Electron probe microanalysis

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Electron probe microanalysis

• Accuracy and Precision in EPMA
• The Role of Standards

Revised 03/26/12
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Whats the point?
EPMAs claim to fame as a microanalytical tool
rests upon (1) faith in a correct matrix
correction and (2) use of good, correct,
true standards. How do you know to trust a
standard?
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Standards

• In practice, we hope we can start out using the
  best standard we have. There have been 2
  schools of thought as to what is the best
  standard is
• a pure element, or oxide, or simple compound,
  that is pure and whose composition is well
  defined. Examples would be Si or MgO or ThF4. The
  emphasis is upon accuracy of the reference
  composition.
• a material that is very close in composition to
  the unknown specimen being analyzed, e.g.
  silicate mineral or glass it should be
  homogeneous and characterized chemically, by some
  suitable chemical technique (could be by EPMA
  using other trusted standards). The emphasis here
  is upon having a matrix that is similar to the
  unknown, so that (1) any potential problem with
  the matrix correction will be minimized, and (2)
  any specimen specific issues (i.e. element
  diffusion, volatilization, sub-surface charging)
  will be similar in both standard and unknown, and
  largely cancel out.

This is based upon experience, be it from prior
probe usage, from a more experienced user, from a
book or article, or trial and error (experience
comes from making mistakes!) It is commonly a
multiple iteration, hopefully not more than 2-3
efforts.
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Standards - Optimally

• Ideally the standard would be stable under the
  beam and not be able to be altered (e.g.,
  oxidizable or hygroscopic) by exposure to the
  atmosphere.
• It should be large enough to be easily mounted,
  and able to be easily polished.
• If it is to be distributed widely, there must be
  a sufficient quantity and it must be homogeneous
  to some acceptable level.
• However, in the real world, these conditions
  dont always hold.

Round Robins
On occasion, probe labs will cooperate in round
robin exchanges of probe standards, where one
physical block of materials will be examined by
several labs independently, using their own
standards (usually there will be some common set
of operating conditions specified). The goal is
to see if there is agreement as to the
compositions of the materials.
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• Sources for standards
• Purchased as ready-to-go mounts from microscopy
  supply
  houses as well as some probe labs (1200-2000)
• Alternately, most probe labs develop their own
  suite of standards based upon their needs,
  acquiring standards from
• Minerals and glasses from Smithsonianwet
  chemically analyzed (Dept of Mineral Sciences
  Tim Rose, free)
• Alloys and glasses from NIST -- certified to
  some level for some elements (100-200 ea)
• Metals and compounds from chemical supply houses
  not certified, caveat emptor (20-150 ea)
• Specialized materials from researchers
  (synthesized for experiments, or starting
  material for experiments) both at home
  institution as well as globally (some , most
  free)
• Swap with other probe labs
• Materials from your Departments collections,
  local researchers/ experimentalists, local
  rock/mineral shop (e.g., Burnies) or national
  suppliers (e.g., Wards). Always must be carefully
  checked/examined.

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USNM Standards

• 1980 Gene Jarosewich, Joe Nelen and Julie
  Norberg at the Smithsonian Dept of Mineral
  Science (US National Museum) published results of
  an effort to develop epma standards for minerals
  and glasses. They had crushed, separated, then
  examined for homogeneity once a subset found, it
  was analyzed by classical methods (wet
  chemistry), and then made available for
  distribution. This list included 26 minerals and
  5 glasses. In 1983, Jarosewich and MacIntrye
  published data on 3 carbonate standards (calcite,
  dolomite and siderite), and in 1987, Jarosewich
  and White published data on a strontianite
  (SrSO4) standard. These all are available at no
  cost to probe labs.
• These are excellent standards. Users must be
  aware of course that the official value
  represents a bulk analysis and individual splits
  may be different. One problem is the small size
  of many grains (100-500 mm).
• Another problem recently discussed (Albuquerque
  MM 2008) is the presence of small inclusions in
  a not insignificant fraction of the grains. This
  requires the prober be very careful.

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Other Mineral Standards

• In the 1960s, Bernard Evans developed a suite of
  silicate and oxide mineral standards (at UC
  Berkeley) that were available for EPMA work. Some
  of these are still around (Gordon Medaris uses
  these).
• 1992, McGuire, Francis and Dyar published report
  on evaluation of 13 silicate and oxide minerals
  as oxygen standards. They included data for all
  elements. Available from Harvard Mineralogical
  Museum for small cost (100-150).
• Here in Madison, I have evaluated several
  minerals from the Mineralogy collection for
  standards and found some very good casserite
  (SnO2), wollastonite (CaSiO3), Mg-rich olivine
  and enstatite. Other minerals from Wards have
  been found to be useful (biotite and F-topaz). On
  the other hand, other efforts have been
  unsuccessful (e.g., ilmenite from Wards --
  zoned/exsolution lamellae)
• Our SIMS lab is developing standards for their
  work, and some of these materials (minerals,
  glasses) turn out to be good EPMA standards also

