1. GSECARS X-ray Microprobe for Earth and Environmental Sciences
Matthew Newville, Peter Eng, Steve Sutton, Mark Rivers
Consortium for Advanced Radiation Sources (CARS)
University of Chicago, Chicago, IL
Objectives for Earth and Environmental Sciences:
Determine chemical associations, speciation, and structure of heavy elements on heterogeneous samples: soils, sediments, aggregates, plant material, isolated inclusions, or contaminants.
X-ray microprobe techniques:
X-ray Fluorescence (XRF), Fluorescence Mapping
abundance and spatial correlations of heavy elements
X-ray Absorption (XANES / EXAFS)
oxidation state of selected element
near-neighbor distances and coordination numbers
X-ray Micro-diffraction
phase identification
ALS Users Meeting Oct-2004

2. GSECARS microprobe: APS 13-ID
Incident Beam:
LN2 cooled Si (111) mono
Sample Stage: x-y-z stage, 1mm resolution
CCD Camera: Bruker 2k area detector
Fluorescence detector:
16-element Ge detector
with DXP electronics
Si-drift detector (shown)
Lytle Detector
Wavelength Dispersive
Spectrometer
Optical Microscope:
5x to 50x objective with external video system.
Focusing: Kirkpatrick-Baez mirrors: Rh-coated Si, typically using 3x3mm spot sizes, at 50mm from end of mirrors.
Entrance Slits: typically 250mm, accepting ~30% of undulator beam
ALS Users Meeting Oct-2004

3. Kirkpatrick-Baez Focusing Mirrors
The table-top Kirkpatrick-Baez mirrors use four-point benders and flat, trapezoidal mirrors to dynamically form an ellipsis. They can focus a 300x300mm beam to 1x1mm.
With a typical working distance of 100mm, and a focal distance and spot-size independent of energy, they are ideal for micro-XRF and micro-EXAFS.
We use Rh-coated silicon for horizontal and vertical mirrors to routinely produce 2x3mm beams for XRF, XANES, and EXAFS.
ALS Users Meeting Oct-2004

4. Arsenic/Iron in cattail roots: XRF tomography
Nicole Keon, Daniel Brabander, Harold Hemond (MIT), GSECARS
The Superfund site at the Wells G+H wetland, Woburn, MA (featured in A Civil Action) contains ~10 tons of arsenic within the upper 50 cm of the sediment. Most of the arsenic is held in the wetland sediments with relatively little As in the groundwater.
Usually an iron-reducing, anoxic environment such as a sediment would be expected to have high As mobility.
Can the metabolic activity of wetland plants, such as Typha latifolia (cattail) explain the sequestration of arsenic in the wetland?
Within ~100mm of the roots, Fe is oxidized to Fe(III) and forms a plaque on the root, even in these sediments. Could As be adsorbed to the ferric oxy-hydroxides formed at the root exteriors?
Where is As in the cattail roots?
What elements (Fe) are associated with As?
What is the As oxidation state in the roots? As Fe Zn Cu
Physical slicing the root for 2D XRF mapping would damage the sample.
Fluorescence tomography can make a virtual slice of the root and show the elemental associations and concentrations in the slice.
ALS Users Meeting Oct-2004

5. X-ray Fluorescence Tomography: Primer
X-ray computed microtomography (CMT) gives 3D images of the x-ray absorption coefficient.
An absorption image is collected as the angle w is rotated through 180o, and the 3D image is reconstructed in software.
In some cases, element-specific images can be made by tuning the x-ray energy above and below an absorption edge.
broad x-ray beam
Phosphor
Sample
CCD camera
visible light w
rotation stage
focused x-ray beam
Fluorescence x-ray tomography use a focused beam, scanned across the sample. The sample is rotated around w and translated in x.
Fluorescence x-rays are collected as for XRF maps. Transmission x-rays are measured as well to give an overall density tomograph.
Transmission detector
Sample
fluoresced x-rays w
fluorescence detector x
can collect multiple fluorescence lines.
rotation and translation stages
data collection is relatively slow –
one slice can be made at a time.
can be complicated by self-absorption.
ALS Users Meeting Oct-2004

6. Fluorescence Tomography: Experimental Setup
Fluorescence detector:
multi-element Ge detector
Sample stage:
x-y-z-q
Sample, mounted on silica fiber, or in ‘shrink-wrap’ tube, on a goniometer head
KB mirrors,
with Pb tape shield
Optical microscope
ALS Users Meeting Oct-2004

7. Fluorescence Tomography: Sinograms
The raw fluorescence tomography data consists of elemental fluorescence (uncorrected for self-absorption) as a function of position and angle: a sinogram. This data is reconstructed as a virtual slice through the sample by a coordinate transformation of (x,w)  (x, y). The process can be repeated at different z positions to give three-dimensional information.
Fluorescence sinograms collected simultaneously for Zn, Fe, and As for a cross-section of As-contaminated cattail root (photo, right): x: 1100mm in 10mm steps w: 180 in 3 steps Zn Fe As w x
ALS Users Meeting Oct-2004

8. Fluorescence Tomogram Slices of Cattail Roots
Wells G&H Typha latifolia root: reconstructed slices from fluorescence m-tomography, showing As concentrated on the root exterior, associated with Fe. As Fe
Quantitative XRF analysis of the As and Fe concentrations from these slices give an As/Fe molar ratio of ~10 ppm, consistent with the average from bulk, wet chemical techniques.
Though only a few virtual slices could be made, this gives us confidence that the slices made are representative of the average.
As and Fe are both at root plaque, not in the root interior. As and Fe are ~98% correlated.
Cu, Zn, and Pb (not shown) are less uniform on plaque, suggesting they are not co-precipitated with or sorbed onto the Fe phase.
Bulk XAFS of Fe shows Fe(III). Zn Cu
ALS Users Meeting Oct-2004

