Isotope fractionation speciation

This simple two component model for the Fe isotope composition of seawater does not consider the effects of the Fe isotope composition of dissolved Fe from rivers or from rain. Although the dissolved Fe fluxes are small (Fig. 19) the dissolved fluxes may have an important control on the overall Fe isotope composition of the oceans if they represent an Fe source that is preferentially added to the hydrogenous Fe budget that is ultimately sequestered into Fe-Mn nodules. In particular riverine components may be very important in the Pacific Ocean where a significant amount of Fe to the oceans can be delivered from rivers that drain oceanic islands (Sholkovitz et al. 1999). An additional uncertainty lies in how Fe from particulate matter is utilized in seawater. For example, does the solubilization of Fe from aerosol particles result in a significant Fe isotope fractionation, and does Fe speciation lead to Fe isotope fractionation ... 

Matthews A, Morgans-Bell HS, Emmanuel S, Jenkyns HC, Erel Y, Halicz L (2004) Controls on iron-isotope fractionation in organic-rich sediments. Geochim Cosmochim Acta, in press Millero FJ, Yao W, Aicher J (1995) The speciation of Fe(II) and Fe(III) in natural waters. Marine Chemistry 50 21-39... 

Dissolution of minerals, such as may occur during dissimilatory Fe(lll) reduction, or precipitation of new biominerals during reductive or oxidative processing of Fe, represent important steps in which Fe isotope fractionation may occur. We briefly review several experiments that have investigated the isotopic effects during mineral dissolution, as well as calculated and measured isotopic fractionations among aqueous Fe species and in fluid-mineral systems. In some studies, the speciation of aqueous Fe is unknown, and we will simply denote such cases as Fe(lll)jq or Fe(ll)aq. 

We wish to stress that comparison of the isotopic effects in biologic and abiologic systems will be most meaningful if experimental conditions are identical, where the only difference is the presence or absence of bacteria. The wide variety of buffers, growth media, and others conditions that are involved in biological experiments raise the possibihty that spurious results may be obtained if these factors are not carefully controlled. Because speciation may exert a strong control on Fe isotope fractionations (Schauble et al. 2001), even small differences across experimental studies may be significant. 

Doe BR (1994) Zinc, copper, and lead in mid-ocean ridge basalts and the source rock control on Zn/Pb in ocean-ridge hydrothermal deposits. Geochim Cosmochim Acta 58 2215-2223 Ehrlich S, Butler I, Halicz L, Rickard D, Oldroyd A, Matthews A (submitted) Experimental study of copper isotope fractionation between aqueous Cu(II) and covellite, CuS. Chem Geol Finney LA, O Halloran TV (2003) Transition metal speciation in the cell insights from the chemistry of metal ion receptors. Science 300 931-936... 

Plasma MS is usually based on quadrupole mass analysers. The atmospheric ICP, optimised for ion formation, is placed on its side facing a sample cone (Fig. 4.3). The mass spectrometer operates at reduced pressure and therefore a two- or three-stage differentially pumped interface is needed to transfer the ions from the plasma to the mass analyser. The interface for GC-ICP-MS is generally the same as for ICP emission systems. In one of the earliest GC-MS speciation studies (Chong and Houk, 1987) a packed GC column was used to obtain mass spectra of organic compounds with detection limits in the range 0.001-500 ngs The effects of isotopic fractionation by natural physico-chemical processes were also studied. 

Boron. There is relatively little information on the coordination and speciation of B on mineral surfaces. Palmer et al. (77) using B isotopic fractionation in the pH range of 6.65 to 8.45, suggested that B is tetrahedrally coordinated with clay surfaces. Su and Suarez (18) examined coordination of B adsorbed on Al and Fe oxides, allophane, and kaolinite using ATR-FTIR. The dominant B species in solution are boric acid, B(OH)3 and borate anion, B(OH)4, with a pK. of 9.23. These two species can be readily distinguished since the B atom in boric acid is trigonally coordinated while B in the borate anion is tetrahedrally coordinated. 

A sample of sediment from a given depth below the water/sediment interface can be dated from its ( Th/ Th) activity ratio if this has stayed constant for at least several hundred thousand years in the water body next to the sediment in any given ocean basin and also if there was no migration of the isotopes in the sediments. In addition, their chemical speciation must have been the same in seawater with no isotopic fractionation between this and the mineral phases involving thorium in the sediment. Furthermore, the ionium and thorium found in detrital mineral particles must be excluded from the analysis. Assuming that the contribution made by uranium-supported ionium is negligible, the activity of a sample given as an activity ratio may be related to its age as... 

Due to the various health risks of different element species, there are a multitude of applications for natural water samples in this field (e.g., Cr and Sb speciation or Br and I determination).19 The investigation of heavy metal complexes with humic substances by isotope dilution SEC-ICP-MS has been described, for example, by McSheehy and Mester.20 Copper, zinc and molybdenum were found to form complexes with similar size fractions of humic substances in seepage water samples from soils. Sturgeon s group proposed the use of solid phase microextraction (SPME)... 

Sulfur isotopes can effectively be used to examine important geochemical processes associated with redox changes in sedimentary environments. The speciation of sulfur is strongly affected by redox potential, pH, productivity, microbial sulfate reduction, and iron availability (Berner, 1984). More details are provided on the sulfur cycle in chapter 12. In general, during microbial dissimilatory sulfate reduction there is fractionation of sulfur... 

According to International Union of Pure and Applied Chemistry (IUPAC), the terms speciation and chemical species should be reserved for the forms of an element defined as to isotopic composition, electronic or oxidation state and/or complex or molecular structure (Templeton el al, 2000). This classical definition, appropriate to speciation in solution samples, would exclude most speciation studies on solid materials, such as soils and sediments, more properly defined as fractionation studies. The terminology used in this chapter is based on the broader definition of speciation given by Ure and Davidson (2002), which encompass the IUPAC s narrow definition and includes the selective extraction and fractionation techniques of solid samples. 

Studies in human adults using radioisotopes or stable isotopes of Zn showed that Zn absorption is higher from human milk (41 percent) than from cow (28 percent) or soy formulas (14 percent) [77]. In any case, it seems that there is no need to fortify cow-based formulas with Zn, since cow milk contains much more Zn than breast milk [77]. However, Zn bioavailavility from breast milk is clearly higher than from formulas, which indicates a different composition in Zn species. Experiments in speciation of Zn in formula milk whey carried out by SEC showed that Zn was mostly found in the LMW (< 10 kDa) fraction, as a complex of citrate and only a small fraction of the metal was bound to whey proteins (435 kDa) [15, 18, 21]. Although such species are detected in human milk, important differences have been found between the distribution patterns of Zn in breast milk and in cow-based formula milks (see Fig. 17.11 and compare with Fig. 17.4). The levels of Zn in formulas are higher than in human milk and in colostrums [18]. 

A. Wimschneider, K. G. Heumann, Iodine speciation in size fractionated atmospheric particles by isotope dilution mass spectrometry, Fresenius J. Anal. Chem., 353 (1995), 191-196. 

Boron has two stable isotopes, 10B and UB, with an average relative abundance of20% and 80%, respectively. The fractionation of 10B/nB (expressed as 8nB, in per mil) relates to boron s speciation as either B(OH), " or B(OH)3j rather than any strong temperature or redox dependence. Most continental (e.g. granitic) rocks, and therefore rivers draining therefrom, have 8nB values close to 0%o, whereas ocean water has a 8nB value of +39%o, due to preferential adsorption of the B(OH)3 species onto marine clays and carbonates. 

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