Accred Qual Assur (2000) 5 :429–436Q Springer-Verlag 2000 GENERAL PAPER

Bernard King The practical realization of the

traceability of chemical measurements

standards

Abstract Metrology is based onthe concept of traceability. Tracea-bility provides a means of relatingmeasurement results to commonstandards thereby helping to en-sure that measurements made indifferent laboratories are compara-ble. Good progress has been madein the application of metrologicalprinciples to chemical measure-ment, but there remains confusionabout how you actually achievetraceability in a practical way.

This paper elaborates on themeaning and application of much

used phrases such as ‘the value ofa standard’, ‘stated references’,‘unbroken chain of comparisons’,and ‘stated uncertainties’. It alsoexplains how traceability can be es-tablished in a practical way for dif-ferent types of stated references,namely pure substance referencematerials, matrix reference materi-als, and primary and referencemethods. Finally, traceabilitychains for some typical examplesof chemical measurement are de-scribed.

Received: 17 March 2000Accepted: 30 May 2000

Presented at 3rd EURACHEMWorkshop “Status of Traceability inChemical Measurement”, 6–8 September1999, Bratislava, Slovak Republic

B. KingNational Analytical ReferenceLaboratory, 1 Suakin Street, Pymble,NSW 2073, Australia

Introduction

Analytical chemistry has served society well over thepast 100 years, but the concepts and systems that un-derpin chemical measurements are cracking under thestress of increasing requirements. The demands beingplaced on chemical measurements are increasing as so-ciety requires more complex, quicker, and cheapermeasurements. A sustainable future depends on relia-ble, comparable, and traceable measurements and fail-ure to provide such measurements will be costly in fi-nancial and human terms.

Although in its infancy, metrology in chemistry is al-ready accepted by a number of governments as themeans of ensuring that measurements made in differentlaboratories are comparable and traceable to commonstandards [1]. The ultimate aim is to develop a struc-tured chemical measurement system which will lead tomutual recognition and trust in measurements made inlaboratories all over the world. This in turn will facili-tate international trade, wealth creation through innov-ation, and enforceable and trusted regulations.

Metrology is based on the concept of traceability,which is defined as follows [2]: Traceability is the prop-erty of a result of a measurement or the value of astandard whereby it can be related to stated references,usually national or international standards, through anunbroken chain of comparisons all having stated uncer-tainties.

Enormous progress has been made in applying thisconcept to chemical measurement and there is a grow-ing amount of published work and associated confer-ences [3–5]. This includs work on concepts, terminolo-gy, and primary methods and has led to the new ap-proach of ‘metrology in chemistry’, which is a synthesisbased on input from physical measurement metrolog-ists and analytical chemists.

The topic is complex with many interrelated strands.Despite successes, there remains some confusion in thedebate and little clear guidance on how you actuallyachieve traceability in a practical way. Paradoxically,there is both ‘loose talk’ about what traceability meansand a lack of recognition of the many elements of tra-ceability that already exist. The definition of traceabili-

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ty succinctly describes the key issues and concepts andthis paper will attempt to elaborate on their meaningand application.

Traceability; a deconstruction of the definition

The value of a standard

This phrase often means the purity or amount of major,minor, or trace component, or a physico-chemical prop-erty of a reference material (RM). It can also be theresult obtained under carefully controlled conditionsusing a primary or reference method. It should benoted that it is not the RM that is traceable but theproperty value associated with the certification. By thesame token, a primary method is not traceable, but thevalue(s) produced using the method is (are).

Related to stated references

For a value to be traceable it must be related to statedreferences. By definition and convention the stated ref-erences are taken to include SI [6] reference values(e.g., atomic mass values), reference materials (RMs),as well as primary, reference, and standard methods. Itis sometimes stated that chemical measurements aretraceable to the mole. This is an incomplete statementas chemical measurements are simultaneously traceableto a number of references, inter alia, the mole, kg, me-ter, etc. Whilst it is considered desirable to employ highlevel references, such as the SI, where feasible, this isnot always necessary in terms of ‘fit for purpose’ crite-ria. Neither is it possible to relate all types of analyte(fat, fiber, protein, pH, etc.) to the SI. The key issue isthat the references should be stated and fit for pur-pose.

