X-ray Spectroscopy

Both the SEM and the TEM generate characteristic X-rays
due to beam/sample interactions. When an electron im-
pacts with an atom, probabilities are that a collision with
an inner-shell electron will result. If this inner-shell elec-
tron is ejected from the atom, an energy deficiency exists
within the atom, and an adjacent shell electron will drop
into the space vacated by the ejected electron. When this
transfer occurs, X-rays of energies and wavelengths spe-
cific to the atomic number of the atom are emitted. These
wavelengths and energies can be detected and are, there-
fore, extremely helpful in terms of chemical element iden-
tification. In some cases, compound identification can be
enhanced by quantification.
Two different systems presently are utilized to detect
and measure X-ray generation in electron beam instru-
ments: wavelength dispersive spectrometers (WDS) and
energy dispersive spectrometers (EDS).
Energy dispersive systems are more likely to be incor-
porated with a TEM or SEM, and wavelength systems, or
crystal spectrometers, are more commonly the basis for
an analytical TEM.
The wavelength dispersive X-ray analyzer is more diffi-
cult to operate and requires a higher energy electron beam.
Analysis time can be lengthy (minutes rather than seconds
for EDS) and multiple detectors must be used if more than
a few elements are to be analyzed. The positive aspectsof WDS are a better than 10-fold increase in resolution
(<10 eV), very low background, and analysis of light el-
ements as low as boron.
The EDS is simple to operate and easy to maintain. With
most samples, preselected instrument parameters and de-
tector position rarely need be changed. Of course, each
sample presents a unique set of aspects which must be
taken into account prior to analysis. Most qualitative tests
are quite simple and results are obtained in a matter of
seconds for all elements above sodium in atomic number.
The major drawback of EDS is the poor spectral resolu-
tion (145–180 eV), which can cause peak overlaps and
difficulty in analysis. For example, a sample containing
both Al (1486 eV) and Br (1480 eV) would have severe
overlap in that region of the spectrum if the unknown were
analyzed at low accelerating voltage (kV). If the voltage is
raised, and if Br is present, additional bromine lines would
appear at 11,907 and 13,287 eV, confirming the presence
of Br, since Al will display only the two lower energy
lines. In WDS, this would not be seen as a problem be-
cause the two low-energy lines would not overlap but be
clearly separated. Another drawback of EDS is the relative
difficulty in accurately and routinely analyzing elements
below atomic number 11, making carbonate or oxide anal-
ysis impossible. This is primarily due to detector design;
however, special detectors are currently available which
circumvent some of these problems.
Almost all X-ray analyzers are powerful computer-
based systems which allow for extensive data storage,
rapid access of files, and simple quantitative analysis of
samples. The combining of these spectrometers with elec-
tron beam microscopes has allowed routine detection of
<10−18 g of some elements, and since these areas are also
imaged by the instrumentation, this elemental analysis can
be related to microstructure.