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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.7, #7)
Evaluation of measurement uncertainty for analytical procedures using a linear calibration function by L. Brüggemann; Rainer Wennrich (pp. 269-273).
In the EURACHEM/CITAC draft ”Quantifying uncertainty in analytical measurement” estimations of measurement uncertainty in analytical results for linear calibration are given. In this work these estimations are compared, i.e. the uncertainty deduced from repeated observations of the sample vs. the uncertainty deduced from the standard residual deviation of the regression. As a result of this study it is shown that an uncertainty estimation based on repeated observations can give more realistic values if the condition of variance homogeneity is not correctly fulfilled in the calibration range. The complete calculation of measurement uncertainty including assessment of trueness is represented by an example concerning the determination of zinc in sediment samples using ICP-atomic emission spectrometry.
Keywords: Keywords Measurement uncertainty; Linear calibration; Method validation
Appropriate rather than representative sampling, based on acceptable levels of uncertainty by M. H. Ramsey (pp. 274-280).
Appropriate sampling, that includes the estimation of measurement uncertainty, is proposed in preference to representative sampling without estimation of overall measurement quality. To fulfil this purpose the uncertainty estimate must include contribution from all sources, including the primary sampling, sample preparation and chemical analysis. It must also include contributions from systematic errors, such as sampling bias, rather than from random errors alone. Case studies are used to illustrate the feasibility of this approach and to show its advantages for improved reliability of interpretation of the measurements. Measurements with a high level of uncertainty (e.g. 50%) can be shown to be fit for some specified purposes using this approach. Once reliable estimates of the uncertainty are available, then a probabilistic interpretation of results can be made. This allows financial aspects to be considered in deciding upon what constitutes an acceptable level of uncertainty. In many practical situations ”representative” sampling is never fully achieved. This approach recognises this and instead, provides reliable estimates of the uncertainty around the concentration values that imperfect appropriate sampling causes.
Keywords: Keywords Representative sampling; Uncertainty of measurement
Linearity of calibration curves: use and misuse of the correlation coefficient by Joris Van Loco; Marc Elskens; Christophe Croux; Hedwig Beernaert (pp. 281-285).
The correlation coefficient is commonly used to evaluate the degree of linear association between two variables. However, it can be shown that a correlation coefficient very close to one might also be obtained for a clear curved relationship. Other statistical tests, like the Lack-of-fit and Mandel’s fitting test thus appear more suitable for the validation of the linear calibration model. A number of cadmium calibration curves from atomic absorption spectroscopy were assessed for their linearity. All the investigated calibration curves were characterized by a high correlation coefficient (r >0.997) and low quality coefficient (QC <5%), but the straight-line model was systematically rejected at the 95% confidence level on the basis of the Lack-of-fit and Mandel’s fitting test. Furthermore, significantly different results were achieved between a linear regression model (LRM) and a quadratic regression (QRM) model in forecasting values for mid-scale calibration standards. The results obtained with the QRM did not differ significantly from the theoretically expected value, while those obtained with the LRM were systematically biased. It was concluded that a straight-line model with a high correlation coefficient, but with a lack-of-fit, yields significantly less accurate results than its curvilinear alternative.
Keywords: Keywords Linearity; Goodness of fit; Correlation coefficient; Lack-of-fit; Calibration
Quantification of total petrol hydrocarbons (TPH) in soil by IR-spectrometry and gas chromatography – conclusions from three proficiency testing rounds by R. Becker; Matthias Koch; Sigrid Wachholz; Tin Win (pp. 286-289).
Due to the utilisation of 1,1,2-trichlorotrifluoroethane (CFE) as extraction solvent the IR-spectrometric determination of total petrol hydrocarbon (TPH) in soil according to ISO/TR 11046 has been replaced by gas chromatography/flame ionisation detection (GC/FID) after extraction with a halogen-free solvent according to ISO/DIS 16703:2001. The results obtained with both methods by field laboratories in three proficiency testing (PT) rounds are compared. The consensus means obtained with GC/FID are typically 10%–20% (ranging between 0% and 25%) higher than those found with IR-spectroscopy. On the contrary, coefficients of variation (CV) are roughly double in case of GC/FID and are briefly discussed against the background of the Horwitz equation.
Keywords: Keywords Hydrocarbons; Soil; IR spectrometry; Gas chromatography; Interlaboratory comparison
Accreditation of medical laboratories in South Africa by G. S. Dhatt; Sean Peters (pp. 290-292).
Accreditation of medical laboratories is a well-established process in many parts of the world. We describe this process in South Africa under the South African National Accreditation System. The elements of accreditation, the accreditation process for medical laboratories and future prospects are outlined.
Keywords: Keywords. Accreditation; South Africa; SANAS; Medical laboratories
Analytical procedure in terms of measurement (quality) assurance by R. Kadis (pp. 294-298).
In the ordinary sense the term ”analytical procedure” means a description of what has to be done while performing an analysis without reference to quality of the measurement. A more sound definition of ”analytical procedure” can be given in terms of measurement (quality) assurance, in which a specified procedure to be followed is explicitly associated with an established accuracy of the results produced. The logic and consequences of such an approach are discussed, with background definitions and terminology as a starting point. Close attention is paid to the concept of measurement uncertainty as providing a single-number index of accuracy inherent in the procedure. The appropriateness of the uncertainty-based approach to analytical measurement is stressed in view of specific inaccuracy sources such as sampling and matrix effects. And methods for their evaluation are outlined. The question of a clear criterion for analytical procedure validation is also addressed from the standpoint of the quality requirement which measurement results need to meet as an end-product.
Keywords: Keywords Accuracy; Analytical procedure; Measurement assurance; Measurement uncertainty; Quality assurance; Validation
Flexibilization of the scope of accreditation: an important asset for food and water microbiology laboratories by A. Leclercq (pp. 299-304).
Beltest, the Belgian accreditation body, has investigated flexibilization of the scope of accreditation for chemistry laboratories and food and water microbiology laboratories. This flexibilization, synonymous with test-type accreditation, allows a laboratory to add new test methods or retry previous test methods without having to undergo a new audit by Beltest. It has been used for nearly ten years by German and Swiss accreditation bodies. Flexibilization permits the validation of methods and results, given that the competence of the particular laboratory is already well established. This new concept in microbiology allows client’s needs to be adequately met, and facilitates the quick establishment of a method in several laboratories at once in case of a public health crisis. The first laboratory to participate at this investigation on the flexibilization concept, as a test of the concept, was the Belgian reference laboratory for food microbiology.
Keywords: Keywords Accreditation; Food and water microbiology; Validation; Type of tests