What is energy dispersive spectrometer?
Energy dispersive spectrometer (EDS)
is an instrument for analyzing the elements of a substance. It is often used in conjunction with a scanning electron microscope or transmission electron microscope to bombard the surface of a sample with an electron beam under a vacuum chamber to excite the substance to emit characteristic x-rays
EDS records all X-ray spectra simultaneously and is used to measure X-ray intensity as a function of X-ray energy. It is a rapid micro-area composition analysis method that does not damage the specimen. Qualitative analysis of elements is performed by measuring the characteristic X-ray energy at which the material is excited, and quantitative analysis is performed by measuring the characteristic X-ray intensity.
Application of energy dispersive spectrometer
Energy dispersive x-ray spectroscopy
is widely used in metals and oxides, semiconductor materials, new energy materials, thin film materials, ceramic materials, and other fields.
EDS spectrometer is used to determine the quantitative or qualitative analysis of a specified point on a sample. The method is quantitatively accurate, preferred for low content elements, and better for microstructure analysis of components, such as precipitated phases, precipitates, inclusions, etc.
Example: Point analysis of a sample.
EDS energy dispersive spectrometry is used to determine the distribution of the content of an element along a given straight line. By fixing the energy spectrometer at the position of the characteristic X-ray signal of the element to be measured and scanning the electron beam point by point along the specified direction, the variation of the characteristic X-ray intensity of the element along the line is obtained, thus reflecting the variation of the content of the element along the line.
Example: Line scan analysis of a sample
The characteristic X-ray count output from EDS is used to modulate the image of elemental distribution formed by the brightness of the pixel spot corresponding to the electron beam scan specimen on the monitor as the surface distribution image. The greater the brightness in the region, the higher the elemental content. This method has the lowest quantitative accuracy and is usually used to analyze elemental deviations in materials, etc.
Example: Facet analysis of a sample
Electrical conductivity meter is a scientific device used in lab
How does energy dispersive spectrometer work?
In EDS, the characteristic X-rays are special because the X-ray energies emitted by different elements are different, just like a person's fingerprint, and are unique. The elemental analysis using different energies of characteristic X-rays is called the energy dispersion method.
The specimen is excited with characteristic X-rays that irradiate directly through the window onto the Si(Li) semiconductor detector, causing the Si atoms to ionize and produce a large number of electron-hole pairs, the number of which is proportional to the X-ray energy, i.e.
N = E / ε, where ε is the energy generated by producing one electron-hole pair (3.8 eV). For example, FeKα with an energy of 6.403 keV can produce 1685 electron-hole pairs.
By biasing the Si(Li) detector (typically -500 to -1000 V), the electron-hole pairs can be separated and collected, converted to current pulses by a preamplifier, then converted to voltage pulses by a main amplifier, and sent to a multi-channel pulse height analyzer. The output pulse height is determined by N, forming the horizontal coordinate of the EDS profile: energy. The intensity of the different elemental X-rays can be determined from the number of characteristic X-rays recorded in different intensity ranges, forming the vertical coordinate of the EDS profile: intensity.
Energy dispersive spectrometer advantages
The use of energy dispersive X-ray spectroscopy has the following advantages:
(1) Detection efficiency
The lithium drift silicon detector in the energy spectrometer is significantly larger than the wave spectrometer for the stereo angle of the X-ray emission source, so the former can receive more X-ray signals, and therefore the detection efficiency of the energy spectrometer is higher.
(2) Spatial analysis capability
Because of high detection efficiency, energy spectrometers can work under smaller electron beam flow, so that the beam spot diameter is reduced and spatial analysis ability is improved. At present, the smallest micro-area analyzed by the energy spectrometer in the micro-beam operation mode in the analytical electron microscope has reached the order of nanometers.
(3) Resolution capability
The best energy resolution of an energy spectrometer is 149 eV, and the wavelength resolution of a wave spectrometer is equivalent to 5-10 eV when expressed in the form of energy, so the resolution of a wave spectrometer is an order of magnitude higher than that of energy spectrometer.
(4) Analysis speed
An energy spectrometer can detect and count the energy of all X-ray photons in the analysis point at the same time, and it only takes a few minutes to get the full-spectrum qualitative analysis results.
