1.3 Metrological Requirements in Chemical Measurements
11
1.3 Metrological Requirements in Chemical Measurements
The evaluation of the value of the measured quantity or—as an analytical chemist
would refer to it—the determination of the content of the analyte in the sample usually
requires the sampling of a suitable amount of the matter from the representative object
delivered to the laboratory. This is followed by the conduct of a whole set of physical
and chemical processing enabling, among other things, the isolation of the analyzed
substance from the matrix, if necessary its possible concentration and/or change of its
chemical form, and ending in the measurement itself for the prepared sample. All the
stages involved have a significant influence on the final result of the determination.
Evaluating the uncertainty of the final result requires a detailed knowledge of all the
steps of the applied measuring procedure, which enforces the need to describe all of
its components. According to the principles of metrology, a critical examination of
the individual steps of the measuring procedure is one of the more important aspects
allowing the evaluation of the quality of the result. It is one of the most important
proofs that confirms the competence of the laboratory. The application of the basic
principles of metrology to the chemical measurement is not an easy task, most of all
because chemical measurements are different from the physical measurements (as
discussed earlier). In many cases, it is not possible to directly fulfil all the metrological
requirements, hence the need to use the best conduct in a given area, such that it can
be and is accepted by all interested parties.
In practice, it means that, for example, if there is a lack of reference material ideally
aligned with the test object, carefully selected reference material is used that differs
to an acceptable extent. It is allowed and justified only when all laboratories that
carry out such tests use the same reference material, traceable to the same standard
and the client (e.g., a ministry) is informed about it and accepts such an agreement.
However, the laboratory staff should always do their best to carry out the
test/measurements to the best of their knowledge in the area of metrology principles, and try to always follow these principles to the greatest possible extent. In case
of the need to accept deviation, staff should have full awareness of their influence on
the final result of the measurement procedure.
Two important definitions (ISO/IEC Guide 99)
Quantity: property of a phenomenon, body, or substance, where the property
has a magnitude that can be expressed as a number and a reference
A reference can be a measurement unit, a measurement procedure, a reference
material, or a combination of such. [Clause 1.1; ISO/IEC Guide 99]
Measurand: quantity to be measured.
The specification of a measurand requires knowledge of the kind of quantity,
description of the state of the phenomenon, body, or substance carrying the
quantity, including any relevant component, and the chemical entities involved.
[Clause 2.3; ISO/IEC Guide 99]
11
1.3 Metrological Requirements in Chemical Measurements
The evaluation of the value of the measured quantity or—as an analytical chemist
would refer to it—the determination of the content of the analyte in the sample usually
requires the sampling of a suitable amount of the matter from the representative object
delivered to the laboratory. This is followed by the conduct of a whole set of physical
and chemical processing enabling, among other things, the isolation of the analyzed
substance from the matrix, if necessary its possible concentration and/or change of its
chemical form, and ending in the measurement itself for the prepared sample. All the
stages involved have a significant influence on the final result of the determination.
Evaluating the uncertainty of the final result requires a detailed knowledge of all the
steps of the applied measuring procedure, which enforces the need to describe all of
its components. According to the principles of metrology, a critical examination of
the individual steps of the measuring procedure is one of the more important aspects
allowing the evaluation of the quality of the result. It is one of the most important
proofs that confirms the competence of the laboratory. The application of the basic
principles of metrology to the chemical measurement is not an easy task, most of all
because chemical measurements are different from the physical measurements (as
discussed earlier). In many cases, it is not possible to directly fulfil all the metrological
requirements, hence the need to use the best conduct in a given area, such that it can
be and is accepted by all interested parties.
In practice, it means that, for example, if there is a lack of reference material ideally
aligned with the test object, carefully selected reference material is used that differs
to an acceptable extent. It is allowed and justified only when all laboratories that
carry out such tests use the same reference material, traceable to the same standard
and the client (e.g., a ministry) is informed about it and accepts such an agreement.
However, the laboratory staff should always do their best to carry out the
test/measurements to the best of their knowledge in the area of metrology principles, and try to always follow these principles to the greatest possible extent. In case
of the need to accept deviation, staff should have full awareness of their influence on
the final result of the measurement procedure.
Two important definitions (ISO/IEC Guide 99)
Quantity: property of a phenomenon, body, or substance, where the property
has a magnitude that can be expressed as a number and a reference
A reference can be a measurement unit, a measurement procedure, a reference
material, or a combination of such. [Clause 1.1; ISO/IEC Guide 99]
Measurand: quantity to be measured.
The specification of a measurand requires knowledge of the kind of quantity,
description of the state of the phenomenon, body, or substance carrying the
quantity, including any relevant component, and the chemical entities involved.
[Clause 2.3; ISO/IEC Guide 99]