Certified Reference Materials definition

The definition of a certified reference material (CRM) is given in ISO Guide 30 (1992) and it forms the root of all other ISO Guides ... 

Much of the early work with certified reference materials was linked to the derivation of reference methods and there was a period in which primary or definitive (i.e. very accurate but usually very complex) and secondary (or usable) methods were reported e.g. steroid hormones (Siekmann 1979), creatinine (Siekmann 1985), urea (Welch et al. 1984) and nickel (Brown et al. 1981). Although there are some application areas, such as checking the concentrations of preparations listed in a pharmacopoeia, where a prescribed, defined method has to be used, in practice such work is limited. However, this approach to chemical analysis is no longer widely used and will not be further discussed. The emphasis now is placed on using RMs to demonstrate that a method in use meets analytical criteria or targets deemed to be appropriate for the application and to develop figures of merit (Delves 1984). 

Certified Reference Materials. Certified Reference Materials are materials whose properties have been guaranteed or certified by recognized bodies. The certified analyses of these materials can be used as an estimate of the "true" value for assessment of accuracy. The United States National Bureau of Standards (NBS) provides an inventory of various materials whose compositions (and properties) have been measured using definitive and reference methods. These materials, Standard Reference Materials (SRM s), when used in conjunction with reference methods, i.e., one of demonstrated accuracy, make it possible to transfer accuracy between measurement protocols. 

The traeness is determined by comparing the arithmetic mean, x, of the value that the laboratory assigns to the Certified Reference Material with the referenee value Xref of the CRM, which is (by definition) accepted as sufficiently trae. If it is found to be within the eonfidence interval of the laboratory s results, then this is judged to be a satisfaetory outcome. 

The presentation gives some definitions that are important to the selection use and production of reference materials and provides doeumentation for further reading and study. There is discussion on the different types of reference materials with some insight into their production process. This is followed by information relevant to selection and use of reference materials with a discussion on the relation between reference materials and traceability, which is very important in the context of this presentation. Finally some internet addresses are given of the major certified reference materials producers. 

This definition is also accompanied by notes. The first says that certified reference materials are a sub-class of measnrement standards (or etalons ). 

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... 

According to the definition [1] the traceability chain is the unbroken chain of comparisons or calibrations from the result of a measurement or the value of a measurement standard to the national or international standards, all having stated uncertainties. The uncertainty of each link in this chain (measuring analytical instrument, reference material or other measurement standard) changes over the course of time. Therefore, the calibration intervals of measuring equipment used in testing (analytical) laboratories [2, 3] and of measurement standards used for their calibration are very important. The same applies to the shelf-life of a certified reference material (CRM) as a measurement standard. So, taking into account these... 

The current definition of Certified Reference Materials CRM ( CRM are refer-... 

ISO Guide 30 1992 Terms and definitions used in connection with reference materials ISO Guide 30 1992/Amd 1 2008 Revision of definitions for reference material and certified reference material... 

Definitions 2 and 3 allow an evolution in the different techniques and methods as definitive methods for the same analyte (Leijnse, 1982). Indeed, even though systematic errors were investigated during the initial research work, later technical advances may uncover errors that were undetected during the original measurements. The end use and end purposes of the definitive method include the evaluation of the accuracy of reference methods and its application to the quantitation of analytes in certified reference materials present in a biological matrix. 

The ISO definition of a certified reference material (CRM) is Reference material, accompanied by a certificate, one or more of whose property values are certified by a procedure, which establishes its traceability to an accurate realisation of the unit in which the property values are expressed, and for which each certified value is accompanied by an uncertainty at a stated level of confidence . ... 

The most recent definitions of reference materials (RM) and certified reference materials (CRM) were given in the International vocabulary of metrology - basic and general concepts and associated terms (VIMS) [1]. 

Fig. 1 Terminology reference material (RM) vs. certified reference material (CRM). Inconsistent terminology is a fertile base for various interpretations of definitions definitions given in Table 1 can be interpreted as given in case 1 (CRMs being a part of a family named RM) or as given in case 2 (several different types of RM, one of them being CRM). Intended use of reference standard depends on accepted interpretation of definitions...

