Traceability of values carried by reference materials

Abstract Traceability is a property of the result of a measurement. Since values carried by (reference) materials must also have been obtained, of necessity, by measurement, the definition of traceability also applies to reference materials. It is extremely helpful to give the traceability (of the origin) of a reference material a separate name, i.e. trackability . An analysis of the function of values carried by reference materials, shows that they can fulfill different functions, depending on the intended use. One of the functions located outside the traceability chain - and hence not very relevant for establishing traceability - is evaluating the approximate size of the uncertainty of the measurement of an un- 

Key words Traceability chain Metrology in chemistry Reference material Amount standard Validation 

Traceability is defined in the International Vocabulary of Basic and General Terms in Metrology, (VIM)[1] as the 

Traceability is therefore clearly a property of the value of a result of a measurement. So what is a measurement The same VIM defines it as a 

the notion of value is very outspoken. Mostly, we obtain an unknown value by comparing the unknown value to a known value (a simple definition of how a measurement is carried out), i.e. measuring the ratio of an unknown value to a known value. 

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De BiLvre P (2000) Traceability of (values carried by) reference materials. Accred Qual Assur 5 224 230... 

They are absent from the literature with the exception of publications by R Dybkaer [8], Pictures of such traceability chains are almost completely missing. Yet such pictures provide simple, transparent insights. Probably international thinking and discussion ought to concentrate more on chains of values of the measurement results, as well as on chains of values carried by reference materials, rather than on materials, methods, instruments or institutes. This would also be closer to the spirit of the SI, which is to become independent of man-made artefacts and to anchor measurement results as far as possible in inalterable properties of nature and in units (with their values) derived from these. However, there are numerous chemical measurements where the latter is not (yet) possible. In those cases, it is useful... 

The one key to traceability that must be supplied from outside the laboratory is the traceability of values carried by references, especially by certified reference materials. As these values are also established by measurements, the same features required from analytical laboratories also apply for producers of reference materials. Additionally information on the stability and homogeneity of the reference material in form of an expiration date or by equivalent means is required. 

We will see later [4] that it is this similarity which can be exploited by analysts to reduce their workload. Analysts could attempt to establish the complete traceability chain all by themselves in Fig. 3a,b (left-hand chain), but cannot do that in practice too much work, too expensive. They can literally buy such values carried by reference materials. They add a known value from a reference material to their unknown sample and measures the ratio of these two values, - unknown to known - in the one and only measurement they must make. That was exemplified in Fig. 2 by means of the method wherein an internal standard is added to the unknown sample and measured under the same circumstances as the unknown sample. How the analyst can buy help by purchasing values carried by reference materials, is described elsewhere [4],... 

Traceability to a common unit, or to a common measurement scale, makes the results of measurements comparable, regardless of their magnitude. The very -and only - reason for the existence of the requirement traceability is to enable us to compare our measurement results (i.e. achieving comparability) measurements of the same measurand, carried out at different places and/or different times, yield concordant results (meaning that they fall within each other s stated uncertainty). It is emphasized again that traceability need not to be to (values of) SI units but can be to values of other (including arbitrary) units with which measurement scales can be built. As Fig. 2 shows, a measurement scale can be constructed from a calibration curve based on a number of values carried by commonly agreed reference materials (in this case 5) [2],... 

All values of the IRMM Isotopic Reference Materials are traceable to the SI (the international system of base quantities and base units). Isotopic measurement results corrected by means of these Isotope Ratio Reference Materials have reduced (ISO/BIPM) uncertainties. Isotopic measruements carried out against these Spike Reference Materials are traceable to the SI, if carried out properly. Further details are available from IRMM website see Chapter 8. 

The key elements of a traceability chain, values and links between values, have already been described [4], More complete traceability chains are presented in Fig. 1. The symbols used are b = amount content [5, 6] in amount (mol) per mass (kg) of element (or compound) E in material X. Note that the chain is constituted by values linked by operations called measurements , defined as above. The analyst could attempt the establishment of a complete traceability chain as shown (Fig. 1, left chain), but that would require a huge amount of work, or may not be possible, e.g. because the chain may be broken (in the upper part under chemical operations ). The same reasoning applies to the value carried by a reference material (central chain or right chain in Fig. 1). Every time we use a reference material, two traceability chains are involved as illustrated in Fig. 1 one for the measurement result obtained on the unknown sample (left chain), and one for the value carried by the reference material, (either the central or the right chain). They must, by their very nature, be similar. The first one must be demonstrated by the analyst. The second one must be demonstrated by the reference material producer. They sell the product and therefore must be accountable for the product. 

