X-rays have a number of unique properties as radiation that go beyond their very short wavelength. One of their important properties for science is elemental selectivity. By selecting and examining the spectra of individual elements that are located in unique places in complex molecules, we have a localized "atomic sensor". By examining these atoms at different times after excitation of the structure by light, we can trace the development of electronic and structural changes even in very complex systems, or, in other words, we can follow the electron through the molecule and through the interfaces.
History
The inventor of radiography was Wilhelm Conrad Röntgen. Once, when a scientist was investigating the ability of various materials to stop rays, he placed a small piece of lead in position while a discharge was taking place. SoThus, Roentgen saw the first x-ray image, his own shimmering ghostly skeleton on a screen of barium platinocyanide. He later reported that it was at this point that he decided to continue his experiments in secret because he feared for his professional reputation if his observations were erroneous. The German scientist was awarded the first Nobel Prize in Physics in 1901 for the discovery of X-rays in 1895. According to the SLAC National Accelerator Laboratory, his new technology was quickly adopted by other scientists and doctors.
Charles Barkla, a British physicist, conducted research between 1906 and 1908 that led to his discovery that X-rays could be characteristic of certain substances. His work also earned him the Nobel Prize in Physics, but only in 1917.
The use of X-ray spectroscopy actually began a little earlier, in 1912, starting with the collaboration between father and son of British physicists, William Henry Bragg and William Lawrence Bragg. They used spectroscopy to study the interaction of X-rays with atoms inside crystals. Their technique, called X-ray crystallography, became the standard in the field by the following year, and they received the Nobel Prize in Physics in 1915.
In action
In recent years, X-ray spectrometry has been used in a variety of new and exciting ways. On the surface of Mars there is an X-ray spectrometer that collectsinformation about the elements that make up the soil. The power of the beams was used to detect lead paint on toys, which reduced the risk of lead poisoning. The partnership between science and art can be seen in the use of radiography when used in museums to identify elements that could damage collections.
Working principles
When an atom is unstable or bombarded by high energy particles, its electrons jump between energy levels. As the electrons adjust, the element absorbs and emits high-energy X-ray photons in a manner characteristic of the atoms that make up that particular chemical element. With X-ray spectroscopy, fluctuations in energy can be determined. This allows you to identify particles and see the interaction of atoms in various environments.
There are two main methods of X-ray spectroscopy: wavelength dispersive (WDXS) and energy dispersive (EDXS). WDXS measures single wavelength X-rays that are diffracted on a crystal. EDXS measures X-rays emitted by electrons stimulated by a high-energy source of charged particles.
The analysis of X-ray spectroscopy in both methods of radiation distribution indicates the atomic structure of the material and, therefore, the elements within the analyzed object.
Radiographic techniques
There are several different methods of X-ray and optical spectroscopy of the electronic spectrum, which are used in many fields of science and technology,including archaeology, astronomy and engineering. These methods can be used independently or together to create a more complete picture of the analyzed material or object.
WDXS
X-ray photoelectron spectroscopy (WDXS) is a surface-sensitive quantitative spectroscopic method that measures the elemental composition in a range of parts on the surface of the material under study, and also determines the empirical formula, chemical state and electronic state of the elements that exist in the material. Simply put, WDXS is a useful measurement method because it shows not only what features are inside the film, but also what features are formed after processing.
X-ray spectra are obtained by irradiating a material with an X-ray beam while simultaneously measuring the kinetic energy and the number of electrons that emerge from the upper 0-10 nm of the analyzed material. WDXS requires high vacuum (P ~ 10-8 millibars) or ultra-high vacuum (UHV; P <10-9 millibars) conditions. Although the WDXS at atmospheric pressure is currently being developed, in which samples are analyzed at pressures of several tens of millibars.
ESCA (X-ray Electron Spectroscopy for Chemical Analysis) is an acronym coined by Kai Siegbahn's research team to emphasize the chemical (not just elemental) information that the technique provides. In practice, using typical laboratory sourcesX-rays, XPS detects all elements with an atomic number (Z) of 3 (lithium) and higher. It cannot easily detect hydrogen (Z=1) or helium (Z=2).
EDXS
Energy Dispersive X-Ray Spectroscopy (EDXS) is a chemical microanalysis technique used in conjunction with scanning electron microscopy (SEM). The EDXS method detects X-rays emitted by a sample when bombarded with an electron beam to characterize the elemental composition of the analyzed volume. Elements or phases as small as 1 µm can be analyzed.
When a sample is bombarded with an SEM electron beam, electrons are ejected from the atoms that make up the surface of the sample. The resulting electron voids are filled with electrons from a higher state, and X-rays are emitted to balance the energy difference between the states of the two electrons. X-ray energy is characteristic of the element from which it was emitted.
The EDXS x-ray detector measures the relative amount of emitted rays depending on their energy. The detector is usually a silicon drift lithium solid state device. When the incident X-ray beam hits the detector, it creates a charge pulse that is proportional to the energy of the X-ray. The charge pulse is converted into a voltage pulse (which remains proportional to the X-ray energy) by means of a charge-sensitive preamplifier. The signal is then sent to a multichannel analyzer where the pulses are sorted by voltage. The energy determined from the voltage measurement for each incident X-ray is sent to a computer for display and further evaluation of the data. The X-ray energy spectrum versus count is estimated to determine the elemental composition of the sample size.
