Electron%20Probe%20Microanalysis%20EPMA

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Electron Probe MicroanalysisEPMA
Revised 10-18-10
UW- Madison Geology 777

• WDS II
• Spectrometers and Spectra

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Key points
UW- Madison Geology 777

• X-rays are dispersed by a crystal with only one
  wavelength (nl) reflected (diffracted), with
  that one wavelength (nl) passed to the
  detector.This is a monochrometer. Recall that n
  can be gt1, so other elements can cause
  interference if their wavelengths are at an
  integral fraction of the desired wavelength.
• The detector is a gas-filled (sealed or
  flow-through) tube where gas is ionized by
  X-rays, yielding a massive multiplication factor
  (proportional counter)
• X-ray focusing assumes geometry known as the
  Rowland Circle
• Key features of WDS are high spectral resolution
  and low detection limits

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Spectrometers- Orientation
UW- Madison Geology 777
The typical electron microprobe has
vertically mounted spectrometers (a), i.e. the
Rowland circle is vertical. This orientation
permits up to 5 spectrometers to be mounted
around the column, as well as having room for an
EDS detector. In this orientation, X-ray
intensities are susceptible to small differences
in the Z position of the sample ( X-ray
defocusing, not good). We use polished planar
samples, and focus the sample Z with reflected
light.
There are some applications (e.g., industry)
where the probe is utilized in production, and
samples are rough surfaces. For this specialized
situation, the spectrometer can be inclined (b),
which then permits X-ray diffraction from a range
of heights, all now on the inclined Rowland
circle. However, only 2 inclined spectrometers
will then fit around the column.
Reed, 1993, Fig. 6.9, p. 70.
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Focussing Geometry
UW- Madison Geology 777

• The point source of X-rays in the electron
  microprobe is not optimally diffracted by a flat
  crystal, where only a small region is in focus
  for the one wavelength of interest.
• X-ray intensities are improved by adding
  curvature to the crystals
• Johann geometry the crystal is bent/curved to a
  radius 2r (diameter of Rowland circle)
• Johansson geometry the crystal is curved to 2r,
  and the surface is ground to r. This is a
  difficult process, so most crystals have Johann
  geometry.

Reed, 1993, p. 67.
According to users on the sx50 listserve in 2003,
both Cameca and JEOL probes use Johann crystals
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Temperature Dependence-1
UW- Madison Geology 777

• We attempt to keep a constant temperature in the
  probe lab, around 68F (20C). This is also the
  temperature of the chilled water that circulates
  both through the electronic cabinet as well as
  the column (and outer jacket of diffusion pump).
• Several aspects of the instrument are sensitive
  to temperature changes the spectrometer
  crystals, the detector P10 gas.
• P10 gas pressure must be constant, as this is
  critical to having reproducibility in counting
  rates between the standards and the unknowns,
  both over short time spans as well as longer
  (e.g. 24 hour) durations. (Loss of air
  conditioning on very hot summer days precludes
  probing.)

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Temperature Dependence-2
UW- Madison Geology 777

• Spectrometer crystals have a linear expansion
  coefficient, and expansion will change the 2d.
  The effect upon PET is 4x worse than upon LIF,
  and increases rapidly with increasing sin q.
• For LIF 200, the linear expansion coefficient is
  3.4 x 10-5. At q 45 (sin q .707107), one
  degree of temperature change causes the sin q to
  change to .707131, about 2.4 sin q units, not a
  lot -- but for say 5 C that is 12 sin q units--a
  major shift.
• For PET the effect is more serious, as the
  coefficient is 1.2 x 10 -4.

