X-Ray Spectroscopy

BY- K. Sai Manogna (MSIWM014)

X-ray spectroscopy is a tool that detects and analyses photons with wavelengths in the X-ray section of the electromagnetic spectrum or particles of light. It is used to help scientists understand an object’s chemical and elemental properties. Many distinct X-ray spectroscopy techniques are used in science and technology, including archaeology, astronomy, and engineering. These approaches are used separately to construct a complete image of the substance or entity being studied. 

History:

1. In 1901, a German physicist, Wilhelm Conrad Roentgen, was awarded the first Nobel Prize in physics for the discovery of X-rays in 1895. 

2. According to the SLAC National Accelerator Laboratory, his new invention was rapidly put to use by other scientists and doctors. 

3. Between 1906 and 1908, Charles Barkla, a British physicist, conducted research that contributed to his discovery that X-rays could be typical of individual substances. He also received a Nobel Prize in physics for his work, but not until 1917. 

4. In fact, the use of X-ray spectroscopy started a bit earlier, in 1912, beginning with William Henry Bragg and William Lawrence Bragg, a father-and-son team of British physicists. 

5. To research how X-ray radiation interacted with atoms inside crystals, they used spectroscopy. 

6. By the following year, their method, called X-ray crystallography, had become the standard in the field, earning the Nobel Prize in physics in 1915. 

X-ray Absorption Spectroscopy (XAS) 

The absorbed photon’s energy lifts an electron from a deeply bound state into unoccupied bound states in x-ray absorption spectroscopy (XAS), or it gains enough energy to exit the atom. Thus, the absorption spectrum provides extensive information on the density of empty states and makes it possible to conclude coordination, the state of oxidation, and much more about the local structure. If the photon’s energy is sufficient to surpass the electron’s binding potential, the likelihood of absorption is affected by the mechanism of electron dispersion from the local atmosphere of the surrounding atoms. This system, called EXAFS, can be used to determine the local structure around the absorbed atoms. 

Instrumentation: 

Main parts of this instrument include:

1. An X-ray source, 
2. The sample holder, 
3. An X-ray monochromator, and 
4. A detector 

An X-ray is produced from the source by bombarding a heavy metal target with high energy electrons. The spectrum of the energy of the released X-rays influences the option of heavy metal. For instance, a tungsten target generates X-rays of more incredible energy than a silver (Ag) target. Most of the energy that drives this bombardment process is lost as heat, so it is essential to cool the target electrode. More modern sources and other X-ray sources, such as the Stanford synchrotron beamline, are more effective. 

a. Source for X-ray: 

1. the X-ray source aims to supply the sample with X-ray radiation so that either X-ray Fluorescence or absorption experiments can be carried out. 

2. Atoms absorb X-rays from the source, and the wavelength of the absorbed X-ray in X-ray absorbance and the strength of that absorbance provides the identity of that atom, and concentration is consumed. 

3. This X-ray absorption causes the electron that absorbs the X-ray to be ionized. 

4. The atomic orbital electrons that absorb this light, in their orbitals, are very similar to the nucleus. 

5. Absorbed X-rays in X-ray fluorescence cause an atomic electron to be expelled, that is, atomic ionization, and the void is subsequently filled by an electron from an orbital further from the nucleus. 

6. The outer electron must emit energy in order to drop down to fill the hole. This emitted light is fluorescence from X-rays. 

b. Samples are exposed to X-rays: 

1. The monochromator in an X-ray instrument is very different from a wavelength grating or prism monochromator that is visible (400 to 700 nm) or UV (200 to 400 nm). 

3. Since the X-ray wavelengths are so short (say, 0.1 to 1 nm), it is not practical to scatter light using a prism or (grating) closely spaced grooves. Instead, contact with crystals of high purity is required; they function like a grating. 

4. The crystals are mounted on a movable stage at which the angle at which the incoming X-rays hit the crystal can be continuously and smoothly varied. 

5. The angle at which X-rays are detected to disperse from the crystal is also varied at twice the angle of entry. 

6. The crystal stage has rotated by 80 degrees in the picture below, so the detector stage has rotated by 160 degrees at this moment. 

