80
6 Reference Materials in Chemical Measurements
accepted uncertainty of measurement was set at 1%, which means that it is
necessary to purchase a RM with the lowest available uncertainty of the certified
value (NIST gives 99.8% ± 0.2% for the RM 8467).
6.11 Proper Use of Reference Materials
The most important features of a RM should be its homogeneity and stability, and
additionally, in the case of CRMs, the reference value with assigned uncertainty.
Homogeneity should cover both the material in the single container, and the materials in various containers with the same batch. The stability of a material primarily
concerns the stability of certain of its properties, and in the case of a CRM also refers
to the stability of the certified value and its uncertainty.
The homogeneity of the material ensures that its subsequent portions taken from
the container carry the same value for that property. Manufacturers must therefore
determine the degree of homogeneity of the material, which in practice means that
it is necessary to state in the certificate the smallest weighed portion of sample that
guarantees the representativeness of the weighed portions of the material. The degree
of homogeneity of the RM means that only using a single portion, no less than the
manufacturer specifies as “the minimum amount of sample to be used,” guarantees
recovery of the reference value with assigned uncertainty.
The stability of the material depends on the type of matrix as well as on the
environmental conditions used for its storage (e.g., temperature, humidity). The RM
should be stable during the period of its validity, according to the manufacturer’s
statement. The producer should provide the required storage conditions of the material to guarantee the stated stability. These requirements typically involve the storage
temperature of the material, for example, below −20 °C; in the range of from 1 to
5 °C; below 20 °C. They may also relate to humidity, especially for those materials
for which an increase in humidity can change its properties. In the case of chemicals
responsive to light radiation, storage of the material may also be required to be in a
dark and/or light-tight container. The manufacturer may also provide special requirement for processing before using the material (e.g., drying under certain conditions,
agitation).
Maintenance of the properties of the reference material is only guaranteed
when the material is stored in the conditions specified by the producer.
The validity period given by RMPs guarantees the characteristics of the material
at the first opening of the sealed container. After opening, the responsibility for the
proper handling of the material falls on the user. It is important to ensure that, each
time, only a portion of the necessary material is taken. In any case, the residues
should not be re-introduced to the original packaging.
6 Reference Materials in Chemical Measurements
accepted uncertainty of measurement was set at 1%, which means that it is
necessary to purchase a RM with the lowest available uncertainty of the certified
value (NIST gives 99.8% ± 0.2% for the RM 8467).
6.11 Proper Use of Reference Materials
The most important features of a RM should be its homogeneity and stability, and
additionally, in the case of CRMs, the reference value with assigned uncertainty.
Homogeneity should cover both the material in the single container, and the materials in various containers with the same batch. The stability of a material primarily
concerns the stability of certain of its properties, and in the case of a CRM also refers
to the stability of the certified value and its uncertainty.
The homogeneity of the material ensures that its subsequent portions taken from
the container carry the same value for that property. Manufacturers must therefore
determine the degree of homogeneity of the material, which in practice means that
it is necessary to state in the certificate the smallest weighed portion of sample that
guarantees the representativeness of the weighed portions of the material. The degree
of homogeneity of the RM means that only using a single portion, no less than the
manufacturer specifies as “the minimum amount of sample to be used,” guarantees
recovery of the reference value with assigned uncertainty.
The stability of the material depends on the type of matrix as well as on the
environmental conditions used for its storage (e.g., temperature, humidity). The RM
should be stable during the period of its validity, according to the manufacturer’s
statement. The producer should provide the required storage conditions of the material to guarantee the stated stability. These requirements typically involve the storage
temperature of the material, for example, below −20 °C; in the range of from 1 to
5 °C; below 20 °C. They may also relate to humidity, especially for those materials
for which an increase in humidity can change its properties. In the case of chemicals
responsive to light radiation, storage of the material may also be required to be in a
dark and/or light-tight container. The manufacturer may also provide special requirement for processing before using the material (e.g., drying under certain conditions,
agitation).
Maintenance of the properties of the reference material is only guaranteed
when the material is stored in the conditions specified by the producer.
The validity period given by RMPs guarantees the characteristics of the material
at the first opening of the sealed container. After opening, the responsibility for the
proper handling of the material falls on the user. It is important to ensure that, each
time, only a portion of the necessary material is taken. In any case, the residues
should not be re-introduced to the original packaging.