Application of Normal and Second-Order Derivative Spectroscopy in Identifying Organic Acids and Sugar Acids in Liquid Chromatography with On-Line Photodiode Array Detection

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Abstract
The UV-spectra of 16 compounds including organic acids and sugar acids were recorded at pH 2.5 using photodiode array detection in three different chromatographic systems: an ion exchange column with phosphate buffer as eluent (I), a C-S reversed-phase column with sulphuric acid as eluent (II) and a C-8 reversed-phase column with hydrochloric acid as eluent (III). Differences in the normal spectra in each of the systems were evaluated by calculating match factors between the spectra. Normal spectra and the second-order derivative spectra gave match factor ranges of 603–1000 and 0.1–999. respectively. The similarity between the normal spectra was very good and excellent between chromatographic systems I and II On the other hand, the similarity between the second-order derivatives of the spectra was not as good. Thus the normal UV-spectra can be used, together with the retention times, for reliable identification despite of the spectral similarity of some acids. Molecular connectivity indices up to the sixth order were correlated with the retention parameters. The correlation was only moderate in all the systems.