7. By the collision of X-rays with high purity argon (Ar) gas, the detector mentioned here produces an electronic signal, an X-ray photon transducer. 

8. The collision causes Ar ionization and the development of free electrons that flow to a positive electrode. The detector’s current flux between the electrodes is proportional to the incoming X-rays: a signal, Voilà. 

c. Detector for X-ray: 

1. this double-stage rotation generates the X-ray spectrum with a wavelength on the x-axis and absorbance on the y axis as the detector signal is captured. 

2. Energy plotted for a fluorescence spectrum is on the x, and fluorescence emission is on the y axis. 

3. An absorbance spectrum is given below. K-edges are considered the shortest wavelength (highest energy) absorbance of elements studied by X-ray. 

4. Longer absorbances for wavelengths are L-edge, M-edge. The absorbance edge shape is very typical of the atoms involved in the absorbance when the function is closely examined. 

5. To assess the oxidation state of heavy metal atoms and whether the heavy metal atom is bound to carbon or hydrogen, modern K-edge X-ray spectra can be used. 

6. In other words, X-ray spectroscopy can be used to determine the chemical environment of heavy metal atoms in complex samples by spectral fitting to the available specifications. 

7. The atmosphere here means the environment for atomic bonding. 

:

One of the pioneers who helped in the production of X-ray emission spectroscopy was Karl Manne Georg Siegbahn from Uppsala, Sweden (1924 Nobel Prize). He painstakingly developed numerous diamond-ruled glass diffraction gratings for his spectrometers (also called X-ray fluorescence spectroscopy). He measured high precision X-ray wavelengths of several elements, using high-energy electrons as a source of excitation. 

With synchrotrons, intense and wavelength-tunable X-rays are now usually produced. In a material, relative to the incoming beam, the X-rays can suffer a loss of energy. This energy loss of the re-emerging beam reflects the atomic system’s internal excitation, an analogous X-ray to the well-known Raman spectroscopy typically used in the optical field. 

Highly accelerated electrons are bombarded with a piece of metal wire called an anticathode. The metal piece becomes a source of radiation from X-ray. With a crystal spectrometer, this radiation can be analyzed. 

The spectrum of emissions is composed of two parts: 

(a) Continuous spectrum 

(b) Line spectrum 

It consists of a line spectrum with a continuum of history. X-ray fluorescence generates X-radiation that only has a line spectrum without a continuous spectrum background. 

(a). Continuous spectrum: 

1. The continuous spectrum depends little on the metal used for the anticathode; with the increase of the metal’s Z, the curve’s height increases, but the curve’s form is independent of z. ν_max is entirely independent of the anticathode metal used. 

I (v) = constant Z (v_max– v)

2. The curve depends heavily on the voltage V used for electron acceleration. 

3. With the voltage V, the maximum frequency increases proportionally. 

eV = hv_max 

4. Since the continuous spectrum is highly dependent on the velocity of the incident electron, it can be concluded that these electrons emit the corresponding X radiation. 

Classical Explanation: 

1. They are subjected to intense electrostatic forces arising primarily from the nuclei of the constituent atoms as electrons traveling at high velocities enter the anticathode. 

2. The electron is enormously accelerated, and the electrostatic charges emit electromagnetic waves, according to classical radiation theory, and the higher the acceleration, the higher the frequency. 

3. It is the sudden slowing down of the electrons responsible for the continuous spectrum when they penetrate the anticathode; this can be defined as deacceleration radiation, but the German term Bremsstrahlung is also used. 

The characteristics: 

1. The spectrum of the line primarily depends on the material from which the X-rays come, either the X-ray tube anticathode or the absorbing material used in a fluorescence experiment. 

2. The spectral lines’ frequencies are independent of the electron-accelerating voltage and the incident radiation frequency. It only depends on the chemical components of which the substance is composed. 

3. The frequencies are properties of the chemical elements’ atoms.

Several Applications :

In science and technology fields, including archaeology, astronomy, engineering, and health, X-ray spectroscopy is used today. 

