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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 300x300m beam to
1x1m - a flux density gain of 105.
With a typical working distance of 100mm, and an
energy-independent focal distance and spot size, they
are ideal for micro-XRF and micro-EXAFS.
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XANES, and EXAFS.
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Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
X-ray Absorption Fine-Structure in Fluorescence Mode
Matt Newville
Consortium for Advanced Radiation Sources
University of Chicago / Advanced Photon Source
EXAFS, XANES, and x-ray fluorescence measurements
Energy and Wavelength Dispersive Detectors for fluorescence
Combined XRF measurements/mapping and XANES/EXAFS
Examples:
EXAFS: coordination and elemental association of Sr in coral
aragonite used for sea temperature determination, using Energy
Dispersive detector.
XANES: speciation of trace levels of Au in FeAsS with a
microfocussed beam and Wavelength Dispersive Spectrometer
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
X-ray Fluorescence Spectroscopy
X-ray Absorption
Incident x-ray is destroyed.
Core-level electron destroyed.
Photo-electron ejected from atom.
X-ray Fluorescence
Higher energy core electron fills
empty electron level, and ejects an
x-ray of fixed energy.
Ka : L electron fills K hole
Kb : M electron fills K hole
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
X-ray Fluorescence Spectroscopy
X-ray Fluorescence: Measure characteristic
x-ray emission lines from de-excitation of
electronic core levels for each atom.
Element Specific: Elements with Z>16 can
be seen (at the APS), and it is usually easy
to distinguish different elements.
Quantitative: precise and accurate
elemental abundances can be made. x-ray
interaction with matter well-understood.
Low Concentration: concentrations down
to a few ppm can be seen.
Natural Samples: samples can be in
solution, liquids, amorphous solids, soils,
aggregrates, plant roots, surfaces, etc.
Small Spot Size: measurements can be
made with spot sizes of a few microns.
Combined with Other Techniques:
XANES, EXAFS, XRD
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
X-ray Absorption Spectroscopy: XANES and EXAFS
X-ray Absorption Spectroscopy: Measure
energy-dependence of the x-ray absorption
coefficient (E) [either log(I0 /I) or (If / I0 )]
of a core-level of a selected element
Element Specific: Elements with Z>20
can have EXAFS measured at the APS.
Valence Probe: XANES gives chemical
state and formal valence of selected
element.
Local Structure Probe: EXAFS gives
atomic species, distance, and number of
near-neighbor atoms around a selected
element..
Low Concentration: concentrations down
to 10 ppm for XANES, 100 ppm for EXAFS.
Natural Samples: samples can be in
solution, liquids, amorphous solids, soils,
aggregrates, plant roots, surfaces, etc.
XANES = X-ray Absorption Near-Edge Spectroscopy
Small Spot Size: XANES and EXAFS
measurements can be made on samples
down to ~5 microns in size.
EXAFS = Extended X-ray Absorption Fine-Structure
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
X-ray Absorption
An atom absorbs and x-ray of energy E, destroying a core electron with energy E0 and
creating a photo-electron with energy (E-E0). The core level is refilled, causing a
fluorescence x-ray or an Auger electron to be ejected from the atom.
The fluorescence probability is proportional to the absorption probability.
The absorption probability
(E) depends on the spatial
overlap of the core-level
and photo-electron wavefunctions. The core-level is
localized, so the overlap is
determined by the photoelectron wave-function at
the absorbing atom.
For an isolated atom, this
is a smooth function of
energy.
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
X-ray Absorption Fine-Structure Spectroscopy
With another atom nearby, the photo-electron can scatter from the neighbor atom.
The interference of the outgoing and scattered waves alters the photo-electron
wave-function at the absorbing atom, modulating (E).
XANES: the near-edge (E)
depends on
the electronic
overlap between central and
bonding atoms:
valence
coordination chemistry.
EXAFS: the oscillations in (E)
depend on:
near-neighbor distance
near-neighbor species
coordination number.
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
The EXAFS Equation
EXAFS: the fine-structure oscillations in (E)
where k is the photo-electron wavenumber:
The EXAFS results from an outgoing photoelectron scattering from a neighbor atom, and
returning to the core atom to interfere with the
core level:
Outgoing photo-electron
Scattering from neighbor atom
(amplitude and phase depend on
Z of neighbor atom)
Returning photo-electron
The EXAFS Equation
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
EXAFS Analysis
Measured EXAFS has a smooth background
removed, and converted to k-space:
Fourier Transformed to R-space
numerically modelled with empirical or
theoretical calculations of f(k) and d(k).