Typo corrected 9/29/08
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Temperature Dependence-3
UW- Madison Geology 777
There is also a potential thermal effect, in that
the motors on the stage and associated
electronics put out heat, and can cause the stage
height to slowly drift upward over hours. So if
points are stored when the stage is cold, the Z
will could be 10-15 microns out of focus 8 hours
later -- thus autofocus is recommended for
overnight automated runs. As mentioned in the
first slide, cooling water is constantly
circulating. If there is a failure of either the
second floor red water heat exchanger, or of
the master heat exchanger in the sub-basement,
the chilled water no longer is chilled and
problems can develop. The probes electronic
cabinet also needs to be kept cool and there are
3 squirrel cage fans that draw hot air past the
chilled water tubing. If one or more fails, one
or more circuit boards overheat and problems
develop with operation of the probe.
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Temperature Dependence-4
UW- Madison Geology 777
This figure shows how PET (top curve) is much
more sensitive to temperature changes than
LIF200. No curve for TAP is shown, but it perhaps
is similar to ADP. Si Ka on PET is at a sinq of
.8, which is an angle of 54 so 2q is 108. Si Ka
on TAP is at a sinq of .277 or an angle of 16 so
2q is 32. This is the reason that it is
generally preferable to use TAP for Si Ka
measurement in EPMA.
Temperature sensitivities of crystal analyzers
correlations between change in 2q value/C. (X
axis is in units of 2q, commonly used in XRF Y
axis is the peak shift due to thermal expansion
of the crystal) From Jenkins and DeVries, 1967,
Fig 2.4 (p. 38)
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Spectral Resolution
UW- Madison Geology 777
WDS provides roughly an order of magnitude higher
spectral resolution (sharper peaks) compared with
EDS. Plotted here are resolutions of the 3
commonly used crystals, with the x-axis being the
characteristic energy of detectable
elements. Note that for elements that are
detectable by two spectrometers (e.g., Y La by
TAP and PET, V Ka by PET and LIF), one of
the two crystals will have superior resolution.
When there is an interfering peak and you want to
try to minimize it, this knowledge comes in very
handy.
Reed, 1995, Fig 13.11, in Williams, Goldstein and
Newbury (Fiori volume)
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Spectrometer Efficiency
UW- Madison Geology 777
The intensity of a WDS spectrometer is a function
of the solid angle subtended by the crystal,
reflection efficiency, and detector efficiency.
Reed (right) compared empirically the
efficiency of various crystals vs EDS. However,
the curves represent generation efficiency
(recall overvoltage) and detection efficiency.
Reed suggests that the WDS spectrometer has 10
the collection efficiency relative to the EDS
detector. How to explain the curvature of each
crystals intensity function? At high Z, the
overvoltage is presumably minimized (assuming
Reed is using 15 or 20 keV). Low Z equates larger
wavelength, and thus higher sinq, and thus the
crystal is further away from the sample, with a
smaller solid angle.
Reed, 1996, Fig 4.19, p. 63
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Spectrum Characteristic continuum
UW- Madison Geology 777
Recall that the X-rays generated by electron
bombardment are both characteristic of each
element present in the specimen, as well as the
broadband continuum. And recall that the
background level increases with increasing mean
atomic number of the specimen. In EPMA, several
steps must occur. First, the peak position must
be precisely found.
Low bkg Lower side
Then, trustworthy background positions must be
found in order to model the background level at
the peak position. Above, the Mg ka peak
position is at the white (center) line and we
need to determine where adjacent to it would be
the best places to measure the background level,
in order to calculate (model) the continuum level
under the peak..
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Probe for EPMA Peak and Background displays
UW- Madison Geology 777
Our software provides handy ways to store
wavescans. The standard procedure is to peak the
