1. Department of Analytical Chemistry, Lund University, P.O. Box 124, 221 00 Lund, Sweden, Department of Electrical Measurements, Lund Institute of Technology, Box 118, 221 00 Lund, Sweden, and Bioanalytical Chemistry, Preclinical Research and Development, Astra Draco AB, Box 34, 221 00 Lund, Sweden