Semiautomated identification of the phase diagram for enantiotropic crystallizations using ATR-FTIR spectroscopy and laser backscattering

Abstract

A semiautomated procedure for measuring the phase diagram for enantiotropes in a dimorphic system was developed using Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) spectroscopy and laser backscattering (Focused Beam Reflectance Measurement, FBRM) for in situ measurement of solute concentration and the particle counts, respectively. The approach is demonstrated using l-phenylalanine, an enantiotropic pseudodimorph. The procedure involves the determination of the anhydrate-form solubility from in situ infrared spectroscopy of an equilibrated slurry, followed by the dissolution of the anhydrate form by heating. The monohydrate form is then recrystallized, and its solubility determined from slow heating until complete dissolution is detected by FBRM. The cycle is repeated for higher temperatures after addition of anhydrate crystals to create a slurry. The solubility of the monohydrate form was determined differently from the anhydrate form due to the interference on the infrared measurements from small needle-like monohydrate crystals. This single-experiment approach is expected to be applicable to other enantiotropic dimorph systems for the measurement of the phase diagram in a more efficient manner. © 2010 American Chemical Society.