For trace elements

The relative simplicity of tlie method and the penetrative nature of the x-rays, yield a technique that is sensitive to elements with Z > 10 down to a few parts per million (ppm) and can be perfonued quantitatively from first principles. The databases for PIXE analysis programs [21, 22 and 23] are typically so well developed as to include accurate fiindamental parameters, allowing the absolute precision of the technique to be around 3% for major elements and 10-20% for trace elements. A major factor m applying the PIXE teclmique is that the bombardmg energy of the... 

Vandecasteele, G. Block, G. B. Modern Methods for Trace Element Determination. Wiley Ghichester, England, 1994. 

National Institute of Standards and Technology (NIST). The NIST is the source of many of the standards used in chemical and physical analyses in the United States and throughout the world. The standards prepared and distributed by the NIST are used to caUbrate measurement systems and to provide a central basis for uniformity and accuracy of measurement. At present, over 1200 Standard Reference Materials (SRMs) are available and are described by the NIST (15). Included are many steels, nonferrous alloys, high purity metals, primary standards for use in volumetric analysis, microchemical standards, clinical laboratory standards, biological material certified for trace elements, environmental standards, trace element standards, ion-activity standards (for pH and ion-selective electrodes), freezing and melting point standards, colorimetry standards, optical standards, radioactivity standards, particle-size standards, and density standards. Certificates are issued with the standard reference materials showing values for the parameters that have been determined. 

E. A. Noble, N. Dakota Geol Survey Bull 63, 80—85 (1973) D. G. Wyant and E. P. Beroni, Reconnaissance for Trace Elements in North Dakota and... 

The analytical chemistry of titanium has been reviewed (179—181). Titanium ores can be dissolved by fusion with potassium pyrosulfate, followed by dissolution of the cooled melt in dilute sulfuric acid. For some ores, even if all of the titanium is dissolved, a small amount of residue may still remain. If a hiU analysis is required, the residue may be treated by moistening with sulfuric and hydrofluoric acids and evaporating, to remove siUca, and then fused in a sodium carbonate—borate mixture. Alternatively, fusion in sodium carbonate—borate mixture can be used for ores and a boiling mixture of concentrated sulfuric acid and ammonium sulfate for titanium dioxide pigments. For trace-element deterrninations, the preferred method is dissolution in a mixture of hydrofluoric and hydrochloric acids. 

Electron Probe Microanalysis, EPMA, as performed in an electron microprobe combines EDS and WDX to give quantitative compositional analysis in the reflection mode from solid surfaces together with the morphological imaging of SEM. The spatial resolution is restricted by the interaction volume below the surface, varying from about 0.2 pm to 5 pm. Flat samples are needed for the best quantitative accuracy. Compositional mapping over a 100 x 100 micron area can be done in 15 minutes for major components Z> 11), several hours for minor components, and about 10 hours for trace elements. 

Compared to EDS, which uses 10-100 keV electrons, PEXE provides orders-of-magnitude improvement in the detection limits for trace elements. This is a consequence of the much reduced background associated with the deceleration of ions (called bremsstrahlun compared to that generated by the stopping of the electrons, and of the similarity of the cross sections for ioiuzing atoms by ions and electrons. Detailed comparison of PIXE with XRF showed that PDCE should be preferred for the analysis of thin samples, surfrce layers, and samples with limited amounts of materials. XRF is better (or bulk analysis and thick specimens because the somewhat shallow penetration of the ions (e.g., tens of pm for protons) limits the analytical volume in PIXE. 

Whenever the appropriate specimens can be prepared, this mode is normally the one preferred for trace-element analysis in geoscience, air polludon and atmospheric science, biology, medicine, water analysis, and forensic science. In this case, the ions pass through the specimen with negligible energy loss and there is minimal absorption of X rays. 

The high-purity water thus produced typically has a conductance of about 0.5 x 10-6fi-1cm-1 (0.5juScm-1) and is suitable for use under the most stringent requirements. It will meet the purity required for trace-element determinations and for operations such as ion chromatography. It must however be borne in mind that such water can readily become contaminated from the vessels in which it is stored, and also by exposure to the atmosphere. For the determination of organic compounds the water should be stored in containers made of resistant glass (e.g. Pyrex), or ideally of fused silica, whereas for inorganic determinations the water is best stored in containers made from polythene or from polypropylene. 

In general, three basic kinds of sorption mechanisms for trace elements in geologic aqueous systems can be distinguished (56). Due to non-specific forces of attraction between sorbent and the solute, a physical adsorption may occur. This sorption mechanism results in the binding of species from the solution in several consecutive layers on exposed solid surfaces. This would be a rapid non-selec-tive and reversible process, fairly independent of nuclide concentration and only little dependent on ion exchange capacity of the solid. 

