Interpretation of spectra and qualitative analysis
Solvent effects
A solvent for UV and visible spectroscopy must be transparent throughout the region of interest and should dissolve a sufficient quantity of the sample to give well-defined peaks. Moreover, consideration must be given to possible interactions with the absorbing species. For example, polar solvents, such as water, alcohols, esters and ketones, tend to suppress vibrational fine structure and should thus be avoided when spectral detail is desired. Nonpolar solvents, such as cyclohexane, often provide spectra that more closely approach that of a gas . In addition, the polarity of the solvent often influences the position of absorption maxima. Consequently, a common solvent must be employed when comparing spectra for the purpose of identification. When using an HPLC-UV system, any effects on the spectral characteristics of a substance brought about by various mobile phases must be considered when comparing spectra generated in this manner with those recorded from acidic, neutral and alkaline solutions. Moreover, potential effects brought about during gradient elution HPLC, in which the composition of the mobile phases is constantly changing, should be borne in mind. With acetonitrile, methanol, ethanol, propan-2-ol and other water-miscible solvents, no essential changes in the spectra may occur and direct comparison with other data bases is feasible. However, changes in pH can have a significant effect on the UV spectra of
compounds involved in an acid–base equilibrium (e.g. carboxylic acids, phenols, thiophenols) and compounds with basic nitrogen atoms. Direct comparison of spectra with other compendia of data, phases have the same pH.
