Electron Probe Microanalysis
What is electron probe microanalysis?
Electron probe microanalysis (EPMA), also known as electron probe X-ray microanalysis, is an instrument that uses the characteristic X-rays produced by the action of an electron beam on a sample to analyze the composition of microregions in thin sections of minerals. Qualitative and quantitative analyses are possible except for a few lighter elements such as H, He, Li, Be, and elements after U. The large volume of electron probe is to use a very narrow electron beam that has been accelerated and focused as a probe to excite a tiny area in the specimen to emit characteristic X-rays, and the wavelength and intensity of the X-rays can be measured for qualitative or quantitative analysis of the elements in the microregion.
Application of electron probe microanalysis
EPMA is mainly used to analyze fine particles or tiny areas on the surface of solid substances with a minimum range of about 1 μm in diameter. The elements analyzed range from atomic number 3 (lithium) to 92 (uranium). The absolute susceptibility can reach 10-14 to 10-15 g. In recent years, a combined scanning electron microscopy-microanalyzer device has been formed, which can analyze the chemical composition and structure of the specimen point by point while observing the morphology of the micro area.
Electron probe is widely used in geology, metallurgical materials, cement clinker research, materials science, mineralogy, metallurgy, criminology, biochemistry, physics, electronics and archaeology, etc. For any kind of solid stabilized in a vacuum, can be used for composition analysis and morphological observation.
Electron probe microanalysis working principle
The working principle of electron probe x-ray microanalysis is to excite the characteristic x-rays of a sample element by incidents the sample surface with a finely focused electron beam.
The wavelength (or characteristic energy) of the characteristic x-rays is analyzed to determine the type of elements contained in the sample (qualitative analysis).
The intensity of the x-rays is analyzed to determine the amount of the corresponding element in the sample (quantitative analysis).
The construction of the barrel part of the electron probe is largely the same as that of the scanning electron microscope, except that in the detector part an x-ray spectrometer is used, which is specifically designed to detect the characteristic wavelength or characteristic energy of x-rays as a means of analyzing the chemical composition of the microregion.
Therefore, in addition to the specialized electron probe instrument, a significant portion of the electron probe instrument is installed as an accessory on the scanning electron microscope or transmission electron microscope mirror brief to meet the needs of the trinity of micro-area tissue morphology, crystal structure, and chemical composition isotope analysis.
There is no essential difference between the barrel and sample chamber of the electron probe and the SEM, so to make an instrument with both morphological and compositional analysis, the scanning electron microscope and the electron probe are often combined.
Advantages of electron probe microanalysis
1. Micro-regionality, microscopic: A few cubic um ranges in the electronic probe microanalyzer can correspond to the micro-region chemical composition of the microstructure. The general chemical analysis, X-ray fluorescence analysis, spectral analysis, etc., is to analyze the average chemical composition of the sample in a larger range, but also can not correspond to the microstructure, and can not study the relationship between the microstructure of the material and material properties.
2. Convenience and speed: EMPA is simple to prepare samples and fast to analyze.
3. Diversification of analysis methods: Electronic probe x-ray microanalysis can continuously and automatically carry out a variety of methods of analysis, such as point, line, and surface analysis of sample x-ray. Automatic data processing and data analysis.
4. Wide range of applications: Electron probe microanalysis can be used for a variety of solid substances, materials, etc.
5. Wide range of elemental analysis: EMPA generally ranges from beryllium (Be4) – uranium (U92). Because H and He atoms have only K-layer electrons, they cannot produce characteristic X-rays, so they cannot be analyzed by electron probe composition. Although lithium (Li) can produce X-rays, the characteristic X-rays produced are too long to be detected usually. electron probe with a large surface spacing saponified film as a diffraction crystal has been able to detect Be elements.
6. No damage to the sample: EMPA can be preserved intact or continue other aspects of analysis and testing after sample analysis, which is especially important for rare sample analysis such as cultural relics, ancient ceramics, ancient coins, and criminal evidence.
7. The high sensitivity of quantitative analysis: the relative sensitivity is generally (0.01-0.05) wt%, the absolute sensitivity of detection is about 10-14g, and the relative error of quantitative analysis is (1-3)%.
8. Analysis while observing: For the phenomena observed under the microscope, all can be analyzed.
Structure of electron probe microanalysis
The basic structure of the electron probe microanalysis consists of an electro-optical system, a crystal spectrometer, an X-ray measurement, and recording device, an optical microscope, and transmission light source, a sample chamber, an automated analysis system, a vacuum, and auxiliary system, and a scanning display system.
Electron optical system
The electron optical system of an electron probe x-ray microanalyzer produces an electron beam of a certain energy, intensity, and smallest possible diameter, i.e., a stable source of x-ray excitation. Electron optical system consists of the following main parts.
