6.12 The Reference Value
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6.12 The Reference Value
In the case of CRMs, it is extremely important to pay attention to information on the
aim of its use. The reference value specified in the certificate of RMs may be used
independently from the procedure of sample preparation and/or the methods of measurement, or may be dependent on the measurement procedure. A good example is
the set of RMs for the determination of metals in soil. It is known that, depending on
the extraction medium used, the efficiency of the extraction varies for different chemical compounds in which the metal is present in the soil. In some cases, RMPs set a
reference value assigned to a defined procedure for sample preparation. Therefore,
sample preparation should be conducted in accordance with the described procedure
guide to restore the value specified in the certificate. For example, in the documentation provided with “Soil CMI 7001 (light sandy soil),” three reference values are
given for the determination of lead. These values (certified value in mg/kg and the
assigned uncertainty in percentage) correspond respectively to the lead leaching:
with nitric acid in room temperature (20.7 mg/kg of ±3%), with nitric acid on heating plate (23.7 mg/kg ± 6%) and using aqua regia (24.1 mg/kg ± 7%). This example
shows the importance of a detailed analysis of the contents of the certificate of the
purchased RM.
6.13 The Shelf Life of Reference Materials
The RMP is obliged to periodically study the material being sold, and if statistically
significant changes are found for a given property, they are obliged to inform their
clients (laboratories). It is expected that RMPs should provide the term of validity,
which means that only in this period is the durability of specific properties assured.
There is a question as to whether it is possible to use a RM after its expiration
date, and if so, under which conditions. A few years ago, it was a common opinion that no material could be used after that date and laboratories were discarding
large amounts of very expensive chemicals. Currently, the commonsense approach
prevails, where the quality control approach can be used to answer this question.
It is recommended to regularly examine the RM used in the laboratory; thus, systematically collected information allows an assessment of the applicability of the
material beyond its expiration date. If the reference value has not changed in terms
of its assigned uncertainty, there is good reason to extend the validity of the material
within the individual laboratory. In this case, it is necessary to carry out the evidence
of confirming the stability of the reference value and to prepare a report confirming
the extension of the validity of the material.
81
6.12 The Reference Value
In the case of CRMs, it is extremely important to pay attention to information on the
aim of its use. The reference value specified in the certificate of RMs may be used
independently from the procedure of sample preparation and/or the methods of measurement, or may be dependent on the measurement procedure. A good example is
the set of RMs for the determination of metals in soil. It is known that, depending on
the extraction medium used, the efficiency of the extraction varies for different chemical compounds in which the metal is present in the soil. In some cases, RMPs set a
reference value assigned to a defined procedure for sample preparation. Therefore,
sample preparation should be conducted in accordance with the described procedure
guide to restore the value specified in the certificate. For example, in the documentation provided with “Soil CMI 7001 (light sandy soil),” three reference values are
given for the determination of lead. These values (certified value in mg/kg and the
assigned uncertainty in percentage) correspond respectively to the lead leaching:
with nitric acid in room temperature (20.7 mg/kg of ±3%), with nitric acid on heating plate (23.7 mg/kg ± 6%) and using aqua regia (24.1 mg/kg ± 7%). This example
shows the importance of a detailed analysis of the contents of the certificate of the
purchased RM.
6.13 The Shelf Life of Reference Materials
The RMP is obliged to periodically study the material being sold, and if statistically
significant changes are found for a given property, they are obliged to inform their
clients (laboratories). It is expected that RMPs should provide the term of validity,
which means that only in this period is the durability of specific properties assured.
There is a question as to whether it is possible to use a RM after its expiration
date, and if so, under which conditions. A few years ago, it was a common opinion that no material could be used after that date and laboratories were discarding
large amounts of very expensive chemicals. Currently, the commonsense approach
prevails, where the quality control approach can be used to answer this question.
It is recommended to regularly examine the RM used in the laboratory; thus, systematically collected information allows an assessment of the applicability of the
material beyond its expiration date. If the reference value has not changed in terms
of its assigned uncertainty, there is good reason to extend the validity of the material
within the individual laboratory. In this case, it is necessary to carry out the evidence
of confirming the stability of the reference value and to prepare a report confirming
the extension of the validity of the material.