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Electron probe microanalysis

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Title: Electron probe microanalysis


1
Electron probe microanalysis
  • Accuracy and Precision in EPMA
  • Understanding Errors and the Role of Standards

2
Whats the point?
How much can I trust the compositions that the
probe computer spits out? Are two analyses
equivalent? Can I compare my numbers with those
published by other researchers using EPMA?
3
Goal and Issues
  • Goal achievement of high accuracy and
    precision in quantitative analyses,
    recognizing sources of errors and minimizing them
  • Issues involved with achieving this goal
  • Standards
  • Instrumental stability
  • Sample and standard physical condition
  • Beam impact on sample complications
  • Spectral issues
  • Counting statistics
  • Matrix correction

4
Standards how good are they? well
characterized? homogeneous? Instrumental
conditions beam stability spectrometer
reproducibility thermal stability detector
pulse height stability/adjustment reflected
light optics (stage Z) Matrix correction any
issues (eg MACs for light elements)? wide range
in Z for binary (eg PbO) Sample and standard
conditions rough surface? polish? etched? tilt?
sensitive to beam? C coat thickness if
used Counting statistics enough counting time?
Spectral issues peak and background
overlaps? Sample size vs interaction volume
homogeneous? small particles? secondary
fluorescence?
5
These can be categorized into random and
systematic errors.
6
Random Errors
  • Random errors include
  • random nature of X-ray generation and emission
  • instrumental (random) instability
  • operator inconsistency (e.g. little attention to
    correct optical focus)
  • sample surface roughness
  • interaction volume intersecting two phases
  • secondary fluorescence from hidden (below
    surface) phases
  • stray cosmic rays

7
Systematic errors
  • Systematic errors include
  • instrumental instability (temperature effect on
    crystal 2d, and on gas pressure stage Z drifts
    as it heats up)
  • inappropriate matrix correction
  • poor electrical ground of either standard or
    unknown
  • beam change/damage to unknown (e.g. Na in glass)
  • difference in peak shape/position (standard vs
    unknown)
  • peak or background interference
  • pulse height depression on standard
  • fluorescence across observed phase boundaries
    (e.g. diffusion couple)

8
Precision and Accuracy in Error Analysis
Precision refers to the reproducibility of the
counts and thus the ability to be able to
compare compositions, whether within a sample, or
between samples, or between analytical sessions.
It is directly tied to counting statistics. It is
a relative description. Accuracy refers the
truth of the analysis, and is directly tied to
the standards used and the matrix correction
applied to the raw data, as well most of the
other variables listed previously that could
affect the X-ray intensities (background and peak
interferences, beam damage, etc). It is an
absolute description. EPMA quantitative error
analysis is a combination of both, the first
being very easy to define, the later more
difficult. Precision for major elements could
easily be lt1, but when combined with accuracy,
total EPMA error probably 1-2 in the best cases
(for major elements).
9
Precision and Accuracy
Low Precision
High Precision
Low Accuracy
High Accuracy
10
Instrumental Errors-1
  • Beam current stability with Faraday cup
    measurements made for each analysis, long term
    drift should not be a problem as the counts for
    each analysis are normalized to a common
    reference current value (could be 1, or 20 nA).
    For long count times (minutes ) for trace element
    work, it is recommended that the peak and
    background counting be constantly cycled so that
    any longer period issues be spread out over the
    whole time period.
  • Spectrometer reproducibility with modern
    microprobes, this should not be a serious
    problem, although problems do crop up with age.
    Where crystals are flipped, in a small fraction
    of cases there is an error generally it is not
    recommended to flip crystals within analyses.
    When spectrometer reproducibility is a problem,
    it is seen as backlash of the gears to minimize
    errors, the peaks should always be approached
    from the same direction. This is set up within
    the software.

