Photooxidation is a potentially significant process in the degradation of crude oil spilled at sea. Moreover, a fundamental understanding of the effect of photochemical degradation on crude oil is a prerequisite for providing an accurate description of the recent history and potential fate of oil spilled in a marine environment. In this report we examine the effect of ultraviolet illumination on crude oil using a variety of techniques including gas chromatography/mass spectroscopy and X-ray absorption spectroscopy. The saturated compounds are resistant, but the aromatic compounds are particularly sensitive to photooxidation. Greater size and increasing alkyl substitution increase the sensitivity of aromatic compounds to photochemical oxidation. The photooxidized products appear in the resin and polar fractions as determined by thin-layer chroma tography. Thus, the effect of photooxidation is distinctly different from that of biodegradation, where larger and more substituted compounds are more resistant to...

Photooxidation of crude oils

A total-electron-yield technique is described in which near-surface extended x-ray-absorption fine-structure (EXAFS) data are obtained from direct measurements of specimen current. Experiments with several model systems---amorphous germanium and crystalline germanium, nickel, and cobalt; and arsenic ion implanted into silicon---demonstrate that this technique can reproduce EXAFS \ensuremath{\chi}(k) functions obtained from transmission and fluorescence measurements. Experiments also reveal that EXAFS amplitudes from total-electron-yield data can be 5--10 % smaller than those from transmission measurements for samples where the very-near-surface structure, at depths of tens to hundreds of angstroms, differs from the bulk structure. Measurements with buried layers confirm that the sampling depth for this total-electron-yield technique is determined primarily by the penetration ranges of Auger electrons emitted from the absorbing atoms. For the model systems listed above, LMM Auger electrons have ranges of hundreds of angstroms and KLL Auger electrons have ranges of thousands of angstroms. Expressions are derived for the sampling depth for total-electron-yield EXAFS experiments. The total-electron-yield technique described here is particularly useful for studying impurities within a few thousand angstroms of the surface of single crystals, where Bragg diffraction complicates the use of fluorescence measurements.

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Total-electron-yield current measurements for near-surface extended x-ray-absorption fine structure.

To understand the many important functions of sulfur, a ubiquitous element in biological systems, in the environment and for industrial applications, detailed analyses are needed. Characterization of the variety of sulfur functional groups in a natural sample, often occurring in a wide range of oxidation states, became possible when the development of dedicated X-ray absorption near-edge structure (XANES) spectroscopy started in the mid-1980s. This tutorial review provides an overview of sulfur XANES spectroscopic investigations into the role of sulfur in all kinds of natural samples, from sediment and oil to marine-archaeological wood and plants.

Sulfur: not a "silent" element any more.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/S0014-5793%2898%2901402-1

Sulfur K‐edge X‐ray absorption spectroscopy for determining the chemical speciation of sulfur in biological systems

In its natural habitat, Astragalus bisulcatus can accumulate up to 0.65% (w/w) selenium (Se) in its shoot dry weight. X-ray absorption spectroscopy has been used to examine the selenium biochemistry of A. bisulcatus. High concentrations of the nonprotein amino acid Se-methylseleno-cysteine (Cys) are present in young leaves of A. bisulcatus, but in more mature leaves, the Se-methylseleno-Cys concentration is lower, and selenate predominates. Seleno-Cys methyltransferase is the enzyme responsible for the biosynthesis of Se-methylseleno-Cys from seleno-Cys and S-methyl-methionine. Seleno-Cys methyltransferase is found to be expressed in A. bisulcatus leaves of all ages, and thus the biosynthesis of Se-methylseleno-Cys in older leaves is limited earlier in the metabolic pathway, probably by an inability to chemically reduce selenate. A comparative study of sulfur (S) and Se in A. bisulcatus using x-ray absorption spectroscopy indicates similar trends for oxidized and reduced Se and S species, but also indicates that the proportions of these differ significantly. These results also indicate that sulfate and selenate reduction are developmentally correlated, and they suggest important differences between S and Se biochemistries.

Chemical Form and Distribution of Selenium and Sulfur in the Selenium Hyperaccumulator Astragalus bisulcatus

X-Ray absorption spectra of the sulfur K-edges of SO42−, SO32−, S2O32−, and S2Ox2− (x=5–8) were measured with synchrotron radiation. The spectra show similarly strong absorption bands which originate in bound-state transitions to antibonding 3p-type orbitals and in shape resonances related to 3d-type orbitals. The K-edge spectrum for the terminal sulfur atom in S2O32−, however, shows only a bound-state transition. Differences in chemical environments and bonding around the X-ray absorbing atoms are discussed to interpret the spectra.

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Sulfur K-edge absorption spectra of Na2SO4, Na2SO3, Na2S2O3, and Na2S2Ox(x=5―8)

The design and performance of an ultrahigh vacuum compatible soft X-ray double crystal monochromator at the Photon Factory in operation is described. This is a compact version of the JUMBO at SSRL with some improved mechanisms. Ge(111), InSb(111), α-quartz (10 1 0), beryl (10 1 0 and GGG(211) were tested as monochromatizing elements with regard to the resolution, intensity and radiation damage.

Design and performance of a UHV compatible soft X-ray double crystal monochromator at the proton factory

We observed the electron emission modulated by a standing wave field from an InP (100) single crystal in the soft X-ray region. Incident photon energy was scanned with a total electron detector, instead of scanning the incident angle to the sample. A remarkable difference of electron yield profiles was observed between below and above the P K edge. This difference can be understood by considering the drastic change of the P K absorption coefficient. It is also suggested that the soft X-ray standing wave method is a promising technique for the structure analysis of semiconductor surfaces.

Effect of the Soft X-Ray Standing Wave Fields on the Total Electron Yield Spectra from an InP Crystal

Initial oxidation of Si(100) and Si(111) surfaces has been investigated by XANES (X-ray absorption near-edge structure) spectroscopy using synchrotron radiation. At room temperature, oxidation of Si(111) surfaces proceeds much faster than that for Si(100) surfaces. The XANES spectra indicate that oxygen is atomically adsorbed onto the Si surfaces in initial stages of oxidation at room temperature. As oxidation on the surface proceeds, the XANES spectra exhibit the formation of a continuum shape resonance. The local electronic states and the local site structure for the progressive oxidation process are discussed.

Hideo Sekiyama

Papers

Sulfur K-edge absorption spectra of Na2SO4, Na2SO3, Na2S2O3, and Na2S2Ox(x=5―8)

Design and performance of a UHV compatible soft X-ray double crystal monochromator at the proton factory

Effect of the Soft X-Ray Standing Wave Fields on the Total Electron Yield Spectra from an InP Crystal

Investigation of the Si Oxidation Process by XANES Spectroscopy Using Synchrotron Radiation