1. Volke, J., Volková, V.: Chem. Listy48, 1031?1038 (1954)
2. Dungen van den, P. W. C. M.: Intern. rapport Food Inspection Service, Den Bosch, The Netherlands. 1972
3. Frodyma, M.M., Lieu, V.T.: Anal. Chem.39, 814?819 (1967)
4. Laboratory methods of the Food Inspection Services of Groningen and Utrecht
5. Brunink, H., Wessels, E.J.: Analyst97, 258?259 (1972)