The phrase ‘related to’ implies that the relationshipis known and valid. This will only be realized if the re-lationship at every step of the process is clearly definedand valid. Hence the requirement for an ‘unbrokenchain of comparisons.’ The parallel between these is-sues and those addressed by method validation is worthnoting. Validation is the process of establishing that amethod is capable of measuring the desired measurand(analyte), with appropriate performance characteristics,such as level of uncertainty, robustness, etc. It shouldalso address systematic effects, such as incomplete re-covery of the analyte, interferences, etc. These latter is-sues can be dealt with by designing a method to elimi-nate any bias, at a given level of uncertainty, or if that isnot possible, to provide a means of correcting for thebias. This may be done at the method level, by applyinga correction factor to all results, or at the individualmeasurement level.

Fig. 1 Essential features of a typical chemical measurement. Theconcentration of analyte in the original sample (C) is given byCpFc!Cs!R, where Cs is the concentration of the calibrationstandard

Through an unbroken chain of comparisons

The ‘comparisons’ may take place every time a meas-urement is made (e.g., calibration of an analytical meas-urement using a standard solution), periodically (e.g.,calibration of the balance), or infrequently (e.g., valida-tion of a method). The reference value is used to eithercalibrate the process or to check its calibration or valid-ity. The number of steps in the chain of comparisonsshould be kept to a minimum as each additional stepintroduces additional errors and increases the overalluncertainty. Interlaboratory comparisons provide evi-dence of comparability and provide confidence in tra-ceability claims; they do not, however, provide tracea-bility directly.

There is of course more than one chain of compari-sons and all the component measurement processes as-sociated with the chemical measurement need to beconsidered. These include physical measurements, suchas mass, volume, etc., and chemical issues, such as iden-tity and amount, which together constitute an ‘amountof substance’ (see later). The traceability of componentmeasurements needs to be established at a level of un-certainty that is consistent with the required overall un-certainty of the final measurement. Components suchas temperature, and even mass and volume measure-ments, often contribute little to the overall uncertaintyand thus can be simply and easily addressed.

Dealing with the chemical issues is usually muchmore demanding. In a typical chemical measurementthere are two main components to the chemical chainof comparisons, as illustrated in Fig. 1. One componentis established by measuring the response from one ormore chemical standard(s), often in the form of stand-ard solutions, prepared from pure substance RMs. Theidentity, purity, and stability of the standards are im-portant issues. The calibration factor Fc and its uncer-tainty, Uc, describe this part of the chain. Such methodsare often called ratio methods and include most of the

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widely used techniques, such as GC, HPLC, MS, NMR,AAS, ICPMS, etc. The advantage of this approach isthat it is not necessary to establish the traceability ofintermediate parameters such as voltages or ion cur-rents providing the sample and chemical standards be-have in the same way within the detection system.

The second component of the chemical chain is therelationship between the original sample and the sam-ple presented for measurement. Analyte loss due to de-composition, incomplete extraction, adsorption on con-tainers or separation media can all affect the recoveryfactor, R. Also, species other than the analyte, thathave not been fully separated, can contribute to themeasurement signal, resulting in a positive bias. Finally,the matrix can enhance or suppress the measurementsignal, further contributing to bias. The combination ofall these factors can lead to a recovery factor, R, that isgreater or less than unity. Ideally, the individual effectsshould be studied, understood, and minimized. This canbe done by studying the inner workings of the method,comparing the value obtained from the method withthat obtained using a reference or primary method anduse of the other strategies illustrated in Fig. 1, such asthe use of closely matched matrix RMs. Nevertheless,obtaining a good estimate of R can be difficult and thusthe associated uncertainty, Ur, can be large. It is inthese cases that consideration needs to be given to es-tablishing traceability to lesser references than SI, forexample, to the values obtained from a standard meth-od or a reference material.

Stated uncertainties

This is an expression of doubt concerning the reliabilityof ’the value’. The process of deriving an estimate ofthe uncertainty is well described [7], even if it is difficultin some cases to produce a good estimate. Whilst tra-ceability is a yes/no issue – you either have it or youdon’t – uncertainty is a matter of degree and describesthe strength of the linkage.

