Quantification of fipronil and its metabolite fipronil sulfone in rat plasma over a wide range of concentrations by LC/UV/MS.

Fipronil is an insecticide extensively used to treat pets, which has been identified as a potential thyroid disruptor in the rat. In this species, fipronil is mainly metabolized to fipronil sulfone and plasma concentrations of fipronil sulfone can be at least 20-fold higher than those of fipronil. Investigations of fipronil and fipronil sulfone exposure in blood remain sparse because of the lack of convenient and suitable analytical methods. We have developed and validated an LC/UV/MS/MS method to quantify both fipronil and fipronil sulfone within a wide range of concentrations in rat plasma. The double detection UV and MS coupled on-line enabled the concentrations to be measured over a 3 Log range (2.5-2500 ng/mL). The volume of sample required for the extraction by solid phase extraction was reduced to 75 microL with a recovery higher than 70%. The two-detection method agreement, evaluated with a Bland-Altman plot, was good for concentrations between 50 and 150 ng/mL. The method was applied to monitor plasma concentrations following a commonly used dosage regimen for the toxicological evaluation of fipronil in rats.