Achieving Metrological Traceability

Section 5.2 introduced the subject of metrological traceability and calibration and the use of pure chemical substances and reference materials in achieving trace-ability. Reference materials are used as transfer standards. Transfer standards are used when it is not possible to have access to national or international standards or primary methods. Transfer standards carry measurement values and can be 

Material, sufficiently homogeneous and stable with respect to one or more specified properties, which has been established to be fit for its intended use in a measurement process. 

Note (4) is very important as it highlights the fact that the reference material used for the method validation cannot be used again when the method is in routine use for calibration purposes. The same type of material can be used, but it needs to come from a different supplier. The same material cannot be used for calibration purposes and then as a quality control material. 

Pure substance RMs pesticides with quoted purities, polycyclic aromatic hydrocarbons with quoted purities and potassium hydrogen phthalate with a quoted purity. 

Standard solutions, a solution of nickel in acid with a quoted mass/volume concentration, a solution of sodium hydroxide with a quoted concentration as a molarity and a solution of pesticides with quoted mass/volume concentrations. Matrix RMs - natural materials, river sediment with quoted concentrations of metals, milk powder with a quoted fat content and crab paste with quoted concentrations of trace elements. 

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The foregoing discussion identifies the need for comparability and its achievement by metrological traceability. Comparability of measurement results is conveniently defined in terms of metrological traceability. If two results are traceable to the same stated metrological reference, then they must be comparable. Please note that in common speech comparable often means about the same magnitude, but this is not the case here. In the laboratory comparability of measurement results means simply that the results can be compared. The outcome of the comparison, whether the results are considered near enough to be equivalent, is not a factor in comparability here. 

There are some existing texts that cover the material in this book, but I have tried to take a holistic view of quality assurance at a level that interested and competent laboratory scientists might learn from. I am continually surprised that methods to achieve quality, whether they consist of calculating a measurement uncertainty, documenting metrological traceability, or the proper use of a certified reference material, are still the subject of intense academic debate. As such, this book runs the risk of being quickly out of date. To avoid this, I have flagged areas that are in a state of flux, and I believe the principles behind the material presented in this book will stand the test of time. 

It is now widely accepted that metrological traceability, i.e. traceability to internationally recognized references like SI units, is an indispensable prerequisite for achieving comparability and hence confidence in, and acceptance of, chemical measurement results in a worldwide framework. 

It should be recognized that, in some cases, it is not difficult to set up a traceable measurement system. The best examples of this are in physical metrology where traceability is often based on direct measurements of the SI units. There is also general agreement that a similar SI fink is highly desirable in the case of chemical measurements, but, for a variety of reasons, direct chemical traceability is difficult to achieve in most of the analytical chemistry applications. Only a very few analytical chemistry procedures exhibit a direct measurement capability that allows the set-up of a traceable measurement pathway such as in physical metrology measurements most of these procedures have been accepted as primary methods if carried out under certain constraints (CCQM 1998). 

Summarizing this chapter, you must remember that metrology in chemistry is still a young discipline thus there is still a lot to be learned, traceability is not an aim by itself, but it is the way to achieve reliable and comparable results. Traceability can only be claimed when the uncertainty statement includes all the uncertainty contributions from the references and the measurement process. 

To achieve this goal, comparability of chemical measurements based on traceability to recognized standards and hence on thorough knowledge of uncertainty, must be established in analogy to the way in which the validity of measurement results is ensured in metrology in general. This task mainly consists of two parts ... 

Abstract Metrology is based on the concept of traceability. Traceability provides a means of relating measurement results to common standards thereby helping to ensure that measurements made in different laboratories are comparable. Good progress has been made in the application of metrological principles to chemical measurement, but there remains confusion about how you actually achieve traceability in a practical way. 

Achieving traceability in chemical measurement - a metrological approach to proficiency testing... 

Abstract ISO/IEC 17025 requires that testing laboratories establish the traceability of their measurements, preferably to the SI units of measurement. The responsibility for establishing traceability lies with each individual laboratory and must be achieved by following a metrological approach. 

Traceability of the assigned value is achievable provided there are direct links to stated references, together with sound estimates of the uncertainty of the links. NARL aims to establish and maintain traceability to SI, where this is technically feasible, but not necessarily at the highest metrological level. This is achieved by establishing and maintaining the following types of links. 