The uncertainty associated with a traceable valuemust be related to a specified measurand (analyte) andbe related to stated references. The following exampleillustrates the effect the choice of stated reference hason the stated uncertainty for the measurement of leadin milk. The uncertainty of a measurement of lead inmilk, measured using a standard method, could besmall, if stated relative to that standard method, wherethe measurand (analyte) is implicitly defined by thestandard method. However, the method is likely to con-tain some additional errors and uncertainties if it wereto be related to a primary method traceable to the SI,and these would need to be included in the estimate ofuncertainty, if the SI was quoted as the stated refer-ence. The interrelationship between uncertainty and

Fig. 2 Interrelationship between traceability and measurementuncertainty

stated references is illustrated in Fig. 2. The above ap-proach has limitations in metrological terms if the re-sults are expressed in SI units, e.g., mg/kg, etc. The useof SI units implies that a measurement is traceable to SIand, if this is not the case, then some form of qualifyingstatement is needed.

The total combined uncertainty of a measurement isa function of the uncertainties associated with the com-ponent measurements, references, and processes. Ref-erences such as RMs and primary methods are, there-fore, important ways of providing uncertainty informa-tion for parts of the traceability chain.

The role of the mole in chemical measurement

Some chemists feel that the mole is an unnecessary SIunit as they make measurements in mass/mass or mass/volume units, using ratio methods. The definition andthe importance of the mole has been discussed else-where [8], and the distinction has been made betweenits importance as a concept, the importance of the re-lated atomic mass values, and the lesser role of themole as a unit for actually reporting results. A distinc-tive feature of the mole is the need to define ‘the enti-ty’. This is an extra dimension compared with other SIunits. For example, it is not necessary to ask, “is this amass” when measuring the mass of an object, in theway that it is critical to ask, “is this lead” before at-tempting to measure the amount of lead. A mole meas-urement thus requires two issues to be addressed,namely identity and amount. It follows therefore thattraceability claims must show unbroken chains coveringboth of these issues. It is because of the existence of avast number of chemical species that it is necessary toclearly specify and separate the specified chemical enti-ties from all other possible chemical entities prior tomeasurement. This leads to complex chemical measure-ment processes, with considerable attention to valida-tion of the measurement method being required.

It is possible to have a mole of lead atoms, a mole ofthe d(c)-isomer of some organic molecule, or at a triv-

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ial level, a mole of bottles of red wine. Any problemsare not in the concept but in the realization of a mole ofpoorly defined entities. As with the definition of themole, it is necessary to specify the entity. The numbermay be absolute or a ratio, as in the definition of themole. The definition of the mole specifies a relativenumber and relates the number to mass. It would bepossible to redefine the mole in terms of an absolutenumber (Avogadro’s number), but the linkage to masswould still exist. The relative atomic masses togetherwith the related issue of chemical equivalence or stoi-chiometry provide the macro links between the differ-ent atomic and molecular entities and allow an amountof any entity to be expressed in terms of mass. It isworth noting, in passing, that the mole is a specific caseof a more basic quantity, which is number, where theunit is one.

Measurement of the mole

For other SI units, the quantity is realized by either anartefact (e.g., the kg), or a measurement process lead-ing to a value. The measurement process has to be cap-able of accurately measuring the quantity and wherethis is achieved by a number of laboratories indepen-dently at the highest metrological level, then the con-sensus value plus its uncertainty is taken as the primarystandard. Others may use the primary standard totransfer traceability to their measurements, using a sin-gle or multi-step series of comparisons (methods/stand-ards). A primary standard is established using a prima-ry method, which by definition is carried out at thehighest metrological level.

The mole can be realized in a similar way but, ofcourse, there are millions of different types of mole. Itis more appropriate to speak of realizing a mole andthis can be done by measuring a specified entity andmaking use of chemical stoichiometry and atomic massvalues to relate the measured property to mass, as de-fined in the definition of the mole, i.e., MxpNx/Anpm/M, where Mxpnumber of moles of entity X; Nxpnum-ber of entities of X; AnpAvogadro’s number; mpmassof X; Mpatomic mass of X.

