieving smaller portions
of the material can lead to erroneous results. As mentioned previously, it is important
to store the material according to the producer’s instructions.
In addition to the described issues relating to the proper use of RMs, it is also
important to evaluate other properties of the used RMs. Several analytes are known
to be present in the form of different chemical compounds, which is the objective
of the study of chemical speciation. The use of RMs containing given element in
various chemical forms can lead to incorrect results, especially where the detector
response depends on the type of compound in which the element is present.
6 Reference Materials in Chemical Measurements
6.15 Summary
There is no doubt that the use of RMs by laboratories is a crucial demand to ensure
the quality of the results of chemical measurements. It is worth noting that RMs
are often very expensive, so it often happens that laboratory management looks for
opportunities to reduce costs by limiting the expenses relating to CRMs. Thus, the
proper selection of the standards is essential to as to ensure the metrologically sound
reference values with the acceptable uncertainty within the economically reasonable
cost of the CRMs.
As previously emphasized several times, when choosing the appropriate RM one
should primarily take into consideration its similarity to the test material in respect to
the matrix, and in the content of the analyte. The uncertainty assigned to the reference
value should be evaluated in respect to the expected uncertainty budget.
Each reference material should be accompanied by the appropriate document,
often called a certificate or a certificate of reference material. According to ISO
17034, this document should include relevant information—namely, it must contain
a reference value and its assigned uncertainty and information on the measurement
traceability for the reference value (in the case of certified material). It is also expected
that the manufacturer should give information on the aim of the use of the material
and the storage conditions. In the case of chemical measurements, it is important
to correctly determine the measured quantity. It is important to know whether the
reference value is independent of the measurement procedure (e.g., determination of
the element after the total digestion of soil samples), or is operationally defined (e.g.,
determination of the content of an element after the extraction of its water-soluble
compounds). The RMP should clearly specify what the reference value refers to, and
the laboratory should be thoroughly familiar with this information. Otherwise, you
can expect that the laboratory will not be able to reproduce the expected reference
value. There have been cases when the manufacturer gives the reference value in
relation to the dried material at a suitable temperature, which can result from a
hygroscopic nature of the substance. This means that the drying of the material
under specified conditions should be included in the measurement procedure.
It is important to also have in mind the physical form of the RM; for example, in
the case of powdered solids, it is the size of grains. The certificate should contain
information about the minimum weight, which guarantees acceptable homogeneity
of the substance. In this case, misplaced saving as well as retrieving smaller portions
of the material can lead to erroneous results. As mentioned previously, it is important
to store the material according to the producer’s instructions.
In addition to the described issues relating to the proper use of RMs, it is also
important to evaluate other properties of the used RMs. Several analytes are known
to be present in the form of different chemical compounds, which is the objective
of the study of chemical speciation. The use of RMs containing given element in
various chemical forms can lead to incorrect results, especially where the detector
response depends on the type of compound in which the element is present.