Infrared spectroscopy involves the interaction of infrared radiation with matter. Molecules absorb specific frequencies that excite vibrational modes. The absorbed frequencies are characteristic of bonds and functional groups within a molecule. Fourier transform infrared spectroscopy (FTIR) has advantages over dispersive instruments as it allows simultaneous measurement of all frequencies using an interferometer. Applications in forensics include identification of materials like paint, fingerprints, and detection of document alterations or counterfeit substances.

1. INFRARED SPECTROSCOPY
Submitted to : Diksha thakur mam
Submitted by : Shabnam
Role no. :1303
Class : Msc forensic science 2nd year

2. CONTENTS
1. Introduction
2. Infrared – region
3. Principle
5. Absorption of IR radiation by molecule
6. Molecular vibrations in IR region
7. Instrumentation
8. FTIR
9. Applications of IR spectroscopy in forensic
science.

3. INTRODUCTION
 Infrared spectroscopy is the spectroscopy that deals
with IR ( infrared region) of electromagnetic spectrum ,
that is light with longer wavelength and lower frequency
than the visible light .
 Definition – It is the study of interaction between
infrared light and matter.The process is based on the
absorption spectroscopy.
 History –IR –Spectroscopy was discovered in 1800 by
“Sir William Hershel”.

4. CONTS.
 IR –Spectroscopy can be very sensitive to
determination of functional groups within the
sample since different functional groups absorbs
different particular frequency of IR –radiation.

5. INFRARED-REGION
 IR –Spectroscopy is also classified into three
region because of their characteristics property in
different region of IR.
1. Far-infrared region (400 - 33 cm-1) –Vibration of
molecules that containing heavy atoms, molecular
skeletal vibrations and crystal lattice vibrations.
 2. Mid-infrared region (4000 - 400 cm-1) –Useful for
organic analysis.
 3. Near-infrared region (12820 – 4000 cm-1)-
Overtones, very useful for quantitative analysis.

6. ELECTROMAGNETIC RADIATION
 Electromagnetic radiation is the properties of waves
in many ways.Light waves can be represented as
oscillating perpendicular electric and magnetic
fields.Both are right angle to each other and to the
wave of propagation light.
 Wavelenth is the crest-to-crest distance between
two succesive maxima.SI unit is meter(m),
centrimeter (cm) ,nanometer(nm).
 Amplitude maximum of vector from the origin to
point displacement of oscillation.

7. CONTS.
 Wavelenth
 Frequency (v) no. of crests passing a fixed point
per second.Unit of v is Hertz (Hz) or inversely
second (s-1).
 E =hv
 E= hc/lembda

8. ECLETROMEGNETIC WAVES

9. PRINCIPLE
 In any molecule it is known that atoms or groups of
atoms are connected by bonds. These bonds are
analogous to springs and not rigid in nature.
 Because of the continuous motion of the molecule
they maintain some vibrations with some frequency
characteristic to every portion of the molecule.This
is called the natural frequency of vibrations.
 When energy is applied in the form of IR-radiation
is applied and when
Applied IR frequency=Natural frequency of vib.

10. CONTS.
 Molecules are excited to high energy state on
absorbs IR radiation, selectively absorbs the
radiation resulting in vibration of a molecule of the
compound , giving rise to closely packed absorption
bands which is called as IR absorption spectrum.
 Bands corresponding to the functional groups and
the bonds present in a chemical substance.
 Hence , an IR spectrum of a compound is
considered as the fingerprint for its chemical
identification.

11. IR SPECTRUM

13. IR-ACTIVE AND IR-INACTIVE
MOLECULES
 All the bonds in a molecule are not capable of
absorbing IR energy.
 IR active- Those bonds which are accompanied by
a change in dipole moment will absorb in the IR
region and such transitions are called IR active
transitions.
 IR inactive- The transitions which are not
accompanied by a change in dipole moment of the
molecule are not observed and are considered as
IR inactive.

14. ABSORPTION OF IR-RADIATION BY
MOLECULE
 Molecule with covalent bonds may absorbs IR
radiation .This absorption is quantized , so only
certain frequency of IR radiation are absorbed.
 When the radiation is absorbed , molecules moves
higher energy radiation state.The energy absorbed
which is associated with IR rotation , its sufficient to
cause molecules to rotate and vibrate.
 Energy required to cause change in rotational level
is small compared to the energy required to cause
change in vibrational level.

15. CONTS.
 Hence, each vibrational change has multiple
rotational changes associated with it.
 Molecules absorbs radiation when a bond in the
molecule vibrates at the same frequency as the
incident radiant energy.
 Frequency absorbed depend on the masses of the
atoms in the bond , geometry of molecule , strength
of the bond and several other factors.
 Not all molecule can absorbs IR radiation ,
molecule must have change in dipole moment
during vibrations in order to absorbs IR radiation.

16. ENERGY LEVEL DIAGRAM

17. ORGANIC STRUCTURE DETERMINATION
 How the atoms are connected together ?
 Which bonds are single,double o triple?
 What functional group exist in a molecule?
 Identity of an organic compound can be
established from its fingerprint region by
comparing the sample spectrum with known
spectrum of compound.

