A H de Boer1, M Hoppentocht2. 1. Department of Pharmaceutical Technology and Biopharmacy, University of Groningen, Ant. Deusinglaan 1, 9713 AV Groningen, the Netherlands. Electronic address: a.h.de.boer@rug.nl. 2. Department of Pharmaceutical Technology and Biopharmacy, University of Groningen, Ant. Deusinglaan 1, 9713 AV Groningen, the Netherlands.