Advanced Spectroscopy

1. Advanced Spectroscopy 3. Infrared Spectroscopy

2. 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 • Give an example of an organic species that would not absorb IR radiation. • there isn’t one!

3. Revision • 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

4. 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

5. 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

6. 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)

7. common detectors: • thermocouple • Golay • thermistor • semiconductor

8. Configurations • 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

9. 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

10. flow-through sample cell flexible metal source gas-filled boxes reference cell 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.

11. 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

12. 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

13. 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

14. sample crystal 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

15. 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