Facilities--> What is EPMA
What is EPMA
- Electron microprobe analysis -- or
electron probe microanalysis -- or electron microbeam probe analysis -- is a technique
developed by R. Castaing in the late 1940's as his Ph.D. dissertation in Paris.
- A beam of energetic electrons, of say,
15,000 volts (but there is nothing sacred about 15 KeV; sometimes I use 7 KeV, other times
30 KeV) is focused upon an object of interest. The electrons are scattered elastically,
resulting in an 'excitation volume' much larger than the actual diameter of the beam --
this volume usually having a diameter on the order a several microns.
- A variety inelastic scattering
events also occur, yielding information about the chemistry and structure of the target.
The products that we mainly are concerned with give us chemical information at the ~micron
- Back-scattered electrons
- Light (cathodoluminescence)
- and to some extent, secondary electrons,
which tell us about surface topography.
- I shouldn't need to remind you, but we
can look at x-rays (and other packets of energy) from either their particle description
(so many electron volts of energy) or from their wave-like feature (wavelength, in say
angstroms or nanometers).
- Electron microprobes are outfitted with
wavelength dispersive spectrometers (WDS), and in many if not most cases have an energy
dispersive one (EDS) as well.
- WDS operates with a small fraction of the
x-rays that escape from the sample, reaching a crystal, and a particular wavelength will
be defracted at a particular angle (the Bragg angle) into a gas-filled amplification tube,
where a pulse is produced that is proportional to the energy of the x-ray. A WDS
spectrometer is tuned to one wavelength and sits there and counts the numbers of x-rays.
You might compare it to a serial information source, getting one channel of information at
- EDS, on the other hand, looks at all the
x-rays coming off the sample, and stretching the analogy, is a parallel information
gathering device, collecting all the different wavelengths. The EDS detector is a
solid-state device, where electron-hole pairs are produced for each x-ray that hits it,
with the number of pairs a function of the total x-ray energy divided by the energy needed
to produce one pair. This small signal is amplified, and then becomes one bit of
information in a spectrum gathered for the sample ('multi-channel analyzer').
- WDS and EDS each have their strengths and
their weaknesses (more below)
- Standards and calibration: the
quantitative analytical routine, in a nutshell, is in two parts: first you determine the
x-ray count rate on standards (should be well characterized and homogeneous material),
then you collect x-rays on your unknown usually the same conditions, and then ratio the
two. That gives you the "k-ratio", which is roughly equivalent to the weight
fraction of your unknown, if the standard were pure element. The second part is that there
must be a matrix correction: that is, the x-rays you collect reflect several processes,
with a major one being absorption as they exit the sample. This correction has several
formulations, with 'ZAF'' a standard type.
- Good standards are essential, and you may
be called upon to produce them yourself if we do not have appropriate ones. Always ask
- Images are very important, both for
determining locations to analyze, as well as documenting what you analyzed. Backscattered
electron (BSE) images are one main image collected, and are produced rapidly for areas up
to 1-2 mm in dimension. Larger areas are possible but might take several hours. X-ray maps
can also be obtained, and generally take significantly longer to produce, due to the lower
efficiency of X-ray acquisition by WDS. Alternately, x-ray maps using EDS can also be
obtained, but the thru-put of x-rays in the channels per element is not much better than
WDS, so again it takes some time for larger areas.
- BSE images can give important information
about the spatial relationships of adjacent phases, plus about zoning and inclusions
within phases. BSE intensity is, to a first approximation, a function of the chemical
composition: the brighter an area, the greater the mean atomic number of that area
relative to adjacent areas. Thus ilmenite, magnetite and chromite will be brighter than
most adjacent silicate minerals. Apatite or zircon similarly. Olivine or pyroxene are
brighter than adjacent feldspars, though the Fe/Mg ratio is of course important.
- Spatial resolution of a chemical
analysis: because of the elastic scattering of the electrons, the x-rays are produced in
an 'interaction volume' that can be several microns across and deep. This area is a
function of two factors: the composition itself, so that in high atomic number materials
the scattering is less than in lower atomic number ones; and the accelerating voltage: the
more energy the electrons have, the greater distance they can scatter. It is possible to
utilize minimal accelerating voltage, to minimize the scattering. I may be possible, in
specifica cases, to constrain most of the electrons and thus most of the generated x-rays,
to a layer of say 1 micron. But this is not typical....everything is in the details, like
the exact composition of your sample.
- Light element analysis: this is somewhat
complicated, and is do-able in theory, and in reality in some cases. Appropriate
standards, conductive samples (no carbon coating necessary) and low accelerating voltages
- Trace element analysis: what do you mean
by trace? We can measure down to the hundred ppm level in many samples, depending on their
nature and how well they can stand up to high current. In some samples, 50 ppm or below is
possible, but again "it all depends on what you got". Trace element mapping is
- Coating of specimens -- if your samples
are not conductors, they need to be coated, and ONLY coated in our lab if you are using
our standards. We have had too many problems with samples coated elsewhere.
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