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
XANES
X-ray Absorption Near-Edge Spectroscopy (XANES) is one of the only
techniques that gives a direct measurement of chemical state (valence state)
of an element. In many chemical and environmentally relevant systems, the
valence state is as important as the total concentration of an element.
XANES Analysis is somewhat more qualitative than EXAFS Analysis.
Cr(VI) is highly carcinogenic and
highly mobile in ground water.
Cr(III) is not carcinogenic or very toxic,
and is not mobile in ground water.
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
XRF / XAFS Microprobe Station at APS (Beamline 13)
APS Beamline13-ID-C is a micro-beam x-ray facility for x-ray fluorescence
(XRF) and x-ray absorption spectroscopy (XAS) studies:
Incident Beam:
Monochromatic x-rays
from LN2 cooled Si (111)
Sample Stage: x-y-z stage, 0.1m resolution
Fluorescence detector:
16-element Ge detector
[shown], Si(Li) detector,
Lytle Detector, or
Wavelength Dispersive
o
Spectrometer at 90
to incident beam
Optical Microscope:
(5x to 50x) with
external video system
Data Collection:
Flexible software for
x-y mapping, traditional
XAFS scans, XAFS
scans vs. sample position.
Focusing: Horizontal and Vertical Kirkpatrick-Baez mirrors
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
X-ray Fluorescence Detector: Energy Dispersive
Solid-State Multi-Element Ge Detector for
X-Ray Fluorescence detection
Ge solid-state detectors have energy
resolutions of ~250eV, which separates most
fluorescence lines from different elements.
They allow a full XRF spectrum (or the
windowed signal from several lines) to be
collected in seconds.
Ge detectors are limited in total count rate to
~100KHz, so multiple elements (10 to 30) are
used in parallel to make one large detector.
Detection limits are at the ppm level for XRF.
XANES and EXAFS measurements of dilute
species (~10ppm) can be measured.
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
XRF and XAFS Example: Sr in coral aragonite
Nicola Allison, Adrian Finch (Univ of Brighton, Univ of Hertfordshire, UK)
Motivation:
Sample Preparation:
The abundance of Sr in aragonite
(CaCO3) formed by corals has been
used as an estimate of seawater
temperature and composition at the
time of aragonite formation, and so
used as a paleothermometer.
Sections of coral (P. lobata,
~50years old), were collected in the
South Pacific, and polished into
thin sections (~50m).
Some
sections were C-coated for electron
microprobe measurements.
On a micron-scale (ie, growth-scale),
how constant is Sr/Ca ratio? How is
the Sr incorporated in the coral?
XRF Mapping Measurements:
With an incident x-ray beam (~5m in size) at 16.2keV (just above the Sr K-edge),
Sr Ka and Ca Ka fluorescence (and other trace elements) were measured
simultaneously at with a multi-element Ge solid-state detector. The sample was
rastered (5m step size), and the XRF spectra was collected at each pixel over a
200m X 300m area.
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
XRF Maps for Sr in coral aragonite
Nicola Allison, Adrian Finch (Univ of Brighton, Univ of Hertfordshire, UK)
XRF Maps:
C
a
These false-color XRF maps of Sr and Ca
concentration in the coral
show an
incomplete correlation between Sr and Ca.
Sr
200m
The Sr/Ca ratio varies substantially on this
small length scale, although this section of
aragonite must have been formed at fairly
constant temperature (within several days).
The Sr concentration at the “hot spots” are
well above the solubility limit of Sr in
aragonite: Is the Sr supersaturated in
CaCO3 or precipitated out into SrCO3?
300m
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
Fluorescence XAFS for Sr in coral aragonite
Since the Sr concentration was above
its solubility limit (~1%) in aragonite, it
was not known if Sr would precipitate
out into strontianite (SrCO3: a
structural analog of aragonite), or
remain in the aragonite phase.
Here are 2 XAFS scans from spots of
relatively high and relatively low Sr
concentration.
The two signals are very similar,
strongly suggesting that the two areas
have
Sr
in
the
same
local
coordination.
