A modified method of Crosby (1977) was adopted. One gram of
each oven-dried sample was weighed into a porcelain crucible and
dry-ashed in a muffle furnace by stepwise increase in temperature
up to 500 C within 1 h and then leaving to ash at this temperature
for the next 12 h. The residue was dissolved in 10 mL of 1 M nitric
acid, filtered into a 25 mL volumetric flask using Whatman filter
paper and made up to mark with nitric acid (1 M). The blank
digests was similarly processed. The recovery study of this analytical
procedure was carried out, as described by Onianwa et al.
(2001), by spiking and homogenising several already analysed food
samples with varied amounts of standard solutions of the metals.
The spiked samples were then processed the same way as other
samples. Average recoveries are as highlighted in Table 1.