Rapid quantitative analysis of magnesium stearate in tablets using laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy (LIBS) was evaluated for its potential as a process analytical tool for the rapid determination of magnesium stearate (MgSt) distribution within and between tablets as well as between batches in a typical manufacturing run, and for the comparison of direct-compression and roller-compaction processes. These studies were conducted using a prototype instrument and a commercial PharmaLIBS unit, both based on pulsed Nd:YAG laser radiation at 1064 nm. The intensity of a magnesium spectral line either at 517.27 or 518.36 nm was used, depending on the product, to quantitate the MgSt concentration in the tablets. Using internal standardization, it was possible with the prototype instrument to accurately quantitate MgSt at the 0.5% level in two different products. For eight batches of one product, using 10 tablets from each batch, the intra-tablet, intra-batch, and inter-batch MgSt %RSDs were found to be 13.8%, 5.4% and 7.4%, respectively. Further studies were conducted with the commercial LIBS unit, which showed similar performance as the prototype unit. In particular, it was found that different depth-profile distributions of MgSt were associated with roller-compacted tablets and direct-compressed tablets. These findings illustrate the potential of LIBS to be developed as a process analytics tool for the direct and rapid determination of MgSt content and distribution in tablets.