Before starting speaking about tools, it must be clarified that the words used are understood and used with their proper meaning. Having recalled the most recent versions of definition of terms, it will be necessary to investigate which tools analysts need to achieve their objectives. The ultimate objective is. as said before, to deliver reliable and adapted answers to customers. We will then examine how reference materials and certified reference materials can help to verify the reliability of analytical procedures. We will also see how interlaboratory studies can benefit analytical quality work. 

Analysis of certified reference materials. By definition, a certified reference material (CRM) is a substance for which one or more analytes have certified values, produced by a technically valid procedure, accompanied with a traceable certificate and issued by a certifying body (e.g. Figure 2.3). 

Chapter 2 is concerned with the concept of quality assurance and all that it involves with respect to obtaining reliable data from environmental samples. Particular emphasis is placed on the definitions of accuracy and precision. Finally, details on the use of certified reference materials in environmental analysis are provided. 

The realisation that every laboratory determination that is carried out is associated with both random and systematic errors has had a major impact on laboratory medicine in the last thirty years. It is also at the heart of quality control and quality assurance procedures which are primarily concerned with understanding the sources of such errors and their suppression or minimisation (Whitehead, 1977 Aitio, 1981 Taylor, 1987). However, it has been pointed out by Broughton (1983) that all laboratories may carry out some form of "quality control" but this is often designed to give retrospective reassurance rather than provide prospective action. The dual concepts of bias and precision in laboratory medicine are well known, but not always appreciated even by users of reference materials (Taylor. 1985 Taylor, 1987). By definition an unbiased result should be the "true" result, but in practice this is hardly ever achieved. The nearest approach to a true value is generally obtained by using a certified reference material and a definitive method, but these ideals are unobtainable in the case of most trace metal analyses. 

The International Standards Organization (ISO) has defined a reference material as a material or substance one or more properties of which are sufficiently well established to be used for the calibration of a method, or for assigning values to materials. The ISO definition of a certified reference material is a reference material one or more of whose property values are certified by a technical procedure, accompanied by or traceable to a certificate or other documentation that is issued by a certifying body. 

In order to validate speciation methods more effectively, a number of certified reference materials (CRMs) have been produced to allow laboratories to measure the accuracy of their techniques. A range of sediment and biota materials are available such as the sediment materials PACS-2 from the National Research Council of Canada, NIES-12 from the National Institute for Environmental Studies, Japan and the CRM-462 from Community Bureau of Reference (BCR), EU. These materials have been rigorously homogeneity and stability tested and the levels of organotins have been certified by a range of techniques utilizing either definitive methods or multiple independent methods. 

The use of Shewhart control charts with multiple decision rules provide a more sensitive and reliable quality control than Shewhart control with simple decision rules. However, it should be noticed that the multiple decision rules are only valid if the conventional true value pt and the standard deviation oy are known. In practice this means that (jl-t is traceable to a certified reference material (CRM) or established using a definitive method by a validated laboratory [5], and 0-7 is established by method evaluation with a sufficiently high number of method evaluation samples. 

Definite Method. No definitive method exists for ionized calcium. To date there is no international consensus agreement on a reference method and no certified reference materials available for standardization. Potentiometry with ion-selective electrodes has been selected as the method to be used. The International Federation of Clinical Chemists (IFCC) Working Group on Selective Electrodes is currently preparing a draft document on the reference method and testing a prototype reference cell [20]. 

The use of a definitive analytical method can also be used to establish standard reference materials. Definitive methods are ones that can produce exacting quantitative data without the need to compare measurements to a calibration standard. The gravimetric analysis method is a definitive technique. Isotope-dilution mass spectrometry, which is extensively used by NIST and other agencies producing certified standard reference materials, is also considered to be a definitive method of analysis. As discussed in Chapter 7, isotope dilution quantitation can be effectively used with ICP-MS. Therefore, a laboratory with ICP-MS instrumentation can produce reference materials in specific sample matrices for selected elements by using the isotope dilution technique.These standard reference materials still must be considered secondary standards, because they are usually not traceable to existing certified standards. 

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