Barry M. Gordon

Reduced chromium in olivine grains from lunar basalt 15555 - X-ray Absorption Near Edge Structure (XANES)

The oxidation state of Cr in 200 [mu]m regions within individual lunar olivine and pyroxene grains from lunar basalt 15555 was inferred using X-ray Absorption Near Edge Structure (XANES). Reference materials had previously been studied by optical absorption spectroscopy and included Cr-bearing borosilicate glasses synthesized under controlled oxygen fugacity and Cr-doped olivines. The energy dependence of XANES spectral features defined by these reference materials indicated that Cr is predominantly divalent in the lunar olivine and trivalent in the pyroxene. These results coupled with the apparent f[sub o[sub 2]]-independence of partitioning coefficients for Cr into olivine imply that the source magma was dominated by divalent Cr at the time of olivine crystallization. 29 refs., 8 figs., 1 tab.

Sensitivity calculations for multielemental trace analysis by synchrotron radiation induced X-ray fluorescence

Abstract The application of synchrotron radiation from high energy electron storage rings as an excitation source for X-ray fluorescence analysis of trace elements will greatly extend the sensitivity of the fluorescence technique. The properties of synchrotron radiation that make improvement possible are: high brightness, continuously tunable energy spectrum, relatively low energy deposition, and polarization in the storage ring plane for reduced scattering backgrounds. New wide energy bandwidth monochromators now being developed will provide intense monochromatic beams which will make practical the use of the wavelength dispersive crystal spectrometers. The sensitivity calculations presented are applied to representative thin biological and geological samples. The storage ring parameters used are the design parameters of the National Synchrotron Light Source. The quantitation limits ( σ =10%) for a thin biological sample using a solid state detector is less than 1 ppm for low Z elements and 1–10 ppm for high Z elements for one minutev irradiations. Using fixed multicrystal wavelength dispersive spectrometers quantitation limits range from 3–30 ppb for biological samples and 30–300 ppb for geological samples. Minimum detectable limits are a factor of 3 to 5 lower.

Bridged Ferrocenes. I

Abstract Previously reported correlations between Mossbauer spectra and the number of bridges in trimethylene bridged ferrocene derivatives have now been observed in the electrode potentials and in the electronic spectra. Crystallographic results reported in the subsequent papers of this series are employed to confirm the previous attribution of these effects to ring—ring tilting, iron-to-ring distance, and to a lesser extent the non-planarity of the rings in some compounds. Rearrangement during the bridging reaction is proposed to explain the failure of the formation of the four-bridged compound.

Bridged Ferrocenes VIII. Correlation of redox potentials with structure

Abstract The redox potentials for oxidation of ferrocenes with hydrocarbon bridges and of corresponding ketoferrocenes were measured. For the reduced compound correlations were observed between the potentials and the iron to ring distances. For the ketoferrocenes, the importance of proper alignment of the carbonyl with the cyclopentadienyl rings is noted. Other possible relationships are discussed.

Bridged ferrocenes. VI. Hydrogenation with Pd/C catalyst

Abstract The method of Van Meurs et al. [1] for hydrogenation of ferrocene has been extended to include a variety of bridged ferrocenes. Singly bridged and dibridged ferrocenes containing tri-, tetra-, or pentamethylene bridges were hydrogenated to the corresponding polycyclic hydrocarbons. Conditions required were consistent with a mechanism that involves an initial protonation of the iron atom. Bridge opening to a propylferrocene derivative was observed for the compound with three not-all adjacent trimethylene bridges, and no hydrogenation reaction was observed for the corresponding derivative with tetramethylene bridges. Without catalyst or hydrogen, trifluoroacetylation and a rearrangement of a bridge were observed in the tris(trimethylene) compound. In the presence of catalyst but no hydrogen, an alpha alcohol and the corresponding diferrocenyl ether were obtained. Possible mechanisms are suggested for these reactions.

