High-precision absolute (true) density measurements on hygroscopic powders by gas pycnometry: application to determining effects of formulation and process on free volume of lyophilized products.

As density (free volume) of the amorphous solids should be related to mobility and stability, an attempt was made to develop a simple, sensitive, and reproducible method to evaluate free volume via high-precision gas pycnometry density measurements, and to apply this methodology to study the variation of free volume with formulation and thermal history (i.e., annealing). Annealed samples were prepared either by heating the product after freeze drying (postannealing) or drying at higher temperature in secondary drying than normal (in-process annealing). Density was measured using a gas pycnometer. We find that the key to high-precision density measurements is isolation of the instrument from atmospheric moisture; accordingly, all operations were carried out in a dry box. With suitable care, densities of amorphous freeze-dried products can be measured with a precision of better than 0.5% in a series of independent but nominally identical samples. Density decreased with increasing molecular weight of dextran, but density of proteins was independent of molecular weight. Small but significant increases in density upon annealing were observed for several formulations. Thus, we conclude that accurate density measurements may be made by carefully controlling residual moisture. Density may be a useful parameter to predict long-term stability.