1. Laboratory of Analytical Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland
2. Wihuri Research Institute, Kalliolinnantie 4, FI-00140 Helsinki, Finland
3. Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland
4. Department of Physics, University of Oulu, P.O. Box 3000, FI-90014 Oulu, Finland