In the early stages (within the first
20 min), the absorbance at 412 nm almost linearly increases
over time while the overall absorbance decreases with increasing
the pesticide concentration. The relationship between the absorbance
(at 15 min) and the methyl parathion concentration is
presented in Fig. 1d; clearly the absorbance monotonically
decreases in the concentration range from 0 to 5 μg mL−1. This
result confirms the inhibition of AChE activity by methyl parathion,
and provides a measure for the quantitation of methyl
parathion in the unknown samples by determining either the
absorbance or the yellow intensity (vide infra).