11
Instrumental Errors-2
  • Thermal stability Spectrometers could drift if
    there is a change in the room temperature, though
    this would presumably be noticeable to the
    operator (air conditioning fails in hot spell). I
    have not seen problems with PET nor LIF, though I
    have with TAP which could be thermal. P10 gas
    pressure is sensitive to the temperature change.
    We attempt to keep the room at 68-70C and the
    circulating water temperature in the machine is
    very close to this. Stage height (Z) drifts due
    to motor heating during long (overnight) runs.
  • Detector pulse height adjustment/stability The
    bias (voltage) of the gold wire in the detector
    must be set to the proper value this is a
    function of the energy of the X-ray and gas
    pressure. The operator must verify that the bias,
    gain and baseline are set properly (the last
    particularly where the Ar-escape peak is
    partially resolved).

12
Instrumental Errors-3
  • Dead timeIn WDS, counts are dead time
    corrected. If dead time is not accurately
    determined, there could be a systematic error
    here. Cameca probes operate somewhat differently
    from JEOL and others, in that Cameca introduces a
    hard constant time delay (e.g. 3 msecs)
    automatically into the counting circuitry and
    then uses that value to correct the counts.
  • Probe labs should verify (at least once) that the
    manufacturers official or default dead time
    factors are correct. This is done by counting on
    a metal standard (e.g. Si or Ge) at varying
    Faraday currents, with the dead time correction
    turned off. These data can then be plugged into a
    spreadsheet that which Paul Carpenter (NASA) has
    developed to calculate the most accurate dead
    time actually present on a particular probe.
    Also, in our Probe for Windows software, there is
    an option for an alternate, more complex dead
    time correction equation, for high count rate
    (gt50K cps)

13
Instrumental Errors-4
  • Specimen focus (stage height) Samples and
    standards must be positioned at the same stage
    height, so that they will all be at the same
    position vis a vis the Rowland Circle ( in X-ray
    focus for Bragg defraction). Sometimes it is
    difficult decide within 1-3 um which is the
    best height this small Z difference is not
    critical. It becomes critical when it reaches the
    5 or 10 um out of focus realm, which can occur
    during unattended overnight runs as the sample
    and stage heat up (heat from stage motors) this
    can be addressed by using the stage Z autofocus
    automation (but test it out first, as it must be
    calibrated).

14
Sample/standard Error Physical issues - 1
  • Surface irregularities the matrix correction
    relies upon the correct take off angle to
    calculate the path length for the absorption
    correction, and irregular surfaces will have
    variable path lengths and thus the measured X-ray
    intensities will not be consistent between
    analytical spots. Moreover, in using different
    spectrometers mounted in different directions,
    the path length will vary between spectrometers
    for one analytical spot.
  • Etched samples generally, etching may introduce
    some irregularity, and should be avoided.
    However, I have seen slightly etched samples
    analyzed without apparent problem.
  • Polishing samples should be polished with final
    stage using lt1 mm diamond or alumina or silica.

15
Sample/standard Error Surface Irregularities
These Monte Carlo simulations show the effect on
K and L line X-rays of Ni and Al, of
one-directional V-grooves of height (h) varying
from .1 to 1 mm. The smallest (.1 mm) grooves
have no noticeable effect, but the deeper grooves
clearly have major impacts on Al Ka and Ni La
(due to more or less absorption), with the
greatest impact on the lowest energy line.
Lifshin and Gauvin, 2001,Fig. 4, p. 171.
16
Sample/standard Error Physical issues - 2
  • Specimen homogeneity a key assumption of
    quantitative EPMA is that the interaction volume
    is one phase (is homogeneous).
  • If more than 1 phase is overlapped by the beam
    the matrix correction usually overcompensates and
    produces an erroneous composition gt100 wt. This
    is common for small eutectic (groundmass) phases.
  • If trace elements are being considered, then
    also the adjacent surrounding volume (up to
    50-100 mm away) must not contain phases with
    higher concentrations of the elements of
    interest, which might be secondarily fluoresced.
  • Diffusion couples have similar constraints, in
    that secondary fluorescence across the boundary
    can yield X-ray intensities up to a couple of
    percent (which could also give high totals).
    Users need to either empirically or theoretically
    verify this is NOT happening.

17
Sample/standard Error Physical issues - 3
  • Incorrect geometry (non-orthogonal surface)
    this occurs too often with 1 diameter plugs that
    have been automatically polished. For whatever
    reason, the sample surface ends up at a slant to
    the wall, and when the set screw is tightened in
    the holder, the surface ends up at an angle to
    the horizontal. This introduces an error in the
    take off angle. Also, the area of interest may be
    too low and impossible to reach stage Z focus.

