Iron, speciation analysis

Sojo, L.E. and de Haan, H. (1991) Multicomponent kinetic analysis of iron speciation in humic lake Tjeukemeer comparison of fulvic acid from the drainage basin and lake water samples. Environ. Sci. Technol., 25, 935-939. 

Sequential extraction of anoxic sediments often yields substantial amounts of iron in the exchangeable and carbonate extractable fraction which do not agree with patterns given by direct speciation analysis by Mossbauer spectrometry... 

C. F. Harrington, S. Elahi, S. A. Merson, P. Ponnampalavanar, A method for the quantitative analysis of iron speciation in meat by using a combination of spectrophoto-metric methods and high-performance liquid chromatography coupled to sector t>eld inductively coupled plasma mass spectrometry, Anal. Chem., 73 (2001), 4422D4427. 

An important problem in the speciation analysis of arsenic is the above-mentioned co-precipitation of oxidized forms of arsenic with sparingly soluble compounds of iron or manganese, or the formation of iron and calcium arsenates... 

Work on the speciation of iron and zinc in vegetable products is just beginning (23), and some progress is being made on the speciation of chromium (24) but not enough data have been accumulated to be of interest to the nutritionist. Speciation analysis has produced impressive advances in the toxicology of heavy metals. Similar advances can be expected in trace element nutrition if the diflBculties of methodology can be overcome. 

F. Albertus, A. Cladera, E. Becerra, V. Cerda, A robust multi-syringe system for process flow analysis. Part 3. Time based injection applied to the spectrophotometric determination of nickel(II) and iron speciation, Analyst 126 (2001) 903. 

C. Pons, R. Forteza, V. Cerda, Multi-pumping flow system for the determination, solid-phase extraction and speciation analysis of iron, Anal. Chim. Acta 550 (2005) 33. 

Pons C., Forteza R. and Cerd a V. (2005) The use of anion exchange disks in an optrode coupled to a multi-syringe flowinjection system for the determination and speciation analysis of iron in natural water samples. Talanta, 66, 210-217. 

Kondratenko, E. and P ez-Ramlrez, J. (2006). Mechanism and Kinetics of Direct N2O Decomposition over Fe-MFI Zeolites with Different Iron Speciation from Temporal Analysis of Products, /. Phys. Chem. B, 110, pp. 22586-22595. 

An MPFS-MCFIA set-up for the spectrophotometric determination, solid-phase extraction (SPE), and speciation analysis of iron is shown in Figure 4.15 [53]. In this system, the use of a solenoid valve allows the flow deviation toward the chelating disk to carry out SPE procedures when necessary. Thus, the determination of iron is feasible in a wide range of concentrations. However, micropumps cannot overcome the back-pressure from the SPE disk. For example, deviations as high as 2 pL were found in the indicated stroke volume of 8 pL/stroke micropumps requiring periodic recalibration. 

Relatively few authors have reported the use of this instrumentation for detection in speciation analysis so far. Only five elements (Se, S, Cr, V, As) have been the focus of research papers, and of these, selenium has received the most attention. Very recently, attempts have been undertaken to establish iron speciation with ICP-CC-MS detection. 

On the basis of the preceding discussion, it should be obvious that ultratrace elemental analysis can be performed without any major problems by atomic spectroscopy. A major disadvantage with elemental analysis is that it does not provide information on element speciation. Speciation has major significance since it can define whether the element can become bioavailable. For example, complexed iron will be metabolized more readily than unbound iron and the measure of total iron in the sample will not discriminate between the available and nonavailable forms. There are many other similar examples and analytical procedures that must be developed which will enable elemental speciation to be performed. Liquid chromatographic procedures (either ion-exchange, ion-pair, liquid-solid, or liquid-liquid chromatography) are the best methods to speciate samples since they can separate solutes on the basis of a number of parameters. Chromatographic separation can be used as part of the sample preparation step and the column effluent can be monitored with atomic spectroscopy. This mode of operation combines the excellent separation characteristics with the element selectivity of atomic spectroscopy. AAS with a flame as the atom reservoir or AES with an inductively coupled plasma have been used successfully to speciate various ultratrace elements. 

