1. Department of Food Sciences, Medical University of Gdańsk, 107 Hallera Street, 80-416 Gdańsk, Poland, Department of Analytical Chemistry, Chemical Faculty, Gdańsk University of Technology, 11/12 G. Narutowicza Street, 80-952 Gdańsk, Poland, and Hel Marine Station, University of Gdańsk, 9 Morska Street, 84-150 Hel, P.O. Box 37, Poland