X-Ray Spectroscopy - Principle, Instrumentation, Steps and Applications

X-Ray Spectroscopy is a technique used for study of interaction between X-rays and matter. It is mainly used for determination of elemental composition and chemical state of a sample. It also gives information about physical properties of materials.

It is based on emission and absorption of X-rays by atoms. When a material is excited by high-energy radiation, the inner shell electrons of atom may be removed. Due to this, vacant space is produced in inner shell.

Then electron from outer higher energy shell comes down to fill this vacant space. During this process, extra energy is released as X-ray photon. This emitted X-ray has a particular energy or wavelength.

The energy of emitted X-rays is not same for all elements. It is different because the energy levels of atoms are different in each element. So, the emitted radiation work as a characteristic fingerprint of the element.

In X-ray spectroscopy, this characteristic radiation is measured and studied. From this spectrum, the elements present in the material can be known. The amount of element and their chemical condition can also be studied.

Henry Moseley showed the relation between X-ray frequency and atomic number. This relation helped to identify elements from their X-ray spectra. This is an important basis of X-ray spectroscopy.

Different methods are included under X-ray spectroscopy. Some important methods are X-ray fluorescence (XRF), Energy dispersive X-ray spectroscopy (EDS), Wavelength dispersive X-ray spectroscopy (WDS) and X-ray photoelectron spectroscopy (XPS).

XRF is used for elemental analysis. EDS is used for elemental mapping and analysis with electron microscope. WDS is used for more accurate wavelength based analysis. XPS is used for surface study and chemical state analysis.

The importance of X-ray spectroscopy is high in analytical and material science. It is used for study of metals, semiconductors, geological samples, biological tissues, thin films and environmental samples. It is also used in forensic analysis, corrosion study, industrial quality control and study of pigments in old artworks.

This technique is mostly non-destructive. So, the original sample mostly remain safe after analysis. For this reason, X-ray spectroscopy is very useful in research, industry and different scientific fields.

History of X-ray Spectroscopy

X-rays was discovered by Wilhelm Conrad Roentgen in 1895. He was a German physicist. He first called this radiation as X-light.
The discovery of X-rays had started the basis of X-ray spectroscopy. After this discovery, scientists studied the production of X-rays and their action with atoms.
William Henry Bragg and William Lawrence Bragg had given important early work in X-ray emission spectroscopy. They used X-ray tube and diamond ruled glass diffraction grating for measuring X-ray wavelength of many elements.

In 1912, Bragg’s law was given by them. This law is related with X-ray diffraction, wavelength and crystal plane. Later they got Nobel Prize in 1915 for this work.
In 1913 to 1914, Henry Moseley used X-ray spectrometer for studying characteristic X-ray emission. Pure elements were bombarded with electrons. Then the emitted X-rays were measured.
Moseley found a mathematical relation between frequency of emitted X-rays and atomic number. This relation is called Moseley’s law.

This law proved that atomic number is a measurable physical property of element. It also proved that periodic table should be arranged by atomic number, not by atomic mass.
Henry Moseley was killed during World War I at the age of 27. But his work became a major basis of atomic physics and X-ray spectroscopy.
After Moseley’s work, classification of X-ray spectra was continued. The K-series and L-series were first studied. These are high energy emission series.

Later longer wavelength transitions were searched. In 1922, Václav Dolejšek discovered N-series in heavy elements like Bismuth, Thorium and Uranium.
This work needed sensitive photographic detection and special high-current X-ray tubes. It helped in understanding more atomic shell structure.
In 1940s and 1950s, X-ray spectroscopy became more commercial in United States. Philips and Norelco Electronics started to develop and market complete X-ray diffraction units and spectrographs.

Due to this, the technique was changed from academic custom instruments to industrial analytical instruments. It became available for many laboratories and industries.
By 1961, Jet Propulsion Lab studied the use of X-ray spectrographic unit in Surveyor spacecraft. It was for analysis of composition of Moon surface.
Later detection system was improved. Photographic plates and gas-filled proportional counters were used first. Then modern solid-state detectors like Silicon Drift Detectors (SDDs) were developed.

Today X-ray spectroscopy is advanced with digital detectors, sensitive sensors and synchrotron radiation. It is used for study of elemental structure and electronic structure of different materials.

X Ray Spectroscopy Principle – Principle of x ray spectroscopy

X-Ray spectroscopy is based on the interaction between high-energy radiation and atoms present in the material. When a sample is exposed to intense beam of X-rays or charged particles like electrons, the atoms of sample become excited.

During this process, an electron from inner atomic shell is ejected out. Due to this, a vacant space is formed in the inner shell. This vacant space makes the atom unstable.

To become stable again, electron from higher energy outer shell comes down to fill the empty space. During this transition, excess energy is released from atom. This energy comes out in the form of secondary X-ray photon.

The energy difference between two atomic shells is fixed and different for each element. So, the emitted X-rays have characteristic energy or wavelength. This is used as fingerprint of the element.

