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Mpat 2 1

  2. CONTENTS 2  Discovery of X-rays  Introduction  Types of X-ray techniques  Production of x- rays  Bragg’s law  Crystallography techniques  Applications
  3. Discovery of X-rays • X-rays were discovered by W.C Roentgen (1895) while working on cathode rays. • He named it X-ray because in mathematics “X” is used to indicate unknown quantity. Wilhelm Conrad Roentgen (1845-1923) 3
  4. Introduction • X-ray spectroscopy is based upon measurement of emission, absorption, scattering and diffraction of electromagnetic radiation. • X-ray fluorescence and x-ray absorption are widely used for qualitative and quantitative determination of all elements in the periodic table having atomic number greater than sodium. • X-rays are short wavelength electromagnetic radiation produced by deccelaration of high energy electrons or by electronic transitions of electrons in the inner orbital of atoms. 4
  5. X-ray crystallography X ray crystallography is an importance tool to identify the atomic and molecular structure of crystal. The crystalline atoms causes a beam of incident x rays to diffract into many direction. Crystallography produce three dimensional picture of the density of electron with in the crystals and from crystal density, the mean position of the atom in crystal can be determine. The intensity of each diffracted ray is fed into a computer, which uses a mathematical equation called a ‘Fourier transform’ to calculate the position of every atom in the crystallized molecule. 5
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  7. How does it work ?
  8. • Crystals Must be: o Small in size: o Less than 1 millimeter o Perfect: o No cracks o No Inclusions, such as air bubbles o If the crystals are not perfect then the end image that is formed will have random patterns or have other problems. 8
  9. PRODUCTION OF X-RAYS • Visible light proton and x –ray protons are both produced by the movement of electrons in atoms. • Electrons occupy different energy levels or orbits , around an atom’s nucleus. • When an electron drops to a lower orbital, it needs to release some energy ; it releases the extra energy in the form of a photon . • The energy level of the photon depends on how far the electron dropped between orbitals. 9
  10. TYPES OF X-RAY TECHNIQUES Classified into three major categories: I. X-ray absorption method • This method is similar to other absorption methods of electromagnetic spectrum like uv visible IR spectroscopy etc. • In this method a beam of X-ray is allowed to pass through the sample and fraction of x ray photons absorbed is considered to be a measure of concentration of the sample. • The amount of radiation absorbed is directly proportional to the concentration of absorbing system. 10
  11. II. X-ray fluorescence method • X-rays are generated within the sample , • By measuring the wavelength and intensity of the generated X-rays analyst can perform qualitative and quantitative analysis. III. X-ray diffraction method • Based on scattering of X-rays by crystals . • By using these method, analyst can easily identify the crystal structure of any solid compound(sample) with high degree of simplicity and accuracy. • XRD is extremely important method as compared to other x ray crystallography. • All the three techniques are non-destructive .
  12. X-ray diffraction Diffraction is the slight bending of light as it passes around the edge of an object. X-ray crystallography uses the uniformity of light diffractionof crystals to determine the structure of molecule or atom Then X-ray beam is used to hit the crystallized molecule. The electron surroundingthe molecule diffract as the X-rays hit them. This forms a pattern. This type of pattern is known as X-ray diffraction pattern. 12
  13. BRAGG’S LAW When x-ray beam strikes a crystal surface at some angle θ,a portion is scattered by the layer of atoms of the surface. The unscattered portion of beam penetrates to the second layer of atoms where again a fraction is scattered and remainder passes on the third layer. The cumulative effect of this scattering form the regularly spaced Centre's on the crystals.  nλ = 2d sinƟ Here ,dis the spacing between diffracting planes, Ɵis the incident angle, n is an integer, and λis the wavelength of the beam. 13
  14. X-RAYDIFFRACTION METHODS X-Ray Diffraction Method Laue Rotating Crystal Powder Orientation Single Crystal Polychromatic Beam Fixed Angle Lattice constant Single Crystal Monochromatic Beam Variable Angle Lattice Parameters Polycrystal (powdered) Monochromatic Beam Variable Angle 14
  15. ROTATING CRYSTAL METHOD In the rotating crystal method , a single crystal is mounted with an axis normal to a monochromatic x- ray beam. A cylindrical film is placed around it and the crystal is rotated about the chosen axis. As the crystal rotates, sets of lattice planes will at some point make the correct bragg angle for the monochromatic incident beam, and at that point a diffracted beam will be formed . 15
  16. The arrangement of this method is outlined as follows : a) The X – rays are generated in the X – ray tube the beam is made Monochromatic by a filter. b) From the filter, the beam is allowed to pass through collimating system which permits a fine pencil of parallel X – rays. c) Then the shaft is moved to put the crystal into slow rotation about a fixed axis. This causes the sets of planes coming successively into their reflecting positions, i.e., the value of ᶿ satisfies the Bragg’s relation. d) Each plane will produce a spot on the photographic plate. One can take a photograph of a diffraction pattern upon a photographic plate perpendicular to X - ray beam or upon a film in a cylindrical camera, the axis of which coincides with the axis of rotation of the crystal. 16
  17. In powder method, the crystal sample need not be taken in large quantity but as little as 1mg of the material is sufficient for the study. A is a source of X – rays which can be made monochromatic by a filter. Allow the X – ray beam to fall on the powdered specimen P through the slits S1 & S2. The function of these slits is to get a narrow pencil of X – rays. Fine powder, P, struck on a hair by means of gum is suspended vertically in the axis of a cylindrical camera. This enables sharp lines to be obtained on the photographic film which is surrounding the powder crystal in the form of a circular arc. The X – rays after falling on the powder passes out of the camera through a cut in the film so as to minimize the fogging produced by the scattering of the direct beam. 17 POWDER METHOD
  18. Applications • X-ray diffraction is most widely used for the identification of unknown crystalline materials (e.g. minerals, inorganic compounds). • XRDis a nondestructive technique • Provides information on the molecular structure of crystal. • X ray Diffraction is used to determination of Cis-Trans isomerism • Determine electron density ,the mean positions of the atoms in the crystal their chemical bonds, their disorder and various other information.. • Arrangement and spacing of atoms in crystalline materials • Physical properties determination of metals,polymeric material, and other solids. 18
  19. REFERENCE • Instrumental methods of chemical analysis by Gurdeep R. Chatwal and Sham K. Anand , Himalaya Publication, 5th revised edition page no. 2.303-2.339 • Instrumental method of analysis by Willard, Merritt, Dean, Settle, CBS Publishers and Distributors, 7th Edition, page NO-340-397. • • 19