19. VIBRATIONS IN MOLECULE
 Molecular vibrations are excited by IR radiation
,also takes in various modes of vibrations.Most
common type of vibration is stretching and bending
type.
 Stretch vibration – Involves the change in the bond
length resulting change in interatomic distance.
 Bending vibration –Involves the change in bond
angle or change in the position of the groups of
atoms with respect to the rest of molecule.

22. FUNDAMENTAL VIBRATIONS

23. NON – FUNDAMENTAL VIBRATIONS
 Overtones –These are observed at twice the
frequency of strong bond. E.g. Carbonyl group
 Combination tones- A combination band is the
result of a two frequencies being excited is allow by
symmetry.Overtone is not required a symmetry.
 Fermi-rasonance –It is the interactions between
fundamental vibrations and overtones . e.g. carbon
dioxide.

24.  Number of vibrational modes:
-For a non-linear molecule,
no. of types of vibrations = 3N-6
-For a linear molecules,
No. of types of vibrations =3N-5
 Examples –
HCl 3(2)-5 = 1
BF3 3(4)-6 = 6

25. DISPERSIVE & NON DISPERSIVE
SPECTROPHOTOMETER
 Dispersive
 Sequencing scanning of each wave number
takes place.
 Double beam instrument are mostly used
than single beam instrument.
Non dispersive
Filters are used for wavelength selection.
Having sample specific detectors.

26. INSTRUMENTATION OF IR
 Main part of IR spectrometer are:
1. Radiation source
2. Sample cell and sampling of substance.
3. Monochromator
4. Detectors
5. Recorder
The material used in an IR spectroscopy must be transparent to
IR radiation.Such as glass or quartz use for mid IR instrument
because glass and quartz are not transparent to IR at
wavelength larger than 3.5 micrometer.
Ionic salts,potassium bromide,calcium floride,sodium chloride
and zinc selenide also used in IR radiation.

27. INSTRUMENTAL SETUP

28. INSTRUMENTATION
 2. Radiation source – The intensity of
radiation should be continuous over the
wavelength range used.Cover wide wavelength
range.Constant over the long period.
 Various source of IR radiation:
a. Nernst glower
b. Mercury lamp
c. Tungsten lamp
d. Glober source
e. Nichrome wire
f. Incandescent lamp

29.  Nernst glower (mid range) - It is a cylindrical bar
composed of zirconium oxide, cerium oxide and
thorium oxide that are heated electrically to a
temperature between 1500k to 2000k.Source are
20mm long and 2mm in diameter. On passing
current through it causes heat and glow.
 Globar – It is more intense than nernst
glower,globar is a bar of sintered silicon carbide
which heated electrically to emit continuous IR
radiation.
 Halogen lamp is used as source in NIR (>2000)
region, its also contains a tungsten
wire filament.

30.  Mercury lamp –( far IR )
 High pressure mercury discharge lamp ,it is
constructed of a quartz bulb containing elemental
Hg.
 Nichrome wire is also used , other metals such as
rhodium are well used its heated electrically about
1100 C.
 Laser is also used which emits monochromatic
radiation.Some lasers called tunable lasers which
emit more than one wavelength of light but each
wave length is monochromatic.

31. SAMPLING METHOD
2. Solid-sampling –
a. Include direct sampling.
b. Palletization technique
c. KBr & NaCl (used for preparation of pallet with
sample.
Particle size less than 2 micrometer
Sample should be 1-2% of KBr/NaCl.
 Mulling –
Paste and mull will be present,hexa
chlorobutadiene.
Solid sample for solution ( sample present in volatile
solvent)

32. CONTS.
 Liquid sampling –
Two layers of NaCl pellet usually thin layer is
used.
 Gas sampling-
Gas cells are used.
Length of gas cells are 10 cm.
Made up of NaCl.
 Solution sampling technique-
Chloroform
Carbon tetrachloride
Carbon disulphide

33. MONOCHROMATOR
 Monochromator is an optical device that transmit a
mechanically selectable narrow band of wavelength
of light.The name was come from greek word mono
– “single” and chroma – “color”, ator detonating a
agent.
 Types of monochromator :
1. Prism monochromator
2. Grating monochromator

34. TYPES OF MONO-CHROMATOR
 Prism mono-chromator – Composed of glass
and quartz ,having a property of reflection, coated
by alkyl halide , greater range and simplicity.
It also two types:
Single beam mono-chromator - ( allow
monopass prismatic radiation pass once through
prism).
Double beam mono-chromator – ( allow double
prismatic pass twice through the prism.

35. GRATING MONO-CHROMATOR
 The device is based on separates the radiation by
diffraction method which diffracts the different
wavelengths at different angles represented by the
multicolored line from the grating.
 It consists of a entrance slits , diffraction grating and
spherical mirrors.

36. DETECTORS
 In IR two types of detectors used:
1. Thermal detector -: are following
a) Bolometer : It is very sensitive electrical
resistance thermometer that has been used to
detect and measure weak thermal radiation.
 Bolometer used in older instruments consisted of
thin metal conductor such as platinum wire.
 Incident radiation heats up this conductor which
causes electrical resistance to change.
 Degree of resistance change measure the
amount of radiation fallen on detector.

