Hi Ralf and Jon, Thanks a lot for your help. I will try to follow your advices. Celine -- Dr. Céline Besnard EPFL - SB - IPMC - LCR2 CH-1015 Lausanne Suisse e-mail : [email protected] Tel : +41(0)21.693.05.41 Fax : +41(0)21.693.05.04 http://lcr.epfl.ch