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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.6, #3)
Certified reference materials in analytical chemistry – A century of NIST contribution by Stanley D. Rasberry (pp. 95-99).
Over the course of its first 100 years, the National Institute of Standards and Technology (NIST) has made numerous contributions to advancing the science and practice of analytical chemistry. Contributions to fundamental constants and reference data, such as determination of the Faraday, Avagadro’s number, and atomic masses, began at almost the beginning of the new institution when it was formed in 1901. Instrumentation development, improvement, and reproducible methods for its use have also been an important part of the NIST effort. This paper will describe what may be the organization’s most important and certainly its most unique contribution; namely, certified reference materials. Ultimately these certified reference materials would become known at NIST as standard reference materials (SRMs). It is a contribution that now has been mirrored around the world with reference materials being certified in at least 25 countries and routinely applied in more than twice that number. The result has been more accurate analyses of materials that impact our safety, health, and well-being.
The measurement assurance concept in calibration and traceability at NBS/NIST by Brian Belanger (pp. 100-102).
During the last quarter of the twentieth century, The United States National Bureau of Standards (NBS), later the National Institute of Standards and Technology (NIST), introduced a measurement quality control concept called ”measurement assurance,” and developed measurement assurance programs, or MAPs, for high-level calibration processes. The measurement assurance approach has, over time, become increasingly popular in the metrology community, and in recent years has become well accepted both inside and, to some extent, outside the United States as a rigorous way to ensure the quality of calibrations. The concept has also found application in defining traceability to national standards. This paper traces the history of the measurement assurance concept.
Keywords: Keywords Traceability; Measurement assurance; Calibration; Statistics
The definition of primary method of measurement (PMM) of the ’highest metrological quality’: a challenge in understanding and communication by P. Taylor; Heinrich Kipphardt; Paul De Bièvre (pp. 103-106).
Problems with understanding, explaining and communication of the present definition of primary method of measurement are described and amendments put forward for discussion. The conclusion is drawn that in many cases more attention should be given to the measurement result and its uncertainty statement, rather than to a method. Some cases are discussed where methods might have a fundamental characteristic that other methods do not have, a condition for the epitheton ’primary’.
Keywords: Keywords Primary method of measurement; PMM; Definition; Metrology; Uncertainty; Chemistry
Lifetime of the traceability chain in chemical measurement by I. Kuselman; Boris Anisimov; Irena Goldfeld (pp. 107-110).
Since the uncertainty of each link in the traceability chain (measuring analytical instrument, reference material or other measurement standard) changes over the course of time, the chain lifetime is limited. The lifetime in chemical analysis is dependent on the calibration intervals of the measuring equipment and the shelf-life of the certified reference materials (CRMs) used for the calibration of the equipment. It is shown that the ordinary least squares technique, used for treatment of the calibration data, is correct only when uncertainties in the certified values of the measurement standards or CRMs are negligible. If these uncertainties increase (for example, close to the end of the calibration interval or shelf-life), they are able to influence significantly the calibration and measurement results. In such cases regression analysis of the calibration data should take into account that not only the response values are subjects to errors, but also the certified values. As an end-point criterion of the traceability chain destruction, the requirement that the uncertainty of a measurement standard should be a source of less then one-third of the uncertainty in the measurement result is applicable. An example from analytical practice based on the data of interlaboratory comparisons of ethanol determination in beer is discussed.
Keywords: Keywords Traceability; Uncertainty; Chemical analysis; Regression analysis; Ethanol; Beer
Continuous dynamic-gravimetric preparation of calibration gas mixtures for air pollution measurements by Dorothea Knopf (pp. 113-119).
The preparation of calibration gas mixtures for air pollution measurements by the dynamic-gravimetric method was investigated using sulphur dioxide in nitrogen as a model. The target mole fraction was 200×10–9 mol/mol, with the option of also getting smaller mole fractions. Thermal mass flow meters calibrated with reference mass flows were used to measure the dilution gas flow (nitrogen). The relative standard uncertainty of the dilution gas flows between 10 mg/s (approx. 500 ml/min) and 40 mg/s (approx. 2000 ml/min) was 0.15%. The mass flow of the target component measured as the permeation rate was determined via the quasi-continuous observation of the loss in the permeation tube mass during the measuring time. A magnetic coupling system and an adapted microbalance were used for this purpose. The results presented show permeation rates measured over the lifetime of a tubular permeation source. The measurement cycles took between 3 days and 7 h at least. The relative standard uncertainty of the mixture composition did not exceed 2%. First comparisons with gas mixtures prepared by the static-gravimetric method show compatibility. The applicability of the system is not restricted to the SO2/N2 mixture. It can also be used for preparing other gas mixtures in this field of application.
Keywords: Keywords Gas mixtures; Mass flow; Permeation tube; MFM; Uncertainty determination
The development of gas standards and calibration techniques for measurements of vehicle, aircraft and industrial emissions, natural gas, occupational exposure and air quality by P. E. Holland; Peter T. Woods; Chris Brookes; I. Uprichard; Paul Quincey; R. Robinson; Hansa D'Souza; Martin Milton; Brian Goody; William Bell; Des Alphonso (pp. 120-129).
The National Physical Laboratory (NPL) is involved in the dissemination of nationally traceable standards to which measurements of air quality, occupational exposure and air pollution source emissions, and natural gas analyses, can be referenced. This has required the development of national primary gas standards using absolute gravimetric and other techniques, and the development of dynamic calibration techniques for gaseous species which would be unstable in high-pressure cylinders. The methodology used for preparing gas standards gravimetrically is described, together with the rigorous quality assurance measurements and consistency checks which are used to demonstrate their accuracy and stability. The uncertainty budget assigned to these standards will also be summarised. NPL primary standards are used to certify traceable 'secondary' gas standards which are disseminated so as to ensure the accuracy of gas analysis measurements. Examples of the applications of these secondary standards are presented. The gas standards are employed in proficiency testing of industrial stack-testing organisations, and results of the initial rounds are presented. NPL gas standards are also now being used as the basis of the United Kingdom Environment Agency's new type-approval and certification scheme for continuous industrial stack-emission analysers. A recent important international initiative, in the field of gas analyses, is the agreement by national standards laboratories across the world to demonstrate the equivalence of their calibrations, by means of key comparisons between them. These worldwide key comparisons are complemented in Europe through the EUROMET initiative which seeks to establish the equivalence and comparability of calibration standards held at national standards laboratories across Europe. Examples of these intercomparisons are presented.
Keywords: Keywords Traceability; Gas standards; Calibration; Quality assurance; Proficiency testing; International comparisons