The emitted X-rays are then measured by special detectors. The energy or wavelength gives the information about which elements are present in the sample. The intensity of emitted X-rays gives information about relative concentration of those elements.

X-rays are high-frequency waves on the electromagnetic spectrum. (Image credit: Shutterstock)

How X-ray spectroscopy works

X-ray spectroscopy works by studying the interaction of high-energy radiation with atoms of the material. The atoms have different electronic structure, so the response is also different.
First, the sample is bombarded with high-energy beam of X-rays or charged particles like electrons. This energy enters into the atoms of the sample.
If the energy is sufficient, it removes an electron from inner shell of atom. This may be K-shell or L-shell electron. Due to this, a hole or vacancy is formed.

After removal of inner electron, the atom becomes unstable and excited. This condition is not stable for the atom.
To get stability again, electron from higher energy outer shell moves down to fill the vacant place. This movement is called electronic transition.
During this transition, extra energy is released from the atom. This energy is emitted in the form of secondary or fluorescent X-ray photon.

The energy of emitted X-ray depends on the difference between two atomic shells. This difference is fixed for each element.
So, every element gives its own characteristic X-ray energy or wavelength. This is used as elemental fingerprint of that element.
The emitted X-rays are collected by special detector. The detector measures the energy or wavelength and also the intensity of emitted radiation.

The energy or wavelength tells which elements are present in the sample. The intensity tells about the amount or abundance of those elements in the sample.

Instrumentation of X-Ray Spectroscopy

Excitation source (X-ray generator)– It is used to produce high-energy radiation for excitation of atoms in the sample. In common laboratory instrument, X-ray tube is used as source. It has high-voltage supply, hot cathode and metal anode target. In some advanced instruments, synchrotron radiation or electron beam may be used as excitation source. These sources give very intense radiation. So, they are used for high sensitive and special analysis.
Beam-shaping optics (collimators)– The primary X-ray beam is passed through collimator. Collimator is made up of parallel metal plates or tubes. It is used to make the beam narrow and parallel. Collimator helps in improving the spatial precision of analysis. It controls the direction of X-ray photons. Due to this, the beam can fall properly on selected area of the sample.

Monochromator (diffractor or analyzer)– Monochromator is used to select a particular wavelength or energy of X-ray beam. It removes unwanted radiations from the beam. So, a narrow energy range is obtained. In many instruments, single diffraction crystal is used as monochromator. It works according to Bragg’s diffraction law. Optical grating or metallic filters may also be used in some instruments.
Sample stage (specimen holder)– Sample stage is the platform where the sample is kept. It holds the specimen firmly in the path of X-ray beam. It also helps in proper positioning of the sample. In Wavelength dispersive X-ray spectroscopy (WDS), sample stage may be connected with mechanical system or goniometer. It adjusts the angle of sample and detector. This is needed for accurate wavelength based measurement.
X-ray detector– Detector is used to collect the secondary X-ray photons emitted from the sample. It changes these photons into measurable electrical signals. This is one of the important parts of instrument. Different types of detectors are used in X-ray spectroscopy. Some important detectors are Silicon Drift Detectors (SDDs), scintillation detectors, gas-filled proportional counters and superconducting microcalorimeters. In scintillation detector, X-rays are first converted into light by crystal. Then light is converted into electrical pulse by photomultiplier tube. In SDDs, the emitted X-rays are directly detected with high sensitivity.

Pulse processor and analyzer– The electrical signals from detector are sent to pulse processor. It measures and processes the pulses produced by detector. Then the processed data are sent to computer. The computer shows the final X-ray spectrum. From this spectrum, energy, wavelength and intensity of emitted X-rays are studied. This gives information about elements and their amount in the sample.

Applications of X-Ray Spectroscopy

X-Ray spectroscopy is used for identification of historical paint pigments, authentication of artifacts and detection of forged objects. It is also used for non-destructive examination of ancient mummies, fragile scrolls and hidden paintings present under old canvas.
X-Ray spectroscopy is used for determination of elemental chemistry of extraterrestrial soils, rocks and geological features. It has been used in Mars rover missions like Curiosity, Spirit and Opportunity, and also in lunar rovers and comet landers.

X-Ray spectroscopy is used for environmental monitoring. It helps in tracing air pollution by analysis of airborne particulate matter or aerosols for toxic heavy metals, and also for monitoring contaminants in soil and water systems like rain water, river water and drinking water.
X-Ray spectroscopy is used in medical and healthcare field. It helps in diagnostic imaging like CT scan for detection of cancer, bone breaks and soft tissue damage, and also used for evaluation of biocompatibility of medical devices and drug delivery systems.
X-Ray spectroscopy is used in materials science and electronics. It helps in characterization of surface chemistry and electronic structure of semiconductors, mapping of thin films, and development of high-performance nanomaterials, catalysts and energy storage batteries.

