Advanced Spectroscopy

1. Advanced Spectroscopy 2. UV-VIS Spectroscopy

2. Revision 1. What are the wavelength ranges for the ultraviolet and visible regions of the spectrum? • UV: 200-400 nm (actually < 200 far UV) • Visible: 400-800 nm

3. Revision 2. What molecular or structural features give rise to absorption of ultraviolet/visible (UV/VIS) radiation in organic species? Give an example of an organic compound that would not absorb UV/VIS radiation. • multiple covalent bonds • unbonded electrons (N, O, Cl) • hexane

4. Revision 3. What molecular or structural features give rise to absorption of ultraviolet/visible (UV/VIS) radiation in ionic species? Give an example of an ionic compound that would not absorb UV/VIS radiation. • valence electrons • NaCl

5. Revision

6. Absorbing species - organic • all species absorb < 200 nm • not a practical area for measurement (need vacuum) • near UV is 200-400 nm • N2 should absorb but doesn’t • conjugation increases absorbance and shifts  to higher values • atoms with non-bonded electrons attached to conjugated system add to this • need a lot to get into the visible region

7. Absorbing species - inorganic • some simple metal ions absorb weakly in the ultraviolet or visible region eg Cu2+ and Ni2+ Exercise 2.1 • Why would weak absorption by a chemical species, eg Cu2+, make it not useful for quantitative analysis? • require a very high concentration to get 0.1-1 absorbance

8. polyatomic ions, such as permanganate and dichromate - much stronger absorbance • a combination of factors: multiple bonds and non-bonded electrons • complexes of metal ions and ligands are needed for intense absorption • ligands are known as colour-formingreagents.

9. Cells • cell and solvent should not absorb more than 0.2 at wavelengths of interest • quartz – UV/VIS • plastic – VIS (aqueous) • glass – VIS (organic)

10. Solvents • solvent cutoff  – above which where the solvent absorbs little • can be used for measurements Exercise 2.2 • dimethylbenzene (250-300 nm) • hexane, dichloroethane or trichloroethane • sodium benzoate (250-320 nm) • water • aspirin (280-320 nm) • acetonitrile, methanol or ethanol

11. Spoectrograde solvents • designed for use in UV spectroscopy • not necessarily more pure than AR grade • guaranteed not to have absorbing impurities • eg AR grade hexane might be 99.9% pure, but the impurity could absorb (benzene) • spectrograde hexane might only be 99% pure, but the impurity is non-absorbing (heptane)

12. Radiation sources • two required: • a deuterium discharge lamp for the UV • a tungsten filament globe for the visible • output of the tungsten filament is dependent on the applied voltage • power supply contains a voltage regulator to ensure a constant value • at the changeover  (around 350 nm) difference in intensity of the two lamps dealt

13. Monochromators • prisms require very high quality calibration and optics • diffraction gratings considerably less expensive and optically more efficient

14. output +ve electrode dynode electrons light sensitive –ve electrode photon Detectors - Photomultiplier tubes • limited operating life due to breakdown of the photocathode • cannot be exposed to the sunlight or bright room lighting • a large semi-permanent dark current results

15. Detectors – diode array • bank of joined semiconductors • multi-channel instrument • diode is responsible for detecting a small portion of the spectrum (1-2 nm per diode) • all diodes operate at the one time • fixed resolution determined by number of diodes • not a problem for broad peaks