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Facilities
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Energy Dispersive X-ray Spectrometer (EDS)
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Elemental Analysis Through Energy Dispersive X-ray Spectrometry
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Operating Principles
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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.
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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.
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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.
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Limitations of EDS
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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.
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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.
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Standardized Methods
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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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