(1) Electron gun: Produces an electron beam of sufficient brightness and speed
Filament: produces hot electrons
The gate: electrostatic focusing effect to form a 10μm-100μm cross
Anode: accelerating electron action
(2) Electromagnetic lenses
Concentrator: two or three levels, controlling the beam current to reduce the electron beam diameter by a few tenths to a few hundredths.
Objective lens: Adjusts the electron beam focal length and reduces the electron beam diameter
To block stray electrons with large scattering angles, so that the diameter of the electron beam incident on the sample is as small as possible, the converging lens and the objective lens have an optical diaphragm below.
(3) Scanning coil
Controlled by the scanning generator, the electron beam is scanned on the surface of the sample, while the picture tube is scanned simultaneously. So that the image observed by the tube, and the electron beam in the sample surface scanning area correspond.
(4) Dissipator
When the magnetic or electrostatic field in the electron optical system is not axisymmetric, it will produce image dispersion, so the original should be a circular intersection into an ellipse. The dissipator generates a magnetic field of the same size and in the opposite direction to the one causing the dispersion to eliminate the dispersion.
X-ray spectroscopic crystal spectrometer
The X-ray spectrometer in EMPA is composed of a spectroscopic crystal, an X-ray detector, and a spectrometer mechanism.
(1) Spectroscopic crystal
A grating or thin film material composed of atoms with the same array. Due to the limitations of the spectrometer design, the theta angle can only be changed within a limited range, so a given crystal can only detect X-rays in a certain wavelength range. For EPMA to analyze elements of Be-U, several sets of spectroscopic crystals with different grid spacing must be equipped. Usually, there are 3-5 spectrometers with 2 convertible spectroscopic crystals with different mesh spacing on each spectrometer.
The performance requirements of spectroscopic crystals are high diffraction efficiency, high resolution, large peak-to-back ratio, easy processing, and suitability for long-term use.
Commonly used crystals include isopentanol, thallium hydrogen phthalate, lithium fluoride crystals, lead stearate, etc.
The resolution of EPMA is determined by the resolution of spectroscopic crystals, so finding new crystals with high resolution is an important way to improve the resolution and sensitivity of electron probes.
(2) X-ray detectors
Commonly used positive proportional counter and scintillation counter. The electrical pulse is proportional to the energy of X-rays.
Proportional counter
Features: The output pulse is highly proportional to the photon energy of the input X-rays, with high sensitivity, short dead time, and a small response time of the output pulse.
(3) Scintillation counter
It consists of a scintillator, light pipe, and photomultiplier tube.
Electrons hitting the scintillator emitted by the light tube made of Plexiglas rods passed outside the vacuum chamber through the coupling interface accepted by the photomultiplier tube into an electrical signal.
(4) Principle of spectrometer spectroscopy
The characteristic X-ray wavelength λ produced by different elements can be measured when the crystal moves along the L line (L changes), and this spectrometer is called a direct-entry wave spectrometer.
X-ray measurement recording device
Requirements: Accurately display and record the X-ray pulse signal measured by the X-ray detector. Low noise, wide frequency band, high resolution, etc.
Working process: X-rays generated by the spectroscopic crystal enter the proportional counter tube, the signal from the proportional counter tube enters the preamplifier and the main amplifier (AMP), and the pulse height is analyzed in the single channel analyzer (SCA), the output pulse of the single channel analyzer is sent to the double channel counter for counting, the output signal of the single channel analyzer and the rate table is displayed on the CRT through the image selector to show the X-ray intensity distribution in one dimension (line profile) or two dimensions (X-ray image).
Optical microscopes and transmission illumination sources
Optical microscopes are used for precise positioning and collimation of the analyzed point.
Type: mostly reflective type.
Indicators: Resolution and focal length.
Transmission illumination light source: used for polarized light observation of geological samples, very useful for geological research.
Requirements: high brightness, large field of view.
Type: Transmissive/polarized, extractable.
Sample chamber
For mounting, exchanging, and moving samples. The sample can be moved in X, Y, and Z directions, and some sample tables can be tilted and rotated.
Automated analysis systems
With the development of computers in the late seventies, EPMAs were automatically controlled. The computer controls the sample stage of the electronic probe, the spectrometer, the electro-optical system, the analytical functions as well as the data acquisition and data processing. Analytical data and images can be processed and stored on the computer.
Vacuum system
The vacuum system reduces the chance of collision between cathode electrons and gas molecules to obtain the required electron beam. Improve the insulation between the anode and cathode, so that they can withstand the high potential difference, but not too high voltage breakdown.
The vacuum level is generally 0. 01 Pa-0. 001 Pa, usually with a mechanical pump.