37. THERMOCOUPLE
 Thermocouple :-
 It is made up of welding together at each end two
wires made from different metals.
 One welded joint called hot junction (which exposed
to IR radiation) hotter than other welded joint cold
junction (screened the protective box).
 Potential difference generated in wire is a function
of temperature difference between the junctions
intensity falling of IR-radiation on the hot junctions.
 Cannot be used for FTIR due to slow response.

39. THERMISTORS
 It is made up of fused mixture of metal
oxides. The relationship between
temperature and electrical resistance.
 As temp. increases its electrical resistance
decreases (opposed to bolometer).
 This temp. and resistance difference allows
thermistors to be used as IR detectors in
same way as in bolometers.

40.  It has small hollow cell filled with nano-
absorbing gas such as xenon,in center a black
film is present which absorb the radiation and
causing an increase in temp.
 Thermal expansion causes a internal pressure
of cell to increase.
 Wall of cell was thin convex mirror that is the
part of optical system.
 As pressure increase the mirror is bulged which
is detected by a mirror , the change in intensity
cause change readout from detector.
 Response time is faster than bolometer ,
thermistor or thermocouple.
GOLAY DETECTOR

41. Golay cell

42. PYROELECTRIC DETECTPORS
 Pyroelectric material change their electric
polorization as function of temperature .This
material may be insulator ,ferroelectric or semi
conductor. Pyroelectric detector consist of thin
crystal pyroelectric material placed between two
electrode .Act as temperature dependent capacitor.
Upon exposure of IR radiation the temperature
and polarization of crystal change. The signal
depend on the rate of polarization with
temperature.
 It can be used as FTIR detector

43. Pyroelectric detector

44. PHOTON DETECTORS
 It is semi conductor materials that are insulator
when no radiation falls on them but become
conductors when radiation falls on them.
 Exposure of radiation causes a very rapid change
in their electrical resistance and therefore very rapid
response to the IR signal.
 There is required must change to conductor from
insulator.The IR also detected on the basis of
bands gap.

45. FTIR SPECTROPHOTOMETER
 FTIR stands for Fourier transform infrared, the
preferred method of IR spectroscopy.
 IR passed through the sample ,some of the
radiation is absorbed by the sample and some of it
is transmitted and detect by detector.
 Resulting spectrum represents molecular vibration
and transmission ( in the form of peaks).
 FTIR spectrometry was developed in order to
overcome the limitations encountered with
dispersive instruments.

46. IMPORTANCE
 FT-IR preferred over dispersive or filter methods of
IR spactral analysis.Due to:-
1. It is non-distructive technique.
2. It can increase speed ,in second collecting a
scan.
3. High sensitive.
4. Greater optical throughput.
5. Mechanical simple device.

47. PURPOSE
 The main difficulty was the slow scanning
process.A measuring all the frequencies
simultaneously rather individually was needed.
 The instrument was formed which have a very
simple optical device called an interferometer.
 Interferometer produce a unique type of signal
which has all the infrared frequencies encoded into
it.
 The signal can measured quickly,in seconds.
 Interferometer employ a beamsplitter which divides
the incoming infrared radiation into two optical
beams.

48. INSTRUMENT

49. IR-MIRRORS
 Fixed mirror – one beam reflects off a flat mirror
which is fixed in place.
 Moving mirror – The other beam of light off on flat
mirror which is move in very short distance away
from beamsplitter.
 The beam reflect back from the mirror and
recombined at the beamsplitter.
 Both radiation reflects on beamsplitter of same
wavelength because travels same length which is
known as zero path difference.
 If the move mirror has slightly more away from the
beamsplitter than the fixed mirror.

50. CONTS.
 The reflects back beam is not have a equal
wavelength also called optical path difference.
 The signal which exist in interferometer is the result
of these two beams interfering with each other.
 The resulting signal called interferogram.
 Zero path difference = constructive interfering.
 Optical path difference= Deconstructive
interfering.

51. CONTS.

52. FT-IR SPECTRUM

53. ADVANTAGE FT-IR
 Speed –All the measurements taken in second
rather than minutes.
 Sensitivity –Sensitivity improved with FT-IR for
many reasons optical throughput give much higher
with lower noise levels.
 Mechanical simplicity- There is very simple
mechanism of FT-IR.

54. APPLICATIONS IN FORENSIC SCIENCE
 IR is useful in forensic investigations.
 Forensic expert can also identify paints material
,sweat , fuels, hairs etc. which found on a crime
scene.
 Can be use directly analysis on a scene ( on site
analysis) with the help of portable IR equipment.
 Can be used to identify forged or altered
documents by shining a beam of IR light on a
documents ink.And can be used to analysis of
fingerprint residues,explosives materials and
counterfeit drug also analysed by the IR technique.

55. REFERNCES
 Undergraduate instrumental analysis sixth edition
James W Robinson.
 https://www.slideshare.net/asmarf/7infraredspecros
copy6
 www.wikipedia.com