EDS
Elemental
Analysis Through Energy Dispersive X-ray Spectrometry
Operating Principles
One of the instruments
most commonly used in conjunction with the SEM is the Energy Dispersive
X-ray Spectrometer (EDS). The x-ray spectrometer converts a x-ray photon
into an electrical pulse with specific characteristics of amplitude
and width. A multi-channel analyzer measures the pulse and increments
a corresponding "energy slot" in a monitor display. The location
of the slot is proportional to the energy of the x-ray photon entering
the detector. The display is a histogram of the x-ray energy received
by the detector, with individual "peaks," the heights of which
are proportional to the amount of a particular element in the specimen
being analyzed.
The locations of
the peaks are directly related to the particular x-ray "fingerprint"
of the elements present. Consequently, the presence of a peak, its height,
and several other factors, allows the analyst to identify elements within
a sample, and with the use of appropriate standards and software, a
quantitative analysis can be made of elements with atomic number of
4 (carbon) or greater.
Combining the EDS
system with the SEM allows the identification, at microstructural level,
of compositional gradients at grain boundaries, second phases, impurities,
inclusions, and small amounts of material. In the scanning mode, the
SEM/EDS unit can be used to produce maps of element location, concentration,
and distribution.
Limitations of
EDS
The design of the
equipment makes the technique incapable of detecting elements lighter
than carbon. Sensitivity (ability to detect the presence of an element
above background noise) is 0.1 wt% with the EDS. There is also poorer
sensitivity for light elements (low atomic weight) in a heavy matrix.
Resolution of the x-ray energy levels
limits the positive
identification of certain elements (i.e., molybdenum and sulfur) due
to overlapping energy slots.
Quantitative analysis
is usually limited to flat, polished specimens. Unusual geometries,
such as fracture surfaces, individual particles, and films on substrates
can be analyzed, but with considerably greater uncertainty.
Standardized
Methods
The standard guide
for the performance of energy dispersive x-ray spectroscopy is covered
in ASTM E 1508, "Quantitative Analysis by Energy-Dispersive Spectroscopy."
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