1. Laboratoire de Chimie Analytique, UMR-CNRS 6521, Université de Bretagne Occidentale, 6, Avenue V. Le Gorgeu, BP 809, F- 29285 Brest, France, and Department of Inorganic, Analytical and Applied Chemistry, University of Geneva, Quai Ernest-Ansermet 30, CH-1211 Geneva 4, Switzerland