Gas Chromatography - Tandem Techniques - The Combination of the Gas Chromatograph with Other Spectroscopic Systems > The Measurement of UV and Visible Absorption > Page 16


The upper spectrum give characteristic adsorption peaks for an aromatic ring and can help in identifying the presence of an aromatic nucleus in the structure of a completely unknown substance. In addition, the unique adsorption peaks can often identify the type of ring and also be used for quantitative estimation. In contrast, the lower spectrum for ethyl ethanoate is typical of many aliphatic compounds and is of little use for structure elucidation or even quantitative assessment. Unfortunately, the vast majority of organic compounds give spectra similar to that of ethyl ethanoate and, for this reason, except for identifying the presence of the aromatic nucleus and certain types of unsaturated compounds, UV spectra have very limited use in structure elucidation




Figure 10. UV Adsorption Spectra of Benzene and Ethyl Ethanoate


The Measurement of UV and Visible Absorption


In practice UV absorption detectors respond to those substances that absorb light in the range 180 to 350 nm. As already briefly considered, a number of substances absorb light in this wavelength range, including those having one or more double bonds (p electrons) and substances having unshared (unbonded) electrons, e.g. all olefins, all aromatics and compounds, for example, containing >C=O, >C=S, ŠN= groups.