Potential of flow injection analysis for dissolution tests: application to on-line monitoring of tablets with different dissolution rates

The possibilities of jow injection analysis for the on-line monitoring of dissolution tests are illustrated by using a very straightforward manifold to monitor tablets of ascorbic acid dissolving at different rates. The dissolution kinetics can be monitored at an adequate samplingfrequency as the method allows up to 90 samples to be processed per hour. Flow Injection Analysis (FIA) has proved to be a useful tool for on-line process monitoring 1,2] and pharmaceutical analysis [3-5]. Both applications are of great importance in the monitoring of dissolution tests of solid dosage forms. Dissolution tests on pharmaceuticals are commonplace quality control procedures and facilitate meeting the legal requirements for compendial drugs, and for oral-dosage forms which are not yet listed in compendia. Conventional approaches to the monitoring of dissolution processes occurring simultaneously in the 'controller', according to the requirements established by pharmaco-poeia regulations, include the following: (1) an automated selecting valve sequentially driving a small volume of the evolving system to the single flow-cell of a photometric detector, or to a dedicated detector with six flow-cells, each receiving the stream from one vessel (both alternatives monitoring the native absorbance of an active compound); (2) the use of a sampler as a interface between the dissolution vessel and the instrument (generally a high-pressure liquid chromatograph); and (3) in-line monitoring. These approaches suffer from a series of shortcomings such as: (a) the difficulty of controlling non-specific, or non-selective, active compounds (for example the case of absorption spectra overlapping with those of other components in the oral-dosage); (b) the process is controlled in anything but real-time-very expensive instrumentation and maintenance costs would be involved if real-time was used; (c) measurement errors arise from the presence of variable amounts of suspended solids and these result in complex and imprecise data treatment. The need for new instrumentation and methods for monitoring of dissolution tests increases day by day, fostered by the manufacture of new drugs with widely different dissolution rates, and by the need for stricter quality control as a result of the growing competition in international markets. This paper aims to show the suitability of FIA in adapting existing methodologies to the kinetics of the dissolution process. A photometric method was developed for the determination of ascorbic acid, and then adapted to the monitoring of this analyte in tablets with fast and slow dissolution kinetics.