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Applications of in-situ X-ray Scattering Techniques

Applications of in-situ X-ray Scattering Techniques. Sam Webb SSRL Scattering Workshop May 15, 2007. Overview. Why in situ? Experimental Design Beamlines Sample prep Analysis Reactions with x-ray scattering Example(s). Why Should I Do Scattering When I Have EXAFS Data?. Triclinic

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Applications of in-situ X-ray Scattering Techniques

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  1. Applications of in-situ X-ray Scattering Techniques Sam Webb SSRL Scattering Workshop May 15, 2007

  2. Overview • Why in situ? • Experimental Design • Beamlines • Sample prep • Analysis • Reactions with x-ray scattering • Example(s)

  3. Why Should I Do Scattering When I Have EXAFS Data? Triclinic Birnessite Triclinic Birnessite Hexagonal Birnessite Hexagonal Birnessite d-MnO2 d-MnO2 EXAFS = Local Structure WAXS = Long-Range Structure

  4. Why In-situ • Traditional powder diffraction experiments require dry, fine powders as samples • For many biological and environmental samples: • Drying = artifact • Dehydration, exposure to air • Powder = artifact • Other thoughts to consider… • Sample throughput • Sample textures • Timing/Reactions

  5. Experimental Design • Does my sample need to be wet? • Transmission vs. reflection • Tradeoffs due to backgrounds of sample holder and water • High resolution vs. low • Soller slits vs. analyzer vs. area detector • Data range • Exposure to beam? • Exposure to air?

  6. collimating slits scattered x-rays incident beam sample Diffractometer (SSRL BL 2-1) detector analyzer

  7. Powder Scattering Experiment X-ray source (Synchrotron) • Monochromatic • Sample contains all crystal orientations • Detector and sample angles unchanged Mono Detector Slits Sample Beamstop

  8. Diffractometer (SSRL 11-3) • Tight spaces in hutch • Samples: • Flat plate transmission • Reflection (half of area detector) • Capillary • BL software (Blu-Ice) • 5-10 MB per picture

  9. Diffractometer (SSRL 11-3) • Tight spaces in hutch • Samples: • Flat plate transmission • Reflection (half of area detector) • Capillary • BL software (Blu-Ice) • 5-10 MB per picture

  10. top plate lexan windows sample shim spacer bottom plate Sample Preparation (Flat plate) • Keep sample hydrated to avoid artifacts! • Change in oxidation state/mineralogy • Collapse of hydrated structures • Use transmission geometry • Why? - Better subtraction of background scattering (water, windows) • Window material important • Lexan is a good material for background removal (WAXS) • Water peaks in similar places as silica • Optimize sample thickness depending on l and sample composition. Sample should absorb ~ 20‑50% of incident beam. One “m” is about max. • Other sample holders – goniometer head – sample distance ~ 37 mm (11-3, 7-2)

  11. What if I have a powder for transmission? • Flat plate is poor for dry samples • Particles are not generally stable and settle – even out of beam! • Need a better support – tape! • Kapton not ideal • Scotch Magic tape (translucent)

  12. Data Analysis • CCD to diffractogram (2D to 1D) • Geometry corrections • Background subtraction • Windows, capillary, tape • Water • Other interferences (cotton, etc)

  13. 2q Integration of Powder Pattern • What Can it Tell? • Peak Positions: • Phase identification • Lattice symmetry • Peak Shape & Width: • Crystallite size • Textures (preferential orientation, multiple phases, etc.) • Peak Intensity: • Crystal structure • FIT2D • http://www.esrf.fr/computing/scientific/FIT2D/

  14. t q t cos q Theta Dependent Effects • Absorption • Samples absorb the incident and transmitted beams Abs = (t / cos q) exp(-mt/cos q) • Measure sample absorption at the beamline! • Volume effect • 1/cos q dependence • Compton Scattering • Highest at large q In order to get proper removal of background (windows, water) these corrections must be made. Critical for thicker samples!

  15. Background Subtraction • Background in experiments consists of lexan windows and water

  16. http://www-ssrl.stanford.edu/~swebb/xrdbs.htm RDSUB • GUI for removal of background and thickness corrections • Designed for use with Fit-2D output (chi files)

  17. Reactions • Mineral-solution reactions • Time scale of minutes to hours • Redox reactions • Cation exchange • Colloid transport • Sample prep = miniaturized “columns” (i.e., particles packed in a capillary) • Lexan capillary • Better background (no overlap with water like silica) • Doesn’t break! • Particle size and porosity • Clogging • Flow rate • Stalling of pump

  18. Reaction Flow Setup (not to scale) Tubing Scattered Beam Sample Cotton Gasketed capillary holder Incident beam 120 mL Syringe pump Flow collection system

  19. Beamline Setup (BL 11-3)

  20. Future Improvements… • Peristaltic pump vs. syringe pump • Better flow and ability to change reactant solutions • Development of better column packing materials • Gas impermeable tubing • Improve anaerobic conditions • Injection loop • Easy loading of capillary • Fraction collector • Analysis of post-reaction fluids • Fluorescence detector • Monitor elemental changes in sample if reactions lead to deposition / removal of compounds

  21. Examples • Mn biomineral structures • Compare 2-1 and 11-3 data quality • Real time biogenic Mn oxidation • Area detectors in reactions • MnOxide reactions with metals • Area detectors in reactions • Sulfide mineral oxidation • Wet-dry artifacts for air sensitive minerals • Air exposure

  22. BL 11-3 2 minute exposure 360 degrees are better than 1! BL 2-1 Sum of 4 to 5 scans, ~8 hours total Mn Oxide Biomineral Structure Tradeoff between noise-resolution-time

  23. a a a* Triclinic Hexagonal b Biogenic Mn Oxidation • Mn oxidation in seawater progresses through symmetry changes in oxide structure • Due to the effect of Ca present in interlayers

  24. Manganese in-situ Oxidation Mn(II) Spores Mn(IV) 2d 1d Triclinic peaks Mn(IV) Oxide Q (nm-1) Spores Scan No. (~20 min between scan)

  25. Manganese oxide reaction with metals Co(II) • Co(II) reacts with pre-formed biogenic oxides to oxidize to Co(III). Mn-oxides are reduced • No evidence of new Co(III) minerals Mn(IV) Decrease in (001) amplitude Time (h) Q (A-1)

  26. Biogenic MnOxides + Co(II) • 001 peak broadens with reaction and shifts to larger d-spacings • Changes follow pseudo-first order reaction kinetics • Slow and fast steps of Co(III) incorporation

  27. Wet-Dry Artifacts • Measurements of anaerobic, dried sample lead to formation of peaks with different texture FeS dried FeS paste, anaerobic

  28. ~1.2 mm from end O2 FeS O2 10 8 6 4 2 0 Time (h) Fe-Sulfide oxidation reactions 2-line Ferrihydrite Mackinawite t=7 t=8 t=9 t=10 t=0

  29. Acknowledgements • Anna Obraztsova and Greg Dick (SIO) • Apurva Mehta (SSRL) • Tanya Gallegos (U of M) • Funding: • NSF-CRAEMS • DOE-BER • DOE-BES R/V Knorr, Black Sea, 2003

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