– By studying them with X-ray spectroscopy, anthropologists and archaeologists can reveal secret knowledge about the ancient artifacts and remains they discover. For example, to determine the sources of obsidian arrowheads produced by prehistoric people in the North American Southwest, Lee Sharpe, associate professor of chemistry at Grinnell College in Iowa, and his colleagues used a tool called X-ray fluorescence (XRF) spectroscopy. 

– X-ray spectroscopy also allows astrophysicists to learn more about how space phenomena function. 

– Researchers at Washington University, for instance, are preparing to observe X-rays that come from interstellar phenomena, such as black holes, for the future prospectus. 

– The team, led by an experimental and theoretical astrophysicist, Henric Krawczynski, is preparing to launch a form of X-ray spectrometer called an X-ray polarimeter. 

– The instrument will be suspended in the Earth’s atmosphere by a long-term, helium-filled balloon beginning in December 2018. 

– Yury Gogotsi, a chemist and materials engineer at Drexel University in Pennsylvania, uses materials analyzed by X-ray spectroscopy to create spray-on antennas and water desalination membranes. 

– The invisible spray-on antennas are only a few hundred nanometers thick but can relay radio waves and steer them. 

A technique called X-ray absorption spectroscopy (XAS) ensures that the fragile material composition is right and helps assess the conductivity. To study the surface chemistry of complex membranes that desalinate water by filtering out particular ions, such as sodium, Gogotsi, and his colleagues also use X-ray spectroscopy. 

– X-ray spectroscopy can also be used in various medical research and practice areas, such as modern CT scanning machines. 

– According to Phuong-Anh T. Duong, Director of CT at Emory University Department of Radiology and Imaging Science, Phuong-Anh T. Duong, Director of CT at Emory University Department of Radiology and Imaging Science, Collecting X-ray absorption spectra during CT scans (via photon counting or spectral CT scanner) may provide more accurate information and contrast on what is going on inside the body, with lower radiation exposures from the X-rays and fewer or no need to use contrast materials (dyes)

X-rays advantages: 

a. Cheapest

b. most convenient and commonly used tool.  

c. X-rays are not absorbed by air, so the specimen does not have to be in an evacuated chamber. 

Disadvantages of X-rays: 

With lighter elements, they do not interact very strongly.

Atomic absorption spectroscopy

BY- K. Sai Manogna (MSIWM014)

Atomic absorption spectroscopy (AAS) is a method in analytical chemistry for determining the concentration of a specific metal element in a sample. The process can be used in a solution to analyze the concentration of over 70 different metals. While atomic absorption spectroscopy dates from the nineteenth century, a team of Australian chemists primarily developed the modern form during the 1950s. They were headed by Alan Walsh and served in the Chemical Physics Division of the CSIRO (Commonwealth Science and Industry Research Organisation) in Melbourne, Australia. 

By applying characteristic wavelengths of electromagnetic radiation from a light source, atomic absorption spectrometry detects elements in either liquid or solid samples. Wavelengths can be absorbed differently by individual components, and these absorbances are calculated against expectations. In effect, AAS takes advantage of the various wavelengths of radiation that different atoms absorb. In AAS, analytes are first atomized so that their characteristic wavelengths are emitted and registered. When those atoms consume particular energy during excitation, electrons go up one energy level in their respective atoms. 

These atoms emit energy in the form of light as electrons return to their original energy state. There is a wavelength of this light that is characteristic of the element. According to the light wavelength and intensity, relevant elements can be detected, and their concentrations determined according to the light wavelength. 

Principle: 

The approach uses absorption spectrometry to determine an analyte’s concentration in a sample. Thus it relies heavily on the Beer-Lambert rule. In short, by consuming a given amount of energy, the atoms’ electrons in the atomizer can be promoted to higher orbitals for a short period. This quantity of energy is unique to a specific transformation of electrons in a particular element, and each wavelength corresponds to only one element in general. This gives its elemental selectivity to the process. 

