Flow-injection fluorimetric method for the determination of dimethylarsinic acid using on-line photo-oxidation Original Research Article

A flow-injection (FI) fluorimetric method for the determination of dimethylarsinic acid (DMAA) is proposed. The assay is based on the reaction of the herbicide with peroxydisulphate in UV light to produce arsenate, which reacts with molybdate and thiamine to form thiochrome. The thiochrome is fluorimetrically monitored at 440 nm with excitation at 375 nm. The calibration graph is linear in the range 0.14–14 μg ml−1 with an accuracy of 0.5%. The limit of detection is 0.014 μg ml−1 and the sample throughput is 60 samples per hour. The proposed method can be used for the determination of DMAA in the presence of inorganic arsenic and for the speciation of arsenic compounds making use of a strong cation exchange resin. DMAA can be pre-concentrated on a cation exchange resin in the H+-form. In this way, it can be determined over the range 5.6–560 ng ml−1 with a detection limit of 0.6 ng ml−1. The usefulness of the method is demonstrated by determining DMAA residues in irrigation waters and vegetables.