1. CESAM and Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
2. Department of Complementary Sciences, Superior School of Health Technology of Coimbra, Rua 5 de Outubro S. Martinho do Bispo Apartado 7006, 3040-854 Coimbra, Portugal
3. BAM Federal Institute for Materials Research and Testing, Department of Analytical Chemistry, Reference Materials, Richard-Willstaetter-Strasse 11, D-12489 Berlin, Germany
4. Department of Plant Nutrition, INRES-Institute of Crop Science and Resource Conservation, University of Bonn, Karlrobert-Kreiten-Strasse 13, D-53115 Bonn, Germany