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Advanced Spectroscopy. 3. Infrared Spectroscopy. Revision. What molecular or structural features give rise to absorption of infrared (IR) radiation? covalent bonds Give an example of an inorganic species that would not absorb IR radiation. NaCl , KBr

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Advanced spectroscopy

Advanced Spectroscopy

3. Infrared Spectroscopy


  • What molecular or structural features give rise to absorption of infrared (IR) radiation?

  • covalent bonds

  • Give an example of an inorganic species that would not absorb IR radiation.

  • NaCl, KBr

  • Give an example of an organic species that would not absorb IR radiation.

  • there isn’t one!


  • What method of sample preparation would be appropriate for the following?

    • a pure organic liquid

    • film between NaCl plates

    • a pure organic solid, soluble in most solvents

    • reflectance, CHCl3 solution in sandwich cells

    • a pure insoluble organic solid

    • reflectance, KBr disc

Regions of the ir
Regions of the IR

  • stretches from the red end of the visible spectrum (12500 cm-1, 800 nm) to the microwave region (20 cm-1, 500 m)

  • most familiar is 4000-600 cm-1 – mid infrared

    • structural determination of organic species

  • closest to the visible – near infrared

    • quantitative analysis – food, plastics

  • the low wavelength end – far infrared

    • limited use

Advanced spectroscopy

Radiation source

  • an inert solid, electrically heated to temperatures approaching 2000K

    • materials include silicon carbide (Globar), mixed rare earth oxides (Nernst glower) and nichrome wire


  • reflection gratings once used

  • modern IR spectros use non-dispersive Fourier transform


  • sense the increase in temperature by an absorbing material with a very small heat capacity

    • gives a significant temperature rise for small amounts of heat energy absorbed

  • temperature rises are 0.00x

  • problem of heat from the surroundings

  • detector must be well insulated (vacuum)

    • from lab & other components

  • chopper (alternating block to measure bkgd T)

Advanced spectroscopy


  • until last 10-15 years, conventional monorchromator-based

  • now all scanning IRs are FT-based

  • superior in all ways

  • some non-scanning devices used for pollution monitoring

Fourier transform
Fourier transform

  • alternative to multi-channel detectors (same advantages but better)

  • mathematical technique which can separate complexes waveforms into individual freqs

  • no monochromator – replaced by interferometer

  • more in AIT

Non dispersive


sample cell

flexible metal


gas-filled boxes

reference cell


  • many pollutant gases absorb IR strongly, eg CO2, CH4

  • can’t put normal instrument in chimneys, on top of buildings

  • some NDIRs don’t even use a normal filter

  • reference cell filled with “normal” air + other absorbing gases

  • detector senses differences in beams by T => P var.


  • use metal halide salts (no covalent bonds)

  • loose plates or fixed pathlength “sandwich” cells

  • spacer between plates determines pathlength

  • determined exactly by measurement of spectrum of empty cell

    • interference pattern of waves

Exercise 3 1

2286 cm-1

2752 cm-1

Exercise 3.1

  • n = 12

  • f1-f2 = 466

  • d = 0.128 mm

  • if the RI of a plastic is known, its thickness can be measured in the same way


  • gas spectra require long pathlength cells

    • low concentration of gas molecules

  • water solubility of most salts is a problem

  • AgCl cells can be used

Attentuated total reflectance



IR beam

Attentuated total reflectance

  • means of recording “difficult” samples with no sample prepn.

    • aqueous solutions, fabrics, powders, plastic sheets

  • IR beam bounces off surface of sample

  • requires very good contact no air) between crystal and sample

  • spectrum intensities distorted but correctable

Near ir
Near IR

  • analysis of a range of commercially important materials

    • eg cereal grains, oils and plastics

  • analytes such as protein, water, sugar

  • reflectance mode most common

  • important peaks due to O-H, N-H and C-H bonds

  • distinctive frequencies from different compounds

    • water frequency different to ethanol

  • main drawback is interactions between species

  • can’t do simple standard calibration graph

  • require known matrix-matched materials (lots of them)

  • instruments often designed for one analysis (eg moisture in flour) and pre-calibrated