The residue was dissolved in 1 M nitric acid, filtered
into a 25 mL volumetric flask using Whatman filter paper and
made up to mark with the 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.