Spectrochemical
Analysis
EFFECT OF ELECTROMAGNETIC RADIATION
ON MOLECULES
Range of IR
Near IR: 0.8 to 2.5μm (12000cm-1 –4000cm-1)
• Analyzing mixtures of aromatic amines
• Determination of protein, fat, moisture, oil content.
• Middle IR: 2.5 to 15μm ( 4000cm-1 – 667cm-1)
• Also known as vibration- rotation region.
• This region is divided into:
1. Group frequency region: 4000cm-1 – 1500cm-1
2. Fingerprint region: 1500cm-1 – 667cm-1
• Far IR: 15 to 1000μm (667cm-1 –10cm-1 )
• Study of inorganic or organometallic compounds
• Sensitive to changes in overall structure of the molecule
Wavenumber
The unit used on an IR spectrum is Wavenumbers û
• IR radiation does not have enough energy to induce electronic
transition as seen with UV
• IR spectroscopy works because chemical bonds have specific
frequencies at which the vibrate corresponding to energy levels
• When energy in the form of IR radiations is applied then it causes
vibrations between atoms of the molecules
• Then the absorption of the IR takes place, and the peaks/spectra is
observed
• Different functional groups absorb characteristics frequencies of IR
radiation and provide the specific peak value
• Therefore, IR spectra of a chemical substance is fingerprint of the
molecules for its identification
Hook’s law
f-Force
m- mass of molecule
Strong bond = High frequency/wave
number
Lighter molecule = High frequency/wave
number
4000-1000 cm- 1 known as the functional group region, and < 1000 cm- 1 known
as the fingerprint region. Fingerprint region in IR spectroscopy, are the regions
where all the bending vibrations are seen in spectroscopy
CLASSIFICATION OF IR BANDS
IR bands can be classified as strong (s), medium (m), or
weak (w), depending on their relative intensities in the
infrared spectrum. A strong band covers most of the y-
axis. A medium band falls to about half of the y-axis,
and a weak band falls to about one third or less of the y-
axis.
In FTIR, an increase in the peak intensity usually
means an increase in the amount (per unit volume)
of the functional group associated with the
molecular bond , whereas a shift in peak position
usually means the hybridization state or electron
distribution in the molecular bond has changed.
Sources Other than the optics and detector system, the source and the
sample cell are also important to be considered. For IR
radiation the common source is an inert solid heated to a
temperature between 1500 and 2000 K when continuous
radiation approximating to that of a black body is omitted
from it. Heating is usually done electrically and the materials
are
(a) Globar, which is bonded silicon carbide,
(b) Nernst glower, which is a mixture of rare earth oxides, and
(c) coil of nichrome wire.
(a) Globar is made in the form of a rod about 5 to 7 mm in
diameter and about 50 mm long. It is electrically heated at a
temperature between 1300 and 1500 K. Globar has positive
temperature coefficient of resistance. For preventing arc
formation it often is centrally watercooled. It covers a
radiation range of 600 to 2600 nm.
(b) For Nernst glower, usually zirconium, thorium and yttrium
oxides are sintered in the form of a hollow rod of 1 to 2 mm
outer diameter (OD), and of length varying from 20 to 50
mm.Platinum leads are sealed into the rod for electrical
heater supply. Temperature is raised up to 2000 K from about
1200 K. It has a negative temperature coefficient of resistance
and hence current limiters are required to be used. The range
is almost the same as that of globar.
(c) Nichrome wire gives longer life and for IR radiation it is
heated to about 1100 K. It has a range covering almost the
entire IR range. A rhodium wire sealed in a ceramic cylinder
shows similar characteristics as that of a nichrome wire insofar
as IR source is concerned. Another material is tungsten fi
lament which at a temperature of about 3000 K provides IR
radiation from 750 to 2500 nm
Monochormators
Prisms Gratings
Single pass
Double pass
Sample cells & Sampling techniques
Gases
• Gas cell – 10cms
• Multi pass gas cells
Liquids
Thin film squeezed between 2 IR
transparent windows.
