Electron probe
microanalysis
EPMA
Difficult/Tricky
Materials for EPMA
What’s the point?
Some materials are somewhat to extremely
difficult to do EPMA on.
Guidance and passed on experience is very
helpful
And: How do you know something is tricky?
Topics
• Thin films and particles (discussed separately)
• Alkali-rich glasses
• Hydrous alkali-rich glasses
• Carbonates
• Hydrous phases (clays, micas, hydroxides, zeolites…)
• Apatite and similar non-isometric materials
• Porous materials
• Magnetic materials
• “Hot” (radioactive) materials
• “Hard to handle” (matrix correction) element pairs
What makes it difficult?
• Counts change over time
– Due to heating
– Due to charging
– Due to oxidation
• Counts change depending on crystallographic orientation
• Difficult to determine the element abundance
• Impossible to detect the element
Drop in Na Ka with counting time: weathered Ethiopian volcanic glass
The prior slide shows an example of “mobility” of some elements in
insulating samples, in response to high energy electron
bombardment in the electron probe. Alkali elements in glasses are
particularly mobile. Jack Lineweaver at Corning in 1963, in
studying the degradation on the inside surfaces of TV tubes,
collected oxygen released due to the electron bombardment, seeing
that it changed with time.
J. Lineweaver (1963) Oxygen outgassing caused by electorn bombardment of
glass, J. Applied Physics, 34, 1786-1791.
Lineweaver hypothesized
that the 10 to 28 keV
electrons travel thru the
Al coating on the glass
surface and are implanted
into the glass at some
depth, which then creates
an electrical field which
the Na atoms migrate
toward.
Non-bridging oxygen atoms then are freed to migrate to vacated
positions nearer the surface and/or lose their electrons to the Al
“electrode” and be released into the vacuum
J. Lineweaver (1963) Oxygen outgassing caused by electorn bombardment of
glass, J. Applied Physics, 34, 1786-1791.
Hydrous alkali glasses: worst case scenario
Morgan and London examined
hydrous aluminosilicate glasses,
which many volcanologists study,
as “melt inclusions” trapped in
minerals.
Na
Here, the dramatic drop in Na Ka
counts versus time is shown. It is
worst at 20 nA, and less at 2 nA
(though still creates errors).
Potassium also drops, though not
as dramatically.
G. Morgan and D. London, 1996, Optimizing the electron microprobe analysis of hydrous
alkali aluminosilicate glasses, American Mineralogist, 81, 1176-1185.
Al
The related phenomenon of
“grow in” is shown to the right,
with Al Ka showing the increase
in counts with time more than Si
Ka.
Presumably, as the alkali atoms
migrate deeper into the material
toward implanted electrons, Si
and Al atoms either migrate
upward -- or maybe there just are
just less “other” atoms for the
beam to interact with.
Si
G. Morgan and D. London, 1996, Optimizing the electron microprobe analysis of hydrous
alkali aluminosilicate glasses, American Mineralogist, 81, 1176-1185.
So how do you deal with this???
There are several approaches:
• Defocus the beam (reduce the
impact) and do no correction
(but you can’t do this with
small melt inclusions)
Na
• Use a low current (reduce the
impact) and do no correction
(statistics will be poor)
• Use a focused beam and high
current and do a time
• In a later paper*, Morgan and London
dependent correction (but you further consider this; one conclusion being
have to have the software).
that 20 kV is better than 15 kV.
* G. Morgan and D. London, 2005, Effect of current density on the electron microprobe
analysis of alkali aluminosilicate glasses, American Mineralogist, 90, 1131-1138.
OK, but how to quantify the Water (in the Glass)?
There are several ways to go: don’t use EPMA, use FTIR or
SIMS. But if you want EPMA, there are two options:
• “Water by difference” where you assume the difference
from 100 wt% is water + other volatiles (CO2, F, Cl and any
other unanalyzed elements). This is not very precise and
embodies all errors.
• Measure oxygen, as suggested by Nash. However, this
requires careful attention to carbon coating (must have same
thickness as standards) plus problems with 2nd order Na Ka
overlap on the O Ka peak.
* B. Nash, 1992, Analysis of oxygen with the electron microprobe: applications to hydrated
glass and minerals, American Mineralogist, 77, 453-457.
