A study of the crystallisation of amorphous salbutamol sulphate using water vapour sorption and near infrared spectroscopy

The crystallisation of amorphous salbutamol sulphate prepared by spray drying was monitored using a humidity controlled microbalance (Dynamic Vapour Sorption apparatus, Surface Measurement Systems) combined with a near-infrared probe. Amorphous salbutamol sulphate was prepared by spray drying from a solution in water. The particles were then analysed using scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, powder X-ray diffraction, isothermal microcalorimetry and water vapour sorption analysis combined with near-infrared spectroscopy (NIR). Isothermal microcalorimetry and water vapour sorption combined with NIR spectroscopy were able to detect the transition from the amorphous to crystalline state. However while the isothermal microcalorimeter showed only a classic crystallisation exotherm when the material was exposed at 75% RH, the DVS-NIR results at the same humidity highlighted a more complex process. When exposed at 75% RH, the uptake of water was followed by crystallisation that was detected using NIR. The expulsion of water after crystallisation was very slow and at a constant rate whether the material was exposed to 75 or 0% RH. The NIR and DVS studies indicated that the material had crystallised very soon after exposure to high RH. The water that was expelled during crystallisation was not displaced from the particles and remained associated with the particles for many days. This study showed that the use of gravimetric analysis together with NIR spectroscopy provided valuable information on the dynamics of the crystallisation of salbutamol sulphate. The retention of water within recently crystallised salbutamol is potentially important to the behaviour of dosage forms containing the amorphous (or partially amorphous) form of this drug.