This Monte Carlo simulation shows that a 5 tilt
of the sample will alter the K ratio of Al Ka by
.01, which equals a 8 relative error before
matrix correction. An Al ZAF of 1.5 would thus
increase the error to 12.
Lifshin and Gauvin, 2001, Fig 3., p. 170.
18
Sample/standard Error Physical issues - 4
  • Incorrect geometry - edge effects materials
    mounted in epoxy and then polished with loose
    polishing compound commonly have differential
    erosion at the epoxy-material interface,
    producing a moat or channel in the epoxy,
    resulting in a rounding of the material at the
    edge. Efforts to do quantitative EPMA of the edge
    (rim) will be in error as the absorption path
    length will be non-uniform and different from the
    nominal length. Special polishing technique will
    minimize or eliminate this problem.

Epoxy
Specimen
Common erosion problem, rounding of specimen edge
Epoxy
Specimen
Desired geometry no rounding of specimen edge
19
Sample/standard Error Physical issues - 5
  • Oxide coating/film this can be a significant
    problem for metals that oxidize (e.g., Al, Mn,
    Mg, Ti, etc.), particularly for standards. These
    can reach fractions of a mm in depth, and
    significantly alter the X-ray intensity of the
    line being acquired for the standard, resulting
    in an overestimate of the element in the unknown.

This plot shows the effect of a thin oxide skin
(TiO2) on reducing the characteristic X-rays from
a pure metal standard (Ti), and is most severe
for lower E0. (Modeled with GMRFilm software).
20
Sample/standard Error Physical issues - 6
  • Smear coat soft materials may smear and cross
    contaminate other materials that are being
    polished either in the same holder, or in a
    subsequent sample, producing a thin smear coat.
    I have seen one reference in the literature to Pb
    or Sn smearing. It is not normally considered a
    major problem, at least for major element
    analysis.
  • Polishing artifacts Diamond and alumina
    polishing particles can get caught in pores in
    the material been polished. I have seen mm
    fragments of brass from a brass sample holder
    become lodged in feldspar and biotite.
  • Charging this will reduce the effective E0 in a
    random manner. Conductive samples in epoxy must
    be grounded with conductive tape (preferred
    rather than paint). Semi-conductors conduct ok.
    Non-conductive samples need to be coated (C, Al,
    Ag, Be...).
  • Porosity There could be 2 errors in porous
    material the electron range will be greater
    (absorption path longer) than in non-porous
    material of same composition) and in
    non-conductive material, there could be problems
    with charging as the electrons travel between
    pores (vacuum) and material.

21
Sample/standard Error Physical issues - 7
  • Carbon coat the conductive coating on the
    samples should be of the same thickness as on the
    standards being used. This can be evaluated
    experimentally or with the GMRFilm modeling
    program. Kerrick et al. (1973) measured the
    effect and showed it affected the light elements
    most strongly, and was worst at lower E0 a
    difference of 200 Å between sample and standard
    translated to a 4 difference in F Ka intensity.
    There is some antidotal evidence that old (many
    years-decade?) carbon coats may go bad
    (oxidize? delaminate?) and lose conductivity.

Kerrick et al, 1973, American Mineralogist, 58,
920-925.
22
Sample/standard Error Physical issues - 8
  • 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
    (volatile element correction in our Probe for
    Windows software)
  • Glasses with alkalis (esp. Na), particularly
    hydrous glasses Na drops precipitously, K
    somewhat, with Al and Si counts increasing .
  • Alkali feldspars, particularly albite Na counts
    drop
  • Carbonates and anhydrite easily decompose with
    10 nA
  • Apatite not as fragile, but some grains will
    crater with moderate currents (60 nA) after 40-60
    seconds.

23
Sample/standard Error Physical issues - 9
Na
Al
Above, the dramatic drop in Na Ka counts versus
time is demonstrate. It is worst at 20 nA, and
much less at 2 nA. 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.
Si
Morgan and London, 1996, American Mineralogist,
81, 1176-1185.
24
Sample/standard Error Physical issues - 10
  • 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 La/Lb as
    indicator of Fe-oxidation state.
  • Sample orientation Stormer et al. showed that F
    and Ca Ka intensities in apatite could vary with
    time if the electron beam was perpendicular to
    the c axis.

