IR Spectroscopy Red to microwave 0 7 500m

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 • • 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, Ca. F 2, Na. Cl, etc. ) • • 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=1100 C). - Eg Nichrome coil (forms light emitting oxide Reliable, simple, rugged inexpensive) - light source less intence

(ii)Nerst Glower: Brighter and hotter (1500 C) -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 – – – 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<35 Hz, response time 30 msec • 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 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 • • 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