Advanced Spectroscopy 2. UV-VIS Spectroscopy

Revision 1.What are the wavelength ranges for the ultraviolet and visible regions of the spectrum?  UV: nm (actually < 200 far UV)  Visible: 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 AnalyteRegionSolventCell copper sulfateVISwaterplastic copper sulfateUV/VISwaterquartz methylbenzeneUVhexanequartz yellow NP dyeVIShexaneglass

Absorbing species - organic  all species absorb < 200 nm  not a practical area for measurement (need vacuum)  near UV is 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

Absorbing species - inorganic  some simple metal ions absorb weakly in the ultraviolet or visible region eg Cu 2+ and Ni 2+ Exercise 2.1  Why would weak absorption by a chemical species, eg Cu 2+, make it not useful for quantitative analysis?  require a very high concentration to get absorbance

 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-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 ( nm) hexane, dichloroethane or trichloroethane  sodium benzoate ( nm) water  aspirin ( 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 light sensitive –ve electrode dynode photon electrons 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