In many chemical measurements, however, we (must) use measurement scales which are based on values carried by commonly agreed reference materials. These enable the analyst to establish a calibration curve through which observed signals from an instrument used in a measurement procedure, are converted to a concentration [8], Any measurement result which is shown to be traceable to such a common scale, is comparable to any other measurement result which is traceable to the same scale, i.e. established by the values of the same reference materials. 

Using the value, carried by a reference material, in the traceability chain of an unknown value in a sample ... 

Fig. 1 Traceability chains of a) the result of a measurement val- by chemical operations on the unknown sample c) of a value ue obtained on an unknown sample b) a value carried by a (ma- carried by a reference material intended to be part of the tracea-trix) reference material intended to reduce the uncertainty caused bility chain of the measured value in an unknown sample...

We will now look at the traceability chain of the value carried by a matrix reference material , in order to understand its function in the measurement process. That is needed to produce reference materials which are adequate for the intended use ( fit-for-purpose ). 

Abstract Talking about traceability means talking about a property of the result of a measurement , about the value of a standard , about stated references and about an unbroken chain of comparisons . It describes by which comparison, and to which other value, the result of a measurement has been obtained, i.e. is traceable to . It is about the underlying structure of the measurement process of the result of a measurement and therefore about the authority of the result. Since values carried by (certified) reference materials have also been obtained by measurement, the definition of traceability equally applies. Traceability in the context of reference materials is also about the authority of the values carried by the (certified) reference materials and is, therefore, of key importance for the authority of the reference materials themselves. Hence, values of results of measurements constitute part of the traceability chain and their uncertainties are an intrinsic accompanying phenomenon. Uncertainties need a traceability chain against which they can be evaluated, and a traceability chain is an... 

Traceability of the value carried by a reference material is to be distinguished clearly from traceability of a reference material which we describe as trackability (e.g. to the place it comes from) [4-6], The basic idea that traceability chains, even in the case of the well-known weights , are a matter of traceability of values, has been discussed before [7],... 

In Fig. 3a,b (right-hand chain), a full traceability chain of the value of a synthesized matrix reference material is shown. The left-hand chain is the traceability chain of the value resulting from the measurement of an unknown sample. The right-hand chain is the traceability chain of the value carried by the reference material and provided by the producer of the reference material. Both chains consist of successive comparisons from one value to another, all the way down to known values (of base units or of fundamental constants). The chain is constituted by values (in the rectangles) linked by operations called measurements (in the ovals), defined as above. Establishing the sequence value-meas-urement-value-measurement-value, etc, is establishing... 

A traceability chain of the value carried by the IHRM to the value carried by the reference material with higher metrological status and sufficiently similar matrix (for example, a certified reference material CRM according to ISO Guide 30) is helpful in such a case. The chain is realized when the IHRM samples are analysed simultaneously with the CRM samples under the same conditions. This and other traceability chains necessary for the IHRM development are examined as the measurement information sources. 

The traceability of the value carried by a reference material (RM) should be demonstrated by the RM producer [13]. The producer shall provide the traceability of results of its measurements to the national or international measurement standards. Where this is not possible, the correlation of results with the values of national or international certified reference materials (CRMs) is required. Ideally, the values of the CRMs should themselves be traceable [2]. 

None the less, questions may arise concerning the traceability of the values carried by chemical reference materials. The... 

A reference material certificate is defined by ISO 31 2000—Guide to the Contents of Certificates and Label of Reference Materials—diS a document accompanying a certified reference material stating one or more property values and their uncertainties, and confirming that the necessary procedures have been carried out to ensure their validity and traceability . For example, this would include information relating to the batch number, a description of the material, the concentration of the material, storage conditions, and expiry date. The content of the certificate is discussed in detail in ISO 31 2000— Guide to the Contents of Certificates and Label of Reference Materials. 

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