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Synthesized Standards

• 1971, Art Chodos and Arden Albee of Caltech
  contracted Corning Glass to produce 3 Ca-Mg-Al
  borosilicate glasses (95IRV, W and X) containing
  a number of (normally) trace elements, at 0.8 wt
  level, to be used as EPMA trace element
  standards. They are available now from the
  Smithsonian.
• 1971, Gerry Czamanske (USGS) synthesized 73
  sulfides and 3 selenides/tellurides (for phase
  equilibria studies). Some of these were made
  available to EPMA labs. We have them here.
• 1972, Drake and Weill (U. Oregon) synthesized 4
  Ca-Al silicate glasses each with 3-4 REE
  elements.
• 1991 Jarosewich and Boatner published data on a
  set of 14 rare-earth (plus Sc and Y)
  orthophosphates (synthesized by Boatner). These
  are also available at no charge from the
  Smithsonian. (A recent study by Donovan et al.
  showed that many have some unreported Pb
  impurities, a problem for monazite age dating.)
• John Hanchar (Memorial University, NFLD) has
  been working on synthesizing zircon, hafnon,
  thorite and huttonite some are now available for
  standards.
• There are other synthetic standards available,
  usually in limited quantities one discovers
  these sources by asking around.
• Have skilled users (who have experimental
  equipment) make up some compounds of elements for
  difficult analyses (e.g. Al, Mg, Ti, Mn where
  pure metal standards oxidize)

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Evaluation of synthetic glasses
Recently Paul Carpenter et al did a rigorous
evaluation of the 95IRV, W and X glasses. Shown
here are the results for one of the glasses,
95IRW. This is a very valuable study, and is
unusual in its thoroughness, as demonstrated in
the
X-ray maps, a few of which are shown here. The
glasses have the trace oxides at .8 wt , and
with good homogeneity (200-300 ppm range) for all
but Cs, which has a much wider (1000 ppm) range.
From Carpenter et al NIST-MAS presentation, 2002.
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From Carpenter et al NIST-MAS presentation, 2002.
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NIST Standards
The National Institute of Standards and
Technology (previously National Bureau of
Standards) began to develop EPMA standards over
30 years ago. SRM Standard Reference Material
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NIST Standard SRM 482 Example... and problem
To the right are the documentation as well as
examples of the materials supplied when one
purchases a NIST standard here, a set of 6 wires
in the Cu-Au binary. At the recent (April 2002)
NIST-MAS workshop on accuracy in EPMA and the
role of standards, Eric Windsor of NIST presented
the results of a study into these Cu-Au
standards. For some time, there had been some
reports of small levels of impurities in these
standards. It turns out that there are
micron-size Cu-oxides present, and the abundance
is a function of the type of surface
preparation/polish.
From Eric Windsor, NIST-MAS presentation, 2002
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Supply House Standards

• Some pure elements and compounds purchased from
  chemical suppliers may be good epma standards.
  However, it pays to pay close attention and be
  careful and test them carefully. It is apparent
  that many materials are processed and sometimes
  have two phases present, whereas they are
  certified as one phase. They get away with this
  error because the one of the phases is an oxide
  of the first, and the compositions are stated to
  be pure to some level (e.g. 99 on a metal
  basis). This in fact can be a benefit, and
  provide 2 standards-in-one, provided the second
  phase is easily distinguished.
• Cr2O3 (99.7) turned out to have small Cr blebs
• CuO (99.98) grains turned out to have cores of
  Cu2O
• Cr fragments and Re and Ir rods seem to be pure
• MgAl2O4, FeTiO3 MnTiO3 (99.9) were not
  homogeneous at all!

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How do you evaluate your Standards?
The traditional answer is that decide your
standards are good by testing if they give you
the answers you think you should be getting, i.e.
you run other standards as secondary standards
and see if you get the correct composition for
them (optimally they havent been used in
calibration). This is done one-by-one, comparing
one pair of primary and secondary standards.
However, we now have a powerful rapid technique
that compares the functioning of several
standards against each other at the same time,
e.g. you acquired Si counts on your forsterite,
fayalite, plagioclase, pyroxene, garnet, and
sillimanite standards. You can then plot up the
official compositions against the count rates
that have been adjusted for the matrix effects in
each standard. If they all plot up on a straight
line, then they all are good. If one is ok,
there is a good chance there is something amiss
about it (could be slightly different composition
from the official value). I suggested to
Donovan that this would be a useful addition to
the Probe for Windows software 2 summers ago, and
he soon developed the Evaluate program.
The line is pinned at the high end by the
standard with the highest concentration of the
element in question (which could be pure element
or oxide), and should go through the (0,0) origin
at the low end.
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Evaluate Standards
Here 2 standards (Al-Fe-Si alloys) synthesized by
Fanyou Xie (MSAE) are plotted with Si defined by
1100, Al2Si4F2. Note that 614 is above the
line, suggesting its real composition may be
higher (shift to right).
Al can be a problem (oxide layer). Here I was
testing std 9979, Al-Mg alloy (98 wt Al) and
9978, Al-Si alloy (99 wt Al) against other
standards including 13 (Al2O3) and Mg-Al alloy
(8903). Fanyous standards are better for
unknowns with 60 wt Al.
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Evaluating Silicate Standards
SiO2
CaSiO3
NaAlSi3O8
Al2Si4F2
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Virtual Standards
Occasions arise when there is no standard
available, for one reason or another. Above is a
case where a low total in a specimen led to a
search for the missing elements, and after some
leg work, it was learned that the specimen had
been produced by sputtering in Ar. A wavescan
showed an Ar Ka peak.
However, I had no Ar standard. This led to
discussions with John Donovan, and he
subsequently developed the Virtual Standard
routine now in PfW.
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EPMA Standards

• Bottom Line
• Must be homogeneous at the sub-micron level (so
  that any interaction volume will have the same
  x-ray intensities for characteristic lines as any
  other in the sample. Note that this allows for
  nanometer-scale differences.
• Must have an accurate (true) known chemical
  composition