1. Department of Analytical Chemistry, Department of Applied Microbiology, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden, Institute of Microtechnology, Université de Neuchâtel, Rue Jaquet Droz 1, 2007 Neuchâtel, Switzerland, and Faculty of Health and Society, Malmö University, 20506 Malmö, Sweden