GSECARS X-ray Microprobe for Earth and Environmental Sciences

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Transcript GSECARS X-ray Microprobe for Earth and Environmental Sciences

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
19-Oct-2004
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: Rhcoated Si, typically using 3x3mm spot
sizes, at 50mm from end of mirrors.
Entrance Slits: typically
250mm, accepting ~30%
of undulator beam
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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.
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19-Oct-2004
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.
As
Fe
Zn
Cu
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 oxyhydroxides 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?
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
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X-ray Fluorescence Tomography: Primer
Phosphor
CCD
camera
visible
light
Transmission detector
fluoresced x-rays
fluorescence
detector
broad
x-ray
beam
Sample
w
rotation
stage
focused
x-ray
beam
Sample
w
x
rotation and
translation
stages
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.
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.
• can collect multiple fluorescence lines.
• data collection is relatively slow –
one slice can be made at a time.
• can be complicated by self-absorption.
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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
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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
x
ALS Users Meeting
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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.
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.
As
Fe
Zn
Cu
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).
ALS Users Meeting
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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
19-Oct-2004
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
EAs3+ As3+ concentration
would show spatial dependence of
the As oxidation state.
ALS Users Meeting
19-Oct-2004
As oxidation tomograms for Cattail Roots
The As3+ / As
heterogeneous
areas).
As5+
correlated with
(Fe, Cu, Zn).
ratio is
(boxed
appears
metals
N. Keon, et al., Environ Sci & Tech,
Oct 2004 (on line already….).
As3+
There does not seem to
be a systematic spatial
As oxidation gradient.
As3+
As3+
total
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
19-Oct-2004
Ni in hyperaccumulating Alyssum murale
David H. McNear Jr*, Edward Peltier, Univ of Delaware
Ni
Zn
Fe
Mn
Leaf cross-section
Ni
high
Zn
Fe
Stem cross-section
low
Ni
Zn
Ca
Fe
Root cross-section
ALS Users Meeting
19-Oct-2004
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.
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
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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).
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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
EXAFS analysis in progress….
Spot #1
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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
More work needed, but a good start
for 2 days of beam time.
Spot #7
Spot #1
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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
19-Oct-2004