Nondestructive analysis of tablet coating thicknesses using terahertz pulsed imaging

An understanding of the finished structure of complex pharmaceutical coating is becoming desirable, because tablet coatings are now one of the preferred routes to control the release of active pharmaceutical ingredients. There are few nondestructive techniques capable of examining the coatings of compressed tablets; for example laser induced breakdown spectroscopy has been used but this is a destructive method. Terahertz pulsed imaging offers a potential technique to examine coatings quickly and nondestructively. In the study reported herein, it was possible to distinguish between two brands of across-the-counter ibuprofen tablets. The terahertz maps obtained were compared with obtained photographs of cut-through sections; there was good agreement. The technique is fast: a waveform can be obtained in <20 ms allowing the technique to be considered as a candidate for on-line or at-line analysis in a process analytical environment. The lateral resolution of the technique is limited by diffraction of the terahertz focus to about 150 microm at 3 THz, whereas the axial resolution is limited by the terahertz pulse duration, which is <200 fs, to about 30 microm.