Certification of RMs

A large number of considerations and factors must be entertained for the conception, development, preparation, assessment, characterization, and certification of RMs, including (a) end use requirements, (b) selection of materials, (c) preparation, (d) physical characterization, (e) chemical characterization, (f) certification, (g) documentation, and (h) distribution. Most of these have an overwhelming impact on the finally developed RM and on its credibility. This section deals with the steps, collectively denoted as collection and preparation, occurring early in the scheme of RM development. It treats general collection and preparation principles, and provides specific examples of preparative procedures. 

The European Commission (1994) has prepared a detailed and most useful guide for the production and certification of RMs. The following is a summary of items relating to certification ... 

An extremely wide variety of analytical methods are used by RM producers and developers in the certification of RMs for inorganic elemental content. These methods range from the classical, through current instrument based methods to highly specialized definitive methods. 

Tab. 3.3 Methods for the certification of RMs for elemental content Initial stages pretreatment, preconcentration, isolation and separation...

Tab. 3.4 Methods for the certification of RMs for eiemental content determination methods used by various RM producers for biologicai, environmental, clinical, agricultural, food, geological, occupational materials, and pure compounds ...

RM producers could obtain greater support for research, production and certification of RMs. Increasingly, laboratories must convince policy level managers that these RMs serve major objectives of human welfare. Instead of presentations which detail the minutiae of a somewhat arcane technical field, presentations (directed at those who control resources) are needed which discuss the external driving forces and expand on the contributions of RMs to the most fundamental and serious problems facing the world (Rasberry 1998). 

Improvement schemes can be defined as a succession of individual interlaboratory studies in which several laboratories analyse the same test samples for the same characteristics (usually the content of an analyte), following a similar protocol to validate each individual step of their own analytical method (Quevauviller, 1999a). They enable laboratories to develop and validate all steps of new or existing analytical procedure(s) in adequately organised successive exercises which may be considered as preliminary studies for laboratory or method performance studies or certification of RMs (Griepink and Stoeppler, 1992 Quevauviller, 1998b). Such programmes are particularly valuable in the case of speciation studies since the analytical procedures include several complex and critical steps. 

Following a feasibility study, it was found that trimethyllead (TML) would be of sufficient stability for the purpose of interlaboratory studies and possibly certifications of RMs whereas dialkyllead species and triethyllead were not stable enough. Interlaboratory studies were performed in 1992-1994 on TML in artificial rainwater and urban dust and enabled a significant improvement of the state-of-the-art to be achieved (Quevauviller et al., 1995b). These exercises were followed by a certification campaign aiming to certify TML in candidate RMs of artificial rainwater and urban dust. The between-laboratory agreement obtained was very satisfactory and both materials were proposed for certification. 

However, depending on the intended application of the RM, larger uncertainties may sometimes be acceptable. The smaller uncertainty provided by IDMS-certi-fied data may not always be necessary in routine environmental monitoring work, where a consensus-certified material may well be fit-for-purpose and provide a cost-effective approach to RM certification. The resource-intensive approach of IDMS may then be properly confined to the certification of RMs for critical applications. 

Preparation and Certification of RMs for Chemical Composition Intended for Inorganic Trace Analysis... 

Preparation and certification of RMs is a complicated, multistage, and time-consuming task. Attention to detail at all stages of this task is obligatory because one failure at any stage could shatter previous achievements and make the candidate CRM useless. Correct cooperation of many experienced analysts and constant supervision by the coordinator from the beginning to the end of the whole campaign is necessary for final success. 

With regard to the hierarchy of methods, primary methods or definitive methods frequently represent the highest level of reliability. Such concepts, coming from fundamental physical metrology, may find their interest in particular within discussions on traceability concepts (see section 2.1.3). In chapter 5, on the certification of RMs, these aspects are discussed in some more detail (section 5.1.4.1). 

In principle, the certification of RMs for microbiological parameters does not differ from the approach used by BCR for chemical parameters. They fall into the category of method-dependent CRMs certified for an activity. In fact, the microbes will behave differently from one type of method (in particular the culture medium) to another. For selective media the resuscitation rate of the microbes in the RM will change. Injured bacteria will have difficulties to form colonies on these media. This reflects the biological activity of the strain after stabilisation. As the method will strongly affect the result, it must be verified that the method was applied in a similar way by all participants. This means that accurate protocols have been prepared and tested beforehand in a feasibility study. The first step of the evaluation of the certification study will be to verify that the protocol was strictly followed. This protocol will be made available to the users of the CRM. 

General methods for the certification of RMs for elemental content are based on atomic absorption spectrometry (FAAS, ETAAS, HG-AAS, CV-AAS), atomic emission spectrometry (FAES, ICP-AES, HG-ICP-AES, DCP-AES), atomic fluorescence spectrometry (CV-AFS), mass spectrometry (IDMS, SSMS, NAMS, ICP-MS), nuclear methods (IPAA, PAA, INAA, RNAA), X-ray emission (EDXRF, WDXRF, particie induced techniques), light-absorption spectrometry (LAS, FL), electrochemistry (ASV, CSV, DPP, ISE) and other methods (Kjeldahl, combustion elemental analysis, volumetry, chromatography, gravimetry) [32]. Certification of BCR-680/681 was carried out by sixteen participating laboratories using a variety of common as well as highly specialised techniques (Table 8.10). 

---