Material certification studies

Material certification study Study that assigns a reference value ( true value ) to a quantity (concentration or property) in the test mate-... 

Material certification study Method performance study Proficieucy test Z Score... 

A certification study is an interlaboratory study in which a group of selected laboratories analyze a candidate reference material by a method judged most likely to provide the least biased estimates of concentration and the smallest associated uncertainty. The purpose is to provide a reference value of the analyte concentration in the material. Certification studies are organized by, e.g., BCR and NIST [7]. 

Principles and Characteristics According to the lUPAC definition, an interlaboratory study is one in which several laboratories measure a quantity in one or more identical portions of homogeneous materials under documented conditions, the results of which are compiled into a single report. Three types of interlaboratory studies are distinguished, namely method-performance, laboratory-performance or material-certification studies. The aim of method-performance or collaborative studies is to assess the performance characteristics of a specific method. In laboratory-performance or proficiency studies a homogeneous test material is analysed of which the true concentrations are known or have been assigned in some way. The participants apply whatever... 

After the performance of collaborative studies organized by the RIVM (National Institute of Ihiblic Health and Environmental Protection, Bilthoven, The Netherlands) and SM T certification studies were performed by a group of eleven or twelve qualified European laboratories. These examined simultaneously the materials using standardized methods following a very strict analytical protocol. Table 5.1 below shows data for CRM 506 (Mooijman et al. 1999)-... 

A number of reference materials for vitamins in foods are under development. Extensive analysis and stability testing have been conducted to assess the potential of vitamin-enriched milk powder, wholemeal flour, and margarine as well as lyophilized brussels sprouts, mixed vegetables, and pork liver for use as reference materials (193-196). The certification study for vitamin C in the brussels sprouts reference material has been completed. However, methodology problems continue to have a significant negative effect on the development of reference materials. 

Type of study A = analytical study, C = collaborative trial, M = monitoring study, P = study on the effect of food processing/preparation, R = Reference material certification, O = other studies, including investigations on element uptake/distribution/biotransformation and biological activity. 

When having all individual data available from the certification report, the analyst may wish to have a closer insight into his performance in terms of comparison to the results achieved by a similar method used in the certification. In the case of materials certified for organic trace substances e.g. dioxins in fly ashes, where methods inevitably remain affected by small systematic errors, the uncertainty calculated from the set of data of the interlaboratory certification study may appear small compared to the spread of the accepted sets of data. The half-width of the 95% confidence interval of the set of data is used as uncertainty. The user can adapt the way of determining the value of Sq above. He may wish to replace the 95% confidence interval by a tolerance interval... 

In cases where methods are similar or show common steps, it is essential to examine if no systematic bias is affecting the results. In some few cases, BCR was confronted with situations where participants had to use a common calibration material (certification of dioxins and furans in a fly ash extract). Such a situation offers the possibility of introducing a systematic error into the study and consequently of a bias of the certified value. Situations where suppliers cannot propose reliable calibration substances may oblige the certification body to provide a common material. It should remain an exception and if unavoidable, the producer of the common calibrant should follow a strict preparation procedure similar to the one described above for the certification of calibrants. 

Before examining the results, it is common practice in BCR certification studies to examine the methods applied. For method dependent parameters, e.g. in microbiology, the data are only disclosed to the participants after the examination of the full follow up of the protocols. Besides quality control steps and verification of calibration materials, e.g. with a common verification solution, each method is discussed to evaluate if inadequate handling was performed or risky steps applied. 

The distribution of a limited number of microbes within a solid matrix is essentially inhomogeneous, i.e. for a certain level of intake, differences due to the distribution of the microbes will appear. The more microbes that have been introduced into the matrix, the less these differences will be important and the lower the sample intake will be where differences appear. The theoretical distribution of the microbes in the sample is described by a Poisson distribution [29,30]. The RIVM group, which developed these materials, used a modified Cochran s dispersion test to evaluate the variation within a single capsule and between capsules. In fact overdispersion (more inhomogeneity than theoretically expected) has been noticed for nearly all RMs and CRMs produced [31]. The certification trial revealed that it had no influence on the outcome of the interlaboratory certification study as the between-laboratory reproducibility largely covered this overdispersion factor. 

The results of this preliminary trial are described in detail elsewhere [60]. The fractional contents obtained by the different procedures were in good enough agreement for the sediments to be sufficiently well chacterized for decisions to be made on the management or use of the sediments. There were serious failures in detail and it could be concluded that, while the results were promising, improvements were necessary if reference material certification was to be attempted. Furthermore, any future study should be based on a well specified procedure, which was designed as described in Section 11.4. 

In each case a referee laboratory is responsible for providing the sample and organizing its distribution and the collation and analysis of the results. For the majority of studies the purpose of the study is to test the laboratory (proficiency testing), the test material (material certification), or the method (method validation study). The purpose determines some of the study parameters. For example, a method validation study requires all the laboratories to use the same method, but for other studies the method may not be prescribed. For materials certification the laboratories are chosen for their demonstrated proficiency in analysis of the material. It would not be appropriate if there was any dubiety concerning the assigned quantity value because of a suspect result. 

Roberts, Hewitt, and Gary Robinson. ISO 14001 EMS Implementation Handbook. Oxford, U.K. Butterworth-Heinemann, 1998. - A guide for implementing and maintaining an ISO 14001 EMS. Includes recommendations, checklists, templates, certification tips, case-study materials, and more. 

Further afield, in 1978 the Japanese National Institute for Environmental Studies (NIES) started the production of a series of biological and environmental matrix CRMs, certified for a number of trace elements (Okamoto and Fuwa 1985). Recently also the certification of metal species in some materials was reported (Okamoto and Yoshinaga 1999). 

In those cases where there are any doubts about the feasibility of producing a sufficiently homogeneous and stable reference material, a feasibility study might be needed. For this study, an extra amount of material is needed. Questions regarding the best way of preparing the sample, the stability of the material, or the fitness for purpose might justify the inclusion of a feasibility study in the project. In the BCR projects, it is common practice to have a feasibility study, which usually has as the sole purpose of assessing the performance of the laboratories in the collaborative study in relation to the certification of the reference material. The feasibility study allows the participants to fine-tune their equipment, their methods, and their procedures in view of the characterization measurements. In each of these cases, a considerable extra number of samples is needed. 

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