Study of photodegradation kinetics of melatonin by multivariate curve resolution (MCR) with estimation of feasible band boundaries

Multivariate curve resolution—alternating least squares (MCR‐ALS) has been applied to data collected from UV spectrophotometric analysis of melatonin samples exposed to light with varying irradiance power. MCR‐ALS was able to explain the degradation kinetics of this drug, deducing the pure spectra and concentration changes of the different species present throughout the entire process. Possible rotational ambiguities associated with MCR solutions were investigated and their extent was evaluated. The extent of the rotation ambiguity was estimated from the band boundaries of feasible solutions calculated using the MCR‐BANDS procedure. The use of a non‐linear fitting routine allowed improving kinetic information and provided a method of evaluation of the rate constants of the degradation process. The degradation pathway was found to follow a first‐order reaction model, in which melatonin underwent photo‐oxidation of the indole ring to give a formylamine group. Kinetics of the reaction was shown to be dependent on irradiation conditions, with an increase of the rate constants when light power also increased. Copyright © 2011 John Wiley & Sons, Ltd.