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Accred Qual Assur (2003) 8:448 453
DOI 10.1007/s00769-003-0682-0
Wolfgang Richter
Bernd G ttler
Received: 21 March 2003
Accepted: 26 July 2003
Published online: 16 September 2003
' Springer-Verlag 2003
W. Richter • B. G ttler ( ✉)
Physikalisch-Technische
Bundesanstalt (PTB),
Bundesallee 100,
38116 Braunschweig,
Germany
e-mail: [email protected]
Tel.: +49 531 592 3200
Fax: +49 531 592 3015
W. Richter
Stadeweg 1,
38106 Braunschweig,
Germany
A national traceability system
for chemical measurements
Abstract Current developments in
Germany for establishing a traceability system for chemical measurements are reported. The focus is on a
dissemination mechanism which employs chemical calibration laboratories accredited within the framework
of the German Calibration Service
(DKD) and acting as multipliers
between the national standards level
and the user level by providing the
user with calibration means which
are traceable to the SI via national
standards. At the national standards
level, a network of high-level chemistry institutes coordinated by the national metrology institute, PTB, provides the primary references for
chemical measurements.
The use of the metrological dissemination system provided by the
Introduction
The continuing globalization of trade and economy requires confidence in measurement results of any kind,
including chemical measurements. Chemical measurement results in particular are often the basis for decisions
and agreements, for example in health care, environmental protection and international trade and must therefore
be reliable and trustworthy.
An important prerequisite for confidence in measurement results is knowledge of the measurement uncertainty, based on traceability to recognized references,
ideally to the SI units. Traceability of chemical measurement results has therefore become a key issue in the
last decade, and its establishment in an organized and
structured way is an important goal in all industrialized
DKD also for chemical measurements is a logical extension of a
traceability mechanism, successful
for more than two decades in general
metrology, to metrology in chemistry. In detail, traceability structures
in clinical chemistry, electrochemistry, elemental analysis and gas analysis are described. This system has
become an important part of the efforts made in Germany to support
chemical laboratories in meeting the
traceability requirements of the market and of legal regulations.
Keywords National traceability
system • Chemical calibration
laboratories • Clinical chemistry •
Electrochemistry • Elemental
analysis • Gas analysis
countries. This increasingly also holds for emerging
economies.
Due to the great variety and complexity of chemical
measurement tasks, the establishment of traceability in
the field of chemical analysis is more difficult than in
other areas of metrology and therefore requires concentration of the efforts on the most urgent demands for
traceability. It is the central aim of the CIPM Consultative Committee for Amount of Substance (CCQM),
which today is the leading organization for traceability
issues of chemical measurements, to promote and harmonize an international primary reference framework for
the most important chemical measurement tasks. This is
an ongoing process which recently gained additional impetus from the Mutual Recognition Arrangement for national measurement standards and for calibration and
142
W. Richter • B. G ttler
measurement certificates issued by national metrology
institutes (CIPM-MRA) [1], drawn up by the CIPM under the Metre Convention in 1999 in order to raise the
confidence in measurement results of any kind and hence
their acceptance. The CIPM-MRA is the response of metrology to the globalization of the markets.
In order to disseminate the units of measurement to
the field laboratories in an efficient way, traceability infrastructures are necessary, first within national frameworks. In the field of chemistry the examples of traceability chains available in general metrology, for example, in
length measurement, which consist of a considerable
number of intermediate steps in the form of artefacts arranged in a hierarchy, are not optimally suited because
chemical analysis is largely method oriented. It is, however, very important that there is at least one intermediate
level in a chemical traceability chain which acts as a multiplier to the user level since it is impossible for the small
number of institutes at the primary level to meet directly
the ever increasing demand for traceability of chemical
measurements. Germany started about 10 years ago to set
up a traceability system for chemical measurements including calibration laboratories accredited within the
framework of the German Calibration Service (DKD) as
such multipliers. This is described in the following.
Structural principle of the traceability system
At present the traceability system consists of structures
(traceability chains) in the fields of clinical chemistry,
electrochemistry and gas analysis. A traceability structure for elemental analysis is under development. Figure 1 shows the structural principle of the traceability
system, which is applied to all the fields mentioned.