In line with the objectives of the CCQM Workshop, we have tried to suggest how measurement traceability can be disseminated to field laboratories. An important anticipated evolution in this respect is the fact that the stakeholders dealing with accreditation/metrology/edu-cation/standardization will need to collaborate much more closely to achieve measurable progress. 

Obviously a small absorbance uncertainty is caused by the lowest concentration but there are many other sources of error. In this respect, it is the authors opinion that calibrating and validating the metrological performances of photometric systems is a necessary condition but not on its own sufficient to achieve traceability in this field. In fact, a measurement uncertainty budget takes into consideration all uncertainties due to the way in which instrumentation is used, the CRMs and calibration of the system. 

Comparability is a key property of chemical measurements. While results can be compared directly under repeatability conditions, a more general approach is needed to provide meaningful comparison to results of other measurements made at different times and places. This comparability over space-and-time is routinely achieved by linking the individual measurement results to some common, stable reference or measurement standard. Results are therefore correlated to that reference. This strategy of linking results to a reference is termed traceability [1,2], Traceability is a key property in metrology, and for this reason the traceability of results is even explicitly demanded in the international norm ISO 17025 [3]. 

The mutual recognition of international standards can be achieved by agreements between National Metrology Institutes (NMI). Calibration as an important activity in establishing traceability should be done by competent personnel. Therefore the competence should be assessed by a third party. Consequently agreements are also needed for accreditation bodies. 

The common goal of EA and IRMM is to demonstrate the reliability and comparability of chemical measurements. This will be mainly achieved by the joint organization of ILCs with traceable reference values of high metrological quality, but also by organizing educational and training activities. Therefore, not only does the IRMM provide a tool for NABs to assess the performance of their accredited laboratories, but it also supports EA in monitoring the efficiency of the EA-MLA. In that sense, the IRMM improves the efficiency of accreditation in chemistry. 

Traceability is one major factor that can be achieved via CRMs as main means in the held of chemical metrology. In general, CRMs are applied for the validation of analytical methods. Standard solutions are then used for instrument calibration. Nonetheless, CRMs should not be understood as the solution for all problems in chemical measurement. It goes without saying that the matrix of a CRM should match the analytical problem as exactly as possible. It is clear that there are not CRMs available for all matrices and analytes. Thus, it is important to have the best matrix match. 

The above sections have highlighted the importance of data comparability and trace-ability in the context of WFD chemical monitoring. Let us now examine in detail what references need to be considered for the development of a sound metrological system. Firstly, as a reminder, traceability is defined as the 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 (ISO, 1993). The ways in which these elements can be applied to chemical measurements were discussed some years ago (Valcarcel and Rios, 1999 Quevauviller, 1999 Walsh, 2000) and those discussions still continue. In this context the basic references are those of the SI (Systeme International) units, i.e. the kg or mole for chemical measurements. Establishing SI traceability of chemical measurements may, in principle, be achieved in relation to either a reference material or to a reference method (Quevauviller and Donard, 2001). The unbroken chain of comparison implies that no loss of information should occur during the analytical procedure (e.g. incomplete recovery or contamination). Finally, traceability implies, in theory,... 

As a final consideration, one should keep in mind that achieving traceability of water, sediment and biota chemical monitoring measurements in the context of the WFD will have direct implications for the way in which programmes of measures will be designed and made operational to achieve the good status objectives by 2015. This places a strong emphasis on the importance of metrology since any erroneous data could have tremendous (social and economic) consequences. 

It is possible to produce artificial matrix materials [12]. Such materials can be prepared on a mass basis by weighing all components both to mimic the matrix composition and the content of trace elements or trace organic substances. The materials could help to have matrix materials available for which the exact contents and composition are known. As a consequence it would be, in theory, possible to certify them on a mass basis and validate methods with highly traceable materials. In organic trace analysis this would circumvent the unknown extraction step. In reality, this is much more difficult to achieve than can be expected. The real matrix composition of many materials is unknown — in particular for environment samples. The physico-chemical status of the various substances depends on the history of the material. Therefore, various natural samples of expected similar composition are different in behaviour. In addition, when preparing mixtures of solid components, losses cannot be excluded and unfortunately are not quantifiable. Attempts have been made where losses were demonstrated but not quantified [12]. Therefore, materials certified for matrix composition and analyte content on a mass basis do not yet exist or are not of real use for method validation by routine laboratories. They may be of interest for laboratories active in the field of fundamental research in chemical metrology where smaller quantities of material are handled. 

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