A sensitive determination of paraquat by spectrophotometry.

A sensitive spectrophotometric method for the determination of a widely used herbicide, paraquat using a versatile reducing agent sodium borohydride is described. Paraquat is reduced with sodium borohydride in an alkaline medium to give a blue radical ion with an absorbance maxima at 600 nm. Beer's law is obeyed in the range of 0.05-0.5 mug ml(-1) of paraquat. The molar absorptivity and Sandell's sensitivity are found to be 2.9 x 10(5) l mol(-1) cm(-1) and 0.0006 mug cm(-2) respectively. The important analytical parameters and the optimum reaction conditions were evaluated. The method is free from the interference of other commonly used pesticides and metal ions. The method was applied successfully to the determination of paraquat in human samples, such as blood, urine and mother's milk compared to food and environmental samples.