Since the amount of energy placed into the flame is known and it is possible to calculate the amount remaining on the other side of the detector, it is possible to estimate from the Beer-Lambert law how many of these transitions have occurred and thus obtain a signal proportional to the concentration of the measured product.

The Instrumentation 

There are four components of the standard AAS instrument: the sample introduction region, the source of light (radiation), the monochromator or polychromator, and the detector. 

It needs to be atomized in order to test a sample for its atomic constituents. The light could then illuminate the sample. Finally, the light emitted is measured through a detector. A spectrometer is usually used between the atomizer and the detector to minimize the effect of the atomizer’s emission (e.g., black body radiation) or from the atmosphere. 

Types of Atomizer:

Usually, the method uses a flame to atomize the sample, but other atomizers are also used, such as a graphite furnace or plasmas, particularly inductively coupled plasmas. 

It is side-long (usually 10 cm) and not deep when a flame is used. The flame’s height above the burner head can be adjusted by changing the fuel mixture’s flow. At its longest axis (the lateral axis), a ray of light passes through this flame and reaches a detector. 

Liquid analysis 

A liquid sample is usually converted in three stages into an atomic gas: 

1. The liquid solvent is evaporated (Drying), and the dry sample remains 

2. Vaporization (Ashing)-the solid specimen vaporizes into a gas 

3. Atomization is divided into free atoms by the compounds that make up the sample. 

Sources of Radiation 

The chosen radiation source has a narrower spectral range than that of the atomic transitions. 

Cathode Hollow Lamps 

The most common source of radiation in atomic absorption spectroscopy is hollow cathode lamps. A cylindrical metal cathode holding the metal for excitation and an anode is inside the lamp, filled with argon or neon gas. Gas particles are ionized when a high voltage is applied to the anode and cathode. Gaseous ions gain sufficient energy to eject metal atoms from the cathode as the voltage increases. Some of these atoms are excited, releasing light with the characteristic frequency of the metal. Various modern hollow cathode lamps are selective for several metals. 

Lasers with diodes 

Lasers, especially diode lasers because of their strong properties for laser absorption spectrometry, can also conduct atomic absorption spectroscopy. The method is then either referred to as diode laser atomic absorption spectrometry (DLAAS or DLAS) or, since wavelength modulation is most commonly used, spectrometry of absorption of wavelength modulation. 

Context Methods of Correction:

The spectral overlap is unusual due to the limited bandwidth of hollow cathode lamps. That is, an absorption line from one element is unlikely to overlap with another. Molecular emissions are much larger, so a specific molecular absorption band is more likely to overlap with an atomic line. This can lead to artificially high absorption and an improperly high measurement of the solution concentration. In order to correct this, three methods are usually used: 

Zeeman correction: A magnetic field is used to break the atomic line into two sidebands. To still overlap with molecular bands, these sidebands are close enough to the initial wavelength, but far enough, they do not overlap with the atomic bonds. It is possible to equate the absorption in the presence and absence of a magnetic field, the difference being the absorption of interest atomically. 

Correction to Smith-Hieftje: This was invented by Stanley B. Smith and Gary M. Hieftje. The high current pulses the hollow cathode lamp, creating more significant atoms and self-absorption population during the pulses. This self-absorption allows the line to be broadened, and the line intensity decreases at the original wavelength. 

Deuterium lamp correction: In this case, for the calculation of background emissions, a different source known as a broad-emission deuterium lamp is used. The use of a specific lamp makes this method the least reliable, but this method is most widely used because of its relative simplicity and the fact that it is the oldest of the three.

Advantages of AAS are given below: 

1. Strong throughput of samples 
2. Simple to make use of 
3. High accuracy 
4. Inexpensive methodology 

Disadvantages/drawbacks of AAS are as follows: 

1. It is only possible to evaluate solutions. 
2. Less sensitivity compared to the furnace with graphite 
3. Relatively large quantities of samples are needed (1-3 ml) 
4. Difficulties with refractory components

Types Of Spectroscopy

BY- K. Sai Manogna (MSIWM014)

Introduction:

Absorption spectroscopy:

Absorption spectroscopy is a technique that compares the power of a beam of light determined before and after a sample contact. It is also referred to as Tunable Diode Laser Absorption Spectroscopy (TDLAS) when done with a tunable diode laser. To decrease the device’s noise, it is most often paired with a modulation technique, most often wavelength modulation spectrometry (WMS) and sometimes frequency modulation spectrometry (FMS). 

