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Advanced Spectroscopy 2. UV-VIS Spectroscopy 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 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 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 Revision Analyte Region Solvent Cell copper sulfate VIS water plastic copper sulfate UV/VIS water quartz methylbenzene UV hexane quartz yellow NP dye VIS hexane glass 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 HO3S N N N(CH3)2 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 polyatomic ions, such as permanganate and dichromate much stronger absorbance a combination of factors: multiple bonds and nonbonded electrons complexes of metal ions and ligands are needed for intense absorption ligands are known as colour-forming reagents. Cells cell and solvent should not absorb more than 0.2 at wavelengths of interest quartz – UV/VIS plastic – VIS (aqueous) glass – VIS (organic) 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 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) 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 Monochromators prisms require very high quality calibration and optics diffraction gratings considerably less expensive and optically more efficient Detectors - Photomultiplier tubes output +ve electrode dynode electrons light sensitive –ve electrode photon • 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 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