The University of Oregon
Micro Analytical Facility
Cameca SX50 Operations Page
Table of Contents
What do you want to do?
- If the configuration of the
microprobe is the result of a well thought out analytical
procedure it is much less likely you'll waste analytical
time (... and dollars ...), and much less likely you'll
have to return for analyzing something else . . . or
analyzing it over again.
-
- Thinking about what you want
to analyze goes beyond the research question you want to
answer. For example, your research may simply want an
evaluation of AB:AN:OR endmembers or perhaps thin film
composition which requires more thought and planning.
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- However, which elements will
the probe need measure? . . . Do you know if you're
analyzing alkali-feldspar or plagioclase? . . . What
other elements are important to feldspar chemistry? . . .
How accurate do your feldspar analyses need to be? . . .
Given the list of elements, what is the most efficient
spectrometer configuration?
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The analysis of a specific
phase
- Specific phases (...e.g.,
mineral or glass ...) demand specific attention.
For example, not enough is known about modelling the
electron interaction and the subsequent x-ray generation
to simply use metal standards for silicate minerals.
It is for this reason we always try to use
specific minerals as the most appropriate standards.
-
- Still . . . we can not provide
as many mineral standards as we'd like . . . you'll be
asked to compromise with dis-similar standards for
elements heavier than sulfur, . . . this can be justified
with the sophistication of today's computer modelling.
For lighter elements there exists likely problems
associated with differences in peak locations and peak
shapes . . . for these elements a similar standard is
likely to exist. Feel free to consult with your
EPMA facility manager for appropriately choosing your
standards.
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Choosing the elements
- Choosing the elements you need
to analyze with this technique can be as easy a looking
at someone else's analysis (... for example, the elements
for pyroxene as listed in Deer, Howie and Zussman
...). The list of elements might also include
special elements associated with your research question
(... e.g., Mg in lunar feldspar, or Al and Fe in detrital
quartz ...). Keep in mind however EPMA is not the
best trace element technique ... electron induced
continuum background, reasonable count times and possible
beam damage limit elemental sensitivities to greater than
100ppm. Still, if in situ measurement of trace
elements is required, EPMA may be the only technique.
-
- The list of elements may also
need include elements not actually measured.
Oxygen is an example of an element which is present and
must be included because it is a major constituant and
therefore must be included in the matrix model for proper
correction of the other elemental x-ray intensities (...
e.g., the corrections for average atomic number and x-ray
absorption ...). Assumptions are therefore made
regarding the amount of oxygen present. It is our
practice to assume stoichiometric oxygen (... e.g., 2
oxygens for every silicon atom measured ...), however
oxygen associated with ferric iron, after an initial
assumption of ferrous (... which is the accepted practice
...), may need to be recalculated based on mineral
stiochiometry. Also, be aware oxygen by difference
is also an option and may be sometimes more appropriate.
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- However, your list of elements
shouldn't include elements you really don't need.
Just because someone else analyzed for Ca in olivine at a
sensitivity of 100ppm doesn't mean you need to.
Consider your financial budget and the fact Ca at 0.03
weight percent will add 3 minutes to every analysis and
possibly $50 to $100 to your invoice. (On the
other hand, you don't want, at a later time, to be adding
calcium to your olivine definition and re-analyzing.)
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