Hence the measurement of a mole involves themeasurement of a number of entities by, for example,mass spectrometry and/or measurement of the mass ofthe isolated entity X. For this reason, much of thescience of chemical measurement is concerned with en-suring the identity of X and the isolation of X, so thatits amount can be measured.

Other vital components of the traceability chain areatomic mass values, other fundamental constants, andassociated physical measurements, such as mass, vol-ume, etc. The results can be expressed in moles, molesper mole (mole fraction), or moles per kg, or if pre-

ferred in other related SI units such as mass/mass units.Whilst conversion between units will contribute to in-creased uncertainty at the highest metrological level, atmore practical levels such considerations will be insig-nificant.

Primary methods

A primary method [2] is one that is capable of opera-tion at the highest metrological level, which can becompletely described and for which a complete uncer-tainty statement can be produced in SI units. Theamount of substance can be measured either directly,without reference to any other chemical standard, orindirectly, by use of a ratio method which relates theamount of unknown entity X to a chemical standard.Primary direct methods, such as gravimetry and certainelectrochemical and thermal methods are the excep-tions in chemistry, as the majority of measurements aremade indirectly by comparison with other pure sub-stance RMs as discussed above and below. These ratiomethods include isotope dilution mass spectrometryand chromatographic and classical methods. Hence theimportance of pure substance RMs.

A method that has the potential to be primary canalso be applied in a less rigorous way to provide a di-rect realization of the SI unit, but not at the highest me-trological level. In chemistry, gravimetric analysis,which can be carried out at varying levels of uncertain-ty, is such an example. That is, it can be employed as atraceable routine method or applied at the highest me-trological level, for establishing the purity of RMs.

Pure substance RMs

Following synthesis or isolation, separation and purifi-cation, materials are characterized for stability and ho-mogeneity. The crucial characteristics are the identityand purity, which may be in the range 99% (or less) to99.9999% depending on the material. To be of valuethere must be a high level of certainty concerning thestructure or identity of the material. This is normallyestablished from knowledge of the synthetic route andby using a number of independent characterizationmethods, which provide a mixture of analytical (e.g., el-emental analysis, NMR, etc.) and fingerprint data (e.g.,MS, retention times, etc.). The purity may be estab-lished by one or more of the following approaches: thedirect measurement of the amount of the main ingre-dient; measurement of all possible impurities and theirdeduction from 100%; other methods, such as differen-tial scanning calorimetry, which provide a direct meas-ure of the level of certain impurities. The traceability of

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Fig. 3 Traceability of a pure substance RM of testosterone

the purity certification will be established by describingthe traceability of the contributory measurements.

The references, measurement processes, and the un-certainties associated with the traceability chain of anorganic pure substance RM are illustrated in Fig. 3. Forthe amount measurement to be meaningful, the uncer-tainty of the identity must be close to zero. Although itis not yet possible to quantify this uncertainty, the useof a number of independent sources of information(synthetic route plus three or more independent analy-tical methods) helps ensure that the close to zero condi-tion is met. The more subtle the structural characteriza-tion required, the more complex the task. Hence thedifficulty in unambiguously identifying and measuringspecific isomers of complex organic compounds such assteroids. Some metrologists may challenge the inclusionof identity within the traceability chain, but if it is notincluded then it is not possible to have an unbrokenchain of comparisons to the SI unit, the mole. Somemethods used for the characterization of purity havethe potential to be primary, such as NMR, titrimetry,DSC. It is questionable, however, whether methodssuch as GC and HPLC, which are widely used for es-tablishing the purity of organic RMs are primary meth-ods. Nevertheless, providing the purity is high any er-rors will be small. Although there is still some debateabout how best to assign the purity value and its uncer-tainty when a number of independent methods areused, it is clear that consideration needs to be given toall the information, whilst giving most weight to the‘best’ estimates.

