There is a possible explanation: Surface charging.
See Alexander Scherls article for some details:
A. Scherl, C. G. Zimmermann-Ivol, J. Di Dio, A. Vaezzadeh, P.-A. Binz, M. Amez-Droz, R. Cochard, M. Gl?ckmann, D. F. Hochstrasser, Rapid Commun. Mass Spectrom. 2005, 19, 605?610. Gold coating of non-conductive membranes before matrix-assisted laser desorption/ionization tandem mass spectrometric analysis prevents charging effect. 😀
