Gregory Hirsch

The fern Pteris vittata accumulates unusually high levels of arsenic. Using X-ray absorption spectroscopy (XAS) and XAS imaging, we reveal the distribution of arsenic species in vivo. Arsenate is transported through the vascular tissue from the roots to the fronds (leaves), where it is reduced to arsenite and stored at high concentrations. Arsenic-thiolate species surrounding veins may be intermediates in this reduction. In gametophytes, arsenite is compartmentalized within the cell vacuole. Arsenic is excluded from cell walls, rhizoids, and reproductive areas. This study provides important insights into arsenic hyperaccumulation, which may prove useful for phytoremediating arsenic-contaminated sites, and demonstrates the strengths of XAS imaging for distinguishing highly localized species.

Sulfur has a particularly rich biochemistry and fills a number of important roles in biology. In situ information on sulfur biochemistry is generally difficult to obtain because of a lack of biophysical techniques that have sufficient sensitivity to molecular form. We have recently reported that sulfur K-edge X-ray absorption spectroscopy can be used as a direct probe of the sulfur biochemistry of living mammalian cells [Gnida, M., et al. (2007) Biochemistry 46, 14735−14741]. Here we report an extension of this work and develop sulfur K-edge X-ray fluorescence spectroscopic imaging as an in vivo probe of sulfur metabolism in living cells. For this work, we have chosen onion (Allium cepa) as a tractable model system with well-developed sulfur biochemistry and present evidence of the localization of a number of different chemical forms. X-ray absorption spectroscopy of onion sections showed increased levels of lachrymatory factor (LF) and thiosulfinate and decreased levels of sulfoxide (LF precursor) following cell breakage. In intact cells, X-ray fluorescence spectroscopic imaging showed elevated levels of sulfoxides in the cytosol and elevated levels of reduced sulfur in the central transport vessels and bundle sheath cells.

Abstract A method for manufacturing metal monocapillary optics is described. The fabrication process begins by withdrawing an initially uniform wire from an etchant bath at a variable rate using a computer‐controlled translation stage. This generates a precise taper profile on the wire that corresponds to the bore of a capillary that will be produced in subsequent steps. Paraboloidal, ellipsoidal or other capillary figures can be programmed into the motion controller. Two different methods are discussed for generating the completed optic from the etched wire. In both of these techniques, the wire functions as an expendable mandrel. In the first method, the wire is first coated with a radiation‐reflecting material and then bonded to a rigid substrate. The wire is then dissolved using a chemical process to produce the hollow capillary optic. In the second method, a purely mechanical process imprints the wire figure into a softer material to create the capillary optic. Both manufacturing processes permit the production of capillaries that are accurately figured, extremely straight and have very low surface roughness. Wide latitude is possible in the selection of reflective materials that coat the internal surface of the optics. Experimental measurements using the optics with synchrotron radiation have demonstrated 5–10 µm diameter beams having flux‐density gains near 100. Potential improvements to the capillaries for achieving higher gains and smaller sizes are discussed. In addition to synchrotron radiation experiments, applications with small laboratory instruments are considered. The ultimate limitations of the optics and their comparison with glass capillaries are discussed. Copyright © 2003 John Wiley & Sons, Ltd.

Tapered metal monocapillary optics provide a potential alternative to conventional methods of producing small X-ray beams. This paper presents the initial results of chemically specific imaging using such devices. Cellular resolution of organic selenium is obtained in a longitudinal section of mature Astragalus bisulcatus, a selenium hyperaccumulating plant. This work demonstrates the utility of metal monocapillary optics for imaging dilute levels of target elements in biological tissues.

A new approach for producing tapered-moncapillary optics has been demonstrated. The fabrication process permits the production of metal optics which are accurately shaped, extremely straight, and have very low surface-roughness. Wide latitude in the selection of materials comprising the optics is possible. Preliminary experiments using gold paraboloidal-capillaries have demonstrated flux-density gains approaching 100 in 10-micron focused beams. The fabrication process, testing procedures, and experimental results are described. Potential improvements to the optics for achieving higher gains and smaller spot-sizes are discussed.