Establishment of Measurement Traceability

There are a number of prerequisites for properly using CRMs in these tasks, including established quality control of the laboratory s analytical measurement operations and proven statistical control of the analytical measurement process. Publications describing the use of RMs and CRMs are not as plentiful as those on how CRMs are made but, in addition to the ISO/REMCO Guides 30-35, the ISO/REMCO publication The role of reference materials in achieving quality in analytical chemis-try (ISO 9000 1987), the NIST Handbook for SRM users (Taylor 1995), and the various LGC-VAM publications listed under Further Reading should all be consulted. 

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Matrix SRM s, Matrix SRM s can be used for a variety of functions. Taylor (71) reviewed the uses of matrix standards, which Include (a) method development and evaluation, (b) establishment of measurement traceability, and (c) assurance of measurement compatibility. During certification of these matrix SRM s, the selectivity of fluorescence... 

An important precursor of valid measurement, and the establishment of measurement traceability, is an adequate description of what is to be measured (the measurand), which includes the measurement units and consideration of the acceptable level of measurement uncertainty (MU). Clearly, if different characteristics are measured, or different measurement units are employed, then different measurement results can be expected. Clarity on this issue can be vital to subsequent decision making. For example, in environmental studies, it may be more important to know the amount of extractable pollutant in a geological material, rather than the total amount of the pollutant. Thus, although self evident when we think about it, it is important to remember that, in addition to making traceable measurements, it is also important to make the right type of measurement. 

NBS (12) maintains a comprehensive program on standard reference materials (SRMs) SRMs are well-characterized, homogeneous, stable materials or simple artifacts with specific properties measured and certified by NBS. They are widely used in a variety of measurement applications, including the evaluation of the accuracy of test methods, improvement of measurement compatability among different laboratories, and establishment of measurement traceability to NBS. The Bureau currently has more than 1000 different SRMs available. ... 

In this Chapter we highlight the practical considerations that must be understood by all users of RMs and CRMs we look at some of the issues of traceability and make the CRM user aware of the uncertainty budgets that need to be considered with the use of CRMs. No attempt will be made to advise CRM users on the proper use of statistics in the analytical measurement process and no statistical approaches on the establishment of measurement uncertainty will be given. There are a number of good texts on the subject which should be consulted. These are listed in the Further Reading section of the references at the end of this Chapter and include Miller and Miller (1993) and Taylor s work for NIST (Taylor 1985). 

We have now moved the problem of establishing the traceability of one value to the establishment of the traceability of another known value, which in turn must be compared to another known value, etc. This process is quite acceptable, provided it stops somewhere. It stops when we arrive at a value which we know because we have defined it (consequently, it has an uncertainty of zero). It is the value of the unit in which we want to express the result of our measurement. Thus we can define a traceability chain as follows ... 

Abstract Establishment of the traceability and the evaluation of the uncertainty of the result of a measurement are essential in order to establish its comparability and fitness for purpose. There are both similarities and differences in the way that the concepts of traceability and uncertainty have been utilised in physical and chemical measurement. The International Committee of Weights and Measures (CIPM) have only in the last decade set up programmes in chemical metrology similar to those that have been in existence for physical metrology for over a century. However, analytical chemists over that same period have also developed techniques, based on the concepts of traceability and uncertainty, to ensure that their results are comparable and fit for purpose. This paper contrasts these developments in physical and chemical metrology and identifies areas where these two disciplines can learn from each other. 

Development and publication of 23 manuals of environmental radioactivity measurements including analytical procedures and TLD measurement. Establishment of the traceability for radioactivity measurements of local laboratories by the use of standard samples which are prepared by JCAC and Japan Isotope Association Corp., under the reference of standard of Electro Technical Laboratory of Japan. 

This clearly underlines the necessity of the establishment of a traceability chain so that all measurements can be related to an internationally recognised standard. 

Accurate micro- and nanoscale fluid metering is important since it serves as a basis for evaluating the performance of microfluidic devices and thus is critical to the quality control of those devices and systems using them. Establishment of a traceability chain and evaluation of the measurement uncertainty for flow rate and droplet volume metering instruments are important, especially for levels of liquid quantity that cannot feasibly be dealt with by gravimetric methods. Moreover, reliable methods to measure extremely small liquid volumes, for example, 10 nl or below, are yet to be developed. 

The most important document, accompanying a CRM is its certificate. ISO Guide 31 (1981) provides guidance for the establishment of certificates, labeling of CRMs, and certification reports. The certificate contains among other information the certified values and their respective uncertainties. As important as this information is the traceability statement, which defines to what references the CRM is traceable. Ideally, a CRM is traceable to a suitable (combination) of SI units. This is not always possible, so other stated references may appear here. Especially when certifying matrix reference materials, making the measurements traceable to SI does not imply that the CRM is traceable to SI as well. The steps necessary to transform the sample into a state that can be measured may have a serious impact on the traceability of the values, and thus on the traceability statement. 

Traceability of measurement results is essential in the establishment of a certified reference material. As stipulated in ISO Guides 30 and 35, a certified reference material can only be certified if there is an uncertainty statement with a traceability statement. Basically, traceability means anchoring. In classical analytical chemistry, that SI system is often the best choice as a reference (= anchoring poinf). However, there is a wide range of parameters either defined by a method or defined by the... 