Sample Orientation Intensity Variations: F in Apatite
Stormer et al. also
showed that Ca Ka
intensities in apatite
also vary with time
if the electron beam
was perpendicular
to the c axis,
though in a
somewhat more
complicated way.
J. Stormer, M. Pierson and R. Tacker, 1993, Variation of F and Cl X-ray intensity due to anisotropic
diffusion in apatite during electron microprobe analysis; American Mineralogist, 78, 641-648.
Carbonates - Difficulties
EPMA of carbonates have always been difficult:
• since C is not measured (nor O), there always is a very low
total and so any errors are not immediately obvious. However,
Probe for EPMA software does away with that issue.
• beam currents of 10-20 nA will immediately cause apparent
changes to the surface of calcite (in particular), e.g. black
irregular area; whether this is a hole or build up of
contamination, is not clear to me (I suspect the former)
• I have always “gotten by” with using very low beam
currents (1 nA) and a defocussed (10-20 um) beam which
seemed to give satisfactory results for calcite
• However, I have always wondered “why”…
Carbonates - Difficulties
But with the
TDI ability
with Probe
for EPMA,
I have
started to
investigate
this -- here
15 kV with
20 nA
…hmmm….not at all what I expected, which would have been
dropping Ca…but it is increasing…and the O has a periodic
behavior, dropping over the first 20 seconds then oscillating a
bit…
Carbonates - Difficulties
Dolomite seems somewhat similar, though less extreme at first
glance, both cations Ca and Mg increase (though some of the up and
down behavior), and O drops as before then rises … so similar in
general.
Obviously only by using a very similar standard to the unknown,
would you have any chance of getting a decent EPMA result.
• Beam sensitive samples: require care, such as lower current
(e.g. 1-6 nA) and defocused beam (10-25 mm), or a correction
for count drop (“TDI” correction in our Probe for EPMA
software): In addition to what was discussed previously,
also:
• Alkali feldspars, particularly albite: Na counts drop
• Apatite: not as fragile, but some grains will crater with
moderate currents (60 nA) after 40-60 seconds.
• Oxidation of iron in basaltic glasses: Fialin et al
(2001) reported that high electron dose (130 nA, less
than 30 um diameter) led to oxidation. This was in
reference to a study of Fe L/L as indicator of Feoxidation state. Presumably this is along the lines of the
studies referred to before, where oxygen migrates to the
surface of the glass.
Fialin et al, 2001, Fe3+/Fe vs Fe L peak energy for minerals and
glasses: recent advances with the electron microprobe, American
Mineralogist, 86, 456-465.
• Beam deflection: magnetic specimens (e.g. some Ni-Mn
compounds) apparently deflect the electron beam, as seen by
contamination spots in the optical image offset from
‘normal’ incident position (which would affect the Rowland
Circle orientation). Limited experience suggests that carbon
coating as well as being rigorous in using a constant
magnification (for all standards and unknowns) may help. Not
much has been published on this.
I attempted to make “mu metal” shields to place over the
specimen (with hole for beam to hit the sample), but the metal
was too thick and blocked the take off angle of the x-rays.
Porous Material: Heterogeneous Catalysts
Many catalysts consist of a number of active metals and promoters in a
porous ceramic support material. However, EPMA assumes a dense bulk
material, such as the standard used. Lakis et al studied this, creating two
end member analog materials: one conductive (Ag), one insulating
(Al2O3). Below are 4 versions of the alumina of varying densities: from
left to right: 100%, 87%, 77%, 57%
R. Lakis, C. Lyman and J. Goldstein, 1992, Electron-probe microanalysis of porous
materials, Proc. 50th Ann. Mtg. Electron Microscopy Soc. Am., p. 1660-1661.
Porous Material: Heterogeneous Catalysts
They observed that
with decreasing
density, the K ratio
relative to solid
standard decreased.
To remove the effects
of surface roughness,
the “peak to
background” method
was used; this
assumes both the
characteristic and continuum x-rays are equally affected by geometric
effects. This produced good results for Ag, but not for the alumina, with
Al and O x-rays acting differently. They hypothesized that electric field in
the material was not linear and yielded different x-ray general volumes
for each x-ray.
Even more basic: does coating an insulator really
deal with the fundamental problem of electrons being
introduced into a non-conductive material?
