What is an Electron Microprobe?
The electron microprobe, more formally
called the Electron Probe Micro Analyzer (EPMA), is based upon the electron
optical column of a conventional Scanning Electron Microscope (SEM), but
incorporates a hardware addition specifically designed for the accurate,
quantitative chemical analysis of solid materials. The application of this
instrument can be most easily explained by breaking down the component parts of
its acronym.
The "Electron
Probe" part
Like the SEM, the EPMA uses a primary
electron beam to stimulate signal emission. An important capability of the
EPMA, however, is the ability to fix the beam into an immobile "spot"
or probe of user-defined size and automatically monitored and regulated
current. This permits the selection of single locations for irradiation at a
constant electron flux over time.
The "Micro" part
With our instrument, the diameter of the
fixed spot can be varied in the range of 0.2 to 20.0 m m.
[Although larger spot sizes are available, they typically are not recommended.
Quantitative analysis of larger areas is commonly done by either replicate
analyses of small spots or by moving the sample beneath a fixed beam.] Thus,
the EPMA does not produce a bulk chemical analysis, but rather provides
information on small areas.
The "Analyzer"
part
Chemical analysis with the EPMA is performed
by the detection and counting of fluorescent x-rays that are produced by
electron transitions (from outer to inner orbitals) in atoms of the sample, the
transitions being stimulated by electron bombardment (by the primary beam).
Because the energy levels of electron orbitals for the atoms of a given element
are intrinsic, the fluorescent x-rays also have characteristic energies. As a
form of electromagnetic radiation, x-rays exhibit both particle- and wave-like
properties, permitting two different methods of detection. The particle-like
properties allow separation on the basis of energies, using a solid state
detector in a device known as the Energy-Dispersive X-ray Analyzer (EDXA). Many
modern SEMs, and our microprobe, are equipped with an EDXA, which has the
advantage of rapid analysis stemming from the simultaneous acquisition of the
entire x-ray spectrum. The rapidity of this process makes it an invaluable
qualitative tool for phase identification, and it can be used in a quantitative
capacity as well. Most elements, however, give rise to fluorescent x-rays of
several different energies, and very often the energy of the x-ray emission
from one element is similar enough to that of another that the two cannot be
distinguished (called x-ray "overlap" or "interference") by
EDXA.
The
EPMA also can sort fluorescent x-rays on the basis of their wave-like
properties utilizing one or more Wavelength-Dispersive Spectrometers (WDS):
these are the "added hardware" alluded to above. The WDS resolve
x-rays via diffraction through regular periodic solids in a manner very similar
to the way a prism can separate component colors from white light. Hence by
selecting the position and inter-planar spacing of the diffraction element, a
single x-ray emission line can be resolved and sent to a gas-filled,
"scintillation-type", detector for counting. WDS have far superior
x-ray resolution compared to the EDXA, and thus represent a much better tool
for the analysis of materials having elements with overlapping x-ray lines.
Superior peak/background intensity ratios for WDS also make them the tool of
choice for minor- to trace-level components and for light elements (which emit
low-energy x-rays), and yield minimum levels of detection commonly 1-2 orders
of magnitude lower than by EDXA.
How does the Electron Microprobe work?
double click the link for a demonstration
What is the Electron Microprobe for?
The very nature of the EPMA makes it ideally
suited to quantifying chemical compositions and compositional heterogeneity
within complex solid materials. Among others, this includes such tasks as
determining the compositions of individual phases in fine-grained multi-component
materials or characterizing chemical heterogeneity within large continuous
grains. Combined with capacities to image a material on the basis of its composition
(see Imaging Capabilities) and digital image
acquisition, you get a very powerful tool for characterizing and documenting
both compositions and phase distributions in complex and heterogeneous solids.
In addition, availability of the secondary electron imaging mode used in conventional
SEMs provides topographic (surface morphology) characterization for such applications
as phase discrimination, surface reaction mechanisms, and component failure
analysis.
Go
Back To EMPL Home Page