1. Faculty of Physics, Babes-Bolyai University, Kogalniceanu 1, 3400 Cluj-Napoca, Romania, Institute of Chemical Technology and Analytics, TU Vienna, Getreidemarkt 9-164/AC, 1060 Vienna, Austria, and Department of Electrical Measurements, Lund University, P.O. Box 118, 221 00 Lund, Sweden.