X- ray crystallography

1. PRESENTED BY :- ISHU SHARMA
M.PHARM (PHARMACOLOGY)
SGRRU DEHRADUN

2.  X-ray crystallography is a method of determining the
arrangement of atoms within a crystals , in which a
beam of x-rays strikes a crystal causes the beam of
light to spread into specific directions.
 From the angles and intensities of these diffracted
beams, a crystallographer can produce a three
dimensional picture of the density of electron within
the crystals .

3.  X-ray crystallography is a powerful technique for
visualizing the structure of protein.
 Tool for identifying the atomic and molecular structure of
a crystals.
 The X-ray region of the electromagnetic spectrum consist
of wavelength in the region of about 0.1-100 A .
 For analytical purposes the ranges of 0.7- 2.0 A.

5.  A variety of x-ray technique and methods are in use but
we shall classify all methods into three main categories:-
1. X-ray absorption
2. X-ray fluorescence
3. X-ray diffraction
 X-ray absorption:- These are analogous to absorption
method in the other region of electromagnetic spectrum.

6.  In these method a beam of x-rays is allowed to pass
through the sample and the fraction of x-ray photons
absorbed is considered to be a measure of the
concentration of absorbing substance.
 X-ray fluorescence:- In this method x-rays are generated
within the sample and by measuring the wavelength and
intensity of the generated x-rays one can perform
qualitative and quantitative analysis. X-rays
fluorescence method is non-destructive and frequently
requires very little sample preparation.

7.  X-ray diffraction method:-These methods are based on
the scattering of x-rays by crystals.
 By these methods , one can identify the crystal
structure of various solid compounds.
 These methods are extremely important as compared to
other two methods.

8.  STEP 1st :- The first and most often and difficult step is
to obtain an adequate crystal of the material under
study.
 The crystal should be sufficiently large typically larger
than 0.1 mm in all dimensions.
 Pure in composition and regular in structure with no
significant internal imperfection such as cracks or
twinning.
 Researchers crystallize an atom or molecule because
the precise position of each atom in a molecule can
only be determined if the molecule is crystalized.

9.  STEP 2nd :- Crystal is placed in intense beam of x-rays .
 Usually a single wavelength (monochromatic x-ray) ,
producing the regular pattern of reflections.
 As the crystal is gradually rotated previous reflection
disappear and new ones appears.
 The intensity of every spot is recorded at every
orientation of the crystal.
 Multiple data sets may have to be collected with each set
covering slightly more than half a full rotation of the
crystal and typically containing tens of thousands of
reflections.

10.  STEP 3rd :- In the third step these data are combined
computationally with complementary chemical
information to produce and refine a model of the
arrangement of atoms within the crystal.
 The final, refined model of the atomic arrangement now
called a crystal structure is usually stored in a public
database.

11.  X-rays absorption, X-rays diffraction and X-rays
fluorescence ate the three main field of x-ray
spectroscopy only optical system varies in each case
although components parts of the equipment are the same.
 The main equipment are:-
1. Production of x-rays
2. Collimator
3. Monochromator
4. detector

12.  Production of x-rays:- X-rays are generated when high
velocity electron impinge on a metal target
 Approximately 1% of the total energy of the electron
beam is converted into x-radiation, the remainder being
dissipated as heat.
 Many types of x-rays tubes are available which are used
for producing x-rays.

13.  Cathode which is a filament of tungsten metal heated by
battery to emit the thermionic electrons.
 This beam of electrons constitute the cathode ray stream.
 If a positive voltage in the form of anode having target is
kept near these electrons , the electrons are accelerated
towards the target.
 On striking the target the electrons transfer their energy
to its metallic surface which then gives off x-ray radiation
 Generally the target is very hot in use . This problem has
been solved to some extent by cooling the tube with
water. Another method to solve this problem is to rotate
the target at high speed so that the production of localized
heating is reduced.

15.  The x-rays produced by the target material are randomly
directed.
 They form a hemisphere with a target at the center.
 In order to get a narrow beam of x-rays , the x-rays
generated by the target material are allowed to pass
through a collimator which consists of two sets of
closely packed metal plates separated by a small gap.

17.  In order to do monochromatization of x-rays , two
methods are available:-
 FILTER:- The x-ray beam may be partly
monochromatized by the insertion of a suitable filter .
 A filter is a window of material that absorbs undesirable
radiation but allows the radiation of required
wavelength to pass.
 This method makes use of the large difference in the
mass absorption co-efficient on either side of an
absorption edge.

