Production of the reference material

The yield of between 21 and 23 kg of the final product was stored under argon prior to sample division in a rotary sample divider. This operation yielded 20 sub-samples each of approximately 1 kg. Each sub-sample was protected from oxidation by an argon atmosphere. The samples for analysis were transferred to glass ampoules, previously dried in nitrogen at 28°C in a special box over dry (blue) silica gel. The method of filling the ampoules varied with each coal as follows  

Coal 180 by manual filling after re-mixing the sample in a mixing drum filled with dry argon, which was replaced every 3 h  

Coal 181 incremental manual sampling (50 increments per ampoule) from the coal spread out in a thin layer on glazed paper  

Coal 182 sample division by rotary sample divider and direct transfer to the ampoule. 

The filled ampoules were evacuated, argon (cleaned over gold) being then admitted to restore the pressure to atmosphere. This procedure was repeated. After cleaning the neck of the ampoule with a cotton wool swab, it was sealed with a small gas torch. The 

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CRMs for water analysis 8.12.2. Production of the reference material... 

Stability of calibrants and analytes is another frequently overlooked aspect of quality assurance, which is particularly relevant to surfactants. This aspect is discussed in Chapter 4.4. Very few intercalibration studies have been performed for the surfactant types of analytes (cf. Chapter 4.5). Currently, no certified reference material is available for surfactants. The European Commission has recently tendered for production of a reference material with certified surfactant concentrations [2]. We can conclude that quality assurance in quantitative surfactant analysis is still in its infancy when compared to analysis of PCB or chlorinated dioxins. Notwithstanding this, several important achievements have been made during recent years regarding improvement of the accuracy and reliability of qualitative analysis of surfactants, which will be the subject of the following chapters. 

Since one of the main goals of reference material production is to provide a stable reference material, tests for stability begin early in the production process. Ideally, these should be conducted over the expected lifetime of the reference material prior to its distribution however, these tests can be conducted concurrently if required. 

A STRATEGY FOR THE PRODUCTION OF NEW REFERENCE MATERIALS FOR THE OCEAN SCIENCES... 

Passing from the theoretical part of the presentation to the more practical one, the production of certified reference materials will now be discussed. 

It can be seen that complex reactions often produce more than one product. In most industrial processes, one particular product (or group of products) is usually considered more desirable than the rest. Efforts will be made to choose reaction conditions and reactor types which favour the production of the desired material. Also, if more than one reactant is involved, attempts will be made to reduce the relative consumption of the most expensive reactant. In order to make quantitative comparisons between various courses of action, it is convenient to have some way of expressing relative product yields. This may be achieved by defining a reaction selectivity which refers to the comparitive formation rates of reaction products or by relating the appearance of a particular product to the consumption of a specified reactant. Various definitions have appeared in the literature the choice of terms is arbitrary. The use of terms in this chapter can be illustrated by an example. Consider the reactions... 

A Boenke. The food and feed chains—a possible strategy for the production of certified reference materials (CRMs) in the area of mycotoxins Food Chem 60 255-262, 1997. 

The primary element standards especially the primary pure elements are used for the production of other reference materials (Element-solutions for PTB/Merck and EMPA/Fluka as well as isotopic standards for IRMM [Joint European Project for Primary Isotopic Measurements=JEPPIM]). 

Sometimes the need for sample digestion can be circumvented. One example is the extraction with HC1. The method is based on hot HC1 and is specifically developed for extraction of Sn from canned fruit and vegetables. It is quite rapid and particularly useful for screening purposes. Another example is solid sampling. This is a technique specifically suited for ET-AAS and therefore not generally applicable. It is useful when homogeneity at the milligram level is of interest, for example, in the production of Certified Reference Materials (CRMs). 

Hopefully, in the near future both areas will be endowed with new CRMs for speciation analysis, the lack of which has been a substantial drawback over the past years. A ground-breaking CRM in the held of Se speciation, a CRM certified for SeMet, has been recently introduced [136], while the first CRM intended for As speciation was released in 1999 [113]. Moreover, an EU-funded project named SEAS has further prompted the development of speciation analytical techniques to produce and use CRMs for As and Se speciation [137], It is understandable that the bottleneck for any further progress is financial as the scientific community involved in speciation has already confirmed that there is a keen need for such QC materials [5, 138-140]. For the time being, either the production of working reference materials for speciation analysis [28, 61, 141] or the speciation-oriented recertification of already available CRMs [117] may provide viable alternatives. 

Gawlik, B.M., Linsinger, T., Kramer, G.N., Lamberty, A., Schimmel, H. Organic contaminants in water - conceptual considerations for the production of liquid reference materials in support of the new water framework directive. Fresen. J. Anal. Chem. 371, 565-569 (2001)... 

The practical meaning of this measurement structure has been discussed by Tietz. The highest quality methods, the definitive methods, are to be used to validate reference methods and primary reference materials. The highest quality reference materials, namely primary reference materials, should be used in the development and validation of reference methods, the calibration of definitive and reference methods, and the production of secondary reference materials. Reference methods should be used to validate field methods. Secondary reference materials should be used to provide working calibrators for field methods and to assign values to control materials. Control materials are used only to monitor field methods. 

Guidelines for the production of BCR reference materials, Doc. BCR/48/93 Rart A Recommendations to proposers of BCR certification projects, European Commission DG Xll-C, rue de la Loi 200, 1049 Brussels, Belgium (1994). 

A further important requirement in selecting the starting material is that, after all the processing has been done to produce the end product, a large amount should remain to permit meaningful use of this material over a period of several years. Most producers do not provide details of how much is available for distribution. The IAEA, for its part, has adopted the criterion that, for a trace element reference material, at least 50 kg of the final end product should be produced. In practice, however, some of the reference materials already issued by the IAEA have been produced in amounts as small as 15 kg, and sometimes less (for human hair, HH-1, only 300 g was produced, with the consequence that stocks were exhausted very quickly). 

According to ISO (1981), the period of validity of the reference material should be stated by the issuing organization. In practice, however, this is usually not done for the simple reason that the producer has no reliable means to determine the lifespan of the product. 

McLaren et al. (1995) analyzed water samples for production of standard reference materials and determine 16 elements with detection limits in the subng/g range. Garbe-Schoenberg (1993), Guo and Lichte (1995) and Guenther et al. (1997) determine a number of elements in aerosols, soils and rock standard reference materials. Detection limits for lanthanides reached 0.002 p,g/g and were 0.01 p.g/g for the rest of the elements. 

Setting up a pedigree material to be used repeatedly in order to calibrate each laboratory or field test is strongly recommended. This will ensure reproducibility of the test procedure. This material should be thoroughly characterized as to its chemical composition, process history, and microstructure. The size and shape of the reference material should reflect that of the candidate product forms being screened. 

An application of technical interest is the determination of different thiophene derivatives, the major sulfur impurities in petroleum products. S-labded thiophene, dibenzothiophene, and 4-methyldibenzothiophene spikes in hexane were used to determine these sulfur compounds in different gasoline samples, naphtha, diesel fuel, gas oil, and heating oil by species-spedfic GC-ICP-IDMS [55]. The accuracy of the results thus obtained was demonstrated by successftd analysis of the reference material NIST SRM 2296. The LOD was limited to 7 ng of sulfur per gram of sample by the background noise in the isotope ratio chromatograms. 

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