It consists of three levels. At the top of the structure a
network of national laboratories provides the primary
chemical measurement standards and ensures that these
are linked up with the international reference framework
for chemical measurements. Via primary reference materials and reference measurements, a secondary level consisting of accredited chemical calibration laboratories,
including verification authorities in the regulated area, is
connected to the national standards level.
This secondary or intermediate level has an important
multiplier function. It is firmly linked to the national
standards and provides traceable calibration means
(mainly certified reference materials) and test samples to
the workshop level, which consists essentially of chemical testing laboratories (including medical laboratories)
which are required to give evidence to their customers
that their measurement results are traceable to recognized references. In the case of medical laboratories, the
traceability requirement also has a legal background.
This multiplier function is of growing importance because it will not be possible in future for the national lab-
Fig. 1 General structure of the traceability system for chemical
measurements
Fig. 2 Structure of the national standards network for chemical
measurements. The double arrows indicate formal agreements between PTB and the three network partners, in which the division
of labour and the contributions to the network are defined
oratories at the top of the traceability chain to serve the
users directly, due to the growing demand for traceability
in the field of chemistry. It was the growing demand for
traceability in metrology in general that led to the establishment of the calibration services some 20 years ago
after the calibration workload had become unbearable
for the national metrology institutes. Now we are facing
a similar situation in metrology in chemistry, although
here other paths of dissemination exist as well.
National standards network for chemical
measurements
The network at the top of the traceability system providing the primary standards for chemical measurements
consists at present of four institutes as shown in Fig. 2.
The national metrology institute, PTB, coordinates
the network on the basis of its legal mandate and its
competence in those parts of chemical analysis which are
relevant to the major demands for traceability. The contributions to the network as listed in the boxes are based
on agreements between PTB and the three network part-
A national traceability system for chemical measurements
ners, the Federal Institute for Materials Research and
Testing (BAM), the Federal Environmental Agency
(UBA) and the German Society for Clinical Chemistry
(DGKC), represented by the Reference Institute for Bioanalysis. In these agreements parts of PTB s responsibility for the national measurement standards, for which
PTB does not have the necessary resources, are transferred to partner institutes, at which these resources are
available. The division of labour is permanently under
review.
With the joint capabilities of this network, the following sectors corresponding to the CCQM priority list of
areas where traceability of chemical measurements is
particularly important, can be addressed:
Health care
Environmental protection
Advanced materials
Commodities
Forensics
The sector Food which is also on the CCQM priority
list is, however, not yet covered by the network. One reason is that this sector is strongly regulated by legislation
which restricts the possibilities of adapting to new developments, even if these are regarded as useful improvements.
An important task of the network is also to ensure that
the national references are firmly linked up with the international reference framework for chemical measurements, represented by the results of the CCQM key comparisons listed in the BIPM key comparison database
(KCDB). For this purpose, every network member takes
part in CCQM key comparisons on its own responsibility
and submits its own calibration and measurement capabilities (CMC) to the international evaluation procedure
for entry into the KCDB. The key comparisons and the
CMCs form the technical basis of the CIPM-MRA.
To have a network of laboratories at the top of a traceability system for chemical measurements instead of just
the national metrology institute seems to be a requirement typical of metrology in chemistry and is under consideration in many industrialized countries, because the
competence for chemical analysis in most countries (except U.S.A.) largely lies outside the domain of the metrology institutes. Another example that underpins this
view is the development of metrology in chemistry in
Switzerland, where the Swiss Federal Office of Metrology and Accreditation (METAS) and the Swiss Federal
Laboratories for Materials Testing and Research (EMPA)
jointly provide the national references for chemical measurements [2].
143
Accredited chemical calibration laboratories
as “multipliers”
The purpose of the national standards network is to provide the primary measurement standards as references
for the measurements carried out on the workshop floor.
In order to reach the working level in an efficient way, a
dissemination mechanism is required.
It is obvious to think of accredited calibration laboratories as the most important link to the working level
also in the field of chemical measurements, after the
concept of the dissemination of the national measurement standards via accredited calibration laboratories
has been successfully applied for more than two decades
in metrology in general.