elements of interest and use either default
background positions or ones previously
chosen. The displayed wavescans have a central
line that is white. This is the peak of interest
(here, Mg Ka). Background positions are shown by
yellow lines. If a
background position has been changed, the old
position is shown in magenta. The background
model is a line, here red (normally yellow).
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Background model
UW- Madison Geology 777
Determining the background offset positions is
important, as other peaks may accidentally fall
where you originally plan to take backgrounds.
Above, the low default background (purple) was
found upon examination to be near a peak, and was
moved further to the left. There are many
background models that can be used above, the
background (red) is linear extrapolation from 2
points on the spectrum (yellow locations).
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Curved background-1
UW- Madison Geology 777
There are some cases where the background has a
curvature, particularly at low sin theta A
linear model (below) results in too high
calculated background.
.
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Curved background-2
An exponential curved background, however,
provides a better result.
Note the presence of several other background
models in the right box.
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Congested backgrounds
UW- Madison Geology 777
In some (many) cases, adjacent peaks can
interfere with either high or low background
position, requiring same side backgrounds here,
average of 2 measurements on the high side.
Highest peak is 3rd order Ca Ka.
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Background Offsets
How far away from the peak should the background
offsets be? Reed demonstrates mathematically
(adjacent figure) that a small overlap on the
tails of the peak ends up making no significant
difference.
However, he warns (and it is my experience) that
being too close to the peak is not good practice,
and should only be done in extreme cases where it
is impossible to find a free area in the
adjacent background.
Reed, 1993, p. 150
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Backgrounds and Absorption Edges-the EDS
perspective
UW- Madison Geology 777
This is an EDS view of the absorption edge. To
translate to WDS (wavelength or sinq), just
reverse the axes.
The background level to the higher energy side of
the edge will be depressed because these
continuum X-rays have energies great enough to be
used up causing secondary fluorescence of the
element in question, and thus produce a
misleadingly low background.
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Backgrounds and Absorption Edges - the WDS
perspective
UW- Madison Geology 777
You need to be aware of potential for error if
you position your low background too far to the
left (low sinq, short wavelength), below the
absorption edge of the element you are measuring.
For the K edges, this is at the Kb position.
THIS ALL IS IN THE SAMPLE!!
Fig 12.9. Step in continuum (CaSiO3) caused by Ca
K absorption edge (dashed horizontal line
represents continuum under peaks).
The background level to the low sinq side of the
edge will be depressed because these continuum
X-rays have energies great enough to be used up
causing secondary fluorescence of the element in
question, and thus produce a misleadingly low
background. If a low background position between
the peak and absorption edge cant be taken, then
only measurements on the high side should occur.
These considerations do not apply to small peaks
for which the associated absorption step is
negligible.Reed, 1993, p. 151
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Ar Absorption Edge
UW- Madison Geology 777
Analysts need to understand the possible
implications of the Ar absorption edge (recall
Ar is in the detector P10 gas). X-rays with
energy lower than the argon K edge (3.202 keV)
will produce a certain amount of ionization of
the Argon. However, for X-rays with energies gt
3.202 keV, approximately twice as many will
interact with the gas and be detected because now
additional ionization occurs.
From Paul Carpenters talk at April 2002
NIST-MAS EPMA workshop
The Ar Edge results from GAS!! Not sample

