Mary Johnson Consulting: How
the Electron Microprobe Works
(in
progress)
What is an electron microprobe?
This older technique is still best
for quantitative analysis of small regions. The surface of a sample is hit by a
focused beam of electrons, and gives off X-rays characteristic of the elements
present. These X-rays are focused onto X-ray detectors with curved crystals,
allowing relative amounts to be counted for specific wavelengths. Additional
processing is done to convert X-ray counts to quantitative chemical analyses.
Electron microprobes are expensive
(usually millions of dollars) and somewhat finicky machines, with many moving
and electronic parts. Often they required dedicated operators.
Parts/how it works:
The surface of a sample is hit
by a focused beam of electrons…
The electrons are given off from a
filament, once typically tungsten metal, and later often replaced by LAB6,
lanthanum hexaboride. Both these substances emit
electrons when a strong current is run through them. Since the electrons are
charged, they can be focused and accelerated through a hole in a charged plate.
The assemblage is referred to as an electron “gun.”
Electrons travel through an
evacuated column, shielded from outside magnetic fields, and through various focusing electron optics, so that they will be at a focus at the sample surface. The
focusing region at the sample surface is called the spot, and the spot size (diameter) is often measured; however,
X-rays are produced from a larger volume than this spot
alone, and excitation depth increases with increasing beam voltage.
The column ends in a central
chamber, also in a vacuum, containing the stage
on which the sample is held and several detectors
The
sample gives off X-rays characteristic of the elements present…
To
get the best results, the sample
being examined must have several characteristics. It must be flat and
well-polished. It should be dry and stable to exposure to vacuum. It needs to
be conductive, since otherwise it builds up a surface charge that can deflect
the incoming electron beam.
Samples
may not be stable under exposure to the electron beam; sodium is especially
known as a mobile element under electron bombardment. (Three techniques are
used to get the best results for sodium: use of a lower-voltage electron beam,
use of a larger spot size, and collection of sodium data as early as possible
in the analysis.)
How
do you know where you are on the sample? Microprobes usually can image in light
(either reflected light or cathodoluminescence –
luminescence from the electron beam) or in secondary
(re-emitted) or backscattered electrons,
with appropriate detectors.
To
change from one spot to another, either the electron beam can be moved (in two
dimensions, usually only over short distances), or the stage can be moved (in
three dimensions). Stage positions can be programmed in advance.
These X-rays are focused onto X-ray detectors
with curved crystals, allowing relative amounts to be counted for specific
wavelengths…
X-ray
paths cannot be bent like light rays or electrons; X-rays must be diffracted to
concentrate them on a detector. Depending on the energy range, specific curved
crystals are used to focus these X-rays so that they can be counted. The
crystals are tilted to select specific X-ray wavelengths. Traditional curved
crystals include KDP (potassium dihydrogen
phosphate), PET (pentaerythritol) and TAP (thallium
acid phthalate); these are used for X-rays given off by sodium and higher-atomic-number
elements.
More
recently, layered metal-oxide semiconductors have been grown by ion deposition,
and these can be used to focus the lower-energy (longer wavelength) X-rays from
the light elements boron, carbon, nitrogen, oxygen, fluorine and argon.
Additional processing is done to convert X-ray
counts to quantitative chemical analyses…
The
number and peak position of X-rays produced by a specific element in a sample
depend on several factors. Equipment factors include: the voltage and current of
the electrons reaching the sample, the gun-to-sample-to-detector geometry, and
the detector efficiency. Sample chemistry factors include the local chemistry
of the element in question (Is it a cation? An anion?
What ionic charge does it have? Is it in a metal or semi-metal?), the mean
atomic number of the sample, and the presence of other elements that can
preferentially absorb X-rays from the element in question. Other sample factors
include the quality of its polish, thickness of the sample, and the possibility
of decomposition under the electron beam.
More to come…
Comments? Send them to mlj@cox.net.