Scanning display system
The electron beam is scanned on the surface of the sample and the fluorescence of the observed image, and the signals such as secondary electrons, backscattered electrons, and X-rays are sent to a CRT for image display or recording after the detector and signal processing. Now the instrument displays digital images and can be processed.
Types of electron probe microanalysis
Wavelength dispersive spectrometer(WDS)
Wavelength dispersive spectrometer uses different wavelengths of characteristic X-rays to spread the spectrum and realize the detection of different wavelengths of X-rays separately.
WDS mainly consists of a spectroscopic crystal, an X-ray detector, and a data processing system; the working principle of the wave spectrometer is that, according to Bragg’s law, the characteristic X-rays emitted from the specimen are spectroscopically separated by a crystal with a certain crystal surface spacing d. The characteristic X-rays with different wavelengths λ will have different diffraction angles θ. By continuously changing θ, the characteristic X-rays with different wavelengths λ can be measured at a position 2 θ from the X-ray incidence direction. at different wavelengths λ.
Energy dispersive spectrometer(EDS)
Energy dispersive spectrometer that uses different energies of characteristic X-rays to spread the spectrum and realize the detection of different energies of X-rays separately, referred to as energy spectrometer.
EDS mainly consists of X-ray detectors, preamplifiers, pulse signal processing units, analog-to-digital converters, multi-channel analyzers, minicomputers, display, and recording systems, etc. The key component of the energy spectrometer is the lithium drift silicon semiconductor detector, which is customarily referred to as the Si (Li) detector. The working principle of the energy spectrometer is to classify each pulse based on the height of charge pulses into “channels” with different energy spans, with the “channel”, i.e., energy, as the horizontal coordinate, and the number of charge pulses entering the “channel” as the horizontal coordinate. The number of charge pulses entering the “channel” is the horizontal coordinate, and the number of charge pulses entering the “channel” is the vertical coordinate to obtain the energy spectrum of the sample.
Comparison of the main performance of WDS and EDS
| Compare content | WDS | EDS |
|---|---|---|
| Elemental analysis range | 4Be－92U | 4Be－92U |
| Quantitative analysis speed | slow | Quick |
| Resolution | High (≈5eV) | Low (130 eV) |
| Detection limit | 10-2（%） | 10-1（%） |
| Quantitative analysis accuracy | High | Low |
| X-ray collection efficiency | Low | High |
| Peak-to-back ratio (WDS/EDS) | 10 | 1 |
Electron probe microanalysis analytical methods
There are two basic analytical methods for electron probe microanalysis: qualitative analysis and quantitative analysis.
(1) Qualitative analysis
Qualitative analysis is a qualitative component analysis of a selected point (area) of a specimen to determine the elements present in the point area.
The principle of qualitative analysis: the point to be analyzed is selected with an optical microscope or on an image displayed on a fluoroscope, and a focused electron beam is shone on the point to excite the characteristic X-rays of the elements of the specimen at that point. The X-ray spectrum is detected and displayed with an X-ray spectrometer, and the elements present in the analyzed point specimen are determined according to the wavelength of the peak position of the spectral lines.
The corresponding energy of X-rays can be expressed as E=hv, where: E is the energy of X-rays; h is Planck’s constant. It can be seen that there is a one-to-one correspondence between the energy and frequency of X-rays, which is the basic principle of qualitative analysis by energy spectrometer.
(2) Quantitative analysis
Under the irradiation of stable electron beam, the intensity value of similar characteristic spectral lines (commonly used Kα lines) of each element should correspond to their concentration after the X-ray spectrum is deducted from the background count.
Electron probe microanalysis scanning methods
EPMA has two scanning methods: line scan analysis and surface scan analysis
(1) Line scan analysis
Line scan analysis is a slow scan of a focused electron beam along a selected line (through a particle or interface) within the observation area of a specimen.
The X-ray spectrometer is in the state of detecting the characteristic X-rays of a known element and derives the distribution of the characteristic X-ray intensity along the scan line of the specimen reflecting the change in the content of the element.
The X-ray spectrometer is in the state of detecting an unknown element, resulting in a distribution of elements along the scan line, i.e., which elements are contained in the line.
(2) Surface scan analysis
The focused electron beam is scanned as a two-dimensional raster; the X-ray spectrometer is in the state of detecting a certain element characteristic X-ray, and an image consisting of many bright spots is obtained, i.e. the X-ray scan image or the elemental surface distribution image.
The elemental content is high and the bright spots are dense. The distribution of the element in the specimen is determined according to the density and distribution of the bright spots on the image: bright areas represent high element content, gray areas represent low content, and black areas represent very low or non-existent content.
Hyperfine splitting of 57Fe excited and ground states
(Schematic representation of the six-fold peak of the magnetic hyperfine splitting in the 57Fe absorber energy level / corresponding Musburger spectrum)
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