Traceability, metrological

Keywords Traceability Metrological Chemical Proficiency Testing... 

Key words Traceability Metrology in chemistry Vocabulary in metrology... 

It may be realised by means of definite complex of intereonneeted and interrelated common rules and norms direeted to the assurance of traceability and uniformity of measuring equipment, NDT equipment including. In another words NDT equipment must be metrologically supported. 

The so-called standard instrument is used for interpolation between the fixed points and for the calibration of other thermometers lower in the metrological hierarchy. The standard instrument in the moderate temperature range is a special platinum resistance probe, as it has to fulfill set requirements. It is important in all calibration that traceability to a primary normal, here the fixed-point ITS-90 scale, exists. 

The key difference between a CRM and an RM is the traceability. In order to play any role at aU in metrology, traceability is a key property. Traceability refers to a property value of the CRM, and thus to the underlying measurements. Insufficient traceability of these measurement results will eventually lead to a RM that cannot be certified, as the property value cannot be related to other standards. In the ideal case, traceability is realized up to the International System of Units, SI, but this is only feasible for a very small number of CRMs. 

To the users of CRMs, the concept of traceabDity" is very closely related to the statistical considerations in the measurement process and the quality of the measurements in the users laboratory. Traceability is defined in the international vocabulary on metrology (VIM) as ... 

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). 

The producers need to establish, in general terms or for specific applications of CRMs, dear instructions for the user on how to establish traceability to the stated reference and (implied in the use of CRMs of national metrology organizations) the SI. But the authors experience is that, as the workload in commercial analytical laboratories increases (as analytical laboratories seek to demonstrate their competence and quality to an increasingly discriminating... 

Associated with method validation, but not part of it, are two properties of results that have been previously mentioned. These parameters are measurement uncertainty and metrological traceability. Measurement uncertainty is covered in Chapter 6 and metrological traceability in Chapter 5. If considered at the planning stage of method validation, the information obtained during validation is a valuable input into measurement uncertainty evaluation. Traceability depends on the method s operating procedures and the materials being used. 

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... 

A reference material characterized by a metrologically valid procedure for one or more specified properties, accompanied by a certificate that states the value of the specified property, its associated uncertainty and a statement of metrological traceability. 

Traceable equipment, e.g. balances, thermometers, glassware, timepieces, pipettes, etc. are appropriately calibrated, and the corresponding certificates or other records demonstrating metrological traceability to National Standards are available. 

In conclusion, one must insist on the importance of the main metrological characteristic, the traceabilility (generally of a result), ensuring a clear (unbroken) relationship between the final result and the complete measurement scheme by using appropriate procedures, standards and calibrated equipment. However, for chemical metrology and particularly for on-site measurement, some adaptations are needed for a wider meaning of traceability [66]. 

Metrology in Chemistry and Traceability of Analytical Measnrement Results... 

This gives an overview of metrology in analytical chemistry as well as describing the role of measurement traceability. The traceability part of the presentation is based on the CITAC position paper on measnrement traceability (Traceability in Chemical Measurement, Accred. (Jual. Assm. (2000) 5 388-389)... 

First we will discuss today s needs for metrology, then the fundamentals of metrology will be presented followed by some information about the international measurement system and the presentation will end with discussion about traceability of analytical measurement results. 

Before continuing, let s see the official definition of traceability as given in the ISO international vocabulary of basic and general terms in metrology. 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. 

Metrological traceability Property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty... 

Detailed information relating to metrology in chemistry and traceability can be obtained from these web-sites. The addresses include international and regional organizations as well as some national metrology institutes. 

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. 

EURACHEM / CITAC (2008) Metrological Traceability of Analytical Results. Leaflet available from www.eurachem.org... 

Details of the metrological traceability and measurement uncertainty of any assigned value... 

Traceability in Practice The practical establishment of traceability is based on a step-by-step implementation of the definition. The ISO definition of traceability, originating from metrology (see above), can be translated into three basic steps ... 

Each of these principles will arise in some guise or other in this book. For example, principle 5 relates to metrological traceability (chapter 7) and measurement uncertainty (chapter 6). These principles will be revisited in the final chapter. 

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