X-Ray Spectroscopy- Definition, Principle, Steps, Parts, Uses
X-ray spectroscopy is a general term for several spectroscopic techniques that use x-ray radiation to characterize the materials by their chemical and elemental properties. X-ray radiation is a form of electromagnetic radiation that has wavelengths in the range of 0.01 to 10 nanometers, which corresponds to frequencies of 30 petahertz to 30 exahertz. X-ray radiation can penetrate through many types of matter and interact with the atoms and molecules within them.
X-ray spectroscopy is based on the principle that when an atom is excited by a high-energy photon or a charged particle, it can emit or absorb x-ray photons with characteristic wavelengths that depend on the atomic number and the energy level of the electron involved. By analyzing the spectrum of x-ray photons emitted or absorbed by a sample, one can identify the elements present and their relative abundances, as well as obtain information about the chemical bonding, crystal structure, electronic configuration, and physical state of the sample.
X-ray spectroscopy has many applications in various fields of science and technology, such as physics, chemistry, biology, geology, materials science, engineering, medicine, archaeology, and astronomy. It can be used to study the composition and structure of solids, liquids, gases, plasmas, and even stars and galaxies. It can also be used to investigate phenomena such as magnetism, superconductivity, catalysis, corrosion, diffusion, phase transitions, and surface reactions.
There are different types of x-ray spectroscopy techniques that differ in the source of x-ray radiation, the method of detection and analysis of x-ray photons, and the type of information obtained. Some of the most common techniques are:
- X-ray fluorescence (XRF): A technique that measures the x-ray photons emitted by a sample when it is irradiated by an external x-ray source. It is used to determine the elemental composition of a sample.
- X-ray diffraction (XRD): A technique that measures the diffraction pattern of x-ray photons scattered by a sample when it is irradiated by a monochromatic x-ray source. It is used to determine the crystal structure and phase of a sample.
- X-ray absorption spectroscopy (XAS): A technique that measures the absorption spectrum of x-ray photons by a sample when it is irradiated by a continuous or tunable x-ray source. It is used to determine the electronic structure and local environment of a specific element in a sample.
- X-ray photoelectron spectroscopy (XPS): A technique that measures the kinetic energy spectrum of electrons ejected from a sample when it is irradiated by an x-ray source. It is used to determine the elemental composition and chemical state of the surface and near-surface regions of a sample.
- X-ray emission spectroscopy (XES): A technique that measures the emission spectrum of x-ray photons from a sample when it is excited by an electron beam or another x-ray source. It is used to determine the electronic structure and valence state of a specific element in a sample.
In this article, we will discuss the principle, steps, parts, and uses of XRF as an example of x-ray spectroscopy technique. We will also briefly introduce some other types of x-ray spectroscopy techniques and their applications.
X-ray spectroscopy works on the principle of the excitation of core electrons that are orbiting in the lower shell(s) of atoms. As the electron absorbs x-rays, it becomes excited and jumps to a higher energy level. The X-ray region used ranges from 1 to 100 nm.
When x-rays interact with electrons, they can cause different effects depending on the energy of the x-rays and the atomic number of the element. Some of these effects are:
- Photoelectric effect: When an x-ray photon has enough energy to eject an electron from the inner shell of an atom, leaving a vacancy. This vacancy is then filled by an electron from a higher shell, releasing a photon with a characteristic wavelength that depends on the difference in binding energies between the two shells. This process is called X-ray fluorescence and it is the basis of X-ray fluorescence spectroscopy (XRF) .
- Compton scattering: When an x-ray photon collides with an outer shell electron and transfers some of its energy to it, changing its direction. The scattered photon has a lower energy and a longer wavelength than the incident photon. This effect is used to measure the effective atomic number of a material by X-ray transmission spectroscopy (XRT).
- Rayleigh scattering: When an x-ray photon interacts with an atom as a whole and is scattered without any change in energy or wavelength. This effect is negligible for high-energy x-rays and heavy elements.
- Auger effect: When an inner shell vacancy is filled by an electron from a higher shell, instead of emitting a photon, another electron from a higher shell is ejected. This process produces a secondary electron with a kinetic energy that depends on the binding energies of the three shells involved. This effect is used to measure the surface composition of a material by Auger electron spectroscopy (AES).
The principle of X-ray spectroscopy is to use these effects to identify and quantify the elements present in a sample by measuring the energy and intensity of the emitted or scattered photons. Each element has a characteristic X-ray spectrum that can be used as a fingerprint to distinguish it from other elements . By comparing the sample`s spectrum with the spectra of known standards, the elemental composition and concentration can be determined.