Establishing the traceability of the measurement is one of the most important requirements in ISO/IEC 17025. Measurement traceability ensures that the measurements in different laboratories are comparable in space and time all over the world. Calibration and the use of certified reference material (see chapters 9, 10 and 14) is the central tool in establishing trace-ability. Therefore a laboratory must have programmes and procedures for both. 

Let s see what is the influence for the two cases where the certified reference materials are useful in the laboratory (i.e. calibration and vahdation). In the case of calibration, the properly value of the certified reference material is used to calibrate the analytical instrument used for the measurement, thus it is used in order to obtain the measurement result. In this way the properly value of the certified reference material is part of the traceability chain, as shown in the shde, and is directly involved in the establishment of the measurement traceabihty. 

The process of providing an answer to a particular analytical problem is presented in Figure 2. The analytical system—which is a defined method protocol, applicable to a specified type of test material and to a defined concentration rate of the analyte —must be fit for a particular analytical purpose [4]. This analytical purpose reflects the achievement of analytical results with an acceptable standard of accuracy. Without a statement of uncertainty, a result cannot be interpreted and, as such, has no value [8]. A result must be expressed with its expanded uncertainty, which in general represents a 95% confidence interval around the result. The probability that the mean measurement value is included in the expanded uncertainty is 95%, provided that it is an unbiased value which is made traceable to an internationally recognized reference or standard. In this way, the establishment of trace-ability and the calculation of MU are linked to each other. Before MU is estimated, it must be demonstrated that the result is traceable to a reference or standard which is assumed to represent the truth [9,10]. 

FIGURE 4 Traceability chain and relationship between traceability and uncertainty of measurements. The three possibilities for establishing traceability referred to in Figure 5 are indicated in bold [25, 28]. 

This update on analytical quahty issues a common understanding on the topics of method validation, traceabihty, and MU of measurements. The interrelationships between method validation and traceability and MU of results have been elucidated. Throughout the landscape of guidelines and standards, the most relevant information was selected, compiled, and summarized. Different approaches are discussed for establishing traceability and assessing MU of analytical methods in general. The importance of both concepts and the link with method validation and analytical quality assurance are highlighted. 

The results of IMEP-1 clearly pointed to the problem of lack of equivalence of measurement results and demonstrated that studies needed to use independently established quantity values. IMEP operated from the beginning under the auspices and with the support of the International Union of Pure and Applied Chemistry, EURACHEM, the Association of European Metrology Institutes, and Cooperation in International Traceability in Analytical Chemistry. 

Unusual are measurements for which a direct link to the mole is useful. We should probably not talk about traceability in that connection, because that term is defined as a relation between measured values. An acceptable chain of measurements for compound X of established purity, containing element E that has isotope E and that would establish a link to the mole, then would take one of the following general routes the amount of substance (X)->n(E)->n( E)-> (12C) or n(X)->n(E)-> (C)-> (12C). The ratio of atomic masses m( E)lm( 12C) is also involved in the definition, but that ratio is known with a negligible uncertainty compared with the other links in the chain. Clearly, only in a few instances will laboratories attempt to execute such a chain of measurements for a link to the SI unit. Is it fear that such a difficult process is involved in every chemical analysis that has kept so many chemists from using the mole as the way to express chemical measurement values Or is it just habit and the convenience of a balance that subconsciously links amount of substance to amount of mass ... 

In order to establish traceabilities of measurements, we advocate the structure shown in Fig. 1 where many types of linkage can be found, including but not limited to those terminating in SI. 

All non-negligible uncertainties are combined by root-mean-square summations. That sum establishes the quality of the traceability link. A traceable measurement may fail as an appropriate measurement if its uncertainty is too large. Its quality will then be too low for the purpose. Conversely, it should be recognized that there are highly reproducible chemical measurements which are useful, but are not traceable to any standard. 

The concepts of calibration and especially of metrological traceability were elaborated by physicists as mentioned above. The reference or top of the calibration hierarchy preferably is the definition of an SI unit, which is realized or embodied as a primary measurement standard. By direct comparison, the quantity value of a secondary measurement standard can be established. Subsequent comparisons may furnish quantity values of reference measurement standard, working measurement standard, and routine measurement standard with which the object carrying the measure and is compared to obtain its measurement result which then retrospectively is metrologically traceable to the SI unit. The primary measurement standard, as the definition of metrological traceability says, is preferably an international or national measurement standard. 

The definition of metrological traceability (see above) stipulates that each link in the chain has a known uncertainty. Nowadays, this concept and its application have been reformulated by the BIPM and recently detailed in the Guide to the expression of uncertainty in measurement (GUM) [26] parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand . Useful explanations are provided in several other guides [26-30] as well as commentaries [e.g. 31-33], The philosophy is to apply a bot-tom-up approach by formulating a function of all input quantities giving the measurand as output. An uncertainty budget of all sources of uncertainty is established. Important items to consider are ... 

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. 

It is sometimes implied that traceability is a concept that cannot be applied to all types of measurements. But all measurements are traceable to some base, without this it would not be possible to give a value. This base may not be satisfactory, it may not be stable, in fact, the base may not have been identified. The purpose of establishing the traceability is to identify this base and to ensure that it is satisfactory. 

Traceability has also been established for ratio measurements such as reflection coefficient, absorbance and angle measurements. Different techniques to establish traceability are used in different fields of measurement and it is a mistake to look for a one size fits all solution. 

It is possible to establish traceability for the results of measurements on all types of quantities, not just those for which there is an SI unit. 

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