The extent to which an organic molecule absorbs electromagnetic radiation in the ultraviolet region (UV light) depends on the structure of the molecule. Generally, molecules that contain one single double bond absorb weakly in the UV region. However, if multiple double bonds are present in a molecule and they are conjugated, the molecule absorbs more strongly and the absorbance is shifted to longer wavelengths. The pH of the mobile-phase effects the ionization of ionogenic solutes and also the analyte UV response. The change in pH can change the electronic structure of the molecule and result in changes in. | EFFECT OF pH ON UV ABSORBANCE 429 EFFECT OF pH ON UV ABSORBANCE The extent to which an organic molecule absorbs electromagnetic radiation in the ultraviolet region UV light depends on the structure of the molecule. Generally molecules that contain one single double bond absorb weakly in the UV region. However if multiple double bonds are present in a molecule and they are conjugated the molecule absorbs more strongly and the absorbance is shifted to longer wavelengths. The pH of the mobile-phase effects the ionization of ionogenic solutes and also the analyte UV response. The change in pH can change the electronic structure of the molecule and result in changes in the molar absorptivity and the absorption maximum of the molecule 25 . Ionization of aromatic compounds containing a pyridinal nitrogen amino carboxylic acid and phenolic group can lead to significant changes of their UV response. Understanding the effects of charge delocalization and conjugation on the UV response and detection wavelength will allow the chromatographer to choose the proper pH and wavelength of detection to obtain a method with higher sensitivity. Silverstein et al. 26 and Shenk 27 provide a good overview for predicting how the structure of a molecule and its environment will affect its molar absorptivity and the wavelength of the absorption maximum. Most applications of absorption spectroscopy to organic compounds are based upon transitions for n or n electrons to the n excited state. Energies required for these processes bring absorption peaks into the spectral region 200-700nm . n-electrons are further delocalized by effect of this delocalization is to lower the energy level of the n orbital and give it less antibonding character and as a result absorption maxima are shifted to longer wavelengths 28 . UV spectra of aromatic hydrocarbons are characterized by three sets of bands E1 E2 and B bands that originate from n n transitions. Generally the E2 and B bands are of