0.1 - 0.3mm thickness mm thickness
Solids
Four techniques:
1. KBr discs/ pellets/ pressed pellet
technique
2. Mulls
3. Deposited films
4. Solutions
1. KBr discs:
• 0.1 – 2.0% by wt.
• Particle size < 2μm.
• Hydraulic pressure – 10 tons load.
Discs: 13mm- diameter, 0.3mm- thickness.
2. Mulls:
• Grinding sample with a drop of oil.
• Nujol (liquid paraffin)
• Complement: Hexacholorobutadiene &
chlorofluorocarbon.
3. Deposited films:
• Solution in a volatile solvent on a NaCl flat.
4. Solutions:
• Solvent – CCl4 , CS2 , CHCl3
• Complementary pair - CCl4 & CS
Detectors
two main types in common IR instruments
a) Thermal Detectors
1.) Thermocouple
- two pieces of dissimilar metals fused together at
the ends
- when heated, metals heat at different rates
- potential difference is created between two
metals that varies with their difference in
temperature
- usually made with blackened surface (to
improve heat absorption) - placed in evacuated
tube with window transparent to IR (not glass or
quartz)
- IR “hits” and heats one of the two wires.
- can use several thermocouples to increase
sensitivity.
Cont. 2.) Bolometer
- strips of metal (Pt, Ni) or semiconductor that has a large
change in resistance to current with temperature.
- as light is absorbed by blackened surface, resistance
increases and current decreases
- very sensitive
The Bolometer working principle is based on the amount of
power dissipated on a temperature resistive sensing element.
It has a positive or negative temperature coefficient based
on the change in resistance.
Change in resistance with a change in temperature can be
used to detect and measure power or heat of incident
electromagnetic radiation or microwave or RF energy.
A bolometer contains a thermal reservoir, and a layer of thin
metal called absorbing element. The thermal reservoir and
absorbing elements are connected through a thermal link.
When the electromagnetic radiation is incident on the
absorbing element, then it raises the temperature, which is
above the thermal reservoir and results in a high temperature
greater than the absorbed power.
The speed of the instrument is set by the intrinsic thermal time
constant that is the same as the absorbing element’s heat
capacity and thermal conductance of absorbing element
and thermal reservoir. The change in temperature or change
in resistance of absorbing elements can be measured by
using a thermometer.
Cont.
Photoconducting Detectors
thin film of semiconductor (ex. PbS) on a nonconducting glass surface and
sealed in a vacuum.
- absorption of light by semiconductor moves from non-conducting to
conducting state
- decrease in resistance → increase in current
Pyroelectric Detector
Cont. A Pyroelectric detector is an infrared sensitive
optoelectronic component which are
specifically used for detecting electromagnetic
radiation in a wavelength range from 2 to 14
µm
Pyroelectricity or pyroelectric material is an
electric response of polar dielectric with a
change in temperature.
If the temperature in return changes it causes
the movement of atoms from there neutral
position hence the polarization of the material
changes, we observe a voltage across the
material.
Temperature fluctuations produce a charge
change on the surface of pyroelectric crystals,
which produces a corresponding electrical
signal. This temperature gradient can be
created by the absorption of light.
- pyroelectric materials: DLaTGS, LiTaO3, and
PZT.
measure degree of polarization related to
temperature of crystal.
Golay detector The principle of operation of Golay detector is
shown in Fig. 8.25. It is a gas thermometer that
consists of cylindrical chamber filled with xenon
and provided with a blackened membrane.
One end of the cylinder has an IR window and
the other a fl exible diaphragm silvered on the
outside.
In the condition without IR radiation, a beam of
light is allowed to reflect from the silvered surface
and to fall on the cathode of a phototube.
With IR radiation entering the cell, the blackened
membrane gets heated which is conducted into
xenon gas increasing its pressure so that the
silvered diaphragm gets distorted and
consequently the fraction of light reaching the
phototube cathode changes, changing, in turn,
the phototube current.
This change can be related to the infrared beam
strength. This detector is costly but is superior in
performance in the range l > 50 mm.