Stormer et al, 1993, American Mineralogist, 78,
641-648.
25
Sample/standard Error Physical issues - 11
  • Beam deflection magnetic specimens (e.g. some
    Ni-Mn compounds) apparently deflect the electron
    beam, as seen by contamination spots 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.

26
Sample/standard Error Procedural issues - 1
  • Peak interferences If measured peaks are
    overlapped by peaks of other elements, obvious
    errors will result. Such interferences can exist
    both in standards and unknowns. Such errors in
    unknowns can yield high totals. Unavoidable peak
    interferences must be addressed by using
    interference standards, to subtract the correct
    fraction of counts attributed to the interfering
    element.
  • Background position interferences Incorrect
    placement of background counting positions can
    lead to errors, as the background estimate at the
    peak position usually is inflated, yielding less
    than true counts for the element. Wavescans
    should be done on typical phases, and/or Virtual
    WDS used to evaluate the situation.

27
Sample/standard Error Procedural issues - 2
  • Peak shift/shape differences We have discussed
    the issues of peak shifts for S Ka. Al Ka is
    another element with a well documented issue of
    differences between the metal, oxide, and
    alumino-silicate phases. Also F and other light
    elements, and L lines of Co and Ni also have such
    issues. Peak shifts can yield small to
    significant errors.
  • PHA settings Bias, gain, and baselines should
    be checked. Gross errors in them could produce
    significant errors in the analytical results.
    Pulse height depression occurs mainly where there
    is a large discrepancy in count rate between
    standard and unknown, e.g. 50000 cps on std B vs
    500 cps on Mo-Si-B phase) count rates up to
    10-15000 cps should be OK. Dropping the current
    on the B standard from 30 to 1 nA worked.

