Standard reference materials, definition

Standard Reference Materials Definitions. Natic n2il Institute of Science and Technology, U.S. Dep2irtment of Commerce. Downloaded November 30,2003. www.nist.gov... 

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. 

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 validation of an instrument demands a procedure and a number of standards (reference materials included) with the appropriate characteristics. The definition and use of standards and reference materials available to the clinical chemistry is still a problem in this field. Although there are several clinical reference materials, the problems of using them should not be underestimated (stability, preservation, contamination, use of aqueous solutions, etc.). 

Performs zero- and double-blind studies. Intralaboratory reproducibility (including ruggedness and robustness for real samples) should be demonstrated additional validation using an authentic standard reference material of the analyte in the sample matrix. Definition of criteria for revalidation. 

ISO Guide 30 (1992) Terms and definitions used in connection with reference Materials. International Organization for Standardization, Geneva. 

INTERNATIONAL ORGANIZATION FOR STANDARDIZATION, Terms and Definitions used in Connections with Reference Materials - ISO Guide 30, Geneva, ISO, 1992. 

This slide gives the definition of a measmement standard You could also use the term reference material. You will find more information on this topic in the chapters 10 and 14. 

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

A primary measurement standard is expected to have a known quantity value with minimum uncertainty, although this is not specified as such in the definitions. Because the measurement uncertainty is propagated down the calibration hierarchy, you should start with as small an uncertainty as possible and choose methods to establish purity of primary reference materials with this in mind. 

Let us begin with the ISO definition [9] A calibration is a set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system, or values represented by a material measure or reference material, and the corresponding values realized by standards. Applied to amount measurements, the standards would then be the values assigned to the RMs (of defined composition) at the stated uncertainty relative to the true value of the property, expressed in SI units, or relative to an internationally recognized, certified standard RM for the relevant property, range, and matrix composition. 

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... 

For a value to be traceable it must be related to stated references. By definition and convention the stated references are taken to include SI [6] reference values (e.g., atomic mass values), reference materials (RMs), as well as primary, reference, and standard methods. It is sometimes stated that chemical measurements are traceable to the mole. This is an incomplete statement as chemical measurements are simultaneously traceable to a number of references, inter alia, the mole, kg, meter, etc. Whilst it is considered desirable to employ high level references, such as the SI, where feasible, this is not always necessary in terms of fit for purpose criteria. Neither is it possible to relate all types of analyte (fat, fiber, protein, pH, etc.) to the SI. The key issue is that the references should be stated and fit for purpose. 

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 ability to perform the same analytical measurements to provide precise and accurate results is critical in analytical chemistry. The quality of the data can be determined by calculating the precision and accuracy of the data. Various bodies have attempted to define precision. One commonly cited definition is from the International Union of Pure and Applied Chemistry (IUPAC), which defines precision as relating to the variations between variates, i.e., the scatter between variates. [l] Accuracy can be defined as the ability of the measured results to match the true value for the data. From this point of view, the standard deviation is a measure of precision and the mean is a measure of the accuracy of the collected data. In an ideal situation, the data would have both high accuracy and precision (i.e., very close to the true value and with a very small spread). The four common scenarios that relate to accuracy and precision are illustrated in Figure 2.1. In many cases, it is not possible to obtain high precision and accuracy simultaneously, so common practice is to be more concerned with the precision of the data rather than the accuracy. Accuracy, or the lack of it, can be compensated in other ways, for example by using aliquots of a reference material, but low precision cannot be corrected once the data has been collected. 

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