X-Ray spectroscopy is used in industrial manufacturing and metallurgy. It helps in quality control of cement, ceramics and glass production, analysis of alloy composition and segregation, measurement of metallic coating thickness and non-destructive weld inspection.
X-Ray spectroscopy is used in geology and mining. It helps in exploration of natural resources, evaluation of ore grade and identification of mineral phases in igneous, sedimentary and metamorphic rocks.
X-Ray spectroscopy is used in food and beverage safety. It helps in non-destructive inspection of products for foreign materials like glass, bone or metal particles, and also helps in maintaining quality and food safety regulations.

X-Ray spectroscopy is used in security and defense. It helps in scanning cargo and vehicles at borders for detection of contraband materials, and also used for inspection of military materials such as propellants.
X-Ray spectroscopy is used in petroleum industry. It helps in determination of sulfur content in crude oils, diesel fuel and other petroleum products.
X-Ray spectroscopy is used in fundamental physics and chemistry. It played important role in rearranging periodic table by atomic number through Moseley’s law, and also helped in prediction of undiscovered elements like Hafnium and Technetium.

Advantages of X-Ray Spectroscopy

X-Ray spectroscopy is mostly non-destructive analysis technique. It allows the examination of materials without changing or damaging them, so it is useful for study of costly cultural artifacts, historical artworks and sensitive samples.
X-Ray spectroscopy gives useful material characterization. It provides qualitative and quantitative data about elemental identity, amount of elements, oxidation states and local coordination environment, which are not easily obtained by visible light or Infrared spectroscopy.
X-Ray spectroscopy is also useful for determining physical structure of compound. Bond length and bond angle can be studied by this method, especially when other spectral methods do not give proper result.

X-Ray spectroscopy has high sensitivity and precision. Special methods like Wavelength Dispersive Spectroscopy (WDS) and X-ray Fluorescence (XRF) give high spectral resolution and can separate overlapping elemental peaks.
X-Ray spectroscopy can detect trace elements at very low concentration. In some cases, elements can be detected at parts per million (ppm) or sub-ppm level.
X-Ray spectroscopy is fast and efficient technique. It is economical and user-friendly, and many elements can be detected at same time from a spectrum.

X-Ray spectroscopy gives real-time chemical information in many instruments. So, the analysis can be completed in short time and result can be obtained quickly.
X-Ray spectroscopy is versatile in nature. It can analyze different forms of samples like solids, liquids and powders.
X-Ray spectroscopy needs comparatively less sample preparation. It is less tedious than many trace analysis methods like wet chemistry or Mass spectrometry.

Modern X-Ray spectroscopy instruments can be compact and portable. Development of solid-state sensors has made the instruments low-weight and easy to carry.
Some modern X-Ray spectroscopy instruments do not need cryogenic liquid nitrogen cooling. Due to this, low power and easy field operation is possible.
X-Ray spectroscopy is suitable for in-field environmental monitoring, hand-held scanning and space exploration missions. It can be used outside laboratory also.

Limitations of X-Ray Spectroscopy

X-Ray spectroscopy gives mainly elemental composition of sample, but it cannot identify exact crystal structure or molecular structure of compound. It cannot easily differentiate molecular configuration, compound form or crystal phases like anatase and rutile form of Titanium dioxide.
X-Ray spectroscopy is mostly surface-sensitive in many cases because emitted fluorescent X-rays come out only from thin surface layer of sample. The depth of analysis may be from few micrometers to few millimeters, so the exposed surface must represent the whole bulk sample properly.
X-Ray spectroscopy may show spectral peak overlapping because elements having similar emission energies can give peaks at nearly same position. For example, Titanium K-β and Vanadium K-α peaks may overlap, so identification and quantification become difficult, especially in Energy Dispersive X-ray Spectroscopy (EDS).

X-Ray spectroscopy is affected by matrix effect where physical and chemical composition of sample can change the measured result. In self-absorption, sample absorbs its own emitted X-rays, and in secondary fluorescence, emitted X-rays excite other elements, so some elements may be overestimated or underestimated and correction like ZAF correction is required.
X-Ray spectroscopy has poor sensitivity for light elements like Beryllium, Carbon and Oxygen because they produce low energy X-ray photons. These low energy radiations are easily absorbed by sample itself, causing low count rate, high background interference and less precision.
X-Ray spectroscopy may require tedious sample preparation because solid samples are often ground into fine powder or melted into fused glass beads. This is done to reduce particle size effect and preferred orientation of crystals, but this process may be destructive, time consuming and may cause contamination from grinding tools.

Liquid sample analysis is difficult by X-Ray spectroscopy because direct liquid analysis can give high background scattering, solvent evaporation and bubble formation. So, liquid samples often need preconcentration steps like freeze drying or precipitation, which may take time and can introduce contamination.
Wavelength Dispersive Spectroscopy (WDS) is slower than EDS because it measures wavelengths one by one and not simultaneously. It can resolve peak overlap better, but the instrument is mechanically complex and also sensitive to vertical position or Z-height of the sample.
Advanced X-Ray spectroscopy instruments are costly. High-resolution WDXRF systems need very high financial investment, so these instruments are not easily available for small laboratories.

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X-Ray Spectroscopy – Principle, Instrumentation, Steps and Applications