1. Institute for Analytical Sciences, Bunsen-Kirchhoff Strasse 11, D-44139 Dortmund, Germany, National Institute of Standards and Technology, Electronics and Engineering Laboratory, 100 Bureau Drive, MS 8120, Building 225, Gaithersburg, Maryland 20899, and The Manchester Interdisciplinary Biocentre, University of Manchester, 131, Princess Street, Manchester. M1 7ND, U.K.