Trace-element-compound determination

Evaporation of the leach water from 1000 g of phosphogypsum and X-ray diffraction of the resulting crystals did not identify any compounds of trace elements. Trace elements may exist as sulfates, such as mercuric sulfate, or as calcium salts, such as calcium selenate, in equilibrium with a saturated solution of calcium and sulfate ions from gypsum. Further isolation of trace elements by extraction, precipitation, ion exchange, or other means would so alter this equilibrium that the original amounts and types of compounds present would not be determined. Literature studies of phase equilibria in saturated gypsum solutions were used to identify possible trace element compounds. 

DETERMINATION OF TRACE ELEMENTS AND TRACE ELEMENT COMPOUNDS THAT ARE CONVERTIBLE TO VOLATILE DERIVATIVES... 

Fig, 2.5. Schematic representation of hydride generation systems for the identification and determination of hydride-forming trace elements and trace element compounds with element-specific and general detectors. 

R. A. Stockton, Graphite furnace atomic absorption and inductively coupled argon plasma emission sjjectrometers as element-specific detectors for the determination of trace elements and trace element compounds , Ph.D. Dissertation, Department of Chemistry, Texas A M University, College Station, Texas, U.S.A., August 1985. 

Direct Process. Passing methyl chloride through a fluidized bed of copper and siUcon yields a mixture of chlorosilanes. The rate of methylchlorosilane (MCS) production and chemical selectivity, as determined by the ratio of dimethydichlorosilane to the other compounds formed, are significantly affected by trace elements in the catalyst bed very pure copper and siUcon gives poor yield and selectivity (22). 

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. 

The list of elements and their species listed above is not exhaustive. It is limited to the relatively simple compounds that have been determined by an important number of laboratories specializing in speciation analysis. Considering the economic importance of the results, time has come to invest in adequate CRMs. There is a steadily increasing interest in trace element species in food and in the gastrointestinal tract where the chemical form is the determinant factor for their bioavailability (Crews 1998). In clinical chemistry the relevance of trace elements will only be fully elucidated when the species and transformation of species in the living system have been measured (ComeUs 1996 Cornelis et al. 1998). Ultimately there will be a need for adequate RMs certified for the trace element species bound to large molecules, such as proteins. 

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,... 

Since the concentrations of the various compounds or oxidation states in which trace elements can occur are always lower than the total content of the analyte, speciation analysis is normally an ultratrace determination in the ng L-1 range for solutions and the ng g 1 range for... 

CPAA may be employed to determine trace element concentrations in bulk solid material, but its importance in our present context is that it permits the characterization of a thin surface layer, i.e. the mass of the analyte element per surface unit, with a good detection limit and outstanding accuracy. For example the composition of a surface layer (or foil) of known thickness can be determined, or, conversely, the thickness of a surface layer of known concentration. Depth profiling or scanning is not possible, and a disadvantage of the method is that heating occurs during irradiation. It is also not possible to discriminate between different oxidation states of the analyte element or between different compounds. 

In the following discussion, three types of air pollutant analytical data will be examined using principal component analysis and the K-Nearest Neighbor (KNN) procedure. A set of Interlaboratory comparison data from X-ray emission trace element analysis, data from a comparison of two methods for determining lead In gasoline, and results from gas chromatography/mass spectrometry analysis for volatile organic compounds In ambient air will be used as Illustrations. 

Stick RV (1997) The Synthesis of Novel Enzyme Inhibitors and Their Use in Defining the Active Sites of Glycan Hydrolases. 187 187-213 Stutz AE, see de Raadt A (1997) 187 157-186 Stumpe R, see Kim JI (1990) 157 129-180 Suami T (1990) Chemistry of Pseudo-sugars. 154 257-283 Suppan P (1992) The Marcus Inverted Region. 153 95-130 Suzuki N (1990) Radiometric Determination of Trace Elements. 157 35-56 Tabakovic I (1997) Anodic Synthesis of Heterocyclic Compounds. 185 87-140 Takahashi Y (1995) Identification of Structural Similarity of Organic Molecules. 174 105-134 Tasi G, Palinkd I (1995) Using Molecular Electrostatic Potentid Maps for Similarity Studies. 174 45-72... 

Another field with a large potential for improvements concerns aluminosilicate minerals, which are of great importance in determining the chemistry of water in many types of rock. In backfill clays, aluminosilicates are responsible for the retention (sorption, incorporation) of trace elements and may affect both oxidation potential (incorporation of Fe(II)/Fe(III)) and pH (hydrolysis of silicate and/or exchange of H+). Related classes of compounds (i.e., calcium silicates and calcium aluminates) form the chemical backbone of cementitious materials. The thermodynamic properties of these substances are still largely unexplored. 

The addition of anticoagulents and/or preservatives, a practice that may be tolerated for total element determination, should be avoided for two reasons. First, the compounds used are usually complexing agents. They therefore could bind various trace elements. Second, they could destroy some species. For example, the addition of potassium dichromate to urine in the presence of nitric acid could destroy methylmercury If the use of such compounds is unavoidable, then there should be experimental evidence to show that the speciation of the element imder study has not been adversely affected. 

It is not possible to prescribe specific pretreatment procedures here because these can only be decided upon when the system and the purpose of the experiments has been properly defined. However, a wealth of information exist in various biochemical reference books on how to isolate various biological compounds. The recommended techniques and methods could be used as part of the trace element speciation protocol often after slight modification, taking into consideration the following points First, the trace element blank levels have to be low, less than 10% of the total concentration in the sample. Second, the regents used should not interfere with subsequent analytical determinations. Third, the experimental conditions should not deviate markedly from those found in vivo, especially the pH and ionic strength of the medium. 

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