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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.9, #6)
Reliability of measurements of pesticide residues in food by Arpad Ambrus (pp. 288-304).
This paper accounts for the major sources of errors associated with pesticide residue analysis and illustrates their magnitude based on the currently available information. The sampling, sample processing and analysis may significantly influence the uncertainty and accuracy of analytical data. Their combined effects should be considered in deciding on the reliability of the results. In the case of plant material, the average random sampling (coefficient of variation, CV=28–40%) and sample processing (CV up to 100%) errors are significant components of the combined uncertainty of the results. The average relative uncertainty of the analytical phase alone is about 17–25% in the usual 0.01–10 mg/kg concentration range. The major contributor to this error can be the gas-liquid chromatography (GLC) or high-performance liquid chromatography (HPLC) analysis especially close to the lowest calibrated level. The expectable minimum of the combined relative standard uncertainty of the pesticide residue analytical results is in the range of 33–49% depending on the sample size.The gross and systematic errors may be much larger than the random error. Special attention is required to obtain representative random samples and to eliminate the loss of residues during sample preparation and processing.
Keywords: Measurement uncertainty; Measurement reliability; Pesticide residue analysis
Certified reference materials as a quality tool in food control: much used—often misused—sometimes abused by Lars Jorhem (pp. 305-310).
Certified reference materials (CRMs) are important tools in the quality control of food analyses. There are, however, many ways in which CRMs can be abused and misused. It can be due to ignorance, as well as to overuse of expensive materials. The major drawback of CRMs is probably that the analyst knows the level of the analyte. The statistical evaluation of CRM results in reports and publications is often limited to a comparison between the found and the certified levels, which yields little, and sometimes erroneous, information. Recoveries based on CRMs often give a picture that is far too bright, with little consideration of uncertainties. The way the use of CRMs is described in most scientific journals is often very crude and shows that CRMs are seldom used to their full capacity. The objective of this paper is to try to summarise the ways in which a CRM can be misused and thereby put into focus how to make better use of such materials. It also gives examples on how to evaluate CRMs, using a procedure that was recently introduced by the Nordic Committee on Food Analysis.
Keywords: Certified reference materials; CRM; Food control; Analytical quality control
Measurement uncertainty and doping control in sport. Part 2: Metrological traceability aspects by Adriaan M. H. van der Veen (pp. 311-316).
The assessment of (non-)compliance in doping control and in particular the appreciation of uncertainty of measurement in such an assessment has become a subject of debate. In a previous paper, the requirements for evaluating measurement uncertainty have been addressed. The debate now focuses on the estimation of the standard uncertainty. The completeness of an uncertainty budget is discussed in view of the (metrological) traceability of the result. It is concluded that despite the fact that the new doping rules dictate that A-tests and B-tests be performed in the same laboratory, the uncertainty budget should encompass all factors, including between-laboratory effects. The assessment of (non-)compliance should be based on a one-sided t-test. There is also consensus about the level of confidence to be applied (99%), despite the fact that the regulations state 95% as the appropriate level.
Keywords: Doping control; IOC; Measurement uncertainty; Accreditation; Compliance tesing
Strategic views of accreditation by Ferry Dizadji; Elke Anklam (pp. 317-322).
This paper summarises the experience we gained while going through an accreditation process in accordance with the requirements of ISO/IEC 17025 Standard. We propose in this paper an initial path finding study stage to evaluate the laboratory's predisposition for, and the benefits of, accreditation. We also present a conceptual framework as part of the path finding study stage to identify and classify different groups of activities in a laboratory into separate business domains. The objective of the study is to assess, for each business domain, the approach, the benefits, costs and implications of working with a systematic QA and then design a priori appropriate working systems tailored to the real needs of each business. Such a study should naturally lead to an appropriate definition of accreditation scope.
The interlaboratory comparison "IMEP-19 trace elements in rice"—a new approach for measurement performance evaluation by Yetunde Aregbe; Caroline Harper; Jørgen Nørgaard; Machteld De Smet; Peggy Smeyers; Lutgart Van Nevel; Philip D.P. Taylor (pp. 323-332).
The aim of IMEP is to present objectively the quality of chemical measurements. Participants in IMEP compare their reported measurement results with independent external certified reference values with demonstrated traceability and uncertainty, as evaluated according to international guidelines. IMEP-19 focused on measurements of trace elements in rice aiming to support the Commission Regulation (EC) No. 466/2001 on maximum levels for certain contaminants in foodstuff. Measurement results for the elements Cd, Cu, Pb and Zn were reported by 267 field laboratories involved in food analysis from 43 countries. Performance criteria for the evaluation of the reported measurement results in IMEP-19 are suggested. The chosen performance indicators not only take into account the deviation of the reported measurement value from the certified reference value, but also set criteria for maximum and minimum acceptable uncertainty. The IMEP-19 participants' performance is reviewed by means of using new simple graphical tools, called "Naji plots".
Keywords: Measurement performance evaluation; Interlaboratory comparison; Naji plots; Uncertainty; Food; Trace elements
Proficiency testing schemes: solutions for homogeneity control by Marie E. Lafargue; Sandrine Biogeaud; Douglas N. Rutledge; Max H. Feinberg (pp. 333-339).
