X-ray spectroscopy

X-ray spectroscopy is an extremely valuable spectroscopic technique. It helps in investigating the elemental properties of a chemical substance and is useful for its structural elucidation. We all have heard a lot about X-rays already, especially their ability to penetrate the human body and photograph our bones. But do you know the working principle behind this technique? If not, then this article is definitely for you. In this article, we will study in detail what is X-ray spectroscopy and how is it important for analytical purposes.

What is X-ray spectroscopy

X-ray spectroscopy is a type of atomic spectroscopy. It is based on the interaction of X-rays  (0.01-10 nm) radiations of the electromagnetic spectrum with individual matter atoms.

The matter atoms absorb high-energy X-ray photons. As a result of this energy absorption, their electrons undergo excitation from a low to a high energy level. This excitation is followed by de-excitation consequently releasing energy. The unabsorbed incident X-ray photons are diffracted.  This energy absorption, emission, and diffraction is then collectively used to study the structure of the target compound and its electronic distribution.

What are X-rays

X-rays represent a specific region of the electromagnetic spectrum. X-rays possess an extremely short wavelength (0.01-10 nm) and a high frequency or energy i.e., 3 x 10¹⁶ to 10²⁰ Hz. This is because of this high energy that X-rays can easily penetrate different chemical substances. The highest energy X-rays (5-10 keV) are known as hard X-rays. Contrarily, X-rays of comparatively lower energy (< 3 keV) are called soft X-rays.

What do X-rays measure

The X-ray wavelength is comparable to the size of an atom. Therefore, X-rays are ideal for studying the structural arrangement of atoms and/or molecules in a wide array of chemical substances. For instance, X-rays are suitable for measuring the interatomic distances between crystal atoms and the distribution of electrons at different places in metallic coordination complexes.  

Historical perspective of X-ray spectroscopy

X-rays were discovered for the first time back in 1895 by Wilhelm Conrad Rontgen, a German physicist, and a Nobel laureate. Several research studies were conducted between 1906-1917 on X-rays and their utilization in determining the characteristic properties of chemical substances.

 It was actually in 1912 that William Henry Bragg and William Lawrence Bragg, the father-son duo of British scientists studied the unprecedented interaction of X-ray radiation with the atoms present in a crystal. This technique was called X-ray crystallography at that time. The famous Bragg equation (we’ll talk about later in the article) is named after these two scientists. They received a Nobel prize in Physics in the year 1915, in reward for their service in the discovery of X-ray spectroscopy.

An X-ray spectrum with sharp and well-defined peaks was obtained on a photographic plate by Maurice de Broglie in 1913. Since then, there is no looking back on the advancements in the field of X-ray spectroscopy.

Different types of X-ray spectroscopy

The wider technique of X-ray spectroscopy can be sub-categorized into three different types:

• X-ray absorption spectroscopy: It is based on measuring the absorption of X-ray photons that lead to the excitation of atomic electrons. When an inner shell electron is promoted to a higher energy level, a position at the inner energy level is left empty. This is called a hole.  
• X-ray fluorescence spectroscopy: It is based on measuring the energy emitted by the de-excitation of electrons from a higher energy level to a lower energy level. An empty core is filled by this electronic de-excitation in the targeted atoms.
• X-ray diffraction spectroscopy: It is based on the dispersion or scattering of X-rays when they hit the targeted crystal atoms.  

4 main components of X-ray spectroscopy

An X-ray spectrometer is made up of the following main components:

1. Radiation source

• An X-ray tube is used as the main radiation source.
• Fast-speed electrons are released from a hot cathode. A high voltage is applied in the X-ray tube to accelerate these electrons.
• Intense X-ray beams are produced when these accelerated electrons collide with the oppositely charged anode.

2. Collimator

• A series of parallel metal plates are closely spaced to make a collimator.
• The purpose of the collimator is to align the X-ray beams in a specific direction of motion.

3. Monochromator

• Although X-rays represent a specific wavelength region in the electromagnetic spectrum still it is a range of wavelengths. However, in the spectroscopic analysis, only one wavelength should pass through the sample at a time.
• The purpose of the monochromator thus is to separate and choose a single wavelength from the given wavelength range.  
• Two different types of monochromators that are used in X-ray spectroscopy include a metallic filter type and a diffraction grating type monochromator. 

4. Detector

• Most commonly used detectors for X-ray spectroscopy are scintillation detectors and solid-state detectors.

Working principle of X-ray spectroscopy

Step I: Penetration of X-rays

Some photons transfer their energy to the electrons with which they are colliding while the rest are deflected away from their original travel direction. The absorption is measured using equation 1.

