Preparation of the Candidate Reference Materials

The composition of the artificial rainwater matrix was chosen to reflect that of natural rainwater which falls over continental land masses [123]. A stock solution was prepared at 100 times the required concentration of the artificial rainwater by the addition of inorganic compounds to deionized water. One litre of stock solution was prepared by the addition of the following compounds (Analytical reagent grade) to 1000 mL of Milli-Q deionized water  

The components were dissolved using an ultrasonic bath and the final solution was filtered through a 0.2 pm (47 mm diameter) cellulose acetate membrane in order to eliminate algal and bacterial particles. No problems were encountered with the solubility of any of the components of the stock solution. 

Component Concentration in stock solutionlymol L Concentration in final solutionlymol L  

The concentrations of the components present in the stock solution and in the artificial rainwater are presented in Table 6.4. 

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Pauwels J, Kramer GN, De Angelis L, Grobecker KH (1990) The preparation of codfish candidate reference material to be certified for Pb, Cd, Hg, Fe and Zn, Fresenius J Anal Chem 338 515-519. 

Two 150 L PVC containers (one for each solution) to be used for the preparation and homogenisation of the candidate reference materials were cleaned with a detergent, rinsed with distilled water and further rinsed with ultrapure water. For each of the reference materials, the ampoules were cleaned in a similar way. The ampoules were (air) dried for two days and conditioned for at least 24 h with the solution which they would contain. The preliminary investigations had shown that this procedure was adequate to bring the walls in adsorption equilibrium with the solution. 

The overall coordination of the interlaboratory studies and certification campaign was managed jointly by the Laboratoire de Chimie Analytique et Minerale of the Universite Louis Pasteur in Strasbourg (France) and the CNRS, Service Central d Analyse in Vernaison (France). The Environment Institute of the Joint Research Centre of Ispra (Italy) prepared the candidate reference material of tuna fish. Arsenobetaine calibrant for the certification was synthetised and characterized in the Laboratoire de Chimie Analytique et Minerale in Strasbourg (France). 

Techniques developed for the determination of selenite and selenate involve a succession of several analytical steps (e.g. reduction, separation, detection) which are often far from being validated. In addition, the knowledge related to the stability of the species is still very scarce. A project has hence been launched within the BCR programme with the aim to evaluate the stability of Se-species in solution [42] this feasibility study has been continued by an interlaboratory study for the evaluation of method performance [43]. Both investigations were designed to improve the state-of-the-art of Se-speciation prior to the tentative certification of solution candidate reference materials as described in this section. As a follow-up, artificial freshwater solutions containing inorganic Se-species were prepared (RMs 602 and 603) [40,41]. 

The final determination of the CBs was performed by capillary gas chromatography with electron capture detection mass spectrometry was used as complementary technique to confirm the identity of each of the CBs determined. Each participant had validated its method by performing experiments on recovery, extraction efficiency, procedure blanks and detector linearity. The seven individual CB calibrants were supplied to the participants as pure, crystalline CRMs from BCR (CRMs Nos. 291,293, 294,295, 296. 297 and 298). Each laboratory was requested to prepare separate calibration solutions of the appropriate concentration, in iso-octane, to calibrate the detector and lying within its linear range. The use of at least one internal standard was mandatory the participants, however, were left free to select the internal standard(s) best suited to their methods. They had to verify that the selected compounds did not occur in the candidate reference material or did not interfere with compounds present in the material. A series of pure dichlorobenzylalkyl ethers (DCBEs) was made available to the participants but other internal standards were also accepted of which the list is given in the certification report [21] along with additional details on calibration procedures. 

The project was coordinated by the Danish Technological Institute (Taastrup, Denmark). The candidate reference materials were purchased from a fish market in Venice and the preparation was performed by Ecoconsult (Gavirate, Italy) and the Environment Institute of the EC Joint Research Centre of Ispra (Italy). MeHg calibrants were prepared by the Department of Chemistry of the University of Aberdeen (United Kingdom). The verification of the homogeneity and stability was performed by the Presidio Multizonale di Prevenzione (Venice, Italy). 

The candidate reference materials of artificial rainwater and urban dust were prepared by the School of Chemistry of the University of Birmingham (United Kingdom) which also verified their homogeneity and stability. The two exercises were coordinated by the Department of Environmental Sciences of the University of Plymouth (United Kingdom). 

Kramer GN, Oostra A, de Vos P, Conneely P (1997) The preparation of lyophilized tuna muscle samples for a feasibility study and for BCR candidate reference material CRM 669 rare earths. JRC-IRMM Report GE/R/MRM/14/97. 

ScHiADOT JD, Backhaus F, Burow M, Froning M, Mohl C, Ostapczuk P, Rossbach M (1993) Collection, preparation and characterization of fresh, marine candidate reference materials of the German Environmental Specimen Bank. Fresenius [ Anal Chem 345 137-139. 

Zeisler R, Strachnov V, Perschke H, Dekner R (1993) The preparation of a cabbage candidate reference material to be certified for residues of agrochemicals. Fresenius J Anal Chem 345 202-206. 

Owing to the difficulty of preparing candidate reference materials of estuarine water and the analytical difficulties encountered by the laboratories for trace element determinations, it was decided to carry out preliminary investigations to optimise the sampling and sample pretreatment procedures, and to evaluate the state of the art of estuarine water analysis in Europe. A limited batch of estuarine water sample was therefore collected for this purpose. 

When selecting a site for the collection of the material, it was argued that most monitoring studies would be performed in coastal areas, and as a consequence a coastal seawater was preferred over an open ocean sample which, due to its lower Hg contents would be of use for only a limited number of users. A compromise was found in a sample with a content typically in the lower range of coastal waters. A feasibility study for the preparation of large batches of seawater for this element and results of interlaboratory studies demonstrated that a certification campaign could be contemplated [28], and a candidate reference material was prepared for this purpose [29,30]. This material (CRM 579) was certified for its content of Hg. 

Round robin tests using candidate reference materials revealed that the surface of dispersed solids can alter remarkably during storage and may be seriously affected during preparation for its measurement. Nevertheless, a variety of certified reference materials for the particle size and for the specific surface area is available from national and international standardization administrations and from industrial distributors. Recently, also reference materials for porous materials have been certified. 

Comparative measurements at different laboratories using candidate reference materials revealed that surfaces of highly dispersed materials can be affected remarkably during storing and sample preparation. On account of the fractal nature of these materials, the results depend on the measuring methods. Reference materials of dispersed materials are offered by national and international standardisation administrations and by industrial distributers. 

In order to cope with the lack of CRMs for endogenous species, the practice of laboratory internal QC materials is developing. A Laboratory Reference Material (LRM) was prepared from Brazil nuts (Bertholletia excelsa) for QC of the selenomethionine determinations. The concentration of selenomethionine was 79.9 p,g g-1. The homogeneity and stability of this candidate CRM passed the relevant tests recommended by the Measurement and Testing Programme [97],... 

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