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Vol.
25 No. 2
March-April 2003
Metrological
Traceability of Measurement Results in Chemistry
In
recent years, the concept of "traceability" in chemical measurement
has received an extraordinary amount of attention. It has
been the theme of numerous workshops and symposia. It is frequently
used in the chemical literature as an accepted concept with
a seemingly obvious meaning. Demonstration of metrological
traceability is required in documents from the International
Standards Organization (ISO), International Laboratory Accreditation
Cooperation (ILAC), and the Bureau International des Poids
et Mesures (BIPM), among others, which makes its implementation
almost compulsory for any measurement laboratory. (In this
project, the systematic term "metrological traceability" is
used to distinguish the concept from other types of traceability
such as documentary traceability and material traceability.)
However, many people concerned with metrology in chemistry
tacitly or privately concur that there is no unequivocal,
internationally agreed definition of traceability of a measurement
result in chemistry. This is a rather remarkable state of
affairs because lack of clarity about such an important and
widely used concept makes it difficult to reach world-wide
agreement on its use. There are only a few examples of documented
traceability of chemical measurement results produced by field
laboratories, reference material producers, or even national
metrology institutes. The definition of traceability in the
International Vocabulary of Basic and General Terms in
Metrology (1993) reads:
traceability:
property of the
result of a measurement or the value of a standard whereby
it can be related to stated references, usually national
or international standards, through an unbroken sequence
of comparisons all having stated uncertainties (VIM 6.10ref:
BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, OIML, International
Vocabulary of Basic and Metrological Terms in Metrology,
2nd edition, ISO Genéve, 1993)
The
first step toward clarification of the concept "metrological
traceability" is to state some current problems:
-
the lack of understanding that metrological traceability
to a common stated metrological reference is a precondition
for comparability of measurement results
-
the widespread perception that a unit from the International
System of Units (SI) is the only possible stated metrological
reference in metrological traceability of chemical measurement
results
-
the claim that a measurement result can be metrologically
traceable to an institution
-
the lack of clarity of the distinction between metrological
traceability and measurement uncertainty
-
the confusion between the quantity value carried by an artifact
used as a calibrator and the artifact itself
-
the fact that few certified reference materials or CRM certificates
indicate the metrological traceability of the certified
quantity value(s)
-
the belief that the use of a (C)RM as a so-called "trueness
control" material establishes metrological traceability
-
the assumption that participation in an interlaboratory
comparison or proficiency testing scheme provides metrological
traceability of a measurement result
-
the belief that metrological traceability is a property
of a material, measurement method, or measurement procedure
-
the use of inaccurate language such as "traceability to
the SI"
-
the lack of a definition of the concept "traceability chain"
-
the lack of a definition of the concept "stated metrological
reference"
-
the perception that metrological traceability does not apply
to measurement results in routine laboratories
These
problems hamper communication about measurement. Numerous
interactions with analytical chemists around the world have
revealed that basic concepts in metrology, with very few exceptions,
are not included anymore in the text books of analytical chemistry.
This may be an underlying reason for many of the listed problems.
To
address this situation, IUPAC has approved a project on metrological
traceability, which has the following objectives:
-
to elucidate the concept "metrological traceability" of
a measurement result and list its characteristics, and to
describe the relations between metrological traceability
and other concepts such as calibration, measurement uncertainty,
and comparability
-
to formulate requirements for establishing metrological
traceability
- to
give specific examples of metrological traceability of chemical
measurement results
The
project task group is highly interdivisional and includes
members from the Commission on Isotope Abundances and Atomic
Weights of the Inorganic Chemistry Division; the Subcommittee
on Nomenclature, Properties, and Units in Laboratory Medicine
of the Chemistry and Human Health Division; the Interdivisional
Working Party for Harmonization of Quality Assurance; and
the Analytical Chemistry Division. A draft report will be
circulated extensively within IUPAC for comment. The amended
final version will be published in Pure and Applied Chemistry.
Interested readers are invited to consult the IUPAC Web site
and follow the link to the project home page, or consult with
the Task Group Chairman Paul De Biévre <[email protected]>.
www.iupac.org/projects/2001/2001-010-3-500.html
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last modified 6 March 2003.
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Applied Chemistry.
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