x-ray analysis-shraddha
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X RAY DIFFRACTIONX RAY DIFFRACTION
Shraddha ParmarM.Pharm 1styear
(Quality Assurance)
Prepared by :-
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CONTENT :
Definition
Emission of X-Rays
Classification
Braggs Law
Instrumentation
Applications
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Definition :
Electromagnetic radiations of highenergy having wavelengths shorter
than those in the ultraviolet region.
Wavelength range of X-Ray is -0.1 to 100 A.
The range is 0.7 to 2 A
Thermal X-rays.
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CLASSIFICATION :
a. X-Ray absorption
b. X-Ray fluorescence
c. X-Ray diffraction
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X-Ray absorption:
X-Ray fluorescence :
Sample is irradiated.
Emits X-Ray beam
very little sample preparation
Follows the Beers law.
Determination of single element with high
atomic number in matrix eg. lead in gasoline
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X-Ray diffraction:
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Characterized by the orderly periodicarrangements of atoms.
The unit cell :basic repeating unit that
defines a crystal.
Crystalline material :
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According to Braggs law, for any given
wavelength it will be diffracted by a crystal if it
strikes that crystal at precisely the correct angle
.
Braggs law :
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= angle of the incident radiation
n= an integer
d = distance between two planes
When a known wavelength is used and theBraggs angle can be measured or inferred then
the d-spacing of a crystal of unknown
composition can be calculated.
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A
C B
D
CB = BD = AB Sin
Total distance CB + BD = 2 AB Sin
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Principle of XRD :
The requirements :
The spacing between layers of atoms must
be roughly the same as the wavelength ofthe radiation.
The scattering centers must be spatially
distributed in a highly regular way.
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X-rays appear to be diffracted from the crystal
only if the angle of incidence satisfies the
condition that sin = n /2d.
At all other angles, destructive interference
occurs.
The X-Ray diffraction methods are helpful in
Structure analysis.
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X-ray diffraction methods :
Laue photographic methods
Bragg X-ray Spectrometer methods
Rotating crystal method
Powder method
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Laue photographic methods :
Shows presence of rotation axes and
lines of symmetry.
Determine symmetry of single crystal.
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Rotating Crystal Method :
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Powder crystal method :
Large quantity of powder .
Random orientations.
All the like orientations of the grains due to
reflection constitute a diffraction cone.
Interaction with photographic plate.
Gives Trace .
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X-Ray absorption, X-Ray diffraction and X-Ray fluorescence are the three main fields
of X-Ray spectroscopy.
Only optical system varies in each casealthough component parts of the equipment
are the same.
X-Ray Photometers
X-Ray Spectrophotometers
Instrumentation :
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The main components :
Sources
Collimator
Monochromator
Detectors
Signal processors and devices
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Sources
Three types of sources are in X-Ray
instruments:
X-Ray Tubes,Radioisotopes,
Secondary fluorescent
X-Ray Tube (Coolidge tube):
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Radioisotopes:
55Fe26,57Co27,
109Cd48,210Pb82.
Used in X-Ray fluorescence & absorption
methods.
Excitation of the atoms
Production of secondary
Secondary Fluorescent Sources:
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Collimator
Collimator gapCollimator gap
Consists of two sets of closely packed metal.
All the X-Rays except the narrow beam that
passes between the gap.
To achieve the narrow beam .
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Monochromator
Filter :Target Element -Filter
Co Fe
Cu Ni
Fe Mn
NaCl, lithium fluoride, quartz.
Positioned such that angle of diffraction
satisfy the requirement of braggs equation.
Crystal monochromators :
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Detectors
Gas-filled detector
Scintillation counter Semiconductor detector
Gas-filled detector
Geiger tubes
Proportional counters
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Geiger-Muller tube counter :
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InexpensiveRelatively trouble-free detectorGives the highest signal for a given X-Ray intensity
Disadvantages :
Used for counting low rates
Efficiency falls off rapidly at wavelengths below 1 A.
As the magnitude of the output pulse does not dependupon the energy of the X-Ray.
Cannot be used to measure the energy of the ionising
radiation.
Advantages :
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Proportional counter :
Filled with a heavier gas like xenon or krypton.
The heavier gas is preferred.
Advantages :
Disadvantages :
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Scintillation detector
Sodium iodide, anthracene, naphthalene
& terphenol in xylene
When X-Ray is incident upon the crystal,the pulses of visible light are emitted which
can be detected by a photomultiplier tube
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A schematic diagram of a scintillation
detector:
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Semiconductor Detectors Si(Li) and Ge(Li)
Outer surface of the p-type layer is coated with
a thin layer of gold for electrical contact
Covered with a thin beryllium window .
Signal output is taken from an aluminum layer
coats the n-type silicon
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Under the influences of an appliedvoltage the electrons move toward the
positive charge
Holes toward the negative charge.
The voltage generated is a measure of
the X-Ray intensity falling on the crystal.
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Application of X-Ray diffraction:
Structure of crystal
Determine the types of atoms and position
of atoms
Polymer characterization
State of anneal in metal
Particle size determination
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Determination of Cis-Trans isomerism
Determination of linkage isomerism
Determination of soil structure based on
crystallinity.
Detection of degradation of natural and
synthetic minerals
Examination of tooth enamel and dentine
Identification of corrosion products
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ReferencesPrinciples of Instrumental analysis by Skoog,
Holler, Crouch, Thomson Brooks\ cole, sixthedition, page no: 272-289.
Instrumental methods of chemical analysis by
Gurdeep R. Chatwal and Sham K. Anand,Himalaya publishing house, page no: 2.303-
2.339.
Handbook of modern Pharmacetical analysis
by Satinder Ahuja and Stephen Scypinski,
Academic press, page no: 69-70,219.
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