Fluorescence spectroscopy: 

To excite a sample, fluorescence spectroscopy uses higher-energy photons, which will then release lower energy photons. This method is known for its biochemical and medical applications and can be used for confocal microscopy, energy transfer of fluorescence resonance, and lifetime imaging of fluorescence. 

X-ray spectroscopy: 

When X-rays with appropriate frequency interact with a material, the atom’s inner shell electrons are excited into empty outer orbitals, or they can be entirely expelled, ionizing the atom. Then electrons from the outer orbitals would fill the inner shell “hole.” In this de-excitation process, the energy available is released as radiation (fluorescence), or other less-bound electrons are extracted from the atom (known as Auger effect). The frequencies (energies) of absorption or emission are characteristic of the individual atom. Moreover, there are minor frequency variations for a single atom that is typical of chemical bonding. These specific X-ray frequencies or Auger electron energies can be determined with an appropriate instrument. In chemistry and material sciences, X-ray absorption and emission spectroscopy are used for determining the elemental composition and chemical bonding. X-ray crystallography is a method of scattering; X-rays are dispersed at well-defined angles by crystalline materials. If the incident X-ray wavelength is known, the distances between the atoms’ planes inside the crystal can be measured. The scattered X-ray intensities provide information about the atomic positions and measure the atoms’ arrangement within the crystal structure.

Flame:

Samples of liquid solution are aspirated into a combination of a burner or nebulizer/burner, dissolved, atomized, and often excited to a higher electronic state of energy. During analysis, the use of a flame includes fuel and oxidant, usually in gases. Gases such as acetylene (ethyne) or hydrogen are used as typical fuel gases. Oxygen, air, or nitrous oxide are common oxidant gases used. These methods can also analyze metallic element analytes in the concentration ranges of part per million, billion, or probably lower. In order to identify light with the analysis data coming from the flame, light detectors are required. 

Atomic Emission Spectroscopy: This technique uses the flame’s excitation; atoms are excited to emit light from the flame’s heat. The total consumption burner with a round burning outlet is usually used in this technique. A more significant temperature flame is usually used to induce analyte atoms’ excitation than atomic absorption spectroscopy (AA). Since the flame’s heat excites the analyte atoms, no particular elemental lamps must shine into the flame. A high-resolution polychromator can be used to generate an emission intensity vs. wavelength spectrum over a range of wavelengths exhibiting multiple-element excitation lines, meaning multiple elements can be detected in one run. Alternatively, a single wavelength monochromator may be set to focus on studying a single element at a specific emission line. A more advanced variant of this process is plasma emission spectroscopy. 

Atomic absorption spectroscopy (often referred to as AA) – A pre-burner nebulizer (or nebulizing chamber) is widely used to produce a sample mist and a slot-shaped burner that gives a longer flame pathlength. The flame temperature is low enough that sample atoms are not excited from their ground state by the flame itself. The nebulizer and flame are used to dissolve and atomize the sample, but for each type of analyte, the analyte atoms’ excitation is achieved by using lamps that glow through the flame at different wavelengths. The amount of light absorbed after passing through the flame defines the analyte quantity in the sample in AA. For greater sensitivity, a graphite furnace is typically used to heat the sample for desolvation and atomization. The graphite furnace process can also analyze any substantial or slurry samples. It is still a widely used analysis method for some trace elements in aqueous (and other liquid samples, due to its strong sensitivity and selectivity. 

