Transcript Used wood splitters

IR Spectroscopy
• Red to microwave :0.7-500μm (Near IR,
fundamental IR & far IR)
• Most used: 2.5-50μm
• Involves twisting, bending, rotating, vibrational
motion of atoms in molecule
• Complex absorption patterns :Types & no. of
Atoms, bonds, functional groups etc.
• Vibrations synonymous to mechanical model:
(ball attached by spring)
Molecular structure & IR Specta
• IR spectrum : Superimposition of absorption
bands of functional groups
• Affected by molecular interactions
• Absorption frequency (or its absence) correlates
with bending stretching motion ( spatial
relationship)
• Information : Presence/absence of functional
group
• Discovery of new molecules
Near IR region
• 0.8 –2.5μm
• Absorption bands H associated
• eg O-H, N-H, C-H, Primary aromatic amines
(stretch vibrations)
• Determination of proteins, fats,sugar, oil moisture
in other chemicals like glycerol, acetone etc.,etc.
• Determination of substances in wood,
components of polymers, geological exploration
of aircrafts,etc
Mid IR region
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2.5-15.4μm
Unsaturated states (double & triple bond)
C=O,C=C, C=N N=O, S=O bonds
Single bond stretching frequency &Bending
vibrations : Molecular mass<19
• Identification of compound
•Far IR Region
• 15-1000μm
• Bending vibrations of C,H, N, F with atom
with mol. Mass>19
• Isomeric structures : sensitive to overall
structure
• Organometallic compounds: Coordination
bonds
•Compound Identification
• Available information : Physical state,
Solubility,, melting point, flame test etc.
• Pure or mixture
• Organic or inorganic
• Aromatic or Aliphatic
• Functional group present or absent &
frequency
• Spectrum comparison
IR Instrumentation
• (i) Dispersive (ii) Non dispersive
• (i) Dispersive: Use prism or gratings: Ionic salts
(KBr,CaF2,NaCl,etc.)
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Glass & quartz not transparent to IR
Used for Qualitative analysis
Choice dependent on wavelenght to be used
Strong enough to be shaped and polished to make
optical component
• Problem: Water soluble (must be kept dessicated)
• Alters opacity & light scatter
Dispersive cont…
• Similar to UV dispersive spectrometers
except different source and detectors used
Non Dispersive IR spectroscopy
• Doest use prism or diffraction gratings not used
for light dispersion
• Use interference filters to isolate selected
wavelength regions
– Constructed of multiple layer of different material
– Λ transmitted controlled by thickness & refractive
index of central layer
– Can be constructed for transmission of IR,Visible or
UV region
• Used for quantitative analysis
• High Signal to noise ratio
• Less complex/expensive,rugged,easy to maintain
Non Dispersive IR spectroscopy
• Eg(i) Filter photometer (source:Nichrome
wire,Transducer: Pyroelectric device, and
interference filters)
Nonfilter photometer
– Employ no wave
length restricting
device
– Monitor gas stream
IR Radiation source
• Ideal requirement
– Continuous over wavelength used
– Cover wide wavelength range
– Constant over long time
IR Radiation source (cont…)
• Near IR region:Tungsten filament lamp
• Mid IR source
(i) Wire source : Inert solid
- Heated electrically to emit cont. radn (T=1100C).
- Eg Nichrome coil (forms light emitting oxide
Reliable, simple, rugged inexpensive)
- light source less intence
(ii)Nerst Glower: Brighter and hotter (1500C)
-Mde from fused mixture of rare earth
oxides(Th, Zr,etc)
– Fragile
(iii)Globar source:Silicon carbide rod
– Intermediate
• Far IR region : High pressure Hg arcs
Detectors
• (i) Thermal detectors: IR radiation produces
heating affect altering physical property of
detector
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Maximizes temperature change to minimum IR level
Element blackened and thermally insulated
Usable over wide wave lengths (IR –visible)
Operate at room temperature
Disadvantage : Low response time and sensitivity
Properties affected : Expansion (solid,fluid or gas),
electrical resistance,voltage induced at junction &
electric polarization
Thermal detectors- types
• Thermocouple
– Joining 2 dissimilar metals (Eg. Bismuth & Antimony)
– Produce voltage α to temp. at junction
• Thermophile
– Several thermocouples connected in series
– Convert radiant energy to electrical signal
– Frequency response flat<35Hz, response time 30msec
• Thermistor
– Changes resistance when heated (5% per degree)
– Made of fused mixture of metal oxides
– Slow response time
• Bolometer
– Change in conductance resistance to measure
– Change in temperature depends on intensity of
incident radiation
• Pyroelectric detector: Uses Insulators,
semiconductors or ferromagnetic material
- Have crystal structure exhibiting magnetic field
Pyroelectric detector (Cont…)
- Heating produces thermal alterations in crystal
lattice
lattice spacing leading to electrical polarization
- Effect depend on rate of change of detector
temp
- Fast response time
- Choice for Fourier transform Spectrometer
- Responds to changing radiation (ignores steady
radn)
Photon detector
• Relies on interaction between photon &
semiconductors ( insulate when no radn. But
conductors when radiation falls on it)
• Radiation induces change in electrical resistance
• Rapod respose and sensitive to IR
• Exhibit cutoff towards far IR region
• Used for FT spectrometer
• Made of Lead selenide, Indole antimonide,etc.
Fourier Transform spectrometer
• Intensity of light determined by simultaneously
measuring all regions by array of detectors
• Measures light at all wavelength to reconstruct
intensity vs wavelength curve to give spectrum
• Overlapping information sorted out by
mathematical procedure called Fouriers
transformed
• Intensity vs wavelength expressed as sum of sine
or cosine functions varying with time
• Require data in digital form
Fourier Transform spectrometer
• SourceMonochromatic radiationbeam
splitter Fixed mirror
Mobile mirror ( moves at contant
velocity) Sample holder  detector
• Interference between beam for fixed and moving
mirror
• Constructive if in phase (max. if path diff. Is
integral multiple of lamda)
• Destructive if distance multiple of half of lamda
No signal
Fourier Transform spectrometer
• In between: Partial destructive interference
• Signal reaches detector in cyclic pattern
• Signal (Intensity vs pathlength) is a cosine
finction called interferogram
• Converted to IR plot of intensity vs wave
length
Advantages of FT system
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Better signal/Noise ratio
Fewer optical elements (lesser noise)
Higher level of intensity reaching detector
All measurements simultaneous (Time to collect
data reduced)
• Higher reproducibility and accuracy
• S/N ratio is background corrected
• More expensive due to precision of mirror
movement