The organisation of the homogeneity control of samples prepared for proficiency testing schemes (PTS) is a challenge for the PTS organisers and engenders logistical and financial problems. Samples used in PTS need to be homogeneous in order to be sure that if a laboratory has a result different from the other laboratories, its error can be attributed to its analysis method and not to its sample. The present paper describes the solutions found by the Bureau Interprofessionnel d’Etudes Analytiques (Bipea) in order to answer the homogeneity control requirement. The methodologies of homogeneity control are adapted to specific characteristics of the products proposed in Bipea’s PTS. The first methodology of control, the “preparation approach”, concerns specific PTS where the samples are formulated and where the assigned values are known and chosen. The second methodology, the “target approach”, concerns mainly the field of the environment and implies the choice of homogeneity targets. The third methodology, the “global approach”, concerns the agricultural and agro-food fields and involves comparison of near-IR spectra samples. Each methodology of control has its advantages and disadvantage and is suited to specific products prepared with specific preparation procedures. It has to be stressed that the solutions developed answer quality requirements but are not a substitute for the use of adapted and detailed preparation procedures with validated and regularly checked devices and for the a posteriori control of the results of the laboratories.
Keywords: Proficiency testing schemes; Control of homogeneity
Estimation of uncertainty in electrochemical amperometric measurement of dissolved oxygen concentration by Lauri Jalukse; Ivo Leito; Aleksei Mashirin; Toomas Tenno (pp. 340-348).
A procedure for the estimation of measurement uncertainty of dissolved oxygen (DO) concentration measurement based on the ISO approach is presented. It is based on a mathematical model that involves 14 input parameters. The uncertainty of DO concentration strongly depends on changes in experimental details (temperature difference between calibration and measurement, the time interval between calibration and measurement, etc.). The relative measurement uncertainty is, however, practically independent of the DO concentration itself. The uncertainty is the lowest if the calibration and the measurement are done at the same temperature and on the same day. A calculation tool is provided (in the form of a GUM Workbench file) for practitioners that can be used for uncertainty calculation of DO concentrations at very different experimental conditions.
Keywords: Measurement uncertainty; Uncertainty budget; Dissolved oxygen; Galvanic sensor
A model for the evaluation of uncertainty in routine multi-element analysis by Thomas Ruth (pp. 349-354).
A model for calculating uncertainty in routine multi-element analysis is described. The model is constructed according to the principles of GUM/EURACHEM. Control chart results are combined with other existing data and results from the actual measurement into a concentration-dependent estimate of combined standard uncertainty. Since possible sources of bias are included in the calculation, overall bias as estimated from the data is used only as a control to identify needs for modification of the model and/or the analytical procedure. For each individual sample, uncertainty can be calculated automatically based on two pre-calculated parameters together with measured concentration and instrumental standard deviation. As an example, the model is demonstrated for inductively coupled plasma-mass spectrometry (ICP-MS) analysis of sewage sludge including laboratory sub-sampling, sample preparation, and instrumental determination.
Keywords: Uncertainty; ICP-MS; Quality control; Control chart
Parametric or non-parametric statistical tools applied to marine aerosol sampling by M. João Nunes; M. Filomena Camões; Frank McGovern; Frank Raes (pp. 355-360).
Atmospheric aerosol sampling has been performed in parallel with both high and low air flows. A comparison of the results requires proper statistical design and analysis. The presence of several outliers and the increase of residuals with increasing concentration have led to the recommendation of handling the data with non-parametric methods rather than parametric statistical analysis.Conversion equations were obtained for each of the ionic species examined for prediction of low air flow sampling from long-term observations with high air flow sampling.
Keywords: Atmospheric aerosols; Sampling; Parametric; Non-parametric statistics
Method validation procedures for environmental radiochemical measurements at AECS by M. S. Al-Masri; A. Shakhashiro; Aba. Y. Amin (pp. 361-368).
The present paper describes the experience of the Atomic Energy Commission of Syria in relation to the application of Eurachem Guide on method validation for environmental radiochemical measurements. Methods validated include gamma and alpha spectrometry for natural and artificial radionuclides determination, and fluorometry determination for total uranium. Documents and records were first set to meet trackability and traceability requirements, where internal quality control mechanisms have been adopted. Methods stability was checked by means of Z-score control charts. Internal method validation parameters including method detection limits, repeatability limits, reproducibility limits, recovery coefficient and relative error were estimated. External method validation has been achieved by participating in international intercomparison exercises and proficiency tests organized by EML, IAEA and WU; some results of these activities are presented. Moreover, the application of both internal and external method validation gave the analysts in our laboratories more confidence in their skills, and it was of great assistance for our customers including regulatory authorities to evaluate the fitness of the method for their applications according to internationally agreed procedure. The steps followed here can be used for method validation in other laboratories with similar applications.
Keywords: Method validation; Radiochemical analysis; Eurachem; Syria
Measurement uncertainty in sports drug testing by Bernard King (pp. 369-373).
This paper discusses some of the contentious issues raised by van der Veen (Accred Qual Assur 8:334, 2003[1]), and Van Eenoo and Delbeke (Accred Qual Assur 8:477, 2003 [2]) concerned with the evaluation of the uncertainty of measurement and compliance decisions based on regulatory limits. Also, the criticisms of accreditation by van der Veen are considered and some general remarks about the strengths and limitations of accreditation provided. It is concluded that a position between those adopted by van der Veen and by Van Eenoo and Delbeke would be more appropriate.
Keywords: Sports drug testing; Measurement uncertainty; Compliance decisions; Accreditation
Author’s response to the Letter to the Editor of Van Eenoo et al. concerning the paper “Measurement uncertainty and doping control in sport”
by Adriaan M. H. van der Veen (pp. 375-377).
Reply to Jarl Forsten’s article “Future direction for measurement, testing and conformity assessment”
by Peter S. Unger (pp. 378-379).
Response to the letter to the editor by P. Unger (see previous letter)
by Jarl Forstén (pp. 380-381).
Role of measurement uncertainty in conformity assessment in legal metrology and trade
by Håkan Källgren; Margreet Lauwaars; Bertil Magnusson; Leslie Pendrill; Phillip Taylor (pp. 384-384).