1. Department of Ecological and Environmental Sciences, University of Helsinki, Niemenkatu 73, FIN-15140 Lahti, Finland, Laboratory of Analytical Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 University of Helsinki, Finland, and VTT Technical Research Centre of Finland, VTT Processes, P.O. Box 1602, FIN-02044 VTT, Finland