Synchrotron Radiation and Its Application to Chemical Speciation

Chapter 23

Synchrotron Radiation and Its Application to Chemical Speciation Β. M . Gordon and Κ. W. Jones

Downloaded by COLUMBIA UNIV on August 1, 2012 | http://pubs.acs.org Publication Date: December 26, 1991 | doi: 10.1021/bk-1991-0445.ch023

Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973

Synchrotron radiation can be used in Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) experimental modes to extract information concerning chemical speciation of elements at trace concentration levels. The structure and relative position of an absorption spectrum at high energy resolution in the absorption edge region provide information regarding the oxidation state of the element and symmetry of the molecule in the immediate vicinity. Speciation is elucidated by comparison of spectra with those of model compounds. Examples of such studies in the literature are presented. The technique, which generally requires no chemical preparation, is sensitive in the mg/kg concentration range in biomedical tissue samples. The recent proliferation of intense and dedicated synchrotron radiation sources provides wide access to the technique.

Trace elements have long been recognized as playing an important role in the functioning of living organisms. This realization has been fostered by the rapid development of analytical techniques capable of quantitation at ever improving sensitivities and decreasing spatial resolutions. Trace elements have been shown to be both essential and harmful to the well-being of organisms (1). For some elements the range between deficiency and toxicity is indeed narrow. The advanced capabilities for trace element determination has also brought the realization that knowledge of the chemical speciation of the trace elements is possibly the most important component of data that can be collected. For example, in the field of nutrition, the bioavailability of an essential element requires that it be in a reactive form rather than one that is incapable of being metabolized. Many elements, particularly first row transition metals, are components of biologically important compounds such as proteins, enzymes, etc. Each of these compounds has specific

0097-6156/91/0445-0290$06.00/0 © 1991 American Chemical Society

In Biological Trace Element Research; Subramanian, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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functions such as metabolism, detoxification, oxidation-reduction catalysis, transfer reactions, etc. Specific techniques for speciation include N M R spectroscopy, dialysis, gel filtration, electrophoresis, anodic stripping voltammetry, and radioactive tracers. Many of these methodologies require sample processing for separation of and identification of the desired elemental species from matrices such as food, serum, tissue, etc. In some cases species identity may be sacrificed and lead to incorrect results. Another method which is less susceptible to species alteration involves the excitation of K- and L-shell x-ray fluorescence where the energy of the fluorescence is determined using high resolution crystal spectrometers (2). The resultant spectra are compared with those of model compounds for possible speciation. Recently, the x-ray fluorescence technique has been improved using a high-intensity x-ray source which has been made possible by the recent development of synchrotron radiation sources (3). The method is carried out by scanning the energy of a highly collimated x-ray beam through the absorption edge of the element in question with a narrow energy interval ( Δ Ε / Ε -10" ). The spectra are taken by measuring the transmittance of a major or minor constituent and byfluorescenceχ ray detection for trace constituents. Again, the spectra obtained are compared with those of model compounds. These comparisons can provide, as illustrated below, information concerning the oxidation state, symmetry, molecular bonding and surrounding structure of the absorbing atom. Under the most favorable circumstances, these measurements can be made in situ at 1 mm spatial resolution and 5 mg/kg concentration level (4). 4

2

THEORY The type of x-ray spectroscopy (5) used to elucidate structural and speciation information in an atomic environment have been termed EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near Edge Structure). Fig. 1 illustrates schematically a scattered radiation spectrum from a fictitious sample containing a trace amount of zinc and shows the major features of the photoelectric effect (PE) resulting in K-shell fluorescence, Compton inelastic scattering and Rayleigh elastic scattering. The line widths shown for the fluorescence and Rayleigh lines are actually much narrower, but are widened for ease of viewing. The width of the Compton scattering reflects the large acceptance solid angle of a typical ion chamber since the energies of the scattered photons are angle dependent. In the case shown the excitation energy is above the Zn-edge. If the excitation energy were below the Zn-edge, the Zn fluorescence would not exist. In addition, but not shown, the spectrum includes K-shell fluorescence lines of elements present with Ζ

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ENERGY (keV) Figure 8. Absorption spectra of Zn model compounds for analysis of aerosols. (Reproduced with permission from Ref. 18. Copyright 1980 American Chemical Society.)


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RESEARCH

resulting in float and sink fractions. The concentration of vanadium was the order of 1000 ppm. The XANES spectra of model compounds are shown in Fig. 9. The spectra b and c represent octahedral coordination, d represents tetrahedral coordination, and e and f represent square pyramidal coordination. Note the strong pre-edge peaks for the structures without a center of symmetry. The pre-edge peak in d, as well as other V(V) compounds occurs at 5 to 6 eV. Pre-edge peaks of V(IV) compounds with nitrogen-bound ligands, such as vanadium phthalocyanine and porphyrin occur at 3.5 to 4.0 eV. Fig. 10 shows the XANES spectra for unseparated coal, the float and sink fractions, and a liquefaction residue, the result of treatment with stannous chloride, tetralin, and hydrogen at high pressure and temperature. The pre-edge peaks are at 4.5 ± 0.2 eV, in good agreement with V(IV) coordinated to oxygen, as in V 0 . The heavy fraction vanadium content seems to be made up of V 0 with some entrainment of V 0 . 2


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ELEMENTS OF BIOMEDICAL INTEREST. The trace elements that have important beneficial roles in human processes, in addition to the first row transition elements mentioned previously, include As, F, I, Mo, Se, Si, and Sn. Other trace elements that can be toxic and that are commonly found in the environment include Be, Cd, Hg, Pb, Pd, and Tl (1). A majority of the essential elements become toxic at levels greatly exceeding the ideal levels. A n extensive literature of EXAFS and XANES studies on biologically interesting compounds has accumulated over the past fifteen years. These studies concerned structural and bonding information of metal interactions in enzymes and proteins. This literature can be part of a data base of model compounds for XANES speciation studies. There is an effort to gather this literature into a centralized and computerized data bank. The compounds of these elements with the more common inorganic ligands have been reported and need not be further discussed here. A brief mention of classes of structures that have been studied for iron and copper follows and is representative of studies with a majority of the beneficial elements. Iron is one of the most studied elements using EXAFS, in part because of many studies performed by other techniques, including Mossbauer spectroscopy (20). The studies of Fe-S cluster sites include rubredoxin, ferredoxin, aconitase, and the Fe-Mo cofactor. There is an extensive literature on hemoproteins, where the iron is bound to four planar nitrogen atoms and to a variety of axial ligands (20). These include c-type cytochromes and related cytochrome oxidases. Ferritins and iron-tyrosinate proteins have had structures elucidated in the Fe vicinity by EXAFS (2Q). Copper proteins and enzymes have also been studied, among them the "blue" copper proteins including azurin, stellacyanin, and plastocyanin (26). There is considerable controversy over the structure and oxidation state of Cu in cytochrome oxidase preparations. Studies of superoxide dismutase enzymes, which catalyze the disproportionation of the superoxide radical, show the existence of Cu(II) and Zn(II). Again, Cu(I) with a filled 3d shell would not show a Is -> 3d transition. Metallothioneins of copper and other metals have also been studied. They function as regulators and storage media for metals and can also be used as detoxification agents for metals (20). Studies have been made on metal compounds used as therapeutic agents, among them cw-Pt(NH ) Cl , an anticancer drug (21); GaCl , which decreases calcium resorption in bone cancer (22); and gold-based drugs in the treatment of arthritis (23). 3

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23.

GORDON AND JONES

Synchrotron Radiation

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Synchrotron Radiation and Its Application to Chemical Speciation

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