9. ALS Users Meeting 19-Oct-2004
As XANES
XANES measurements on the Typha latifolia cattail roots show mixed As oxidation state. The As3+ fraction did vary between different root samples, and along a single root, ranging from ~10% to ~60% As3+.
The spectra here shows roughly equal portions As3+ and As5+, which is higher As3+ than average.
Porewaters in the wetland have ~50% As3+: the As is generally more oxidized in the root plaque, which is consistent with adsorption onto ferric oxyhydroxides.
ALS Users Meeting Oct-2004

10. As XANES, Oxidation State Tomograms
XANES measurements on the Typha latifolia cattail roots show mixed As oxidation state. The As3+ fraction did vary between different root samples, and along a single root, ranging from ~10% to ~60% As3+.
The spectra here shows roughly equal portions As3+ and As5+, which is higher As3+ than average.
Fluorescence tomograms made at 2 different energies:
EAs total As concentration
EAs As3+ concentration
would show spatial dependence of the As oxidation state.
ALS Users Meeting Oct-2004

11. As oxidation tomograms for Cattail Roots
N. Keon, et al., Environ Sci & Tech, Oct 2004 (on line already….).
The As3+ / As ratio is heterogeneous (boxed areas). As5+ appears correlated with metals (Fe, Cu, Zn).
There does not seem to be a systematic spatial As oxidation gradient.
As3+
As3+
total
As3+
total
As total
As5+ appears at location with high Fe.
As5+ appears at location with high Cu and Zn.
More detailed spatial and oxidation state information would need faster data collection rates.
ALS Users Meeting Oct-2004

12. Ni in hyperaccumulating Alyssum murale
David H. McNear Jr*, Edward Peltier, Univ of Delaware
ALS Users Meeting Oct-2004

13. Sr distribution in Artic fish ear bones
Ken Severin, Tom Trainor, Univ of Alaska, Fairbanks, Randy Brown, US Fish and Wildlife Service
Artic migratory fish, such as sheefish (stenodus leucicthys), are difficult to track, which complicates fisheries management. For the sheefish, it is not even known how much time is spent in fresh and brackish waters. (Sheefish from the Selawik river in northern Alaska).
One promising approach is to study the composition of otoliths (ear bones) which are records of the water chemistry.
Sr is particularly interesting, as it seems to be strongly correlated with salinity, and can be incorporated directly into the carbonate phases.
Bulk XRD shows a mixture of vaterite and aragonite.
One promising approach is to study the composition of otoliths (ear bones) which are records of the water chemistry.
Sr is particularly interesting, as it seems to be strongly correlated with salinity, and can be incorporated directly into the carbonate phases.
Bulk XRD shows a mixture of vaterite and aragonite.
Thin section of sheefish otolith: the dark inclusion is presumed to be vaterite while the lighter area expected to be aragonite. The otoliths are ~96% CaCO3 (mostly aragonite) and ~4% protein.
Electron microprobe results: strong Sr heterogeneity, consistent with aragonite / vaterite (aragonite can incorporate more Sr).
ALS Users Meeting Oct-2004

14. Sr distribution in Selawik Sheefish otoliths
Ken Severin, Tom Trainor, Univ of Alaska, Fairbanks, Randy Brown, US Fish and Wildlife Service
We start with XRF analysis:
A line scan (a 1-dimensional map) shows a strong variation in Sr concentration (from ~2000ppm down to ~200ppm), across the aragonite (high Sr) to vaterite (low Sr) regions.
There is a strong heterogeneity in Zn (at 10s of ppm level), while Cr, Mn, Fe, Co, and Ni are uniformly distributed across the section.
We then proceed to doing XAFS on selected spots (#1: high Sr and #2: low Sr / high Zn).
ALS Users Meeting Oct-2004

15. Sheefish Sr micro-XAFS
Ken Severin, Tom Trainor, Univ of Alaska, Fairbanks, Randy Brown, US Fish and Wildlife Service
mXAFS (with ~10x10mm spot size) were collected on selected spots:
#1: high Sr
#2: low Sr / high Zn
Also shown is data from Sr in coral aragonite from Allison et al. (earlier GSECARS mXAFS measurements).
Spot #2
Spot #1
EXAFS analysis in progress….
ALS Users Meeting Oct-2004

16. ALS Users Meeting 19-Oct-2004
Sheefish Sr micro-XRD
Ken Severin, Tom Trainor, Univ of Alaska, Fairbanks, Randy Brown, US Fish and Wildlife Service
mXRD (with ~10x10mm spot size) were collected on several spots. Spots #1 and #7 are confirmed to be aragonite and vaterite structures, but there are questions about spot #2 (high Zn).
Spot #2
Spot #7
Spot #1
More work needed, but a good start for 2 days of beam time.
ALS Users Meeting Oct-2004

17. Conclusions, Future Directions and Improvements
The GSECARS microprobe station is running well and productively, combining mXRF, mapping, tomography, mXRF, XANES, and EXAFS for a wide range of problems in geological, soil, and environmental sciences.
Areas for Improvement:
Beam positional stability, especially during EXAFS. This has improved, but needs more work.
Ease of focus to below 2mm and of reliable defocus / refocus to a desired beam size.
Data collection speed and efficiency. We’re generally limited in time by the solid-state fluorescence detectors.
Ease of use for novice users: data collection and on-line analysis software for quantitative XRF, XANES, EXAFS, and XRD.
ALS Users Meeting Oct-2004