Spectrophotometric techniques combined with flow injection analysis (FIA) and on-line preconcentration can meet the required detection limits for natural Fe concentrations in aquatic systems (Table 7.2) by also using very specific and sensitive ligands, such as ferrozine [3-(2-bipyridyl)-5,6-bis(4-phenylsulfonic acid)-l,2,4-triazine], that selectively bind Fe(II). Determining Fe(II) as well as the total Fe after on-line reduction of Fe(III) to Fe(II) with ascorbic acid allows a kind of speciation.37 A drawback is that the selective complexing agents can shift the iron redox speciation in the sample. For example, several researchers have reported a tendency for ferrozine to reduce Fe(III) to Fe(II) under certain conditions.76 Most ferrozine methods involve sample acidification, which may also promote reduction of Fe(III) in the sample. Fe(II) is a transient species in most seawater environments and is rapidly oxidized to Fe(III) therefore, unacidified samples are required in order to maintain redox integrity.8 An alternative is to couple FIA with a chemiluminescence reaction.77-78... 

Laboratory methods. Huorimetry has been used widely for selenium analysis in environmental samples but is being superseded by more sensitive instrumental methods. Some of the instmmental methods used for arsenic speciation and analysis can also be used for selenium. In particular, HPLC and HG can separate selenium into forms suitable for detection by A AS, AFS (Ipolyi and Fodor, 2000), or ICP-AES (Adkins et al, 1995). Only Se(IV) forms the hydride, and so Se(VI) must be prereduced to Se(IV) if total selenium is to be determined. This is normally achieved using warm HCl/KBr followed by co-precipitation with La(OH)3 if necessary (Adkins et al., 1995). KI is not used, since it tends to produce some Se(0), which is not reduced by HG. La(OH)3 collects only Se(IV), so the prereduction step to include the contribution from Se(VI) is required before co-precipitation. Other methods of preconcentration include co-precipitation of Se(IV) with hydrous iron oxide or adsorption onto Amberlite IRA-743 resin (Bueno and Potin-Gautier, 2002). 

Similar demands for speciation of trace elements exist for food analysis. Substantial differences in the biological availability are known for several essential elements and depend on the form in which they are present in the diet. The chemical bases for these differences are known for cobalt, iron, and chromium but not for zinc, copper, and selenium. The importance of speciation in food analysis is best demonstrated by the example of iron. That element, when part of heme compounds, is well absorbed, and there is little influence on the absorption by other factors in the diet. Nonheme iron, on the other hand, is not readily absorbed and, in addition, is subject to many influences from dietary ingredients those influences are poorly understood and probably not completely known (14). 

An analysis of the waste pond water was used for calculating speciation and solubility of the injectate (Table 8.4). The modeling results show supersaturation of calcite and dolomite (and other carbonates), as well as iron oxides, quartz, and a number of unlikely silicates. 

Welter E, Calmano W, Mangold S, Troger L (1999) Chemical speciation of heavy metals in soils by use of XAFS spectroscopy and electron microscopical techniques. Fresenius J Anal Chem 364 238-244 Westre , Di Cicco A, Filipponi A, Natoli CR, Hedman B, Solomon El, Hodgson (1995) GNXAS, a multiple-scattering approach to EXAFS analysis Methodology and applications to iron complexes. J Am Chem Soc 117 1566-1583... 

T. P. Lynch, N. J. Kernoghan, and J. N. Wilson, Speciation of Metals in Solution by Flow Injection Analysis. Part 2. Determination of Iron(III) and Iron (II) in Mineral Process Liquors by Simultaneous Injection into Parallel Streams. Analyst, 109 (1984) 843. 

A. T. Faizullah and A. Townshend, Application of a Reducing Column for Metal Speciation by Flow Injection Analysis. Spectrophotometric Determination of Iron(III) and Simultaneous Determination of Iron(II) and Total Iron. Anal. Chim. Acta, 167 (1985) 225. 

The Pb dust analysis demonstrated the applicability of XAFS, p-XRF and p-XRD to determine the speciation of trace metals in heterogeneous samples. The X-ray techniques successfully identihed the speciation and potential sources of Pb in house dust samples, and provided an explanation for the observed high bioaccessibility. Linear combination fitting of the EXAFS i (k) k data of a Pb-elevated house dust sample show that Pb is complexed in two environments, as both a mineral (hydrocerussite) and an adsorbed species (adsorbed onto an iron oxide). The main findings from the Pb dust analysis can be summarized as follows ... 

Bagheri H., Gholami A. and Najafi A. (2000) Simultaneous preconcentration and speciation of iron(II) and ironflfl) in water samples by 2-mercaptobenzimidazole-silica gel sorbent and flow injection analysis system. Analytica Chimica Acta, 424, 233-242. 

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