How to determine a coating level.
Evaluating different methods to gain reliable data
In pharmaceutical film-coating applications, various approaches are known to evaluate the coating level (1-3). Yet huge differences in accuracy and reliability can be seen when comparing methods such as weight, colour and photometrical measuring. The determination of a tracer added to the coating formulation (photometrical measuring) was found to be the best method. A huge advantage is seen in the fact that the amount of coating material applied onto an individual tablet can be determined. This allows a precise correlation of a tablet’s dissolution profile and the individually borne coat. Gravimetric measuring showed a standard deviation too high for reliable testing and colour measuring couldn’t indicate the coating level at all.
In the present case study of a solid oral dosage form, the taste masking functionality and the dissolution profile in phosphate buffer respectively could clearly be correlated with the coating level determined photometrically. It could be shown that 3.5 mg/cm² of applied coating formulation already offer maximum functionality.
In the field of functional film-coating, the coating level typically correlates clearly with the resulting dissolution profile. Hereby, one has to differentiate two principles:
a) Functionality strongly dependent on layer thickness
(e.g. sustained release, moisture protective coats)