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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