All white shrimp tissue and feed samples were freeze-dried to
constant weight and finely ground. To achieve total dissolution,
approximately 0.1 g of freeze-dried, ground sample was digested
in 5 ml concentrated nitric acid at 135 8C for 4 h. One millilitre
hydrogen peroxide (30%) and 1 ml concentrated perchloric acid
were then added, and the temperature maintained at 150 8C until
the liquor was clear and all particles turned white or grey (Zhou
et al., 1998). The solution obtained was filtered (Albet 240 filter),
and diluted to 25 ml. To avoid possible contamination, all
glassware and equipment used were acid-washed.