Rapid at-line analysis of coating thickness and uniformity on tablets using laser induced breakdown spectroscopy

Because of the functionality of controlled release tablet coatings, it is desirable to have a rapid means of optimizing coating conditions and predicting the performance of a batch at-line, prior to exhaustive lab analysis. In this paper, Laser Induced Breakdown Spectroscopy (LIBS) was utilized for the first time as a rapid means of simultaneously determining the thickness and uniformity of an enteric coating on compressed tablets. In these studies, the core tablets contained a high concentration of calcium, and the coating contained titanium, silicon, and magnesium, all of which are excellent analytical targets for LIBS. The emission spectra of all four elements were simultaneously monitored as a function of the number of laser pulses in the same spatial location, literally drilling through the coating and into the core tablet. The depth penetration of individual laser pulses was calibrated using profilometry, and the thickness of the coating was determined by the incidence of calcium signal and simultaneous decrease in emission from the other probe elements (titanium, silicon and magnesium) as the laser penetrated the coating. Coating thickness measurements were evaluated by sampling coatings ranging from 5 to 21% (100 mg tablet weight basis). Additionally, LIBS spatial resolution capabilities were exploited to evaluate the film coat thickness uniformity across the tablet faces and edges. Furthermore, tablet to tablet uniformity differences were readily assessable. The results indicate that a change in coating application of less than 2 wt.% on a 100 mg tablet can be easily detected. The rapid analysis times for this technique (10 tablets were analyzed in under 15 min) makes it applicable for in-process evaluation of coating thickness, as well as intra- and inter-tablet coating uniformity on compressed tablets.