Pure substance RMs are used to derive the calibra-tion factor Fc (see Fig. 1) and its uncertainty Uc forchemical measurements. Traceability of the amount ofRM in a prepared standard solution is determined bythe traceability of the purity certification, the massmeasurement, and the volume measurements. Formany trace chemical measurements the uncertainty(Uc) of the calibration derived from the pure substanceRM is a small component of the final measurement un-

certainty, because other factors, such as interferences,incomplete extraction. etc. (Ur) dominate the uncer-tainty budget. Thus, for many trace analysis methods,reagent grade materials are adequate for use as calibra-tion standards. An exception is with complex organicsubstances, where purity of commercially available ma-terials can be low and often many similar substancesexist, leading to confusion concerning identity duringmeasurement. In many cases the impurities are struc-turally similar to the main component making it diffi-cult to purify and further confusing multicomponentmeasurements. In these cases RMs with a high level ofcertainty concerning identity (and purity) are essen-tial.

Matrix RMs

Whilst pure substance RMs are mainly used for calibra-tion purposes (determining Fc and Uc), matrix RMs aremost often required to validate a measurement ormethod (determining R and UR). To be of use matrixRMs must closely match the real sample in terms ofanalyte, matrix type, and concentration. In addition,the analyte must be incorporated into the matrix in thesame way as in the real sample. RMs may be preparedby gravimetrically mixing the components or by charac-terizing the amount of the analyte(s) of interest in nor-mal production or naturally occurring material. Theformer provides a more ready route to traceability, butin many cases such materials do not sufficiently closelymatch the real samples.

In sectors as diverse as metallurgy, food science, andenvironmental control, it is necessary to characterizecomplex materials and eliminate or control a widerange of potential interferences and incomplete recove-ry of the analyte during digestion, extraction, or separa-tion. This presents a serious challenge to analyticalscience. The traditional strategy has been to take apragmatic approach and to standardize the method as away of achieving comparability. Interlaboratory preci-sion statements are used to characterize the range ofresults that can be expected, but it is implicitly acceptedthat there are likely to be additional unidentified syste-matic effects and uncertainties.

Alternatively, interlaboratory consensus valuesbased on a range of different methods are used to try toaddress systematic effects. Such approaches leave openthe question concerning traceability, or how closely thecertified value agrees with the true value. The estab-lishment of traceability to SI requires the use of prima-ry methods, such as isotope dilution mass spectrometry,or the use of other well understood and validated meth-ods, where any systematic effects have been fully evalu-ated and corrected for. The uncertainty budget must in-clude appropriate allowance for any suspected residual

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systematic effects. It is also worth remembering that iftraceability to SI is not possible or appropriate, thentraceability to some lesser reference should be specifiedand demonstrated.

Reference measurements

Although it is not common practice, another way of es-tablishing traceability is by comparing measurementsmade using a primary or reference method with resultsobtained using a working level method. This can bedone on an individual measurement basis or on a largerscale through interlaboratory studies, where the as-signed value is based on a metrologically traceable val-ue. Given the cost of such work it can be expected thatsuch measurements will, when possible, be associatedwith more widely used and durable RMs.

Classification of reference materials and methods

Hierarchies, such as primary, secondary, and workinglevel, or certified RMs and RMs are extensively used indescribing traceability chains. Whilst such terms can beuseful in explaining processes and links, they can alsobe confusing. For this reason their use has been limitedin this paper. It is considered preferable to describehierarchies in terms of the associated uncertainties. Itcan also be noted that, whereas in physical measure-ment it is common to have a hierarchy of references ofthe same basic type (e.g., a series of mass standards),this is rare in chemical measurement where the chainusually contains only one chemical RM, linked to ahigher reference by a measurement process.

The practical realization of the traceability of routinechemical measurements

Providing the measurand can be defined in SI units,then in principle its measurement can be made tracea-ble to SI. It would be a matter of convenience to stopthe traceability chain at a lesser reference, such as a ref-erence method. The associated uncertainty would bemade relative to the stated reference, which would beassigned an uncertainty of zero. That is, relative to SIsome systematic effects would be ignored. Increasingly,however, there is a drive to establish traceability to SIwhere feasible and to accept that this will result in alarger uncertainty.