18.  CRYSTAL MONOCHOMATOR:- A crystal
monochromator is made up of suitable crystalline material
positioned in x-ray beam so that the angle of the reflecting
planes satisfied the Bragg’s equation for the required
wavelength.
 The beam is split up by the crystalline material into the
component wavelength in the same way as the prism splits
up the white light into a rainbow.
 Such a crystalline substance is called as analyzing crystals.

20.  The x-ray intensities can be measured and recorded
either by photographic or counter methods.
 PHOTOGRAPHIC METHOD:- In order to record the
position and intensity of x-ray beam a plane or
cylindrical film is developed .
 The film after exposing to x-rays is developed.
 The blackening of the developed film is expressed in
terms of density unit D given by
D = Io
I

21.  Where Io and I is intensity of incident and transmitted
intensity of x-ray.
 D = Related to the total x-ray energy that causes the
blackening of the photographic film.
 The value of D is measured by the densitometer
 The photographic method is mainly used in diffraction
studies since it reveals the entire diffraction pattern on a
single film.
 This method is time consuming.

22.  COUNTER METHOD:-
A. GEIGER-MULLER TUBE COUNTER:-
It is field with an inert gas like argon and the central wire
anode is maintained at a positive potential of 800-2500V
When an x-ray is entering the tube this ray undergoes
collision with the filling gas, resulting in production of an
ion pair, the electron produced moves towards the central
anode while the positive ion moves towards the outer
electrode.
The electron is accelerated by the potential gradient and
causes the ionisation of large no. of argon atoms, resulting
the production of an avalanche of electron that are
travelling towards the central anode.

23.  This results in an output pulse of 1-10 V which is
measured very easily by employing a simple circuit.
 Inexpensive
 Trouble free detector
DISADVANTAGES:-
Used for counting low rates.
The efficiency of a Geiger tube falls off rapidly at
wavelength below 1A.

25.  SCINTILLATION DETECTOR:- In scintillation
detector , there is a large sodium iodide crystal activated
with a small amount of thallium.
 When x-ray is incident upon the crystal the pulse of
visible light are emitted which can be detected by
photomultiplier tube.
 It is particularly useful for measuring x-rays of short
wavelength .
 Crystal include are sodium iodide, anthracene,
naphthalene and p-terphenol in xylene.

27.  1. STRUCTURE OF CRYSTALS:- The analytical
applications of x-ray diffraction are numerous.
 The method is nondestructive and gives information on
the molecular structure of the sample.
 Perhaps its most important use has been to measure the
size of crystal planes. The patterns obtained are
characteristics of the particular compound from which
the crystal was formed.
 POLYMER CHARACTERISATION:- Powder method
can be used to determine the degree of crystallinity of the
polymer .

28.  The non-crystalline portion simply scatters the x-ray
beam to give a continuous background , while the
crystalline portion causes diffraction lines that are not
continuous.

29.  The amorphous material in the polymer will scatter at all
wavelength and give a scattered pattern; however , the
crystalline material will include crystal structures and
will produce definite diffraction lines or spots.
 The ratio of area of diffraction peaks to scattered
radiation is proportional to the ratio of crystalline to
noncrystalline material in the polymer. The ultimate
quantitative analysis must be confirmed using standard
polymers with known crystallinities and basing the
calculation on the known ratio of crystalline diffraction
to amorphous scattering.

30.  STATE OF ANNEAL IN METALS:- A property of
metal than can be determined by x-ray diffraction is the
state of anneal.
 Well annealed metals are in well ordered crystal form
and gives sharp diffraction lines.
 If the metal is subjected to drilling, hammering or
bending, it become “worked” or “fatigued” , that is its
crystals become broken and the x-ray pattern more
diffuse.
 It is occasionally necessary to check moving parts for
metal fatigue, such as airplane wings, combustion engine
parts, and bridges.

31.  MISCELLANEOUS APPLICATION:-
 Soil classification based of crystallinity :- Different
types of soil , such as various types of clays and sands,
exhibit different types and degrees of crystallinity .
Knowledge of this crystallinity gives valuable
information concerning soil structure.
 X-ray diffraction can also be used to assess the
weathering and degradation of natural and synthetic
minerals. By designed experiments , the factors
responsible for the degradation can be revealed .