The calibration laboratories in question are accredited
within the framework of the German Calibration Service
(DKD). The competence of the DKD calibration laboratories for their dissemination tasks is above all based on
two central requirements for which the laboratories are
thoroughly assessed before accreditation:
1. A firm link must exist to the national institute, in metrology in chemistry to the national standards network, at the primary level via transfer standards (e.g.
reference measurements and/or reference materials)
or other means by which traceability to the national
standards is ensured.
2. The laboratory must demonstrate its capabilities in
comparison measurements with the national institute,
here again the network, on real laboratory samples
and provide a complete uncertainty budget according
to the ISO Guide on the Expression of Uncertainty in
Measurement (GUM) for its calibrations, i.e. value
assignments to the reference materials or other calibrators which, in its capacity as a calibration laboratory, it is going to supply to the field laboratories (e.g.
testing laboratories).
Meeting these stringent requirements enables the chemical calibration laboratories to act as providers of calibration means at the secondary level. This task requires that
the value assignment to the reference materials and other
calibration means provided to the user is more accurate
than the measurement results at the user level need to be.
Chemical calibration laboratories also exist in other
countries, for example in the Netherlands and in the U.K.
At present the state of accreditation of chemical calibration laboratories in Germany within DKD in accordance with EN 45 001, now ISO/EC 17025, is as follows:
Two calibration laboratories for pH measurement.
Two calibration laboratories for electrolytic conductivity measurement.
Two calibration laboratories for measurands of clinical chemistry. Further accreditations are under way.
One accreditation for gas analysis is under way.
144
W. Richter • B. G ttler
The experience so far gained with this approach to an efficient traceability system for chemical measurements is
very positive, although the system, and particularly the
dissemination mechanism, are still in an early stage of
development. In the following the traceability structures
already available are described.
ing level mainly via ring tests on well characterized samples (undisclosed to the participants), which are traceable
to the primary references, within the framework of the socalled external quality assurance as required by the B˜K
guidelines. In the case of measurands for which accredited calibration laboratories exist at the intermediate level
according to Fig. 1, a two-step procedure is used:
Clinical chemistry (laboratory medicine)
1. The calibration laboratory is connected to the national
standards level via comparison measurements on laboratory samples taken from the calibration laboratory,
which are analysed by the national standards laboratory (PTB or DGKC) and the calibration laboratory to
be accredited or re-evaluated. Agreement within predefined limits is required as a proof of the competence of the calibration laboratory. The sample with
the known value (the national laboratory s value) is
then used by the calibration laboratory as measurement standard for its work. It is the advantage of this
kind of transferring standards over the transfer of reference materials from the shelf that these standards
perfectly match the matrices occurring in the calibration laboratory.
2. The calibration laboratory in turn provides the calibrated test samples for the ring tests to the medical
laboratories.
The quality assurance guidelines issued by the Federal
Physicians Council (B˜K) and based on the medical
products legislation is the main driving force behind metrology in clinical chemistry. The aim is to increase the
reliability and recognition of the chemical measurements
carried out for diagnostic and therapeutic purposes by
the numerous medical laboratories as part of the German
health care system. The central goal is to minimize repeat measurements and hence costs and physical strain
on patients. The key to higher reliability and recognition
is demonstrated traceability to recognized standards, as
far as possible to the SI units, and a structured system
ensuring this traceability, in addition to a fully implemented quality assurance system [3]. The traceability requirement is further supported by a new EU directive on
in vitro diagnostics, which requires traceability of the
values assigned to clinical calibrators and control materials to higher-order references.
As a consequence of these driving forces, a traceability structure for clinical chemistry was set up. At the national standards level PTB and DGKC, the latter represented by the Reference Institute for Bioanalysis, are
providing the primary standards and procedures to which
the measurements in the medical laboratories on the
working level are ultimately referred. At present, national references are provided for the following groups of
analytes (concentrations in human blood serum), which
are subject to the quality assurance measures of the B˜K
guidelines. Only the most important analytes are given in
brackets as examples.
PTB
Metabolites and substrates (cholesterol, creatinine,
glucose, uric acid), hormones (cortisol, progesterone),
electrolytes (Li, Na, K, Mg, Ca, Cl)
DGKC
Metabolites and substrates (urea, triglycerides, bilirubine, lactate), enzymes (the measurands are the enzyme activities), hormones (aldosterone, estradiol, estriol, testosterone, thyroxin), drugs (theophylline,
digoxin, digitoxin), total proteinAs far as possible,
isotope dilution mass spectrometry is used for the primary measurements in both institutes (e.g. [4]).
