X-ray Spectroscopy

X-ray spectroscopy is a common analytical technique with a broad range of applications in science and medicine. X-rays are an extremely short wavelength, high energy form of electromagnetic (EM) radiation that can be absorbed by core electrons within atoms. The electromagnetic radiation resulting from inner orbital electron transitions or deceleration of high-energy electrons is referred to as X-rays. The absorption, diffraction, emission, fluorescence and scattering of X-rays are exploited in a variety of X-Ray spectroscopic techniques that reveal useful information about the structure and composition of matter.

Typical applications of X-rays include: measurement of the energy of emitted X-rays during SEM imaging (X-ray Fluorescence) to give elemental information from a sample surface, diffraction of X-rays on crystalline materials to obtain their crystal structure (X-ray Diffraction), using X-rays to knock out core electrons of atoms to provide surface chemical information from samples (X-ray Photoelectron Spectroscopy), and medical imaging (X-rays and CT scanning). This was one of the earliest uses of X-rays, since it was noticed that X-rays passed through the human body, although dense materials such as bone absorb most of the X-rays, creating the image. The discovery of the X-ray photograph for diagnosing bone breakage was one of the most important discoveries in medicine. NASA’s Chandra X-ray Observatory has enabled breakthrough research in understanding how huge amounts of light are generated by supermassive back holes.

The discovery of the X-ray by Wilhelm Roentgen is interesting in that, like many scientific discoveries, it was made by accident. Roentgen was experimenting with uranium, went home for the weekend and left a large piece of uranium ore in a drawer on top of a sealed box of photographic film. When the film was developed after use, he noticed that it had been exposed accidentally. After some puzzling over the cause, Roentgen realized that the uranium ore was emitting g-rays and X-rays which had exposed the film. This lead to his experimenting with this new phenomenon, which led to the use of X-rays for imaging the human skeleton.

X-rays

X-rays are defined as having a wavelength of between 10-5Å to 100Å. Most applications however, use X-rays of between 0.1Å to 25Å. X-rays are generated by large energy transition of electrons within an atom from outer orbitals to core orbitals. Most X-rays are created by bombarding a metal target with energetic electrons. The electron beam typically ejects core electrons from the target metal, making the atoms unstable. The atoms relaxes from this position by dropping an outer valence electron to the core level. This large decrease in energy required for an electron to be able to drop to the core level requires the emission of the excess energy in the form of a photon. These photons have the energy of X-rays. Since X-rays are formed from the core of an atom they do not typically contain molecular information, although techniques such as XPS do contain some limited molecular information.

X-ray spectra are characteristically made up of narrow line spectra, although in most X-ray sources there exists a band of continuous radiation at higher energies, known as Bremsstrahlung (breaking radiation). This radiation is caused by collisions between the electron beam from the source with the atoms of the target material. During collision, the electrons are rapidly deccelerated, creating Bremsstrahlung radiation.

The most common means of generating X-rays involves using an X-ray tube, which can vary in shape and size, although all X-ray tubes contain the same basic elements. The tube is contained in an evacuated chamber to allow the electron beam an unobstructed passage. Electrons emitted by thermionic emission from a tungsten filament cathode are accelerated towards the anode target, typically made of copper, although oter metals can be used depending on the softness or hardness of the X-rays required. The anode is usually water cooled to prevent it melting from the intense heat created by the electron beam. The copper target emits X-rays with a characteristic wavelength. The X-rays pass through a beryllium window, consisting of a very thin piece of Be foil. This window absorbs many of the elastically scattered electrons producedby the target and allows the X-rays to get out of the tube without significant absorption.

X-ray Fluorescence (XRF)

X-ray Fluorescence culminates with the emission of a photon. An atom is induced into an excited state, usually by an electron beam removing a core electron from the atom. Another electron in a higher energy state drops to fill the core level vacancy, causing relaxation of the atom’s excited state. The large energy decrease required to allow this to occur produces the release of an X-ray photon.

There are three main types of XRF instruments in use today. They are wavelength dispersive, energy dispersive and nondispersive. These techniques are one of the most widely used analytical X-ray techniques for elemental analysis of elements of atomic number 8 (ie Oxygen) or higher. The XRF instrument contains a filament which creates the electrons, which are then focussed through the objective and condenser lenses onto the sample. Most instruments also include an optical system so that the operator can visually see the sample and position it.

In single channel wavelength dispersive instruments a diffracting crystal and detector are adjusted to the precise angle for each element which is scanned, and a signal detected until sufficient counts have been achieved. The energy dispersive instrument has no moving parts, and has the Si(Li) detector much closer to the sample, resulting in a stronger signal. As with wavelength dispersive, instruments can be either single channel or multichannel.