Atomic Fluorescence Spectroscopy: A burner with a circular burning outlet is widely used in this technique. To solve and atomize the sample, the flame is used. However, a lamp shines a light into the flame at a particular wavelength to excite its analyte atoms. Then the atoms of some components will fluoresce, emitting light in another direction. For quantifying the amount of analyte component in the sample, this fluorescent light’s strength is used. A graphite furnace is also used for atomic fluorescence spectroscopy. This technique is not as widely used as spectroscopy of atomic absorption or plasma emission. 

Plasma Emission Spectroscopy:

It has virtually replaced in several respects similar to flame atomic emission spectroscopy. 

1. Direct-current plasma (DCP) An electrical discharge between two electrodes creates a direct-current plasma (DCP). It needs a plasma support gas, and Ar is standard. Samples could be deposited on one of the electrodes, or one electrode can be built up by conducting them. 
2. Glow discharge-spectrometry of optical pollutants (GD-OES) 
3. Plasma-atomic emission spectrometry, inductively coupled (ICP-AES) 
4. Laser-Induced Breakdown Spectroscopy (LIBS) (LIBS), also called plasma spectrometry induced by laser (LIPS) 
5. Plasma caused by microwave (MIP) 

Spark or arc/emission spectroscopy – used in solid samples for the study of metallic elements. In order to make it conductive, a sample is ground with graphite powder for non-conductive materials. A sample of the solid was usually ground up and damaged during research in conventional arc spectroscopy methods. The spark or electric arc is passed through the sample to excite the atoms, heating the sample to a high temperature. The excited analyte atoms glow at different wavelengths, producing light that can be detected by standard spectroscopic methods. Since the conditions generating the arc emission are usually not quantitatively regulated, the study is qualitative for the components. Nowadays, under an argon atmosphere, spark sources with controlled discharges allow this method to be considered eminently quantitative, and its use is widely applied worldwide through the production control laboratories of foundries and steel mills. 

Visible Spectroscopy:

Many atoms emit visible light or absorb it. In order to achieve a continuum of fine lines, the atoms must be in the gas phase. It suggests the material has to be vaporized. In absorption or emission, the spectrum is studied. In UV/Vis spectroscopy, visible absorption spectroscopy is mostly paired with UV absorption spectroscopy. While this type may be unusual as a similar indicator is a human eye, it still helps identify colors. 

Ultraviolet light Spectroscopy: 

In the Ultraviolet (UV) field, all atoms are absorbed because these photons are energetic enough to excite outer electrons. Photoionization takes place if the frequency is high enough. In quantifying protein and DNA concentration and protein ratio to DNA concentration in a solution, UV spectroscopy is also used. Several amino acids, such as tryptophan, usually present in proteins, absorb light in the range of 280 nm, and DNA absorbs light in the 260 nm range. For this reason, in terms of these two macromolecules, the 260/280 nm absorption ratio is a good general measure of the relative purity of a solution. It is also possible to make fair estimates of protein or DNA concentration using Beer’s law. 

Infrared Spectroscopy:

The IR absorption spectrum analysis shows what kind of bonds are present in the sample, especially in organic chemistry. The study of polymers and components such as fillers, pigments, and plasticizers is also necessary. 

Raman Spectroscopy:

To study the vibrational and rotational modes of molecules, Raman spectroscopy uses the inelastic scattering of light. An interpretation help is the resulting ‘fingerprints.’ 

Coherent anti-Stokes Raman spectroscopy (CARS) is a recent technique for in vivo spectroscopy and imaging with high sensitivity and robust applications. 

Nuclear Magnetic Resonance Spectroscopy (NMR): 

To determine the various electronic local environments of hydrogen, carbon, or other atoms in an organic compound or other compounds, nuclear magnetic resonance spectroscopy analyses such as atomic nuclei’s magnetic properties. This is used to assist in assessing the compound structure. 

Photoemissions: 

Mossbauer: 

Mossbauer spectroscopy modes of transmission or conversion-electron (CEMS) probe individual isotope nuclei’s properties in various atomic environments by studying the resonant absorption of characteristic gamma-ray energy as the Mossbauer effect.

In the next chapter we will discuss in detail about each spectroscopic methods.