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Energy Dispersive X-ray Spectrometer (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 6 (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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