1. Free University, Department of Analytical Chemistry, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
2. On leave from Instituto da Agua da Região Norte, Rua Aníbal Cunha 164, 4050 Porto, Portugal
3. Landesuntersuchungsamt für das Gesundheitswesen, Veterinärstraβe 2, 85764 Oberschleissheim, Germany