Essentially both techniques provide an elemental analysis, with the height or area of signal being proportional to the concentration of each particular element. The concentration can be made quantitative by analyzing standards prior to analysis of the sample. An interesting use of a microprobe is in obtaining X-ray maps of a sample surface, indicating where on the surface particular elements are located.

X-ray Photoelectron Spectroscopy (XPS)

Also known as ESCA (electron spectroscopy for chemical analysis), XPS is a form of electron spectroscopy where photoelectrons emitted from the top few atomic layers of a sample are sorted according to their kinetic energy by a hemispherical analyzer. XPS is most often used to measure the oxidation state of elements on the surface of a sample. XPS can detect all elements except H and He (owing to the abscence of core orbitals) and has found particular use for measuring the electronic properties of valence electrons in complex molecular structures.

XPS is in many ways the complete reverse of XRF in that an incoming X-ray photon causes the removal of a core or valence electron, although core electrons are more easily removed. These escaping electrons have a kinetic energy which is determined by the original photon and the binding energy of the atom. We can measure the kinetic energy of the electron and find out how tightly the electron was held in the atom. This is then used to find such information as oxidation state and bonding information. Like all X-ray techniques however, the information relates only to the top 100 Angstroms of the sample surface, making it useful for surface science techniques.

The typical instrument is made up of an X-ray tube emitting polychromatic photons which are then monochromatized to within 0.3 eV before being directed onto the sample. The electrons emitted from the sample are then accelerated towards an electron lens system before passing into an analyzer, typically hemispherical, which sorts out the electrons according to their kinetic energy. The top plate of the analyzer is negatively charged which bends the path of the electrons onto an electron multiplier. Many modern instruments contain a multichannel analyzer which are able to detect all kinetic energies simultaneously.

XPS spectra are usually displayed as a plot of intensity versus binding energy in eV. Binding energy is a measure of how tightly the photoelectron was originally held in the atom. It is measured as the difference between the kinetic energy of the X-ray photon, the kinetic energy of the emitted electron and the work function of the spectrometer. Two other forms of electron spectroscopy which are used are UPS (ultra-violet photoelectron spectroscopy), which uses UV photons to emit valence electrons from a sample, and AES (Auger electron spectroscopy) which analyzes photoelectrons produced by the decay of a neighboring electron to a lower empty orbital.

Auger Electron Spectroscopy (AES)

Auger spectroscopy is complimentary with XPS and most instruments allow both types of measurement to be made on a sample. The Auger process occurs in excited atoms which have a core electron removed. As in XRF, an upper electron drops to fill the vacancy, although instead of emitting a photon to release the excess energy, it transfers its energy to a neighboring electron which is then emitted from the atom. Auger electrons are of very low energy and are only emitted from the extreme surface of the sample. This is exploited by etching the surface of the sample with an ion beam, exposing progressively deeper layers into the sample. An electron beam is then directed onto the exposed regions. The Auger electrons produced by the beam come from each new layer exposed, allowing depth profiling to be achieved.

Computed Axial Tomography (CAT)

Computed Tomography(CT) or Computed Axial Tomography(CAT), which was first introduced in 1973 by the British firm EMI Ltd., is used to provide 2D and 3D pictorial views of the internal structure of the body. When a CT examination takes place, the patient is situated on a table which is surrounded by a ring which generates x-ray radiation and images the subject as the body is exposed to x-rays.

A computer combines the x-ray images into cross-sectional pictures, or “slices” of the body. Varying absorption characteristics of radiated tissues affects the intensity of the x-ray beam which produces images showing internal structures such as blood clots, skull fractures, tumors and infections. 2D images, or slices, show cross sectional localized views of internal systems, while 3D images are “reconstructions” of multiple slices which yield detailed pictures of larger systems.

Study Questions

  1. Why are X-rays diffracted by a crystal lattice, and not other forms of radiation?
  2. What is Bragg’s equation and describe each term of the equation
  3. What is the main use of X-ray diffraction in modern chemistry?
  4. What are the most common means of detection in XRD?
  5. X-ray Fluorescence (XRF) is one of the most widely used analytical methods. What is the primary use of XRF?
  6. There are two main types of XRF instrument: wavelength dispersive and energy dispersive. Discuss the differences between these.
  7. Discuss the use of XRF for quantitative elemental analysis.
  8. What elements are generally not detected by XRF?
  9. What type of information is obtained in an X-ray Photoelectron Spectroscopy (XPS) spectrum: elemental and/or bonding information?
  10. XPS was once described as “an all-element NMR.” What is the limitation of this statement?
  11. Describe the XPS process and the means of detection of the photoelectron.
  12. What are the main applications of XPS analysis?