The primary references maintained by PTB and DGKC
are disseminated to the medical laboratories at the work-
A total of about 30,000 ring test measurements are performed every year by approximately 4,000 medical laboratories. For several of the measurands for which external quality assurance is required by the B˜K guidelines,
accredited calibration laboratories do not yet exist.
Here the ring test samples are directly provided by
DGKC. It can be expected that the number of accredited
calibration laboratories operating as multipliers between
the national standards and the user level will increase in
future. As already mentioned, further accreditations are
underway.
Electrochemistry
Traceability structures for pH and electrolytic conductivity measurement have been built up in this field. The
measurement of these quantities is of high relevance to
metrology in chemistry and of great economic and scientific importance. This is demonstrated by the high and
still growing demand for traceability of measurement results for these quantities to national measurement standards. Furthermore, a new series of European standards
on these topics is in preparation within CEN/TC 332.
While traceability of pH measurement has been established as one of the PTB s first actions in metrology in
chemistry [5], the necessary building blocks for a traceability structure for electrolytic conductivity have been
installed only recently [6].
A national traceability system for chemical measurements
pH measurement
The national standard maintained at PTB is a primary
electrochemical measuring system in which the definition of the pH value is very closely realized by the
Bates Guggenheim approximation, the conventional
procedure adopted by the national metrology institutes
leading in this field, and also adopted by IUPAC. At the
secondary level accredited calibration laboratories use
the buffer solutions measured at the primary level for the
multiplication process. A special differential measuring
set-up is used for this purpose which increases the uncertainty only slightly. The calibration laboratories provide
certified secondary buffer materials to the working level
at which the materials are used for calibration purposes
in a great variety of fields. Glass electrode measuring
systems, which require frequent re-calibration, are mostly applied at the working level. It is a special feature of
this structure that traceability does not extend to the SI at
the uncertainty level provided by PTB but to a conventional reference framework which is recognized worldwide. Traceability to the SI can be established if needed,
but with increased uncertainty.
Electrolytic conductivity
Providing traceability for electrolytic conductivity measurements is a new activity of PTB. It is a consequence
of the growing demand for reliable calibrations of electrolytic conductivity measuring cells. The measurement
of electrolytic conductivity is a useful analytical tool often applied in various fields of science and technology,
in particular in the case of aqueous media, for which
electrolytic conductivity is a measure of the concentration of ionized substances. Although it is a non-specific
sum parameter, it can, under given conditions, be used as
an easily accessible quantitative measure of the water
quality, replacing cumbersome and expensive chemical
analyses.
Accurate electrolytic conductivity measurements are
required, for example, in water purity assessment which
is needed by the pharmaceutical and semiconductor industries and in power plants, for the evaluation of the
water quality under regulatory requirements and for water analysis in environmental monitoring.
The national standard for electrolytic conductivity
measurement is a primary measuring set-up developed
and maintained at PTB. Its central element is a measuring cell of exactly known geometry in which the distance
of the electrodes can be changed and exactly measured.
Resistance measurements are carried out with at least
two different electrode spacings with exactly known
shift, with all other conditions kept constant. The measured electrode shift, the cross section of the cell and the
two resistance values allow the electrolytic conductivity
145
to be determined in absolute terms with an uncertainty
comparable to that of leading institutes in the field.
The dissemination of the unit to the users takes place
via DKD-accredited calibration laboratories as described
for pH measurement.
Elemental analysis
Element solutions with mass concentrations of elements
of nominally 1 g/l, either as single or multi-element solutions are among the most frequently used calibrators in
chemical analysis, and traceability to the SI units of the
concentrations stated by the manufacturers is increasingly required. In response to this growing demand a traceability structure for elemental analysis is at present being
set up.
At the national standards level, BAM and PTB jointly
provide the primary references. BAM provides the highpurity elements with known uncertainty for the purity as
primary chemical standards and PTB uses these materials to prepare primary element solutions.
These primary solutions will be used as transfer standards to link up accredited calibration laboratories which
in turn will provide element solutions as CRMs in the required amounts to the chemical testing laboratories. So
far, accredited calibration laboratories do not exist in this
field, but a first accreditation is in preparation.
