The precision of absorbance ratios and peak purity parameter values for two test compounds, promazine and promethazine, were determined over the mass injected range of 6 ng–5.25 µg using a multi-wavelength diode array detector. The reproducibilities of these techniques are dependent on the instrument noise level and in addition, for absorbance ratio data, signal band widths must be optimised to achieve satisfactory signal to noise ratios. For the two compounds selected, which are resolved chromatographically but have similar UV spectra, both parameters offer the same degree of discrimination. However, the compounds are distinguishable provided that 13 ng of each compound are introduced into the system by on-column injection.

For two compounds that co-elute and produce similar UV spectra, e.g., atenolol and metoprolo, it is possible to obtain spectral discrimination between a pure sample of atenolol and a sample containing 1%(m/m) metoprolol using both techniques.