INTERDIVISIONAL
WORKING PARTY ON HARMONIZATION OF QUALITY ASSURANCE
Number: 2001-010-3-500
Title: Metrological traceability of measurement results in chemistry
(revised title)
Task Group
Chairman: P. De Bi�vre
Member: R. Dybkaer,
A. Fajgelj, and D.B.
Hibbert
Objective:
To develop and present to the chemical scientific society a concept
for traceability of chemical measurements as a foundation for the introduction
of metrology in chemistry on which the field practice can be built.
A concept will be underpinned with examples (various scenarios) for
establishing traceability in chemical measurement.
Description:
Traceability as currently defined is a "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 chain of comparisons all having stated uncertainties". (International
Vocabulary of Basic and General Terms in Metrology, ISO, Geneva, 1993)
Traceable measurement results, accompanied with their associated measurement
uncertainty, form the most solid basis for comparability of results
at national or international level.
Problem:
Treaceability concept has been well developed for measurement results
in physics but not for the chemical field. There are various reasons
for the existing situation in chemical measurement practices. They can
be split in at least two types: technical and communicational. Technical
reasons include:
a) a large number of available measurement techniques (methods) for
the same purpose, where at the same time calibration pathways are different
(often instrument dependent);
b) a large number of combinations: measurand - matrix - method influencing
a measurement process;
c) a lack of tools (calibrators, primary methods) for establishing a
complete or direct traceability chain;
d) lack of clear practical examples of complete traceability chains
in chemical measurement;
e) a quantity of interest 'amount of substance' is in chemical practice
realized indirectly through measuring other quantities (mass, electric
current, etc.) and application of appropriate conversions; the approach
which is completely acceptable, but sometimes preventing straightforward
understanding of the related traceability chain;
f) improper definition of the stated reference
Communicational reasons are:
a) misinterpretations of the above definition of traceability;
b) a lack of properly defined, commonly understood and internationally
accepted concepts in establishing traceability chain in chemical measurement,
and
c) terminology: a variety of terms is defined by VIM (standard, reference
material, calibration standard, measurement standard, and many more)
but in communications often not used with their exact meaning;
d) current metrological terminology (VIM) mainly suits measurements
in physics.
In addition a number of commercially available calibration sources
(CRMs, calibration solutions, etc.) is often lacking a proper information
on their traceability.
Traceability, as defined by VIM, is a property of the result of a measurement
or the value of a standard. In both cases traceability applies to values.
Thus traceability chains consist of values. A complete traceability
chain should end in the value of a basic (or derived) unit of the international
system of units (i.e. SI) or of another appropriate unit. Traceability
is to values produced by National Measurement Institutes or to the values
assigned to CRMs under the responsibility of the producers. Traceability
is not established to National Measurement Institutes or CRM producers
themselves. What exists is traceability to and through their values
- stated reference. So these organizations are the first that have to
be able to demonstrate traceability of the values which they assign
to their standards or CRMs.
Solution:
There have already been several attempts to clarify the theoretical
part of the problem as well as some proposals for practical approaches
in establishing traceability (EURACHEM, CITAC, etc.) Unfortunately these
approaches were not harmonized and they mostly fail in describing a
complete traceability chain.
Aim
To develop a concept for traceability of chemical measurements underpinned
with examples (various scenarios) for establishing traceability in chemical
measurement. Project is aimed to provide clarification related to the
term traceability and to smooth its proper practical application in
chemical measurement. The following is the project frame:
- State the problem
- Traceability (importance)
- Characteristics of traceability (calibration hierarchy, stated reference,
value, uncertainty, comparison, measurement procedure, calibrator, measuring
system, calibration, value determination, measurement)
- Requirements for establishing traceability (models, examples)
- Function of RMs in the calibration hierarchy
- Function of reference measurement procedures in the calibration hierarchy
- Current infrastructure (BIPM, NMIs, etc.)
- Relation of traceability to proficiency testing, laboratory comparisons,
etc.
Progress:
A draft document is under preparation, and it is expected that early
2005 the document can be made available for public review and before
publication.
> Nov 2004 report update (pdf
file - 11KB)
An article published in Chem Int Sep-Oct 2005 presents the problem
tackled > see
full text
See also follow-up letter and reply (Vol.
28 No. 2, Feb-Mar 2006)
A presentation was made at the IUPAC-APAT workshop in March 2006 >
link to calendar to access presentations
> May 2006 report update (pdf
file - 14KB)
> Sep 2007 - A draft is submitted to public review comments until
29 February 2008 >> see provisional
recommendations
Last update: 16 October 2007
<project announcement published in
Chem.
Int.
25(2), 2003>