• Lines of the same family that fall on either
  side will have abnormal proportionsnormally
  the La gt Lb, but for Cd, it is reversed due to
  this.
• Trace element studies utilizing the U Ma line
  must utilize only background offsets on the high
  sinq side of the peak (see figure above).

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Holes in the Background-1
UW- Madison Geology 777
In 1987, Bruce Robinson (Australia)
reported on a phenomenon uncovered during
examination of arsenopyrite for trace amounts of
gold. After looking at unknowns, he checked a
reference arsenopyrite that should have had no Au
in it but it showed 100 ppm. High resolution
scan of the background near the Au La peak on the
LIF crystal showed a distinct drop (trough) by
10 relative to the adjacent background. With an
incorrect (artificially low) background, zero
Au had become 100 ppm Au.
From Remond et al, 2002, NIST Journal of Research
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Holes in the Background-2
UW- Madison Geology 777
These holes or negative peaks are caused by
reflections of the continuum X-rays from planes
in the crystal other than the correct plane (the
200 in LIF). These X-rays do not reach the
counter. The point is that when you are looking
for very low detection levels, it pays to pay
very close attention to the shape of the
background and to try to understand it well. And
to have some well characterized secondary
standards to evaluate your procedure with.
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On Peak Interferences
UW- Madison Geology 777
Users must be vigilant for interferences upon the
peaks being measured, both in the unknowns and in
the standards. The figure to the right shows an
interference type seen where peak B overlaps peak
A.
For elements whose wavelengths can be diffracted
by a choice of 2 crystals, one will be better for
spectral resolution (the lower 2d, with higher
sin theta position).
Reed, 1993, p. 153
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On Peak Interference-Slight
UW- Madison Geology 777
REE analysis is particularly tricky, since there
are so many elements present and there are so
many L lines.Here the high side tail of Ce Lb1
interferes with Nd La.
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Interference Correction
UW- Madison Geology 777
We must correct for interferences, by software
options, if we have standards that have the
interfering element (here Ce) but none of the
interfered with element (here Nd).
During count acquisition (calibration or
standardization) on the Ce standard (356), we
also acquire interference counts at the Nd La
peak position. Then during analysis of unknowns,
an appropriate correction is made for the overlap.
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Peak Centering-1
UW- Madison Geology 777
Quantitative analysis requires knowledge of the
peak intensity, which is a function of the
composition of the specimen.We could also use the
integrated area of the peak, but that is
time-consuming. The peak position is first
determined upon either the specimen or the
standard, with the general assumption that the
position is the same (with important exceptions!).
Then that position (and user-determined
background offsets) are utilized in the automated
movement of the spectrometers during analysis.
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Peak Centering-2
UW- Madison Geology 777
There are several methods possible to find the
peak center. In times past, manual searching was
done (aided by an audio device whose pitch
increased as counts increased). Today we have
automated peaking routines.
We have developed a routine of (1) using one of
the top 4 methods (usually the fast ROM) to get
pretty close to the peak, and then (2) a
post-scan across the top of the peak that
allows the operator the final decision of picking
the optimal peak position. (This option was added
based upon detailed peak scans done by 777
students in the Fall of 2002.)
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Peak Centering-3
UW- Madison Geology 777
It is very important to start with the best
peak position, which means the center of the peak
(usually the highest counts though not
necessarily). At the right are two post-scans
where the red vertical line is the position
selected by the ROM automationvery good for the
Si Ka position, but about 7 units too high for Al
Ka. The post-scan option gives the operator the
freedom to over-ride the computers best guess
with an intelligent decision. Best operating
practice calls for picking the center of the peak
centroid, because there may be slight offsets
developed over a probe session due to mechanical
drift and there is also the question of real peak
shifts by starting in the dead center, these
variables should be minimized.
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Results of Automated Al Ka Peaking Options
ROM very reproducible 10 measurements, s.d. of
1.2, range 32374-8
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X-ray Peaks Poissonian
UW- Madison Geology 777
X-ray peaks follow a Poisson distribution, which
describes the counting of events that occur at
random but at a definite average rate. It can be
shown that for a Poisson distribution, the
standard deviation is the square root of the
counts.
In our case, the above figure could make more
sense if we said the following On x axis is
spectrometer reading, y axis is x-ray counts.
What it shows is that for a peak whose true
value is 9 bla units, there is a distribution
around that value, that looks for all the world
like a Gaussian distribution.
