Energy dispersive X-ray spectroscopy

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Energy-dispersive x-ray spectroscopy ( English energy dispersive X-ray spectroscopy , EDX , EDX or EDS , including energy dispersive X-ray analysis , EDA , called) is one for X-ray spectroscopy associated measurement method of material analysis . The atoms in the sample are excited by an electron beam of a certain energy, they then emit X-rays with an energy specific for the respective element, the characteristic X-rays . This radiation provides information about the elemental composition of the sample. A similar method is the energy-dispersive X-ray absorption in which the absorption is evaluated instead of the emission.

Origin of the X-ray emission

Atomic model to explain the origin of X-ray emissions (EDX)

In order to emit characteristic X-rays in the sample, the atom must first be excited. This can be done by bombarding with electrons (e.g. in a scanning electron microscope ) or by exposure to X-rays ( X-ray fluorescence ). An electron is knocked out of one of the inner shells. Such a state is unstable and the resulting “gap” is immediately filled by a higher-energy electron from a higher atomic orbital . The energy difference is released in the form of an X-ray quantum. The resulting X-rays are characteristic of the transition and the atom, i.e. the element.

Different transitions are allowed for an element, depending on which shell the more energetic electron comes from and in which energy state (shell) the “gap” is to be filled. This creates X-ray quanta which are marked with K α , K β , L α , .... The energy of an X-ray line (position of the line in the spectrum) is an indicator of which element it is. The "strength" of the line depends on the concentration of the element within the sample.

Furthermore, the braking of electrons in the Coulomb field of the atomic nuclei creates X-ray braking radiation , which makes up the continuous background of the EDX spectrum.

How the detector works

The detector measures the energy of each incoming X-ray photon. Different variants of semiconductor detectors are often used. Typical are the Si (Li) detector , the silicon drift detector and detectors made of high-purity germanium . If an X-ray photon is absorbed in the sensitive area of ​​such a detector, electron-hole pairs are created there , the number of which is proportional to the energy of the photon. Statistical effects in the detector and electronic noise lead to a broadening of the natural line width, which is why some types of detector must be cooled. The typical energy resolution of a semiconductor detector is 120–140 eV.

EDX spectra and their evaluation

EDX spectrum of iron oxide

In the EDX spectrum, the signal intensity is plotted as a function of the energy of the X-ray quanta. The EDX spectrum consists of element-specific peaks and the broad, unspecific background that is generated by bremsstrahlung .

Peak identification, peak overlay and peak unfolding

For most elements there are multiple lines in the spectrum. When assigning lines, it must be checked whether all lines of an element are present and whether their intensities are in the correct relationship to one another. Possible peak overlaps with other elements must be taken into account. This is particularly important for peak unfolding when there is a superposition of signals from different elements. Alternatively, an additional measurement could be carried out with the higher-resolution wavelength-dispersive X-ray spectroscopy (WDX).

Quantitative analysis

The quantitative analysis of EDX spectra depends on many factors, such as: B. absorption , fluorescence , sample tilting, excitation energy. For most elements with an atomic number greater than eleven (i.e. from sodium ) the detection limit can be roughly estimated at 0.1% by weight. The detection limit becomes significantly worse for elements with a lower atomic number. Theoretically, all elements with an ordinal number greater than four (i.e. from boron ) can be detected with windowless detectors.

Lateral resolution of the analysis

The local accuracy of a measurement in the scanning electron microscope is limited by the depth of penetration of the electron beam into the material. When the beam hits the material, it is scattered in the sample, so that the emitted X-rays are created in a pear-shaped volume with a diameter of 0.1-2 µm. The size of the excitation bulb is smaller in materials with a higher density and with a lower accelerating voltage. If the acceleration voltage is chosen too small, however, peaks of higher energy can no longer be excited and the corresponding elements can no longer be detected.

A higher spatial resolution can be achieved when the EDX detector not having a grid , but with a transmission electron microscope is combined (TEM): Since the sample is prepared as a very thin lamella (<100 nm) for TEM analysis, can the incident electron beam does not spread that far in the volume. In addition, the electrons are scattered much less because of the much higher acceleration voltage. The resolution is then only determined by the diameter of the electron beam and is smaller than 1 nm. However, artifacts from secondary excitations from the scattered electrons or the X-ray quanta (X-ray fluorescence) generated on the rest of the sample, on the holder, on microscope parts or on the Detector occur.

Due to the relatively large range of X-rays in matter, the area analyzed for excitation with X-rays (X-ray fluorescence) is in the millimeter to centimeter range.

application

EDX detectors are used e.g. B. in the following analysis methods:

  • SEM-EDX: Combination with a scanning electron microscope for element analysis on a microscopic scale. The excitation is done by electrons. Due to the widespread use of this process, EDX is often used as the short form for REM-EDX. By combining the imaging raster method in SEM with element analysis (EDX), it is also possible to record element distribution images.
  • X- ray fluorescence analysis: In an energy dispersive X-ray fluorescence spectrometer (EDRFA), the excitation is carried out by X-rays and the sample emits X- rays based on the principle of fluorescence. This method allows a large-area analysis of compact samples.

Comparison with wavelength dispersive X-ray spectroscopy

An alternative is wavelength dispersive X-ray spectroscopy (WDS or WDX). EDX allows the simultaneous measurement of the entire X-ray spectrum of the analyzed sample point and thus the simultaneous analysis of all detectable elements , which brings a significant time and speed advantage. In contrast, the detection sensitivity with WDX is an order of magnitude better and, at the same time, a significantly higher spectral resolution of the X-ray spectrum is achieved.

See also

Web links

literature

Individual evidence

  1. ^ Maria Mulisch, Ulrich Welsch: Romeis microscopic technique . Springer, 2010, ISBN 978-3-8274-2254-5 , pp. 32 .
  2. ↑ Horst Biermann, Lutz Krüger: Modern methods of material testing . John Wiley & Sons, 2014, ISBN 978-3-527-67070-3 , pp. 232 ( limited preview in Google Book search).
  3. ↑ a b c d DC Bell, AJ Garratt-Reed: Energy Dispersive X-ray Analysis in the Electron Microscope . Garland Science, 2003, ISBN 978-0-203-48342-8 ( limited preview in Google Book Search).
  4. ↑ Frank de Groot, Akio Kotani: Core Level Spectroscopy of Solids . CRC Press, 2008, ISBN 978-1-4200-0842-5 , pp. 484 ( limited preview in Google Book search).
  5. ^ A b Günter Gauglitz, Tuan Vo-Dinh (Ed.): Handbook of Spectroscopy . 1st edition. Wiley-VCH, 2003, ISBN 3-527-29782-0 , pp. 386-387 .
  6. ↑ Hermann Salmang, Horst Scholze: Ceramics . Springer Science & Business Media, 2006, ISBN 978-3-540-63273-3 , p. 162 ( limited preview in Google Book search).