AN AUTOMATED CONTROLLED-ATMOSPHERE MICROBALANCE FOR THE MEASUREMENT OF MOISTURE SORPTION

A controlled atmosphere microbalance system has been constructed for the measurement of moisture sorption in pharmaceutical materials. The system is automated, and the user can program arbitrary changes of relative humidity (RH) under computer control. The sample is suspended in an isothermal chamber at 20-25 degrees C, and the RH is regulated from 1 to 98% RH by adjusting the flows of dry and moist gas. A commercial sensor system is used to determine the relative humidity of the flowing gas. Over most of the range, the absolute RH accuracy is +/- 3%, the RH reproducibility is +/- 1%, and the RH stability is +/- 0.2%. Sample masses as large as 1 g can be accommodated. For a typical sample mass of 10 mg, the reproducibility of the weight uptake measurement is typically better than 0.05% of the dry weight over the entire working range of relative humidity. The data collection software employs a weight stability criterion for equilibrium to allow data collection at the maximum rate permitted by the sorption kinetics of the material under investigation. Three examples are presented of applications to pharmaceutical powders.