In each of the three chosen days, 0.5 mL sample of insulin is correspondingly diluted to 50 mL with 0.01 M hydrochloric acid and the solution is spiked with 0.5, 1.0 and 2.0 mL of 10 μg Zn/mL reference stock solutions. The spiked sample solutions are analyzed according to the analytical procedure and the recovery is calculated with the following equation: