x-ray analysis-shraddha

8/7/2019 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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