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
Fluorescence XAFS for Sr in coral aragonite
First and Second shell EXAFS are nearly
the same for both strontianite (SrCO3) and
Sr in aragonite:
9 Sr-O at ~2.59A +/- 0.02A,
6 Sr-C at ~2.98A +/- 0.02A.
Third shell EXAFS shows a strong
preference for Sr-Ca over Sr-Sr, at ~4.0A.
A simple comparison to SrCO3 data and a
theoretical simulation (and fit) of the
EXAFS spectra for Sr substituted into the
aragonite structure.
The coral is able to trap Sr in aragonite
at a super-saturated concentration, as it
forms the thermodynamically less
stable aragonite (compared to the
calcite form of SrCO3 data).
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
Cu speciation in Hydrothermal Fluid Inclusions
John Mavrogenes, Andrew Berry (Australian National University)
Understanding the metal complexes trapped in
hydrothermal solutions in minerals is key to
understanding the formation of ore deposits.
Cu 25oC
Fe 25oC
Cu 495oC
Fe 495oC
XRF and XAFS are important tools for
studying the chemical speciation and form of
these fluid inclusions.
Natural Cu and Fe-rich brine fluid inclusions in
quartz from Cu ore deposits were examined at
room temperature and elevated temperatures
by XRF mapping and EXAFS.
Initial Expectation: chalcopyrite (CuFeS2) would
be precipitated out of solution at low
temperature, and would dissolve into solution
at high temperature. We would study the
dissolved solution at temperature.
XRF mapping showed that the initial expectation was wrong, and
that a uniform solution at room temperature was becoming less
uniform at temperature. This was reversible.
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
Cu speciation in Hydrothermal Fluid Inclusions
John Mavrogenes, Andrew Berry (Australian National University)
XAFS measurements at low and high temperature
were very different. The large change in the XANES
and edge position indicates a change in speciation.
Low temp: Cu2+
O
2.35Å
Cu2+
High temp: Cu1+
O
Cl
2.09
CuÅ1+
1.96Å
Low temp (?)
High temp (?)
Preliminary fits to the EXAFS of the high
temperature phase (below) is also consistent
with Fulton et al: Cu1+ with Cl (or S) at 2.09Å,
and possibly some O at 1.96Å.
These results are consistent with Fulton et al [Chem Phys
Lett. 330, p300 (2000)] study of Cu solutions near critical
conditions: Cu2+ solution at low temperature, and Cu1+
associated with Cl at high temperatures.
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
Oxidation state maps: Mn redox at plant roots
D. Schulze (Purdue University)
Manganese is an essential
nutrient for plants, needed for
photosynthesis and response
to stress and pathogens.
Reduced Mn2+ is soluble and
bio-available in soils but Mn4+
will precipitate (along with
Mn3+) as insoluble Mn oxides.
The redox chemistry of Mn in
soil is complex, with both
reduction
and
oxidation
catalyzed by microorganisms.
Collecting Mn fluorescence
with the incident be at a few
well-chosen energies around
the Mn K-edge, we make 3-d
(X-Y-Energy) maps that give
the spatial distribution of Mn
oxidation states.
Spatially-resolved -XANES is
well-suited for mapping Mn
oxidation state in live plant
rhizospheres to understand
the role of Mn redox reactions
in a plant’s ability to uptake
trace elements.
XRF image of total Mn (left) of soil traversed by a sunflower root
(dashed line) showing heterogeneous Mn and enrichment near the
root. The Mn oxidation state map (right) shows both Mn2+ and Mn4+
in the Mn-rich sites, with reduction near the root.
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
Overlapping X-ray Fluorescence Lines
Fluorescence lines from different atoms can overlap, making XRF and EXAFS
measurements difficult. The resolution of a solid-state fluorescence detector
(~150eV) is sometimes not good enough to resolve nearby fluorescence lines.
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
High Resolution X-ray Fluorescence and EXAFS
The Wavelength Dispersive Spectrometer has much better resolution (~20eV) than
a solid-state detector, and a much smaller solid angle. It uses a Rowland circle, not
electronics, to select energies of interest.
This really needs the brightness of an undulator, but complements the Ge detectors,
and allows XRF and even EXAFS on systems with overlapping fluorescence lines.
A typical XRF spectra with the WDS
spectrometer is able to resolve
fluorescence lines that would be
impossible to separate with a solid-state
detector.
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
Sector Zoning of Rare Earth Elements in Apatites
John Rakovan (Miami University)
.