(5) The range of analyzed elements
The beryllium window of the Si(Li) detector in the spectrometer absorbs X-rays of ultra-light elements and can analyze elements after sodium (Na).
Some people always think that EDS is a semi-quantitative analysis, and the results will be more biased. EDS is the most convenient, fast, accurate, and reliable analysis method for micro-area composition analysis, and the stability and reproducibility of the data are good, its accuracy is second only to WDS, can reach 2-10%, and the median atomic number without overlapping peaks of the main element quantitative error in 2-3%, detection limit of 0.1-0.5%. Reliability usually decreases as atomic number decreases and elemental content decreases. The measurement depth is in the micron range.
Silicon drift detectors (SDD), large stereo angle detectors, and various software processing advances have also led to a further reduction in EDS measurement errors.
The energy spectrometer has no special requirements for the sample surface, which needs to be dry solid and carrier table can be placed, and free of magnetism, radioactivity, and corrosion. If the sample conductivity is very poor, can be sprayed with gold or spray carbon treatment.
Energy dispersive spectrometer analyzes the range of elements
The elements that can be analyzed by the energy dispersive spectrometers are affected by the type of window material. The conventional beryllium window can only analyze elements after sodium (Na) because it absorbs X-rays of ultra-light elements, while the organic film ultra-thin window can analyze all elements between (Be)-Uranium (U).
Energy dispersive spectrometer specifications
|Analysis chamber vacuum||5 X 10-9 mbar
|Resolution|| 0.5 eV
|Sensitivity||4 Mcps@1.0 eV
|Basic configuration|| vacuum sample transfer chamber, atomic and cluster dual mode ion source
1. Energy dispersive spectrometer using advanced algorithms, Direct-to-Phase software can extract and display known phases while the data is still being collected.
2. Allows the EDS system to determine if enough statistics have been collected or to visualize the data being created.
3. Utilizes rule-based identification and peak deconvolution methods to quickly and accurately identify elemental peaks.
FAQ of energy dispersive spectrometer
Why EDS measurement of light elements is not accurate?
Light elements, usually those with atomic numbers less than sodium, face the same problems that affect the accuracy of the analysis results, whether analyzed by wave spectrometer or EDS energy spectrometer.
The characteristic X-ray yields of light elements are low. When collecting the spectra, the counts are insufficient, the spectral peaks are low and the shapes of the spectral peaks are irregular.
The characteristic X-ray energy of light elements is low. They are easily absorbed within the sample matrix, generating a large number of oscillating electrons; and the X-rays emitted from the surface layer are absorbed in the detector window, so the quantitative analysis has to make a large absorption correction, which brings errors.
The efficiency of the Si(Li) detector is wide, accepting X-rays in the energy range of 1.5KeV~15KeV, and the efficiency is close to 100%, however, the detector efficiency decreases significantly at the low energy end of less than 1.5KeV, which is mainly due to the serious absorption of low energy X-rays by the window in front of the detector, and at the high energy end more than 15KeV, because the high energy X-rays may completely penetrate the silicon crystal and run out. The thickness of the Be window is about 8 microns, and only 60% of the low-energy X-rays such as Na pass through the window, while only 1% of the oxygen can pass through. To detect ultra-light elements such as B, C, N, O, F, etc., ultra-thin window detectors are now widely used, using ultra-thin film plastic instead of Be windows, with a thickness of less than 1 micron, simple structure, and easy to use.
The spectral peaks of EDS have many peak positions corresponding to one element, does it mean that the element is very high in content?
EDS is an electron shell layer of electrons by foreign particles or energy excitation, leaving a vacancy, and then the outer electrons leap to this vacancy, and at the same time will release characteristic X-rays, so that the energy difference caused by the transfer of electrons between different shell layers will have different spectral lines, EDS spectral lines are obtained by separating the accumulation of these characteristic X-ray pulses. In this way, the more spectral lines, the more electrons outside occupy the shell layer. The quantitative analysis is based on different elements to choose the intensity of the spectral peak of different lines and the response value of this element to do the calculation, so the spectral peak is not related to the elemental content.
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