A critical study on the zero-crossing method in derivative spectrophotometry

The derivative spectrophotometry by the “zero-crossing” technique exploits the signal drop within the abscissa axis for some components in a given mixture to assign an absorbance signal to the remaining components. The utilization of such a technique succeeds in the quantitative analysis of several complex mixtures. On the other hand its use suffers from limitations as for ordinary spectrophotometry, namely those deriving from signal measurements seldom set over the maximum or minimum values of the spectral curve, but more often along ascendant or descending slopes of absorption peaks. The low accuracy and precision in the measurements of experimental values would lead to less reliable analytical results. The present study has been set up by a critical comparison between the zero-crossing procedure and other analytical applications of spectrophotometric data. The research has been performed on a two-component mixture of Lacidipine (LAC), a photosensitive antiipertensive dihydropyridine drug, and its main photodegradation product (LACd), namely the corresponding aromatic didehydro derivative. The different analytical methods have been defined by selected first order derivative spectra. The zero-crossing diagram has been constructed by measurements on signals centered at 245 and 287 nm, from which it was possible to calculate the LAC concentration, and from signals at 220, 262 and 287 for the calculation of LACp concentration. This method showed to bring to remarkable results only by considering wavelenghts corresponding to a maximum absorance peak or in the case of compounds with high concentration. The zero-crossing technique has been compared with either customary procedures employing absorbance measurements at discrete wavelenghts and multivariate analysis regression methods, such as PLS (Partial Least Squares) and PCR (Principal Component Regression).