The number of samples of reference material needed is a commercial issue in the first place. An important variable is the number of samples likely to be sold during the lifetime ( shelf life ) of the reference material. As the lifetime is a function of the intrinsic stability of the material, this variable also affects the amount of raw material is needed. For instance, microbiological materials have limited intrinsic stability, and therefore their lifetime is expected to be shorter than for a dry sediment certified for trace elements. So, under the assumption of an equal number of sam-... 

Versieck J, Vanballenberghe L, de Kesel A, Baeck N, Steyart H, Byrne AR and Sunderman FW Jr. (1988) Certification of a second-generation biological reference material (freeze dried human serum) for trace element determinations. Anal Chim Acta 204 63-75. 

Versieck j, Hoste j, Vanballenberghe L, De Kesel A, Van Renterghem DJ (1987) Collection and preparation of a second generation biological reference material for trace element analysis. J Radioanal Nucl Chem 113 299-304. 

Restricted List of Chemical Species for Trace Elements and Their Compounds... 

It is clear that the available RMs certified for trace element species are not sufficient to cover present needs. As it is quite unlikely that producers of CRMs will ever be able to meet all demands, users are encouraged to produce their own RMs to guar-... 

International Atomic Energy Agency and United Nations Environment Programme (1995) Survey of Reference Materials, Volume i Biological and Environmental Reference Materials for Trace Elements, Nudides and Micro-contaminants. IAEA-TECDOC-854, IAEA, Vienna. 

IAEA-TECDOC-854 (1995) Survey of reference materials. Vol. i Biological and environmental reference materials for trace elements, nuclides and microcontaminants. IAEA, Vieima. IAEA-TECDOC-88o (1996) Survey of Reference Materials. Vols. i and 2. International Atomic Energy Agency, Vienna, Austria. 

Taylor A (1996) Reference materials and analytical standards to stimulate improved laboratory performance Experience from the external quality assessment scheme for trace elements in biological samples. Mikrochim Acta 123 251-260. 

Foley SF, Barth MG, Jenner GA (2000) Rutile/melt partition coefficients for trace elements and an assessment of the influence of ratile on the trace element characteristics of subduction zone magma. Geochim Cosmochim Acta 64 933-938... 

Jenner GA, Foley SF, Jackson SE, Green TH, Fryer BJ, Longerich HP (1994) Determination of partition coefficients for trace elements in high pressnre-temperature experimental ran products by laser ablation microprobe-inductively conpled plasma-mass spectrometry (LAM-ICP-MS). Geochim Cosmochim Acta 58 5099-5103... 

Taura H, Yurimoto H, Kurita K, Sueno S (1998) Pressure dependence on partition coefficients for trace elements between olivine and the coexisting melts. Phys Chem Min 25 469-484 Taura H, Yurimoto H, Kato T, Sueno S (2001) Trace element partitioning between silicate perovskites and ultracalcic melt. Phys Planet Earth Int 124 25-32... 

Honeyman and Santschi 1989). Therefore, flocculation of colloids to form settling particles in estuaries is an important mechanism for trace element removal (Sholkovitz 1977). This is particularly true of Fe, which is a ubiquitous colloidal species and is removed at low salinities. Additional removal may occur by adsorption onto floes, as demonstrated by mixing of organic-rich waters with seawater in the laboratory (Sholkovitz 1977). 

Decomposition methods are usually classified as melt decompositions, wet decompositions (with liquid decomposing agents) and dry decompositions by combustion. Sample decomposition methods are varied, and involve open and closed systems (at low and high pressure), UV and thermal activation, low or high temperature, and use of conventional convective or microwave heating. Table 8.4 lists the main sample decomposition methods for trace-element determination. 

Table 8.10 Advantages of pressure decomposition for trace-element analysis...

Conventional XRF analysis uses calibration by regression, which is quite feasible for known matrices. Both single and multi-element standards are in use, prepared for example by vacuum evaporation of elements or compounds on a thin Mylar film. Comparing the X-ray intensities of the sample with those of a standard, allows quantitative analysis. Depending on the degree of similarity between sample and standard, a small or large correction for matrix effects is required. Calibration standards and samples must be carefully prepared standards must be checked frequently because of polymer degradation from continued exposure to X-rays. For trace-element determination, a standard very close in composition to the sample is required. This may be a certified reference material or a sample analysed by a primary technique (e.g. NAA). Standard reference material for rubber samples is not commercially available. Use can also be made of an internal standard,... 

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