28
Counting Statistics - 1
We desire to count X-ray intensities of peak and
backgrounds, for both standards and unknowns,
with high precision and accuracy. X-ray
production is a random process (Poisson
statistics), where each repeated measurement
represents a sample of the same specimen volume.
The expected distribution can be described by
Poisson statistics, which for large number of
counts is closely approximated by the normal
(Gaussian) distribution. For Poisson
distributions, 1 sigma square root of the
counts, and 68.3 of the sampled counts should
fall within 1 sigma, 95.4 within 2 sigma, and
99.7 within 3 sigma.
Lifshin and Gauvin, 2001, Fig. 6, p. 172
29
Counting Statistics-2
The precision of the composition ultimately is a
combination of the counting statistics of both
standard and unknown, and Ziebold (1967)
developed an equation for it. Recall that the
K-ratio is where P and B refer to peak and
background. The corresponding precision in the K
ratio is given by where n and n are the
number of repetitions of counts on the unknown
and standard respectively. (The rearranged sK/K
-- with square roots taken-- term was sometimes
referred to as the sigma upon K value.)
30
Counting Statistics-3
From MAC shortcourse volume
Another format for considering cumulative
precision of the unknown is the above graph. A
maximum error at the 99 confidence interval can
calculated, based upon the total counts acquired
upon both the standard and the unknown e.g. to
have 1 max counting error you must have at least
120,000 counts on the unknown and on the
standard you could get 2 with 30,000 counts on
each.
31
Probe for Windows Statistics -1
PfW provides several statistics in the normal
default log window printout for bkg subtracted
peak counts average, standard deviation, 1
sigma, std dev/1 sigma (SIGR), standard error,
and relative std dev. For Si the average is 4479
cps, and the average sample uncertainty (SDEV)
for each of the 3 measurements is 15 cps. The
counting error (1 sigma) is somewhat larger (21
cps), and the ratio of std dev to sigma is lt1,
indicating good homogeneity in Si.
For homogeneous samples, we can define a standard
error for the average here, 8 cps.
Finally, the printout shows the relative standard
deviation as a percentage (0.3, excellent).
NB These measurements only speak to precision,
both in counting variation and sample variation.
32
Probe for Windows Statistics - 2
After the raw counts, the elemental weight
percents are printed, with some of the same
statistics, followed by the specific standard
(number) used. Following that are the std
K-ratio, and std peak (P-B) count rate. Below
that are the unknown K-ratio, the unknown peak
count rate, and the unknown background count.
Below that are the ZAF (ZCOR) for the element,
the raw K-ratio of the unknown, the
peak-background ratio of the unknown, and any
interference correction applied (INT, as
percentage of measured counts).
NB The number of digits after a decimal point in
a printout composition needs to be used with
common sense!
33
Probe for Windows Statistics - 3
PfW software provides for additional optional
statistics. One set relates to detection limits,
i.e. what is the lowest level you can be
confident in reporting.We will deal with them
later, when we talk about trace elements in a few
weeks. The other set of statistics relates to the
homogeneity of the unknowns as well as
calculation of analytical error. We will now
discuss these statistics.
34
Analytical error - single line
This calculation is for analytical sensitivity of
each line ( one measurement), considering both
peak and background count rates (Love and Scott,
1974). It is a similar type of statistic as the 1
sigma counting precision figure, but it is
presented as a percentage.
Love and Scott, 1974
35
Additional analytical statistics
Probe for Windows provides a more advanced set of
calculations for analytical statistics. The
calculations are based on the number of data
points acquired in the sample and the measured
standard deviation for each element. This is
important because although x-ray counts
theoretically have a standard deviation equal to
square root of the mean, the actual standard
deviation is usually larger due to variability of
instrument drift, x-ray focusing errors, and
x-ray production. A common question is whether a
phase being analyzed by EPMA is homogeneous, or
is the same or distinct from another separate
sample. An simple calculation is to look at the
average composition and see if all analyses are
within some range of sigmas (2 for 95, 3 for 99
normal probability).
36
Homogeneity confidence intervals
A more exacting criterion is calculating a
precise range (in wt) and level (in ) of
homogeneity. These calculations utilize the
standard deviation of measured values and the
degree of statistical confidence in the
determination of the average. The degree of
confidence means that we wish to avoid a risk a
of rejecting a good result a large per cent of
the time (95 or 99) of the time. Students t
distribution gives various confidence levels
for evaluation of data, i.e. whether a particular
value could be said to be within the expected
range of a population -- or more likely, whether
two compositions could be confidently said to be
the same. The degree of confidence is given as 1-
a, usually .95 or .99. This means we can define a
range of homogeneity, in wt, where on the
average only 5 or 1 of repeated random points
would be outside this range.
37
Students t distribution
The general problem, where the sample size is
small and the population variance is unknown, was
first treated in 1905 by W.S. Gossett, who
published his analysis under the pseudonym
Student. His employer, the Guinness Breweries
of Ireland, had a policy of keeping all their
research as proprietary secrets. The importance
of his work argued for its being published, but
it was felt that anonymity would protect the
company. (S.L. Meyer, Data Analysis for
Scientists and Engineers, 1975, p. 274.)
Goldstein et al, p. 497
38
Test for homogeneity
39
Recall the original analysis
Olivine analysis Example of homogeneity tests
What this means for Si, at highest level (95),
we can say that there is chance that only 5 of
number of random points will be .14 wt greater
or lesser than 18.89 wt (or as a percent, 0.7).
PfW also provides a handy table to show if the
sample is homogeneous at the 1 precision level,
and if so, at what confidence level.
40
Counting Statistics
Analytical sensitivity is the ability to
distinguish, for an element, between two
measurements that are nearly equal.
So here, at the 95 confidence level, two samples
would have to have a difference in Si of gt .20
wt to be considered reliably different in Si.
41
Numbers of significant figures-1
There have been cases where people have taken
reported compositions (i.e. wt elements or
oxides) from probe printouts and then faithfully
reproduced them exactly as they got them. Once
someone took figures that were reported to 3
decimal points and argued that a difference in
the 3rd decimal place had some geochemical
significance. The number of significant figures
reported in a printout is a mere programming
format issue, and has nothing to do with
scientific precision! (However, a recent added
feature to PfW is an option to output only the
actual significant number of digits. This is not
normally enabled.) Having said that, it is
tradition to report to 2 decimal places.
However, that should not be taken to represent
precision, without a statistical test, such as
given before.
42
Numbers of significant figures - 2
In the example of the olivine analysis above,
where Si was printed out as 18.886 wt, it would
be reported as 18.89 -- but looking at the
limited number of analyses and the homogeneity
tests, I would feel uncomfortable telling someone
that another analysis somewhere between 18.6 and
19.2 were not the same material. Nor would I be
uncomfortable with someone reporting the Si as
18.9 wt (though I stick to tradition.)
Considering silicate mineral or glass
compositions, Si is traditionally reported with 4
significant figures. If we were to be rigorous
regarding significant figures, we would follow
the rule that we would be bound by the least
number of figures in a calculation where we
multiply our measurement (K-ratio, which will
have thousands of counts divided by thousands of
counts) by the ZAF. As you can appreciate there
are many calculations that comprise each part of
the ZAF, and it would be stretching it to argue
that the ZAF itself can have more than 3
significant figures. Ergo, we should not strictly
report Si with more than 3 significant figures.
43
Numbers of significant figures - 3
When we enable the PfW Analytical Option Display
only statistically significant number of
numerical digits for the olivine analysis, heres
the result
Wrong
For comparison, heres the original printout
44
Errors in Matrix Correction
The K-ratio is multiplied by a matrix correction
factor. There are various models alpha, ZAF,
f(rz) and versions. Assuming that you are using
the appropriate correction type, there may be
some issues regarding specific parameters, e.g.
mass absorption coefficients, or the f(rz)
profile. There is a possibility of error for
certain situations, particularly for light
elements as well as compounds that have
drastically different Z elements where pure
element standards are used. The figure above
shows that a small (2) error in the mass
absorption
Lifshin and Gauvin, 2001, p. 176.
coefficient for Al in NiAl will yield an error of
1.5 in the matrix correction. This is a strong
incentive to either use standards similar to the
unknown, and/or use secondary standards to verify
the correctness of the EPMA analysis.
45
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
    homogenous 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.
46
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.
47
  • 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 (Dept of
    Mineral Sciences Ed Vicenzi, free)
  • Alloys and glasses from NIST (100 ea)
  • Metals and compounds from chemical supply houses
    (20-60 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)