Some additional points are:
- X-ray spectroscopy can also provide information about the chemical state, crystal structure, bonding environment, and electronic properties of the elements in a sample by analyzing the fine details of their X-ray spectra .
- X-ray spectroscopy can be performed in different modes depending on the source and detection of the X-rays. For example, energy-dispersive X-ray spectroscopy (EDS) uses a semiconductor detector to measure the energy of incoming photons, while wavelength-dispersive X-ray spectroscopy (WDS) uses a single crystal to diffract and separate the photons according to their wavelengths.
- X-ray spectroscopy can be applied to various types of samples, such as solids, liquids, gases, powders, thin films, surfaces, etc., with different sample preparation methods depending on the technique and the purpose of analysis.
X-ray spectroscopy involves the interaction of X-rays with matter. When a sample is exposed to an intense beam of X-rays, some of the X-rays are scattered by the atoms in the sample, while some are absorbed and cause the emission of secondary X-rays. These secondary X-rays have characteristic wavelengths and energies that depend on the type and number of atoms in the sample. By measuring the intensity and wavelength of these secondary X-rays, one can identify and quantify the elements present in the sample.
The basic steps involved in X-ray spectroscopy are:
- X-ray generation: An X-ray source, such as an X-ray tube or a synchrotron, produces a beam of high-energy X-rays that is directed towards the sample. The X-ray source can have a fixed or variable wavelength, depending on the type of spectroscopy technique used.
- X-ray interaction: The incident X-ray beam interacts with the atoms in the sample, causing various effects such as elastic scattering, inelastic scattering, photoelectric effect, Compton effect, and fluorescence. These effects result in the emission of secondary X-rays from the sample, which have different wavelengths and energies than the incident beam. The intensity and wavelength of these secondary X-rays depend on the atomic number, electronic configuration, and chemical environment of the atoms in the sample.
- X-ray detection: The secondary X-rays emitted from the sample are collected and analyzed by a detector, such as a solid-state detector or a scintillation detector. The detector converts the X-ray photons into electrical signals that can be measured and processed. The detector can be either energy-dispersive or wavelength-dispersive, depending on how it separates and identifies the different wavelengths of X-rays.
- X-ray analysis: The data obtained from the detector is processed and interpreted by a computer software that compares the measured X-ray spectra with reference spectra of known elements. The software can identify and quantify the elements present in the sample based on their characteristic X-ray peaks. The software can also provide information about the crystal structure, chemical bonding, oxidation state, and phase composition of the sample based on the shape and position of the X-ray peaks.
The main components of an X-ray spectroscopy instrument are:
- X-ray generating equipment (X-ray tube)
X-ray generating equipment (X-ray tube)
X-rays can be generated by an X-ray tube, which is a vacuum tube that uses a high voltage to accelerate the electrons released by a hot cathode to a high velocity. The high velocity electrons collide with a metal target, the anode, creating the X-rays. The target material determines the characteristic wavelengths of the X-rays emitted. Common target materials are Rh, W, Mo and Cr .
A collimator is a device that narrows a beam of particles or waves. It causes the directions of motion to become more aligned in a specific direction (i.e., collimated or parallel). Collimation is achieved by using a series of closely spaced, parallel metal plates or by a bundle of tubes, 0.5 mm or less in diameter. A collimator reduces the divergence of the X-ray beam and improves the resolution of the spectrometer .
A monochromator is a device that separates and transmits a narrow portion of the optical signal chosen from a wider range of wavelengths available at the input. Monochromator crystals partially polarize an unpolarized X-ray beam. The main goal of a monochromator is to select a specific wavelength of X-rays for analysis and eliminate unwanted wavelengths. There are two types of monochromators:
- Metallic filter type: This type uses a thin metal foil to filter out unwanted wavelengths. The foil absorbs low-energy X-rays and transmits high-energy X-rays that are characteristic of the element being analyzed.
- Diffraction grating type: This type uses a crystal lattice to diffract the X-ray beam according to Bragg`s law. The diffracted beam has a specific wavelength that depends on the angle of incidence and the lattice spacing of the crystal. By changing the angle, different wavelengths can be selected.
The detector is a device that measures the intensity of the incoming photons and converts them into an electrical signal. The detector can be either energy-dispersive or wavelength-dispersive.
