Preparation of the Candidate CRMs

Before starting the preparation of the candidate CRM, a preliminary test was carried out on a small sample of 50 g wet sediment, which was dried for 15 h at 60°C, ground in a porcelain mortar, homogenized for 10 min in a Turbula mixer and subsequently analyzed for various trace metals by Neutron Activation Analysis (NAA) (see Table 11.1) and for its particle size distribution (Figure 11.1), after which the powder was examined microscopically (Figure 11.2). This semi-quantitative check was carried out to verify the initial trace element levels in order to establish whether the preparation steps listed below could be carried out free of contamination and whether the resulting material was of the required quality ... 

Figure 11.3. Flow-sheet of the preparation of the candidate CRM Murst-ISS Al Antarctic Sediment.

A feasibility study was carried out to investigate the optimal conditions for the preparation of the candidate CRMs of artificial freshwater to be certified for their... 

SUBSTANCES ADDED FOR THE PREPARATION OF THE CANDIDATE CRMS AND CONCENTRATIONS EXPECTED UPON SPIKING... 

The optimal conditions for the preparation of groundwater candidate CRMs were tested in a feasibility study of which the results are published elsewhere [14] two materials were selected, representing typical carbonate and sandstone media. Two batches were prepared from ultrapure water by adding pro-analysis grade chemicals and their homogeneity was verified to evaluate possible effects of the preparation procedure on the sample composition [14]. The stability was also checked at +4°C and +20°C over a period of three months. 

Four 680 L polyethylene containers (two for each solution) to be used for the sampling, preparation and homogenisation of the candidate CRMs were cleaned with detergent, rinsed with distilled water and soaked with diluted pro-analysis nitric acid (1 10). The containers were rinsed again with Milli-Q water and soaked with Milli-Q water for a period of at least one month prior to the use of the containers. Tubings used for filtration and homogenisation were also cleaned at the same time as the container cleaning procedure. 

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],... 

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. 

AMOUNTS OF SUBSTANCES (GRAMS) ADDED TO 150 LITRES OF WATER FOR THE PREPARATION OF THE TWO CANDIDATE CRMS 479 AND 480... 

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. 

IRMM has established dedicated facilities for the preparation of candidate CRMs aiming at ... 

A candidate CRM for trace elements in Antarctic coastal marine sediment was prepared by coarse sieving, drying, jet-milling with ultrafine classification of particles and Turbula mixing. The material was found to be of appropriate particle size distribution, dryness and homogeneity to be certified as a stable and homogeneous CRM. 

The stabilisation of microbial strains in spray-dried milk powder has allowed the preparation of several RM available from RIVM in the Netherlands and six BCR-CRMs. Some additional strains have demonstrated sufficient stability to be candidates for certification. Other types of materials could be or have been developed by various producers (see text). For certification purposes a similar approach to the one used by BCR could be used for these materials. 

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. 

Two 250 L polyethylene tanks were used for the preparation and homogenisation of the two candidate CRMs they were cleaned thoroughly with deionized water (Milli-Q) and subsequently rinsed with the solution they would contain. Polyethylene bottles were carefully cleaned with deionized water (Milli-Q) each bottle was rinsed with the solution they would contain prior to final bottling. The bottle caps were treated in a... 

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 two sediment samples used as candidate CRMs were collected by the University of Pau (France) and prepared at the Institute for Reference Materials and Measurements (IRMM, Geel, Belgium). The verification of the homogeneity and stability was carried out by the University of Pau as described below. The certifications were coordinated by the BCR. 

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