The use of gravimetric studies to assess the degree of crystallinity of predominantly crystalline powders

An accurate humidity and temperature controlled microbalance system (dynamic vapour sorption) has been used to investigate the water sorption properties of mixtures of amorphous lactose (spray dried) and crystalline a-lactose monohydrate. From the shape of the sorption isotherms it could be seen that the first sorption process for each sample showed higher weight gain due to absorption into the amorphous regions, than was observed for the subsequent adsorption events on the sample. For the first sorption process, the weight fell after the humidity reached about 60% RH, as the amorphous material recrystallised (causing expulsion of absorbed water). On desorption there was always residual water after the first cycle, which correlated reasonably well with the residual that would be predicted if the amorphous material had been converted into the monohydrate form. The water sorption technique can readily be used to demonstrate the existence of amorphous material even for mixtures which contain as little as 0.05% w/w, and it is possible that the residual weight change may be used as an approximate quantification of the original amorphous content of the sample.