In the 1991 book Electron Probe Quantitation*, Bastin and Jeijligers
discussed this in their chapter “Nonconductive specimens in the electron
probe microanalyzer -- A hitherto poorly discussed problem”.
After many experiments, some using metallic coatings on insulators, they
warned that using metallic coatings is not a solution, and said where
possible “ try to analyze either without a coating at higher voltages or to
burn a hole ina carbon coating. A closed coating is probably too
dangerous because small differences in (carbon) coating thickness would
produce large differences in oxygen coat rates due to the excessive
absorption of O Ka x rays in carbon.”
They mention Cazaux’s 1986 paper where he tossed out the idea that it
might be better to make thin films of materials and apply conductive
coating to the bottom surface, not the top.
* Published by Plenum Press, Editors K. Heinrich and D. Newbury
EPMA of “hot” materials
Far left: Front view
of a hot cell with
operator working
remotely. (French
Atomic Energy
Commission)
Left: General view
of shielded probe
and  box in
dedicated hot cell
(front wall in
concrete moved to
allow viewing of
the probe)
EPMA is used to characterize nuclear fuel’s fission products produced by
fissile atoms (U, Pu) and the neutrons. However, the special character of the
hot fuel creates problems for the tradition electron probe, and so it must be
modified, both to protect the operator, and to shield the detection system from
the inherent radiation coming from the material.
J. Lamontagne, T. Blay and I. Roure (2007) Microbeam Analysis of Irradiated Materials: Practical
Aspects; Microscopy and Microanalysis, 13, 150-155.
Shielding the detectors
A shielding of heavy metal is used to prevent  and  particles from the samples from
hitting the detectors. Far left: Schematic of a typical electron probe. Left: Inside view
of spectrometer with shielding close to column window to prevent direction radiation
from chamber from hitting the detector. Also shield along back of detector (vs
ricocheting ?)
Problems with ‘hot’ analyses
Xe x-ray map @ 15 kV
There are at least 2 problems for quant analysis: even with shielding there is a
high curved background (above left) and low abundance (~1 wt% ea) elements
of interest (Nd, Xe, Mo, Ru, Te, Tc, Zr, Cs) require high beam currents (eg. 250
nA @ 15 kV) for 10 second counting times. Also, Xe bubbles in the material
are difficult to measure: those at the surface have been popped open and the
gas lost; those below the surface may require higher (25 kV) E0. And there is
no Xe standard, requiring ingenuity!
Hard to figure out which matrix correction is right!
Very little (no?) guidance exists in the literature about the problem
of materials composed of extremes of Z-number elements.
Armstrong and Carpenter reported an issue at the Cal Tech EPMA
lab years ago for a Si-Ir unknown. Using pure Si and Ir
standards, they concluded the sample was either Si55Ir45 or
Si50Ir50, depending upon which one of 8 matrix corrections
was used.
This problem has several parts:
•
Incorrect calculation of the alloy’s mean atomic number (Z), a
point of controversy highlighted for extreme differences in Z
•
Failure to include continuum fluorescence esp. a problem for
high Z elements
•
Possible error in the mass absorption coefficients
Hard to figure out which matrix correction is right!
Solution: Use a std similar to the unknown! Then all 3 issues disappear
Example: A few years ago we had some Ni-Al-Pt alloys to analyze, but
just as the CalTech people had, we got different answers using pure
element stds, for different matrix corrections (top 4 lines below):
Zhu, Zhang, Ballard, Martin, Fournelle, Cao, Chang, 2010, Study of the Ni-rich multi-phase
equilibria in Ni-Al-Pt alloys using the cluster/site approximation for the FCC phases, Acta
Materialia, 58, 180-188.
Hard to figure out which matrix correction is right!
I asked the grad student if there was any single intermediate alloy in
the system that was homogeneous that could be used as a standard,
and he said yes, so we used that as the standard, which yielded the
improved numbers shown at the 2nd half below:
Zhu, Zhang, Ballard, Martin, Fournelle, Cao, Chang, 2010, Study of the Ni-rich multi-phase
equilibria in Ni-Al-Pt alloys using the cluster/site approximation for the FCC phases, Acta
Materialia, 58, 180-188.
Bottom line:
Always hope for the best (simple, easy to do EPMA) but
beware that more often than not, many materials may
have issues, so if you get “poor results”, more time may
be required to unravel the issues.
And always ask questions to those with “more
experience” as they may have some insights….