Element species, trace analysi

Gonzalez, E.B., Sanz-Medel, A. Liquid chromatographic techniques for trace element speci-ation analysis. In Caruso, J.A., Sutton, K.L., Ackley, K.L. (eds.) Elemental Speciation New Approaches for Trace Element Analysis, pp. 81-121. Elsevier Science B.V, Amsterdam (2000)... 

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. 

Mass spectrometry can be specific in certain cases, and would even allow on-line QA in the isotope dilution mode. MS of molecular ions is seldom used in speciation analysis. API-MS allows compound-specific information to be obtained. APCI-MS offers the unique possibility of having an element- and compound-specific detector. A drawback is the limited sensitivity of APCI-MS in the element-specific detection mode. This can be overcome by use of on-line sample enrichment, e.g. SPE-HPLC-MS. The capabilities of ESI-MS for metal speciation have been critically assessed [546], Use of ESI-MS in metal speciation is growing. Houk [547] has emphasised that neither ICP-MS (elemental information) nor ESI-MS (molecular information) alone are adequate for identification of unknown elemental species at trace levels in complex mixtures. Consequently, a plea was made for simultaneous use of these two types of ion source on the same liquid chromatographic effluent. 

For many elements, the atomization efficiency (the ratio of the number of atoms to the total number of analyte species, atoms, ions and molecules in the flame) is 1, but for others it is less than 1, even for the nitrous oxide-acetylene flame (for example, it is very low for the lanthanides). Even when atoms have been formed they may be lost by compound formation and ionization. The latter is a particular problem for elements on the left of the Periodic Table (e.g. Na Na + e the ion has a noble gas configuration, is difficult to excite and so is lost analytically). Ionization increases exponentially with increase in temperature, such that it must be considered a problem for the alkali, alkaline earth, and rare earth elements and also some others (e g. Al, Ga, In, Sc, Ti, Tl) in the nitrous oxide-acetylene flame. Thus, we observe some self-suppression of ionization at higher concentrations. For trace analysis, an ionization suppressor or buffer consisting of a large excess of an easily ionizable element (e g. caesium or potassium) is added. The excess caesium ionizes in the flame, suppressing ionization (e g. of sodium) by a simple, mass action effect ... 

Multi-element trace analysis is an important prerequisite for the quality assurance of foodstuffs with respect to the characterization of non-essential, toxic and essential (nutrient) elements as pollutions or as mineral elements relevant to health. Contamination with heavy metals such as Cd, Pb or Hg has become a serious problem with increasing environmental (artificial) contamination e.g., due to industrial pollution. The increasing use of inorganic mass spectrometric techniques (especially of ICP-MS) in the analysis of foodstuffs for multi-element analysis of trace elements or the detection of selected elements and species at a low concentration level has resulted from advances in very sensitive and quantitative measurements of metals, metalloids and several non-metals, including their speciation. 

The isotopic dilution method can be extended to non-radioactive tracers by using mass spectrometry or NMR to determine the variation in isotopic ratios. This method can be used for the measurement of molecules or elemental species (about 60 elements have stable isotopes). This approach allows ultra-trace analysis because, contrary to radioactive labelling where the measurement relies on detecting atoms that decompose during the period of measurement, all labelled atoms are measured. Isotopic mass spectrometers are well suited for these measurements. 

J. A. Caruso, K. L. Sutton and K. L. Ackley eds., Elemental Speci-ation. New Approaches for Trace Element Speciation Analysis , 1st edition, Elsevier, Amsterdam, 2000—a comprehensive view of the anal)dical speciation and the hyphenated techniques with atomic spectrometry detection. 

This discussion examines the recent progress of nutritional trace element research and its implications for trace element analysis. Elements recently identified as essential are present in low concentrations for which analytical methods are not yet reliable. Biological availability of trace elements depends on chemical form and on interactions with other inorganic and organic constituents of the diet. Therefore, information on elemental species is required, in addition to quantitative data. Finally, the demonstration of essential functions of trace elements previously known only for their toxicity necessitates establishing safe ranges of intake, free from danger of chronic toxicity but sufficient to meet human needs. 

The remark has been made that compounds of tin can be studied by more techniques than those of any other element. The fact that it has more stable isotopes that any other element gives it very characteristic mass spectra, and isotopic labelling can be used to interpret vibrational spectra, and for spiking samples in trace analysis two of the isotopes have spin 1/2 and are suitable for NMR spectroscopy, and their presence adds information to the ESR spectra of radical species. Further, the radioactive isotope 119mSn is appropriate for Mossbauer spectroscopy. The structural complications that are referred to in the previous chapter have therefore been investigated very thoroughly by spectroscopic and diffraction methods, and structural studies have always been prominent in organotin chemistry. 

Some of the difficulties in the unbiased determination of certain trace elements in biological materials may be due to problems of speciation. The range of complex organo-metallic species that can be found in nature is very wide (Frausto da Silva and Williams, 1991). In carrying out an analysis for a particular element in any type of biological fluid or tissues, major assumptions are made concerning the precise chemical composition of element species present. Different analytical techniques will have different sensitivities towards particular element species. Much of the early understanding of the special analytical problems posed by element speciation comes from studies of arsenic (Buchet et al., 1980 Buchet et al., 1981) and mercury (Clarkson, 1983). Problems with other metals remain to be resolved and may require considerable analytical sophistication such as in the analysis of chromium speciation (Urasa and Nam, 1989). 

In mass spectrometric studies, WT have been applied mainly in two areas including secondary ion mass spectrometry (SIMS) and instrumentation design. SIMS is a type of surface technique for trace analysis, determination of elemental composition, and the identity and concentrations of adsorbed species and elemental composition as a function of depth [46]. The application of wavelet denoising techniques to SIMS images has been studied by Grasserbauer et al. [47-50], and details about these studies are presented in another chapter of this book. 

The physico-chemical characterisation of the species in the case of, for example, the determination of the form in which it is present, represents a substantially more demanding field of trace analysis than total trace contents. According to Florence speciation analysis of an element in a water sample may be defined as the determination of the concentrations of the different physico-chemical forms of the element which together make up its total concentration in the sample . 

In aquatic sediments or soils, there are also a range of trace elements species ranging from ions exchanged to particles, to those bound to organic matter or in various inorganic forms (e.g., oxides, carbonates, sulfides) or as more inert crystalline mineral phases. As in waters, speciation studies in soils and sediments are generally undertaken to better understand the bioavailability of toxic substances and to investigate transport pathways to and from other parts of the ecosystem. Sediment and soil pore waters (soil solutions) are of particular interest because they are in equilibrium with the solid phase and are the medium for contaminant uptake by plants and many other biota. The techniques used for speciation analysis in these aqueous samples differ little from those for waters. 

A recently defined and new objective has been trace element speciation, which provides information about the physical/chemical state of the trace elements. Species separated by chemical methods are subsequently determined by instrumental or radiochemical activation analysis. 

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