The Poisson distribution is similar to, but
different from the Gauss distribution. The Gauss
distribution is bell shaped and symmetrical about
its mean value, while the Poisson distribution
has neither of these properties in general.
John R. Taylor, An Introduction to Error
Analysis, 2nd Ed., 1997. p. 245-256
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Satellites
UW- Madison Geology 777
In the X-ray tables, you see lines with an S
in front, e.g SKa3. These are called
non-diagram lines because they are not caused
by the usual process where the normal target
atom loses one electron. Instead they come from a
state where the atom is doubly ionized, by
whatever process (e.g. Auger electron production).
Because there are less electrons present
orbiting the nucleus, there is less screening
of the positive charge of the nucleus, so the
remaining electrons have a slightly higher
binding energy. So a characteristic X-ray
produced by knocking one of them out will have a
slightly higher energy, or lower wavelength, than
a normal X-ray of that atom.
Goldstein et al, 1992, Fig 6.16, p. 363
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Peak Shifts-1
UW- Madison Geology 777
Al Ka Peaks
Shifts in peak shape of certain elements can
occur due to difference in chemical bonding,
between different samples/standards. Some well
understand examples are Al Ka (between metal,
and oxides and silicates), and S ka (between
sulfides and sulfates) as well as P ka, and the
light element K lines. Articles have been
published that show evidence of peak shifts of Si
and Al kb as well as Fe La/Lb in relation to
valence/bonding.
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Peak Shifts-2
UW- Madison Geology 777
These shifts can be understood if you understand
that a characteristic peak is actually a
cumulative sum of several discrete peaks which
the spectrometer may not resolve, though many
times one can discern humps within each peak.
Figure from Remond et al, 2002, NIST Journal of
Research
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Al Ka Peak Shifts
UW- Madison Geology 777
7 units or 0.6 eV
Al metal
TAP sp1
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Peak Shifts-4
UW- Madison Geology 777
If unrecognized, shifts can affect the X-ray
intensity measured at the defined peak center
position, producing errors.
This is predominately an effect on the light
elements (B, C, N, O, F) where the valence
electrons are involved in X-ray production. We
will discuss this further in the section on
Light element analysis, referring to (1)
integration under the peak, and (2) Area Peak
Factors.
Excellent article on this subject is Remond et
al, from the NIST 2202 Journal. A pdf file is on
the Geol 777 web site.
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Identifying Lines in Spectra
UW- Madison Geology 777
In normal WDS quantitataive analysis, the
analyst is concerned with a pre-determined set of
elements defined by previous knowledge. You focus
only upon these elements.
However, there are times when you must identify
unknown elements. EDS is a quick way to search
for elements however, there are limitations with
EDS, and you may then turn to WDS scans. The
easiest is running through a small list of
elements (looking for a peak for each), but if
this fails, you must do full wave scans on all
the spectrometers, and manually identify the
peaks (using the X-ray database in the software
to assist). There is an optimal order for doing
this identification of peaks.
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Guidelines for WDS peak ID-1
UW- Madison Geology 777
1. Always start with the shortest wavelengths,
where you have the highest chance of finding
first order (n1) peaks. These will be on the
crystal with the lowest 2d, the LIF crystal.
Spectra on the other crystals may have higher
order reflections of elements present here. 2.
Identify the highest intensity peak at the low
wavelength end of LIF. It should be a Ka or La
peak. If you find such a peak, you immediately
assume that you will find a related family of
lines, such as Kb or Lb1, Lb2 , Lb3 , Lb4 etc
you immediately look for them on all the
crystals. The rough ratio of Ka to Kb is 101,
and La to Lb1 is 21.You can also use this
relationship to exclude some lines e.g. a peak
at 1.176Å could be either As Ka or Pb La.
Inspection shows there is no As Kb, but there are
other Pb L lines. Thus, the peak is due to Pb.
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Guidelines for WDS peak ID-2
UW- Madison Geology 777
3. If you find a strong K line, you will probably
also find some L lines of that element. And if
you find a strong L line, you could also find
some M lines.They may be on a different crystal.
4. Once all first order elements found initially
from of the LIF spectrum have been identified and
all family member peaks on all spectra checked
off, then all possible higher order peaks need to
be accounted for. 5. Only now should you proceed
to the next unidentified high intensity, low
wavelength peak, repeating steps 1-4. Then
continue repeating until all peaks are identified.
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Virtual WDS
UW- Madison Geology 777
Virtual WDS is a very useful program that
utilizes stored EPMA spectra, to model potential
interferences offline, prior to a probe session.
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Virtual WDS - Ti on V interference
UW- Madison Geology 777
Here we can model the overlap of Ti kb upon V ka
on either PET (left) or LIF (right), visualizing
the individual peaks (top) and a realistic
combined peak (bottom). Clearly, the LIF crystal
should be used.