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  X-Ray Spectroscopy, Chemistry tutorial

Introduction: 

X-rays are electromagnetic radiation ranging in wavelength from about 0.1 - 25 Å.  The shorter the wavelength of the x-ray, the greater the energy and its penetration power. Both light and x-rays are produced by electronic transition. The only difference is that light rays are produced by the transition of the outer electrons, while x-rays are generated via the transition of inner electrons.

X-ray spectroscopy is concerned by the measurement of the absorption of x-rays when it interacts through matter.

Definition

 X-ray spectroscopy is a method that involves learns of the interaction between x-ray radiations through matter.

Sources of X- rays:

There are 3 ordinary sources of X-rays for analytical purposes:

1. Electron bombardment of a metal target.

2. Irradiation of a target (sample) through primary beam of elevated energy x-rays to generate a secondary beam of fluorescent x-rays.

3. Exposure of a example to a radioactive source that produces X-rays. The X-ray emission spectra produced from such sources might be continuous or discontinuous, or an amalgamation of both.

Feature line spectra are generated via an electron beam. Such are attained when a fast moving electron excites an atom to a higher energy level, then x-ray emission follows when an outer electron falls onto the vacancy in the lower energy, inner shell, giving increase to a series of lines.

An X-ray fluorescence spectrum is generated via exciting the target atom by a beam of elevated energy x-ray that is adequate enough to knock out k electrons. After a short time the excited ion returns to the ground state, producing a fluorescence spectrum alike to the emission spectrum. Unlike the line emission spectra, where the lines show as spikes superimposed on the continuous background, fluorescence spectra generate only the line spectrum with no the continuous background. Therefore fluorescence spectra illustrate a much greater signal - background level and are preferred for analytical work.

X-ray Emission Spectrometers:

This is a device or instrument utilized to compute analytically the emission of the x-ray radiation through a sample. This instrument consists of:

1. Source of x-ray radiation called x-ray cooling tube, 

2. Specimen chamber

3. Collimator made of a parallel fine tube

4. Crystal analyzer

5. Auxiliary collimator

6. Detector 

In this instrument, the fluorescence spectrum is conveniently produced through elevated energy x-rays from a Coolidge tube, even though the example could be made the target in an x-ray tube. No suitable transparent materials are available for the fabrication of lenses; therefore x-rays are collimated by passage through a series of slits or a collection of long narrow tubes. Likewise no prisms are available to disperse X-rays, but fortunately crystals of many salts are able to disperse X-rays by diffraction and serves as excellent monochromator. Such crystals are: Topaz, LiF, NaCl, ammonium dihydrogen phosphate (ADP), ethylene diamine d-tartrate, and so on. The huge single crystal analyzer (monochromator) is generally rotated on its axis to attain the spectrum.

X-ray Detector

Three kinds of detectors are utilized to calculate the intensity of an x-ray beam.

1. The simplest detector is a photographic film or plate that will darken when exposed to x-rays. The expanded film is then scanned through a densitometer to record the spectrum.

2. Gas ionization detectors, these as ionization chamber, proportional counters and Geiger tubes utilized to compute radioactivity, are suitable for x-rays.

3. Several crystals fluoresce in the ultraviolet or visible region when exposed to x-rays. X-ray tube Specimen Collimator

1177_Block diagram of X-ray emission spectrometer.jpg

Fig: Block diagram of X-ray emission spectrometer

These crystals may be incorporated in a scintillation counter that as well utilizes a photomultiplier tube to determine the intensity of the fluorescence.

Non dispersive X - ray Spectrometers

Such are dense, relatively inexpensive and present performance comparable to crystal monochromator instruments, except for somewhat poorer resolution of closely spaced lines.

627_Non-dispersive x-ray spectrometer.jpg

Fig: Non-dispersive x-ray spectrometer

Applications:

All kinds of solids are easily handled. If needed, the example might be placed in a thin walled cell or deposited as a film under cellophane. All elements above calcium (Z = 20) are readily detected, those between sodium (Z = 11) and calcium with difficulty, and the lighter elements not at all. X-ray emission spectroscopy is extensively utilized for the analysis of steels and other alloys and for the determination of heavy elements in organic samples (for example lead and bromine in aviation fuels). It is extremely precise through bounds of detect ability as low as a few parts per million.

X - Ray absorption

The absorption of X-rays is alike to the absorption of other electromagnetic energy in the ultraviolet, visible or infrared regions. The only important dissimilarity is the energy involved. Like the emission procedure, the absorption procedure as well concerned through the innermost electron. This procedure identifies the element regardless of its environment. Absorption of the X-ray photon is most probable. If the energy of the incident photon just equivalents the energy needed to throw out the electron. That is, the electron departs through essentially zero kinetic energy. For instance, the absorption spectrum of lead consists of a few broad peaks by sharp discontinuities termed absorption edges. Each absorption edge corresponds to the energy needed to eject a K or L electron. The wavelength of an absorption edge is slightly less than that of the corresponding emission line, since the energy needed to expel electron completely from the atom is greater than the energy connected by an outer electron (already in the atom) falling into the vacancy. Beer's law is valid for the absorption of X - rays, and is generally written as 

2.303 log (Io/I) = µx

Where Io is the incident intensity and I is the intensity transmitted through a sample thickness of x cm. The proportionality constant, µ is termed the linear absorption coefficient. The broad absorption bands as seen above, guides to the interference among neighbouring heavy elements. For the most part, X-ray absorption processes are limited to examples enclosing a single heavy element in an organic matrix (for example lead in gasoline or chlorine in chloro compounds).

Application of X - ray Fluorescence Analysis

X-ray fluorescence analysis has the benefit that it is non-destructive. It can be utilized for the examination of works of art, valuable coins and forensic materials. Several elements can be computed in a few minutes, on only a tiny amount of material. The main disadvantages are

(i) Components lighter than sodium can't be determined readily,

(i) Lower concentrations aren't so readily determined 

(iii) The instruments are comparatively costly and

(iv) The method deals primarily via the surface of the example whereas the composition of the outermost layer of a substance might differ from that of the internal layer.

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