- Energy-dispersive detector: This type uses a semiconductor material, such as silicon or germanium, to detect the energy of each photon. When a photon hits the semiconductor, it creates an electron-hole pair that is swept away by an electric field and collected by an electrode. The amount of charge collected is proportional to the energy of the photon. An energy-dispersive detector can measure multiple wavelengths simultaneously, but has lower resolution and sensitivity than a wavelength-dispersive detector .
- Wavelength-dispersive detector: This type uses a scintillator material, such as sodium iodide or cesium iodide, to detect the wavelength of each photon. When a photon hits the scintillator, it produces a flash of light that is proportional to the wavelength of the photon. The light is then amplified by a photomultiplier tube (PMT) that converts it into an electrical pulse. A wavelength-dispersive detector can measure only one wavelength at a time, but has higher resolution and sensitivity than an energy-dispersive detector .
A monochromator is a device that selects a narrow range of wavelengths from a broad spectrum of X-rays. It is used to improve the resolution and accuracy of X-ray spectroscopy by reducing the background noise and interference from other wavelengths. There are two main types of monochromators: metallic filter type and diffraction grating type.
Metallic Filter Type
A metallic filter type monochromator uses a thin metal foil or film to absorb unwanted wavelengths and transmit the desired ones. The choice of the metal depends on the energy range of the X-rays and the element to be analyzed. For example, aluminum filters can be used to remove low-energy X-rays and enhance the K-lines of elements with atomic number higher than 13. Nickel filters can be used to remove the K-lines of iron and enhance the L-lines of elements with atomic number higher than 26.
The advantages of metallic filter type monochromators are that they are simple, cheap, and easy to use. They also have high transmission efficiency and low scattering. The disadvantages are that they have low resolution and selectivity, and they may introduce errors due to absorption edge effects, fluorescence, and contamination.
Diffraction Grating Type
A diffraction grating type monochromator uses a periodic structure, such as a crystal lattice or a ruled surface, to diffract X-rays at different angles depending on their wavelengths. By adjusting the angle of incidence and the orientation of the grating, a specific wavelength can be selected and focused onto a detector. The choice of the grating depends on the energy range of the X-rays and the desired resolution. For example, lithium fluoride crystals can be used as gratings for low-energy X-rays (below 1 keV), while silicon or germanium crystals can be used for high-energy X-rays (above 10 keV).
The advantages of diffraction grating type monochromators are that they have high resolution and selectivity, and they can cover a wide range of wavelengths. They also have low absorption and fluorescence. The disadvantages are that they are complex, expensive, and sensitive to alignment and temperature changes. They also have low transmission efficiency and high scattering.
X-ray detectors are devices that measure the properties of X-rays, such as their flux, spectrum, spatial distribution, and energy. They are essential for obtaining images or spectra of the samples that are irradiated by X-rays. There are different types of X-ray detectors, depending on their working principle, design, and performance. Some of the most commonly used X-ray detectors are:
Gas-filled detectors: These detectors use a gas-filled chamber that ionizes when X-rays enter it. The ionized gas molecules produce an electric current that is proportional to the intensity of the X-rays. Gas-filled detectors are simple, cheap, and robust, but they have low energy resolution and efficiency. They are mainly used for dose measurement and low-resolution spectroscopy.
Scintillation detectors: These detectors use a scintillator material that emits light when X-rays strike it. The light is then converted into an electric signal by a photomultiplier tube (PMT) or a photodiode. Scintillation detectors have high sensitivity and efficiency, but they also have low energy resolution and suffer from afterglow effects. They are widely used for imaging and high-count-rate spectroscopy.
Semiconductor detectors: These detectors use a semiconductor material that generates electron-hole pairs when X-rays interact with it. The electron-hole pairs are collected by electrodes and produce an electric pulse that is proportional to the energy of the X-rays. Semiconductor detectors have high energy resolution and efficiency, but they also have high noise and require cooling. They are used for high-resolution spectroscopy and imaging.
Some examples of semiconductor detectors are:
Direct detectors: These detectors use a single semiconductor material that directly absorbs X-rays and produces electric pulses. Examples of direct detectors are cadmium telluride (CdTe), cadmium zinc telluride (CZT), and mercuric iodide (HgI2) detectors.
Indirect detectors: These detectors use a scintillator material that converts X-rays into visible light, and a semiconductor material that converts light into electric pulses. Examples of indirect detectors are silicon (Si), gallium arsenide (GaAs), and amorphous selenium (a-Se) detectors.
X-ray spectroscopy is a versatile and powerful technique that can be used for various purposes in different fields of science and industry. Some of the common applications of X-ray spectroscopy are:
- Research in igneous, sedimentary, and metamorphic petrology: X-ray spectroscopy can help to identify the mineral composition, crystal structure, and chemical bonding of rocks and minerals. It can also reveal the origin, evolution, and transformation of rocks under different conditions of temperature and pressure.