It has been shown above how the traceability ofRMs can be established. These RMs can be used tohelp establish the traceability of routine measurementsas illustrated in Fig. 4. It will be noted that the uncer-tainties associated with the high level references are

Fig. 4 Traceability of a trace of lead in blood serum measure-ment

small compared to those associated with measurementof trace quantities of lead in a complex matrix such asblood serum. Where component uncertainties are lessthan one third of the combined uncertainty, then theywill contribute little to that combined uncertainty. Thisdoes not mean that the higher level references are notnecessary, only that they are not the major causes ofdifficulty. Also, although traceability of identity is al-ways important it is not addressed in Fig. 4, as it is not aproblem. This would not be the case with organic anal-ysis, where confusion concerning identity can be a ma-jor problem, as discussed above and in Fig. 3. The ma-jor problem in trace analysis usually is the size of UR,leading to a large combined uncertainty. The weak linkin this chain is the validation of the routine method.Hence the importance of this in determining the overallquality of the measurement. This is typical of traceanalysis involving complex matrices. The absence of theIDMS reference value for serum would reduce the tra-ceability to just the routine method, with no control orknowledge of R and Ur. Estimates of R and UR couldbe made by other means, for example by spiking and/orusing a matrix RM. In trace analysis, UR usually domi-nates the combined uncertainty. This underlines the im-portance of access to good matrix RMs and the impor-tance of method and measurement validation.

In the case of purity analysis or the measurement ofmajor components, the traceability steps and associateduncertainties illustrated in Fig. 3 can be expected. Inthese cases, a small uncertainty is normally requiredand a variety of factors can contribute significantly to it,including mass, volume, and other physical as well aschemical effects. Thus in these cases, both Ur and Uc

may be significant.Measurement of parameters which cannot be related

to SI, such as fat and fiber content of food and pH canbe made traceable to other references according to thesame principles as discussed above.

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Ensuring the equivalence of standardsaround the world

A number of organizations have contributed to theabove developments and programs are being estab-lished at the national, regional, and international levels,to help provide the standards needed to facilitate tra-ceability [1–9]. An important aspect of this work is thedemonstration of the equivalence of the various stand-ards used in different parts of the world.

A key organization is Comité Consultatif pour laQuantité de Matière (CCQM), which since its estab-lishment in 1994 has made considerable progress on theagreement of definitions and the organization of inter-laboratory studies to evaluate primary methods and tostudy the equivalence of national standards. Areas ofwork include gas analysis, trace element analysis, traceorganic analysis, and the characterization of the purityof pure substance standards. The work is focused onthe development of metrological tools and the demon-stration of the feasibility of the metrological approach.CCQM aims to provide a framework for the demon-stration of the equivalence of national standardsthrough interlaboratory comparisons, known as ‘KeyComparisons’, which can be linked through regionaland sectoral networks. It will only be possible to con-duct a limited number of Key Comparisons, due to re-source limitations. Each comparison will be carefullyselected to cover an important measurement area andas far as possible address specific matrix, analyte, andmeasurement technique problems. For example, themeasurement of trace levels of the pesticide metabolitepp’ DDE in fish oil by IDMS is relevant to food safetyand environmental concerns and represents the analy-sis of a complex organic material in a complex matrix,by IDMS.

It is envisaged that about 80 key comparisons will beneeded to cover chemical measurements. Although itremains unclear how far ‘the light will shine out’ from aspecific key comparison to other related areas of meas-urement, the demonstration of equivalence of nationalstandards in selected areas will be of great importancefor international trade. Despite the use of the term ‘na-tional standard’ (which, perhaps, has more to do withhistory than with the future international vision of theworld), it is not envisaged that every nation will haveall the standards. The aim is more to do with demon-strating the equivalence of the different metrology inchemistry capabilities that are growing up around theworld. Also, Key Comparisons will need to be under-pinned by QA systems and accreditation to help trans-fer measurement traceability to the working level. Thecombined strategies will enhance international compa-rability of measurements and facilitate one stop test-ing.

