There is an inherent similarity between UV spectra of widely different types of compounds, and so UV spectra are not very reliable for the identification of most solutes. The technique can be used, however, to determine the homogeneity of a peak (e.g., by comparing spectra taken from both sides of the peak. Both spectra are normalized and either one is subtract one from the other and the difference is shown to be zero, or the ratio of the two spectra is calculated and the result shown to be unity.

A common use of multi-wavelength choice is to enhance the sensitivity of the detector by selecting a wavelength that is characteristically absorbed by the substance of interest. Conversely, a wavelength can be chosen that substances of little interest in the mixture do not adsorb and, thus, make the detector more specific to those substances that do.

Column: Spherisorb Octyl, 25 cm x 4.6 mm I.D., 5 mm particles. Mobile Phase: 0.2 M phosphoric acid. Flow rate 0.8 ml/min. monitored at 210 nm. 1. tartaric acid, 2. lactic acid, 3. malic acid, 4. formic acid, 5. acetic acid, 6. citric acid, 7. succinic acid, 8. fumaric acid.

Courtesy of Supelco Inc.

Figure 29. The Separation of Some Carboxylic Acids Monitored by UV Absorption at 210 nm