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THEORY, INSTRUMENTATION AND
APPLICATIONS OF DISPERSIVE & FTIR
Presented By
Bhavana Vedantam,Dept. Of Pharmaceutical Analysis
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Contents
Introduction to IR Spectroscopy
Dispersive IR Spectroscopy
FT - IR Spectroscopy
Conclusion
It Contains…
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INTRODUCTION Spectroscopy is the branch of science dealing
with the study of interaction of electromagnetic radiation with matter.
IR spectroscopy is Absorption spectroscopy in which molecular vibrations observed due to absorption of IR radiation.
Infrared radiation was discovered in 1800 by William Herschel.
4 The range of EMR between the visible and microwaves region is called INFRARED region(14000-40 cm-1 ).
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IR REGION From application and instrumentation point of view
infrared region is subdivided into
Region Wave length (m) Wave number(cm-1 )
Near infrared 0.75-2.5 14000-4000
Mid infrared 2.5-50 4000-400
Far infrared 50-300 400-40
Mid IR (4000-40 cm-1)
Functional group region (4000-1400 cm-1)
Finger Print/Single bond region (1400-40cm-1)
Stretching vibrations occurs in F.G. region Bending vibrations occurs in F.P. region
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Infrared light is used in industrial, scientific, and medical applications.
• In Telescopes to detect planets• Finding heat leaks from houses
General Purpose Of IR Radiation
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Contd...
• Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatus.
• Night-vision devices• Remote temperature sensing, short-ranged
wireless communication, spectroscopy, and weather forecasting.
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Pharmaceutical Applications
Qualitative determination of substances Structural elucidation by determining the functional groups. Detection of impurities Identification of geometrical isomers for both organic &
inorganic samples Detection of presence of water in sample Quantitative determination of sample by using Beer’s-Lamberts
law
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PRINCIPLE
In any molecule , atoms or groups of atoms are connected by
bonds which are similar to springs and not rigid in nature.
Bonded atoms having different strengths due to different
masses.
Absorption of IR energy will cause vibrational changes in
molecule and a peak will be observed, when
Conditions to obtain IR spectrum
1) Selection rule for IR spectroscopy: Dipole moment of bonds should change during vibration.
2) When Applied infrared frequency = Natural frequency of vibration
3) When covalent bonds are polar in nature
When sample obeys these conditions, then it gets vibrated by absorbing radiation and gives IR spectrum.
Contd…
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Hooke’s law
K = force constant (in dynes/cm)
m = atomic masses of atom 1 & 2
Hooke's law of elasticity is an approximation which states that the extension of a spring is in direct proportion with the load added to it as long as this load does not exceed the elastic limit.
Used to calculate approximate position of band
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TYPES OF VIBRATIONS
Stretching Mode
In plane bending vibrations
Scissoring Rocking
Out plane bending vibrations
Wagging Twisting
Symmetric Asymmetric
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InstrumentationRadiation sources
They must produce intense & steady radiation.
Nernst Glower heated rare earth oxide rod (zirconium, yttria, thoria) (~1500 K)
1-50 µm(mid- to far-IR)
Globar heated Silicon Carbide rod (~1500 K)
1-50 µm(mid- to far-IR)
W filament lamp 1100 K 0.78-2.5 µm(Near-IR)
Hg arc lamp Hg plasma 50 - 300 µm(far-IR)
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Monochromators
Filtration of desired frequency of radiation
Monochromators are 2 types
1) Prismatic Monochromator
2) Grating Monochromator
Prismatic Monochromators: Composed of glass or quartz and coated by alkyl halides (NaCl) These are 2 types Mono pass Prismatic Monochromator: radiation will pass once
through the prism Double pass Prismatic Monochromator: radiation will pass
twice through the prism
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Grating Monochromators
Grating monochromators introduced in 1950’sHigh dispersion and resolution than prismatic
monochromators These are 2 types Reflection Grating Monochromator are common
than Transmittance Gratings
Gratings are linear grooves or lines which are made up of Aluminium.
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Detectors/Transducers
Thermocouple Thermoelectric effect -dissimilar metal junction
cheap, slow, insensitive
Bolometer Ni, Pt resistance thermometer (thermistor)
Highly sensitive <400 cm-1
Golay cell Metal cylinder with Xe gas Faster than others & having wide wavelength range
Pyro electric Tri glycine sulfate piezoelectric material
fast and sensitive (mid IR)
Photoconductive (non-thermal)
PbS, CdS, Pb Se light sensitive cells
fast and sensitive (near IR)
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TYPES OF IR INSTRUMENTS
Non- Dispersive •Filters used for wavelength selection & having sample specific Detector
Dispersive •Sequential scanning of each wave number takes place
FTIR systems •Widely applied and quite popular in the far-IR and mid-IR spectrometry.
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Dispersive IR instruments are introduced in 1940’s.
Double-beam instruments are mostly used than Single beam instrument.
In dispersive IR sequential scanning of wave numbers of light takes place.
Dispersive IR Instrument
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Instrumentation
In double beam spectrometer , beam separates into two and passes to sample & reference. Prismatic monochromators have been replaced with Grating monochromator. Dispersive IR failed due to monochromator containing narrow slits which limit the wave number of radiation.
To overcome all these problems FTIR has been developed
(X) It containing all movable parts which causes mechanical slippage
(X) Slow scan speed
(X) Less resolution, accuracy and sensitivity
(X) Only narrow frequency range can be studied
(X) Involvement of stray light
(X) Atmospheric absorptions by CO,
water also takes place.
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Fourier Transform IR Instrument
FTIR collects all wavelengths simultaneously and scans at once.
FTIR works based on Michelson Interferometer which having
• Beam splitter• Fixed mirror• Movable mirror
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Interferometer
He-Ne gas laser
Fixed mirror
Movable mirror
Sample chamber
Light source
Detector
Beam splitter
FTIR Instrumentation
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When the beams are combined an interference pattern is created
Combined beam reaches detector by passing through sample
Obtained spectrum is referred as Interferogram
This will be amplified and translated into IR spectrum by FTIR
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Advantages
Fast & sensitiveAll frequencies can be modulated at once Simple mechanical design with only one
moving part No stray light is involvedWhen using He-Ne laser as internal
standard, no need of external calibrationAvailability of easy sampling accessoriesAir pollutants like CO, ethylene oxide
etc. can be analysed
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Conclusion
FTIR having significant advantages over Dispersive IR due to its fast and accurate analysis.
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References
Instrumental Methods Of Chemical Analysis; By Gurudeep R. Chatwal, pg No. 2.29-2.82
Infra Red Spectroscopy: Fundamentals And Applications; By Barbara Stuart, pg No. 16-23
Introduction to Spectroscopy, 4th edition, By Pavia, Lampman, Kriz.
Elementary organic chemistry, By Y.R.Sharma; 2007,Pg No.69-137