1. Unit for Macromolecular Mass Spectrometry, FOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ, Amsterdam, The Netherlands; Océ Technologies, P.O. Box 101, 5900MA Venlo, The Netherlands; Akzo Nobel Chemicals Research Arnhem, P.O. Box 9300, 6800SB Arnhem, The Netherlands; RIKILT, P.O. Box 230, 6700AE Wageningen, The Netherlands; and DSM Research, P.O. Box 18, 6160MD Geleen, The Netherlands