Natural matrix reference material

Some of the intended categories of use of radioisotopic reference material have been reviewed recently by Fajgelj et al. (1999). They include assignment of property values, establishing the traceability of a measurement result, determining the uncertainty of a measurement result, calibration of an apparatus, assessment of a measurement method, use for recovery studies and use for quality control purposes. It should be noted however that, in general, natural matrix reference materials are not recommended for calibration purposes. This should preferably be done with pure chemical forms of the element labelled with the isotope of interest. Calibrated isotopic sources of this kind are available from a number of commercial suppliers and are not the subject of this review. 

Natural matrix reference material Reference material Traceability... 

GAU-Radioanalytical has participated in several intercomparison exercises for H-3 and C-14 activities in aqueous and biota samples. These results are in good agreement with the reference values (Table 4). Currently there are no commercially available natural matrix reference materials for total H-3 and therefore the analysis of spiked samples with known certified activities of H-3 is the best possible way of showing that the method is working. 

All of the IAEA reference materials which are currently available have been certified on the basis of previously conducted intercomparison exercises. Natural matrix reference materials with certified values for the activities of various radionuclides are listed in Table 5.18. 

Strachnov, V., Valkovic, V., LaRosa, J., Dekner, R. and Zeisler, R., IAEA Programme of natural matrix reference materials for the determination of radionuclides. Fresenius J. Anal. Chem., 345 (1993) 169-173. 

Bleise AR, Smodis B, Glavic-Cindro D and Parr RM (2001) the updated IAEA database of natural matrix reference materials. J Radioanal Nucl Chem 248 205-209. 

A significant change in geochemical analysis, and in the demands for natural matrix reference samples to support the analysis of geochemical materials, came with the advent of instrumental methods of analysis. The historical roots of these instrumental methods date from the mid-i88o s to the early 1900 s, but only later were the methods used in routine rock analysis. This use of instrumental methods of analysis generated a demand for rock reference samples, that could be used for calibration purposes. The earliest of them were analyzed by classical methods to determine major and minor oxides... 

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. 

Certified reference materials (CRMs) to validate measurements of organic constituents were introduced in the early 1980 s, more than a decade after the development of the first natural matrix CRMs for inorganic constituents. There are three types of CRMs to support measurements of organic constituents ... 

CRMs for Contaminants in Environmental Matrices For nearly two decades NIST has been involved in the development of SRMs for the determination of organic contaminants such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and chlorinated pesticides in natural environmental matrices such as fossil fuels (Hertz et al.1980 Kline et al. 1985), air and diesel particulate material (May and Wise 1984 Wise et al. 2000), coal tar (Wise et al. 1988a), sediment (Schantz et al. 1990, 1995a Wise et al. 1995), mussel tissue (Wise et al. 1991 Schantz et al. 1997a), fish oil, and whale blubber (Schantz et al. 1995b). Several papers have reviewed and summarized the development of these environmental matrix SRMs (Wise et al. 1988b Wise 1993 Wise and Schantz 1997 Wise et al. 2000). Seventeen natural matrix SRMs for the determination of organic contaminants are currently available from NIST with certified and reference concentrations primarily for PAHs, PCBs, chlorinated pesticides, polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofiirans (PCDFs) see Table 3.11. 

A wide variety of reference materials is now available, covering several different kinds of natural matrix such as food (e.g. milk powder), human tissues (e.g. liver), marine biological materials (e.g. tuna fish) and soils and sediments. The radionuclides of interest cover naturally occurring ones (e.g. Ra), fission products... 

The concept of preparing a reference material with property values traceable to SI units is a relatively recent one which has not yet been applied in practice to natural matrix radioisotopic reference materials. This is one of the main reasons why such materials are generally not recommended for calibration purposes. 

Zeisler R (2000a) Investigations by INAA for the Development of Natural Matrix Standard Reference Materials (SRMs) Suitable for Small Sample Analysis. J Radioanal Nucl Chem 245 73-80. 

The revised database holds over 23 000 analyte values for 660 measurands and 1670 reference materials produced by 56 different producers, from 22 countries. The database is restricted to natural matrix materials (i.e. made from naturally occurring materials, excluding calibration standards manufactured from pure chemicals). Information has been extracted from the relevant certificates of analysis, information sheets, and other reports provided by the reference material producers. As a general rule, the authors have only included in the compilation reference materials for which a certificate of analysis or similar documentation is on file. Information included in the survey is on values for measurands determined in reference materials, producers, suppliers, the cost of the materials, the unit size supplied, and the recommended minimum weight of material for analysis, if available. The new searchable database has been designed to help analysts to select reference materials for quality assurance purposes that match as closely as possible, with respect to matrix type and concentrations of the measurands of interest and their samples to be analyzed see Table 8.3. 

Matrix Components The term matrix component refers to the constituents in the material aside from those being determined, which are denoted as analyte. Clearly, what is a matrix component to one analyst may be an analyte to another. Thus, in one hand for the case of analyses for elemental content, components such as dietary fibre, ash, protein, fat, and carbohydrate are classified as matrix components and are used to define the nature of the material. On the other hand, reference values are required to monitor the quality of determinations of these nutritionally significant matrix components. Hence, there is a challenging immediate need for certified values for dietary fibre, ash, protein, fat, and carbohydrate. Concomitantly, these values must be accompanied by scientifically sound definitions (e.g. total soluble dietary fibre, total sulpha-ted ash, total unsaturated fat, polyunsaturated fat, individual lipids, simple sugars, and complex carbohydrates). 

Direct participation of the oceanographic research community will enable collection of matrix-based materials such as natural seawater or sediments in an efficient manner (for example, material collection could be combined with other scheduled research activities). Furthermore, the extensive analytical skills of the oceanographic research community can be harnessed to conduct the necessary testing for homogeneity, stability, and ultimately characterization of the proposed reference materials. At the same time, complete success requires the participation of reference material producers, who bring to the table extensive knowledge and experience in the preparation and stabilization of reference materials, and on occasion access to necessary facilities. 

Several organizations (e.g., NIST, NRC-Canada, and IAEA) provide sediment reference materials containing radionuclides, many of which are only certified for artificial radionuclides ( Cs, Sr, Am, and Pu). Certain specific radionuclides have no certified natural matrix materials, including ocean, lake, and river sediments. Although these sediments are certified for a few naturally occurring and artificial radionuclides, the extent of radioactive equilibrium of the uranium and thorium decay series in these environmental materials is not provided. NIST currently offers an ocean sediment Standard Reference Material (SRM 4357) in... 

The development and improvement of analytical methodologies for mycotoxins has been greatly improved by the increased availability of matrix matched certified reference materials (CRMs) (Boenke, 1995) (Table 11.6). The type of matrix CRMs and concentration of the specified mycotoxin are based on the natural occurrence pattern of the toxin in specific foods and feeds. The recent availability of suitable CRMs, while being a prerequisite for the implementation of regulations and standards, will also be invaluable in many ways for the validation of new methods, solving trade disputes and for harmonising proficiency schemes. 

The main parts that support the traceability in chemical measurements are primary analytical methods, reference materials and valid analytical methods suitable for some available instruments for a group of materials, according to their nature, range of measurements in a specific matrix. These elements should serve to establish an uninterrupted chain of comparisons in chemical measurements and its uncertainty estimation. 

---