X-Ray Fluorescence Microprobe Imaging with Undulator Radiation

Synchrotron x-ray fluorescence experiments were performed using a prototype undulator for the Advanced Photon Source installed on the CESR storage ring at Cornell University during a run in May, 1988. Fluorescence spectra were collected from a number of standard references and unknowns. Thick target minimum detectable limits (MDL) were about a factor of two higher than those obtained using white bending magnet radiation at the NSLS. The higher MDLs could be due to lower polarization and/or imperfect alignment of the Si(Li) detector. Thin target MDLs were about 10 times lower than the NSLS since the undulator produced a usable spot size which was also 10 times smaller. Several one dimensional multi-elemental scans and two dimensional images were made with 10 μm resolution and 30 ppm MDL. These experiments demonstrate that undulators on the proposed Advanced Photon Source will be ideal for a trace element x-ray fluorescence microprobe with excellent elemental sensitivity and spatial resolution.

Survey of chemical speciation of trace elements using synchrotron radiation.

Information concerning the chemical state of trace elements in biological systems generally has not been available. Such information for toxic elements and metals in metalloproteins could prove extremely valuable in the elucidation of their metabolism and other biological processes. The shielding of core electrons by binding electrons affect the energy required for creating inner-shell holes. Furthermore, the molecular binding and symmetry of the local environment of an atom affect the absorption spectrum in the neighborhood of the absorption edge. X-ray absorption near-edge structure (XANES) using synchrotron radiation excitation can be used to provide chemical speciation information for trace elements at concentrations as low as 10 ppm. The structure and position of the absorption curve in the region of an edge can yield vital data about the local structure and oxidation state of the trace element in question. Data are most easily interpreted by comparing the observed edge structure and position with those of model compounds of the element covering the entire range of possible oxidation states. Examples of such analyses will be reviewed.

The Role of High-Energy Synchrotron Radiation in Biomedical Trace Element Research

This paper will present the results of an investigation of the distribution of essential elements in the normal hepatic lobule. the liver is the organ responsible for metabolism and storage of most trace elements. Although parenchymal hepatocytes are rather uniform histologically, morphometry, histochemistry, immunohistochemistry, and microdissection with microchemical investigations have revealed marked heterogeneity on a functional and biochemical level. Hepatocytes from the periportal and perivenous zones of the liver parrenchyma differ in oxidative energy metabolism, glucose uptake and output, unreagenesis, biotransformation, bile acid secretion, and palsma protein synthesis and secretion. Although trace elements are intimately involved in the regulation and maintenance of these functions, little is known regarding the heterogeneity of trace element localization of the liver parenchyma. Histochemical techniques for trace elements generally give high spatial resolution, but lack specificity and stoichiometry. Microdissection has been of marginal usefulness for trace element analyses due to the very small size of the dissected parenchyma. The characteristics of the high-energy x-ray microscope provide an effective approach for elucidating the trace element content of these small biological structures or regions. 5 refs., 1 fig., 1 tab.

An X-Ray Microprobe Beam Line for Trace Element Analysis

The application of synchrotron radiation to a x-ray microprobe for trace element analysis is a complementary and natural extension of existing microprobe techniques using electrons, protons, and heavier ions as excitation sources for x-ray fluorescence. This was first recognized by HOROWITZ and HOWELL [1] in their development of the first synchrotron radiation microprobe at the Cambridge Electron Accelerator. SPARKS, et al. [2] used a miniprobe beam at the Stanford Synchrotron Radiation Laboratory in an attempt to find natural occurring superheavy elements by x-ray fluorescence of characteristic L-lines. The ability to focus charged particles leads to electron microprobes with spatial resolutions in the sub-micrometer range and down to 100 ppm detection limits and proton microprobes with micrometer resolution and ppm detection limits. The characteristics of synchrotron radiation that prove useful for microprobe analysis include a broad and continuous energy spectrum, a relatively small amount of radiation damage compared to that deposited by charged particles, a highly polarized source which reduces background scattered radiation in an appropriate counting geometry, and a small vertical divergence angle of ~ 0.2 mrad which allows for focussing of the light beam into a small spot with high flux. The features of a dedicated x-ray microprobe beam line developed at the National Synchrotron Light Source (NSLS) are described.

The application of a synchrotron radiation microprobe to trace element analysis

Synchrotron radiation is light emitted by electrons when accelerated in a circular orbit. Some properties of synchrotron radiation important to trace element analysis by x-ray fluorescence analysis are: 1) a broad, continuous and tunable energy spectrum for K- and L-shell excitation of all elements; 2) a linearly polarized source reducing the scattered radiation backgrounds; 3) low energy deposition in the target; and 4) an appreciable flux in narrow energy bandwidths for chemical speciation.