XRF
X-ray fluorescence spectroscopy (XRF) is used for routine, relatively non-destructive chemical analysis of rocks, minerals, sediments and fluids. However, XRF typically cannot analyze at small spot sizes (2-5 microns), so it is typically used for bulk analysis of large fractions of geological materials. The relative ease and low cost of sample preparation, as well as the stability and ease of use of X-ray spectrometers, make this method one of the most widely used for the analysis of major trace elements in rocks, minerals and sediments.
The physics of XRF XRF depends on fundamental principles that are common to several other instrumental techniques involving interactions between electron beams and X-rays on samples, including radiography techniques such as SEM-EDS, diffraction (XRD), and wavelength dispersive radiography (microprobe WDS).
The analysis of the main trace elements in geological materials by XRF is possible due to the behavior of atoms when they interact with radiation. When materialsExcited by high-energy short-wavelength radiation (such as X-rays), they can become ionized. If there is enough radiation energy to dislodge the tightly held inner electron, the atom becomes unstable and the outer electron replaces the missing inner one. When this happens, energy is released due to the reduced binding energy of the inner electron orbital compared to the outer one. The radiation has a lower energy than the primary incident X-ray and is called fluorescent.
The XRF spectrometer works because if a sample is illuminated with an intense X-ray beam, known as an incident beam, some of the energy is scattered, but some is also absorbed in the sample, which depends on its chemical composition.
XAS
X-ray absorption spectroscopy (XAS) is the measurement of transitions from the ground electronic states of a metal to excited electronic states (LUMO) and continuum; the former is known as X-ray Absorption Near Structure (XANES) and the latter as X-ray Extended Absorption Fine Structure (EXAFS), which studies the fine structure of absorption at energies above the electron release threshold. These two methods provide additional structural information, XANES spectra reporting the electronic structure and symmetry of the metal site, and EXAFS reporting numbers, types and distances to ligands and neighboring atoms from the absorbing element.
XAS allows us to study the local structure of an element of interest without interference from absorption by a protein matrix, water or air. However, X-ray spectroscopy of metalloenzymes has been a challenge due to the small relative concentration of the element of interest in the sample. In such a case, the standard approach was to use X-ray fluorescence to detect absorption spectra instead of using the transmission detection mode. The development of third-generation intense X-ray sources of synchrotron radiation has also made it possible to study diluted samples.
Metal complexes, as models with known structures, were essential for understanding the XAS of metalloproteins. These complexes provide the basis for evaluating the influence of the coordination medium (coordination charge) on the absorption edge energy. The study of structurally well-characterized model complexes also provides a benchmark for understanding EXAFS from metallic systems of unknown structure.
A significant advantage of XAS over X-ray crystallography is that local structural information around an element of interest can be obtained even from disordered samples such as powders and solution. However, ordered samples such as membranes and single crystals often increase the information obtained from XAS. For oriented single crystals or ordered membranes, interatomic vector orientations can be inferred from measurements of dichroism. These methods are especially useful for determining cluster structures.polynuclear metals such as the Mn4Ca cluster associated with the oxidation of water in the oxygen-releasing photosynthetic complex. Moreover, rather small changes in geometry/structure associated with transitions between intermediate states, known as S-states, in the water oxidation reaction cycle can be easily detected using XAS.
Applications
X-ray spectroscopy techniques are used in many fields of science, including archeology, anthropology, astronomy, chemistry, geology, engineering, and public he alth. With its help, you can discover hidden information about ancient artifacts and remains. For example, Lee Sharp, associate professor of chemistry at Grinnell College in Iowa, and colleagues used XRF to trace the origin of obsidian arrowheads made by prehistoric people in the North American Southwest.
Astrophysicists, thanks to X-ray spectroscopy, will learn more about how objects in space work. For example, researchers at Washington University in St. Louis plan to observe X-rays from cosmic objects such as black holes to learn more about their characteristics. A team led by Henryk Kravczynski, an experimental and theoretical astrophysicist, plans to release an X-ray spectrometer called an X-ray polarimeter. Beginning in December 2018, the instrument was suspended in the Earth's atmosphere with a helium-filled balloon for a long time.
Yuri Gogotsi, chemist and engineer,Drexel University of Pennsylvania creates sputtered antennas and membranes for desalination from materials analyzed by X-ray spectroscopy.
Invisible sputtered antennas are only a few tens of nanometers thick, but capable of transmitting and directing radio waves. The XAS technique helps ensure that the composition of the incredibly thin material is correct and helps determine conductivity. “Antennas require high metallic conductivity to perform well, so we have to keep a close eye on the material,” Gogotzi said.
Gogotzi and colleagues are also using spectroscopy to analyze the surface chemistry of complex membranes that desalinate water by filtering out specific ions such as sodium.
In medicine
X-ray photoelectron spectroscopy finds application in several areas of anatomical medical research and in practice, for example, in modern CT scanning machines. Collecting X-ray absorption spectra during a CT scan (using photon counting or a spectral scanner) can provide more detailed information and determine what is happening inside the body, with lower radiation doses and less or no need for contrast materials (dyes).