In a joint project started in 2001, PTB and BAM are
developing the basis for providing primary reference solutions for the 60 most frequently required elements. At
present, primary solutions are available for ten elements
at PTB.
Gas analysis
The national standards for the various fields in which gas
analysis is of importance are provided by BAM and
UBA. PTB s contribution is the type approval of gas analytical measuring instruments whose metrological control is required by the legal regulations.
The whole traceability structure for gas analysis can
be subdivided into three parts.
1. Gas analysis within legal metrology
Gas analytical instruments for vehicle exhaust emission
surveillance, evidential breath alcohol analysis in road
traffic and calorific value determination of fuel gases are
subject to legal control and require type approval and
initial and subsequent verification. The national standards required in this part of gas analysis are provided
by BAM. PTB uses in-house standards prepared by dynamic blending to ensure traceability of its type approval
146
W. Richter • B. G ttler
measurements. For the type approval of the breath alcohol analysers as well as for the tests of calibrators used
for their initial and subsequent verification, thermodynamic air alcohol mixture generators are used at PTB
for which ethanol water solutions are provided by BAM
as certified reference materials.
The multiplier function at the intermediate level is
fulfilled by the verification authorities of the federal
states. The result is the deployment of a large number of
verified gas analysers whose measurements are traceable
to the SI units through the structure described.
provided by BAM to the user level. A traceability
structure of the kind shown in Fig. 1 does not yet exist
but can be expected within a short time when the first
calibration laboratory for gas mixtures as used in the
automobile sector has finalized its accreditation process. There is now considerable interest in DKD accreditation for gas analysis which will increase the importance of establishing traceability via accredited calibration laboratories.
Conclusions
2. Gas analysis under environmental protection
legislation
For the gas analytical measurements performed by the
air quality monitoring networks, UBA provides the primary standards. In most cases these are low-concentration mixtures of pollutants in air prepared by static or
dynamic blending. A completely different approach is
used for ozone measurement where so-called standard
reference photometers (SRP) operating in the UV spectral range are applied as primary references in several
countries. These SRPs, and also that operated by UBA,
are linked within an international ozone reference network which is coordinated by the BIPM.
The primary standards are disseminated to the air
quality monitoring networks by calibration of their gas
analytical equipment at the UBA Pilot Laboratory at
Langen. Within the networks air quality monitoring laboratories are appointed and provided with the necessary
calibration gases.
3. Gas analysis in the unregulated area
Traceability requirements in this area have so far mostly been fulfilled with calibration gas mixtures directly
It is now generally accepted that traceability of chemical
measurements to recognized standards is an indispensable prerequisite for achieving comparable and trustworthy analytical results. After it has been largely clarified
how traceability can be established for chemical measurements, structured systems are now called for to realize traceability in practice.
A tested example of a practical traceability structure
has been described which has already proved useful in
several fields of chemical analysis. It makes use of a national calibration service as a successful and efficient
dissemination mechanism. An essential part of the structure is a network of high-level chemistry institutes at the
national standards level providing the end points of
traceability and coordinated by the national metrology
institute. The need for a network of competence seems to
be typical for metrology in chemistry for which in most
countries the resources are largely to be found outside of
the national metrology institute.
An important feature of the traceability structure described above is an improved reliability of data as a result of reference measurements rather than the use of reference materials alone. Reference measurements allow
exact matching of the sample matrix. This is very important in clinical chemistry where dif ficult matrices are
quite common.
References
1. BIPM (1999) Mutual recognition of national measurement standards and of
calibration and measurement certificates
issued by national metrology institutes.
Bureau International des Poids et
Mesures (BIPM), Pavillon de Breteuil,
92312 SŁvres Cedex, France. Internet:www.bipm.org
2. Haerri H (2001) metINFO 2:9-14,
Internet:www.metas.ch
3. Dybkaer R, rnemark U, Uldall A,
Richter W (1999) Accred Qual Assur
4:349 351
4. Henrion A, Dube G, Richter W
(1997) Fresenius J Anal Chem
358:506 508
5. Spitzer P, Eberhardt R, Schmidt I,
Sudmeier U (1996) Fresenius J Anal
Chem 356:178 181
6. Spitzer P (2000) DKD aktuell 2:4
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