- Soil surveys: X-ray spectroscopy can provide information on the elemental composition, mineralogy, and geochemistry of soils. It can also detect the presence of contaminants, such as heavy metals, pesticides, and organic pollutants, in soils.
- Mining: X-ray spectroscopy can be used to measure the grade and quality of ore deposits, as well as to monitor the extraction and processing of ores. It can also help to identify and quantify the impurities and trace elements in ores and metals.
- Cement production: X-ray spectroscopy can help to control the quality and consistency of cement products by analyzing the chemical composition and phase distribution of raw materials, clinker, and cement. It can also detect the presence of harmful substances, such as free lime, alkalis, sulfates, and chlorides, in cement.
- Ceramic and glass manufacturing: X-ray spectroscopy can help to optimize the properties and performance of ceramic and glass materials by analyzing their chemical composition, crystal structure, and microstructure. It can also help to identify defects, cracks, and stresses in ceramic and glass products.
- Metallurgy: X-ray spectroscopy can help to improve the quality and reliability of metal products by analyzing their chemical composition, phase distribution, texture, and grain size. It can also help to detect corrosion, wear, fatigue, and fracture in metal components.
- Environmental studies: X-ray spectroscopy can help to assess the environmental impact of human activities by analyzing the elemental composition and speciation of particulate matter on air filters. It can also help to determine the sources and pathways of atmospheric pollutants, such as dust, smoke, aerosols, and metals.
- Petroleum industry: X-ray spectroscopy can help to characterize the quality and composition of crude oils and petroleum products by analyzing their sulfur content, hydrocarbon groups, aromatics, olefins, and naphthenes. It can also help to monitor the refining process and detect adulteration or contamination in petroleum products.
- Field analysis in geological and environmental studies: X-ray spectroscopy can be performed in situ using portable, hand-held XRF spectrometers that can provide rapid and non-destructive analysis of samples in the field. This can save time and cost compared to laboratory analysis.
These are some of the examples of how X-ray spectroscopy can be applied in various domains. However, there are many more potential applications that can benefit from this technique. X-ray spectroscopy is a valuable tool for scientific discovery and industrial innovation.
X-ray spectroscopy is an excellent method to determine the structure of a compound. In the event when other spectral methods fail to reveal a compound’s identity, X-ray spectroscopy is the method of choice for structural determination where the other parameters such as bond lengths and bond angles are also determined.
Some of the advantages of X-ray spectroscopy are:
- It can analyze elements from the entire periodic table, with the exception of H, He and Li.
- It can provide both qualitative and quantitative information about the elemental composition of the specimen.
- It can probe the surface and near-surface regions of the sample, which are often important for understanding the properties and interactions of materials .
- It can produce high-resolution images of the sample by mapping the spatial distribution of elements or chemical states.
- It can operate in various environments, such as vacuum, air, liquids or high pressures.
- It can use different types of X-ray sources and detectors to optimize the performance and sensitivity for different applications .
9. Limitations of X-Ray Spectroscopy
X-ray spectroscopy is a powerful and versatile technique for the analysis of materials, but it also has some limitations that need to be considered. Some of the main limitations are:
- X-ray spectroscopy requires a high-energy source of X-rays, such as an X-ray tube or a synchrotron, which can be expensive and complex to operate and maintain.
- X-ray spectroscopy can be affected by various sources of interference, such as background radiation, scattering, fluorescence, and matrix effects, which can reduce the accuracy and precision of the measurements.
- X-ray spectroscopy may not be able to detect elements with low atomic numbers (Z < 11), such as hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, and oxygen, because their characteristic X-rays are too weak or absorbed by the air or the detector.
- X-ray spectroscopy may not be able to distinguish between different chemical states or bonding environments of the same element, such as oxidation states or coordination numbers, because their characteristic X-rays have similar energies.
- X-ray spectroscopy may not be able to provide information on the molecular structure or crystallography of the sample, because it only probes the electronic structure of the atoms.
- X-ray spectroscopy may require sample preparation, such as grinding, polishing, coating, or mounting, which can alter the physical or chemical properties of the sample or introduce contamination.
- X-ray spectroscopy may damage or destroy the sample by causing ionization, heating, or radiation damage, especially for sensitive or biological materials.
Therefore, X-ray spectroscopy should be used with caution and in combination with other complementary techniques to obtain a comprehensive and reliable characterization of the sample.
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