Preparation of reference materials

By avoiding contamination, one can in principle come close to the ideal goal of producing an appropriate reference material, i.e. one that not only has the same matrix as that of the samples to be analysed but also matches them with respect to the levels of the trace elements of interest. As is also pointed out by Dams (1983) it is desirable that speciation (i.e. the valency and chemical binding of trace elements) should be the same as in the real matrix. In other words, reference materials should preferably be made of natural products with a similar matrix to that of the samples to be analysed, and not out of 

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). 

Statements of the first kind about homogeneity, however, may in future no longer be considered sufficient. The International Standards Organization (1981) is already moving 

The stability of a reference material is of great importance since the same material may be used over a period of many years. At issue is not only the question of whether it continues to be pleasant to handle (biological materials can of course be attacked by bacteria, fungi, insects and other pests), but also that, due to evaporation or chemical reactions, the concentrations and chemical binding of some of the elements of interest may change. This is obviously of greatest concern for elements that can exist in a volatile form such as mercury and arsenic, which could thereby be lost. 

Most biological reference materials may be assumed to be relatively stable if stored in the form of a dry powder. A further extension of lifespan can be expected as a result of radiation sterilization, which is commonly applied to IAEA biological reference materials. Finally, the user is well advised to store his reference materials in a refrigerator, or even a deep-freeze cabinet (though this recommendation is usually not stated in the documentation that accompanies the reference material). 

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Another issue in the preparation of reference material is the required shelf life. The shelf life of reference material is the time that it remains stable under proper storage conditions. Depending on the nature of the mechanisms affecting the stability of the material, various actions can be taken to improve the shelf life. Reduction of the moisture content is one of the first options to be considered. In many cases, moisture plays a key role in mechanisms leading to instability of the matrix and/or parameters. In other cases, sterilization or pasteurization of the material might be considered in order to stop bacterial activity. When preparing solutions, additives may increase the shelf life. Obviously, the shelf life of material is also a function of the storage conditions. 

In summary, the preparation of reference material involves the following steps ... 

IAEA/AL/095 1996). Both methods were suitable, but needed to be repeated several times to produce the small particle size that was required. The particle size reduction, e.g. of IAEA-395 from a median size of 30 pm to 3.5 pm, improved the homogeneity of elements. Sampling constants (the minimum mass that can be used to achieve a random error of i % at the 65 % confidence level) improved from a factor of 1.2 for Sc, up to a factor of 800 for Au. The average improvement was about a factor of 2-10. (Ni Bangfa et al. 1996). From these initial experiences, it is dear that preparation of reference materials is critical with respect to the final particle size distribution, which should exhibit a low maximum (<50 pm) and a narrow range in particle sizes. Milling techniques to meet such criteria are available today, and materials that show intrinsic uniformity are particularly suitable to achieve the desired properties. 

Many of the analytes of interest for solid phase chemical reference materials are the same as those in seawater, but the need for and the preparation of reference materials for suspended particulate matter and sediments is quite different. The low concentrations of many seawater species and the presence of the salt matrix create particular difficulties for seawater analyses. However while sediments frequently have higher component concentrations than seawater, they also have more complicated matrices that may require unique analytical methods. A number of particulate inorganic and organic materials are employed as paleoceano-graphic proxies, tracers of terrestrial and marine input to the sea, measures of carbon export from the surface waters to the deep sea, and tracers of food-web processes. Some of the most important analytes are discussed below as they relate to important oceanographic research questions. 

The multitude of organic compounds present in the environment makes it impossible to discuss each compound, or even compound class. A general rationale for the matrix approach to preparation of reference materials is presented below, as well as a specific discussion of several classes of particular importance. 

The committee recommends, but assigns a lower priority to, the preparation of reference materials from other locations. For example a standard for dissolved iron in a coastal seawater matrix containing high concentrations of dissolved organic material would be particularly useful in addressing matrix effects associated with such materials. 

Preparation of Reference Materials for Proficiency Testing Schemes... 

Sahuquillo A, Carrasco E, Muntau H, Rubio R, Rauret G, 2004. Mat Control a new laboratory for the preparation of reference materials at the University of Barcelona, Spain. Accred. Qual. Assur., 9, 272-7. 

Currendy, measurement of enzymes is based on catalytic-activity measurements, and these depend on the experimental conditions under which the enzyme is measured. It is often difficult to compare enzyme results from different laboratories or to those reported in the literature. Efforts to improve interlaboratory comparability in enzyme measurements (129, 130, 131, 132, 133, 134) have concentrated on two aspects standardization of methodology and preparation of reference materials (135). 

Every analytical laboratory should have its own reference materials for internal quality assurance, and such materials should conform to the same standards of appropriateness, homogeneity and long term stability as are required for certified reference materials (see section "preparation of reference materials). In practice, however, most analytical laboratories do not use their own "in-house" reference materials for internal quality assurance but rather rely on internationally available reference materials. In the opinion of the present author, this is undesirable since much larger amounts are required for internal quality assurance than for externai quality assurance, and thereby the available stocks of expensively prepared certified reference materials will be consumed much too quickly. 

There is still a large need for the preparation of reference materials certified for arsenic compounds in various matrices, such as food, urine, and water. Currently, only standard reference materials prepared from fish are available. These materials will help to prepare quality-controlled data on arsenic compounds and, of course, establish speciation analysis in routine laboratories. 

Preparation of reference materials for following elemental and/or molecular distributions closer to the outer surface, across some interface within a solid substrate, and/or at concentrations exceeding 1 atomic % is faced with greater challenges. These challenges are associated with not only their fabrication but also the validation of the elemental/molecular distributions. These must also exhibit a high degree of stability, i.e. out-diffusion etc. must be minimized. 

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