Apatites have a high affinity for Rare Earth Elements (REE), and are
often used to study petrogenesis. Heterogeneities in crystal surface
structure during apatite growth can strongly alter REE incorporation.
011 vicinal
face
011 vicinal
face
Most REE show sectoral zoning in apatite based on ionic size. Ions
larger than Ca2+ (La3+) preferring growth along the 001 face, and
those smaller than Ca2+ (Sm3+) preferring the 011 face
Subsector
boundry
011 subsector
001 subsector
001 vicinal
face
3000
520.000000
2500
470.000000
420.000000
2000
370.000000
1500
320.000000
270.000000
78.35
78.15
77.95
77.75
77.55
Linescan distance (mm)
1000
77.35
La Concentration (ppm)
Sm Concentration (ppm)
570.000000
Eu is the only REE showing no zonation,
but it has two valence states and two ionic
sizes that straddle the size of Ca2+.
Is there a partitioning of Eu
valence state/ionic size?
Denver X-ray Conference | X-Ray Absorption Spectroscopy
based on
2001-Jul-30
Sector Zoning of Rare Earth Elements in Apatites
Since Eu has two valence states with different
ionic sizes (Eu2+ / 1.2 Å, Eu3+ / 1.3 Å), it was
suggested that there may be a valence/ionic size
variation in different growth zones.
.
X-ray counts
The bad news: There is far too much Mn in the
apatite to separate from the Eu fluorescence line
with a solid state detector.
Using the high resolution WDS and the
microprobe, we measured the Eu XANES
on several spots in the different sectors,
and across a <011> / <001> boundary.
Result: We see almost no change at all in
Eu2+ / Eu3+ across the zone boundary: the
ratio is ~17% Eu2+ throughout the apatite.
Energy (keV)
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
Using the WDS for XANES: 1000ppm Au in FeAsS
Louis Cabri (NRC Canada), Robert Gordon, Daryl Crozier (Simon Fraser), PNC-CAT
1000ppm Au in FeAsS (arsenopyrite):
The
understanding of the chemical and physical state of
Au in arsenopyrite ore deposits is complicated by the
proximity of the Au LIII and As K edges and their
fluorescence lines.
At the Au LIII-edge, As will also be excited, and
fluoresce near the Au La line. Even using the WDS,
the tail of the As Ka line persists down to the Au La
line, and is still comparable to it in intensity.
250x250m image
of the Au La line in
arsenopyrite with a
6x6m beam, 5m
steps and a 2 sec
dwell time at each
point. The
x-ray
energy was 12KeV.
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
Using the WDS for XANES: 1000ppm Au in FeAsS
Louis Cabri (NRC Canada), Robert Gordon, Daryl Crozier (Simon Fraser, PNC-CAT)
CANADIAN MINERALOGIST 38, pp1265-1281 (2000)
The tail of the As Ka line is still
strong at the Au La energy, so
using a Ge detector gave the Au
LIII edge-step as about the same
size as the As K edge-step, and
the Au XANES was mixed with
the As EXAFS.
With the WDS, the As edge was
visible, but much smaller, and so
the Au XANES was clearer.
As K-edge
As Ka line
11.868 KeV
10.543 KeV
Au LIII-edge
Au La line
11.918 KeV
9.711 KeV
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
Using the WDS for XANES: 1000ppm Au in FeAsS
Louis Cabri (NRC Canada), Robert Gordon, Daryl Crozier (Simon Fraser, PNC-CAT)
With a 13-element Ge detector (at PNCCAT: ID-20), the tail of the As Ka line was
still strong at the Au La energy, so the Au
LIII edge-step was about the same size
as the As K edge-step, and the Au
XANES was mixed with the As EXAFS.
With the WDS, the As edge was visible,
but much smaller, and the Au XANES
was much clearer.
Measuring
two
different
natural
samples of FeAsS, both with
~1000ppm of Au, we see evidence for
both metallic and oxidized Au.
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30
Fluorescence EXAFS and XANES
EXAFS and XANES are powerful tools for a wide range of
scientific problems
EXAFS: local coordination
XANES: valence state
EXAFS and XANES measurements made in fluorescence
mode can be combined with traditional x-ray fluorescence
measurements for quantitative determination of minor and
trace concentrations of heavy elements.
There are few restrictions on sample preparation or elements
that can be probed.
Denver X-ray Conference | X-Ray Absorption Spectroscopy
2001-Jul-30