48
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 slightly different. One problem is the
    small size of many grains (100-500 mm).

49
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.
  • 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 lamallae)

50
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.)
  • Recently, John Hanchar (George Washington U) 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)

51
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.
52
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
53
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
54
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

55
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.
56
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.
57
Evaluating Silicate Standards
SiO2
CaSiO3
NaAlSi3O8
Al2Si4F2
58
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.
59
Summary How to know if the EPMA results are
good?
  • There are only 2 tests to prove your results are
    good actually, it is more correct to say that
    if your results can pass the test(s), then you
    know they are not necessarily bad analyses
  • 100 wt totals (NOT 100 atomic totals). The
    fact that the total is near 100 wt. Typically, a
    range from 98.5 - 100.5 wt for silicates,
    glasses and other compounds is considered good.
    It extends on the low side a little to accomodate
    a small amount of trace elements that are
    realistically present in most natural (earth)
    materials. These analyses typically do oxygen by
    stoichometry which can introduce some
    undercounting where the FeO ratio has been set
    to a default of 11, and some the iron is ferric
    (FeO 23). So for spinels (e.g. Fe3O4), a
    perfectly good total could be 93wt.
  • Stoichometry, if such a test is valid (e.g. the
    material is a line compound, or a mineral of a
    set stoichometry.

60
Checking our olivine analysis
  • The total is excellent, 99.98 wt
  • The stoichometry is pretty good (not excellent)
    on the 4 oxygens, there should be 1.00 Si atoms
    and we have .985. The total cations MgFeCaNi
    should be 2.00, and we have 2.03.
  • The analysis is OK and could be published. If
    this were seen at the time of analysis, it might
    be useful to recheck the Si and Mg peak positions
    , and reacquire standard counts for Si and Mg. If
    this were only seen after the fact, you could
    re-examine the

standard counts and see if there are any obvious
outliers that were included and could be
legitimately discarded.
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