Advanced Spectroscopy 3. Infrared Spectroscopy

Revision 1.What molecular or structural features give rise to absorption of infrared (IR) radiation? covalent bonds 2.Give an example of an inorganic species that would not absorb IR radiation. NaCl, KBr 3.Give an example of an organic species that would not absorb IR radiation. there isn’t one!

Revision 4.What method of sample preparation would be appropriate for the following? a)a pure organic liquid film between NaCl plates b)a pure organic solid, soluble in most solvents reflectance, CHCl3 solution in sandwich cells c)a pure insoluble organic solid reflectance, KBr disc

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 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

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 Monochromator reflection gratings once used modern IR spectros use non-dispersive Fourier transform

Detectors 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)

common detectors: thermocouple Golay thermistor semiconductor

Configurations until last 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 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 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. flexible metal gas-filled boxes reference cell flow-through sample cell source

Cells 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 n = 12 f1-f2 = 466 d = mm if the RI of a plastic is known, its thickness can be measured in the same way 2752 cm cm -1

Cells 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 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 sample crystal IR beam

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