A novel Multisyringe flow injection analysis (MSFIA) system combined with a 200 cm long pathlength liquid core waveguide (LCW) has been developed enabling for the first time the sensitive spectrophotometric determination at μg L⁻¹ levels of the herbicide paraquat (Pq²⁺) in drinking waters. The proposed system is a simple, economic and fast alternative for obtaining the first evidence of paraquat pollution prior the use of more complex instrumental techniques.

The proposed methodology is based on the production of a blue free radical by reaction of Pq²⁺ with ascorbic acid (partially oxidized with potassium iodate) in basic medium. Limits of detection and quantification as low as 0.7 and 2.3 μg L⁻¹, were obtained respectively. The working range is linear up to a concentration of 250 μg L⁻¹ of Pq²⁺. The injection throughput of the proposed method is 34 h⁻¹. The results obtained with the LCW are compared with those using a conventional 1 cm flow cell. The automation of standard addition procedures has been studied and implemented for samples causing matrix effects. Finally the proposed system has been applied to the determination of paraquat in drinking water samples.