Transcript Electron probe microanalysis - UW

Electron probe microanalysis
Accuracy and Precision
in EPMA:
The Role of Standards
Revised 03/26/12
What’s the point?
EPMA’s 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?
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.
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 don’t 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.
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 Smithsonian—wet 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 Department’s collections, local researchers/
experimentalists, local rock/mineral shop (e.g., Burnie’s) or national
suppliers (e.g., Wards). Always must be carefully checked/examined.
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 M&M 2008) is
the presence of small inclusions in a not insignificant fraction of the
grains. This requires the prober be very careful.
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 Ftopaz). 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
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)
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.
From Carpenter et al NIST-MAS presentation, 2002.
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
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
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!
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 haven’t been used in calibration). This is done oneby-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.
“Evaluate” Standards
Best fit
Max count forced
thru (0,0)
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). Fanyou’s standards are
better for unknowns with ~60 wt% Al.
“Evaluating” Silicate Standards
CaSiO3
NaAlSi3O8
Al2Si4F2
SiO2
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.
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