Table 1 Examples of proposed topics for CCQM Key Compari-sons

Health: e.g., cholesterol in serumFood: e.g., arsenic in fishEnvironment: e.g., permanent gases in airAdvanced materials: e.g., semiconductorsCommodities: e.g., sulfur in fossil fuelsForensics: e.g., ethanol in airPharmaceuticals to be decidedBiotechnology: e.g., DNA profilingGeneral analytical applications: e.g., pH

To date 3 key comparisons have been conducted, 6are planned for 1999 and the areas listed in Table 1 areon the agenda.

Progress is illustrated by the following statistics. In1998 CCQM organized 4 interlaboratory studies; in1999 over 24 studies are planned. The progress madeby laboratories participating in CCQM studies overtime and their improved performance compared withworking level laboratories is illustrated by the followingperformance data.

Only 2 out of 8 laboratories met the target accuracyby being within B1% of the assigned value in a 1994study of trace lead, compared with 9 out of 10 laborato-ries meeting the same target accuracy in a repeat exer-cise in 1997.

Also, in a 1998 trace element study involving muchlower concentrations (1/1000) 9 out of 10 metrology la-boratories established equivalence to within B2.6%.This performance level can be compared with other la-boratories (routine) where the range of results ex-ceeded B50%.

Developments at the national and regional levelshave been described elsewhere [8].

Sectoral developments

Developments concerned with improving the validity,comparability, and traceability of chemical measure-ments are also taking place in specific sectors, such asfood and agriculture, environment, clinical, pharmaceu-tical, forensic science and some areas of industry. Forexample, the clinical chemists [9] have adopted the me-trological approach, expressing results in SI units andprogressively developing measurement traceability atthe working level. Another sector where the metrologi-cal approach is being pursued is gas analysis concernedwith car exhaust pollution, environmental measure-ments, and drink-driving prosecutions. Internationaltrade and regulation are driving improvements in foodand agriculture analysis but a different approach is oft-en being taken in this particularly difficult sector. Someof the developments in this sector are the same or sim-ilar to the metrological approach, even if different ter-

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minology is used, but some of the champions havemuch less ambitious aspirations with regard to meas-urement accuracy and are content to establish compa-rability rather than traceability. Cross-fertilization be-tween the groups will be important, in order to bringboth approaches to a common focus. Failure to collabo-rate would result in parallel and unconnected measure-ment systems being developed with cost penalties andinferior measurement potential.

Conclusion

Traceability is a property of a measurement valuewhereby it meets certain criteria and in particular is re-lated to stated references at a stated level of uncertain-ty. The nature of the stated references is open to choiceon a fit for purpose basis and the level of uncertaintymust be a reasonable estimate of the actual uncertaintyand appropriate for the purpose. Where feasible, tra-ceability to SI is, however, recommended as it providesstable references, unchanging in time or space. There isno requirement for traceability to imply high accuracyand for many purposes traceable measurements with alarge uncertainty will be adequate. Primary and refer-ence methods and reference materials provide the

transfer standards that can help establish traceabilityfor routine measurements. The measurement methodmust of course be well understood and describedthrough a process of method validation and it must berecognized that this is often the most crucial and diffi-cult part of establishing traceability. It is also clear thatthe traditional analytical chemistry strategies of calibra-tion and validation are embraced by the metrologicalapproach, but that the latter provides a fuller frame-work to describe measurement quality and provides aquality strategy that applies to all types of measure-ment.

In physical measurement, calibration standards areof prime importance, but in chemistry, standards suchas mass standards and pure substance reference materi-als are necessary but not sufficient and often not themost problematic aspect of establishing traceability. Asevery analytical chemist knows, issues such as sampling,sample stability, contamination, interferences, and in-complete recovery of the analyte are usually the majorcontributors to measurement uncertainty. It is being in-creasingly recognized that if we wish to improve thetraceability of chemical measurements, then we need toput the effort where the chemical problems are, and notwhere the problems are in physical measurement. It is asign of maturity that this is now happening.

References

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2. International Vocabulary of Basic andGeneral Terms in Metrology, Secondedition, 1993, ISO, Geneva

3. King B, et al (1999) Accred Qual As-sur 4 :3–13

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