Project Details:

Metrological traceability of measurement results in chemistry (revised title)

Project No.:2001-010-3-500
Start date:2001-12-01
End date:2011-06-15
Division:Analytical Chemistry Division
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:

> project announcement published inChem.Int.25(2), 2003

> 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 until29 February 2008 >> see provisionalrecommendations [abstract07/fajgelj_290208.html]

> October 2008 report update (pdf file - 12KB)

> July 2009 - A revised draft of the provisional recommendations is submitted to public review comments until 31 October 2009 (> provisionalrecommendations at ../provisional/abstract09/debievre_311009.html)

> June 2011 - project completed -  published as Technical Report titled "Metrological traceability of measurement results in chemistry: Concepts and implementation" published in Pure Appl. Chem. ASAP article doi:10.1351/PAC-REP-07-09-39 (online 2011-06-15)

Chairman: