Transformation of metastable forms of acetaminophen studied by thermal Fourier transform infrared (FT-IR) microspectroscopy

A novel Fourier transform infrared (FR-IR) microspectroscopy equipped with a micro hot stage (thermal FT-IR microscopic system) was used to quickly study the phase transformation of acetaminophen polymorphs by a one-step process. Acetaminophen was sealed in KBr disc on the first and second heating processes under this system. The results indicate that the contour IR profile of form I acetaminophen in the first heating process changed dramatically only near 165degreesC, but in the re-heating process exhibited a considerable alteration in peak intensity, band width and position near the temperatures at 85, 118 and 153degreesC. A glassy form of acetaminophen was obtained after rapidly cooling the melted acetaminophen from 200 to 25degreesC. The glassy acetaminophen was recrystallized at 85degreesC to transform to the form III of acetaminophen in the reheating process, and then transformed to its form 11 near 118degreesC. The thermal FT-IR microscopic system is a simple, quick and timesaving tool for investigation of the thermo-dependent molecular structure of acetaminophen polymorphs in the processes of recrystallization and polymorphic transition.