The crystal surface is illuminated with an intense X-ray beam. The X-rays penetrate into the crystal surface and interact with each crystal atom individually.  The distinct packets of energy present in an incident X-ray beam are called photons. The X-ray photons collide vigorously with atomic electrons.

Step II: X-ray photon absorption

Some photons transfer their energy to the electrons with which they are colliding while the rest are deflected away from their original travel direction. The absorption is measured using equation 1.

A=-log l_x/l_o……. Equation 1

where I_x is the intensity of absorbed radiations while I_o is the incident X-ray intensity.

Step III: X-ray diffraction

The diffracted X-ray beams are collected and measured using the Bragg equation (equation 2) to determine the pattern of electronic distribution in the crystal surface under study.

n λ = 2d sin θ……..  Equation 2

where n= integer to show the order of diffraction, λ= wavelength of radiations, d= distance between atomic layers in the crystal, and θ= angle at which diffraction occurs, also called the Bragg’s angle.

Bragg’s angle varies depending upon the structural composition of the crystal such as its geometric shape and surface orientation. Constructive interference occurs at Bragg’s angle while destructive interference occurs at all other angles. Therefore, radiation scatters at only a specific point in the crystal structure.

In this way, the diffraction pattern obtained via X-ray diffraction from a crystalline chemical substance acts like a fingerprint for that substance.

Step IV: Detection                                         

The detection is based on the absorption and/or diffraction of X-rays depending upon the chemistry of the targeted chemical substance and the type of X-ray spectroscopy being performed. The detector response is plotted against the energy of radiations in electron volts (eV). This is called the X-ray spectrum.

Usually, scientists combine both absorption and diffraction data for obtaining the complete structural profile of a chemical substance under investigation.

Why is X-ray spectroscopy important

• X-ray spectroscopy reveals the three-dimensional crystal structures of important chemical substances such as the sodium chloride (NaCl) crystal lattice and the hexagonal arrangement of carbon atoms in diamond.
• It also aids in studying the crystalline phase of a material such as the size of its atoms, size distribution, any defects or strains present in the crystal, etc. 

• X-ray spectroscopy allows both qualitative as well as quantitative structural analysis. Thus, in addition to the structural arrangement of atoms, specific bond lengths and bond angles can also be calculated using X-ray spectroscopy.
• X-ray spectroscopy enabled molecular biologists to study the structure of nucleic acids such as DNA and RNA.
• This technique also helps in the structural characterization of body tissues for disease diagnosis called X-ray radiography.

• X-ray diffraction spectroscopy helps find the degree of crystallinity present in a polymer.
• X-ray spectroscopy also supports the analysis of trace elements present in geological materials such as sediments, rocks, minerals, and ores.

Thus, X-ray spectroscopy is a validated structural characterization and identification technique that offers high accuracy and exceptional resolution.

Also, check out our article what are the uses of analytical spectroscopic techniques in chemistry to learn other interesting spectroscopic applications.

Limitations of X-ray spectroscopy

Like all instrumental analysis techniques, there are a few limitations associated with X-ray spectroscopy as well such as:

• The target compound must be exposed to X-ray radiations as a single crystal.
• Environmental interference may lead to inaccuracies in the results, especially when using X-ray spectroscopy for trace analysis.
• Running an X-ray spectrophotometer requires an adept technician.
• It is a tedious and time-consuming process overall.
• Longer exposure to high-intensity X-rays may damage chemical samples.

However, scientists are making continuous efforts to improve X-ray spectroscopic methods and instrumentation so there is always hope for the future. A spectroscopic technique that allows structural elucidation within seconds overcoming all the challenges associated with X-ray spectroscopy is NMR spectroscopy.

For a more basic understanding of spectroscopy, you may also like our article: Introduction to spectroscopy-Everything you would like to know about spectroscopy.

References

1. Agarwal, B. K. (1979). Interaction of X-Rays with Matter. X-Ray Spectroscopy: An Introduction. Berlin, Heidelberg, Springer Berlin Heidelberg: 121-179.

2. Błachucki, W., J. Czapla-Masztafiak, J. Sá and J. Szlachetko (2019). “A laboratory-based double X-ray spectrometer for simultaneous X-ray emission and X-ray absorption studies.” Journal of Analytical Atomic Spectrometry 34(7): 1409-1415.

3. Czapla‐Masztafiak, J., W. M. Kwiatek, J. Sá and J. Szlachetko (2017) “X‐Ray Spectroscopy on Biological Systems.” X-ray scattering.

4. Feiters, M. C. and W. Meyer-Klaucke (2020). Chapter 7 – X-ray absorption and emission spectroscopy in biology. Practical Approaches to Biological Inorganic Chemistry (Second Edition). R. R. Crichton and R. O. Louro, Elsevier: 229-273.