Analysis of monofluoroacetic acid in urine by liquid chromatography-triple quadrupole mass spectrometry and preparation of the positive sample by the bioconversion from monofluoroacetamide to monofluoroacetic acid in vitro

• Monofluoroacetamide was found to be bioconverted to monofluoroacetic acid in urine in vitro.
• Urine sample containing monofluoroacetic acid was prepared without animal experiment.
• The analyte was cleaned by anion exchange cartridge and analyzed by LC–MS/MS.

Whether as a rodenticide or as a natural product, monofluoroacetic acid (FAcOH) may cause poisoning to humans or animals for its high acute toxicity. Urine is one of the most typical specimens for forensic diagnosis when poisoning case about FAcOH happens. The positive sample containing FAcOH plays a key role for the development of an accurate and reliable analytical method. The bioconversion from monofluoroacetamide (FAcNH2) to FAcOH in urine in vitro was studied for the preparation of positive urine sample containing FAcOH without standard spiking or animal experiment. The average bioconversion rates were 0%, 18.6% and 41.3% when incubated the FAcNH2 spiked urine in vitro for 21 days at −20 °C, room temperature (RT) and 37 °C, respectively. Afterwards, a fast and sensitive analytical method was developed for determination of FAcOH in urine. Samples were diluted with water containing formic acid and cleaned with polymeric anion exchange (PAX) cartridge. The acid eluate was neutralized with ammonium hydroxide and directly measured by hydrophilic interaction liquid chromatography–triple quadrupole mass spectrometry (LC–MS/MS) using basic mobile phase condition. The limit of detection and limit of quantification of FAcOH in urine were 2 and 5 ng mL−1, respectively. The linear range was 5–1000 ng mL−1 with a correlation coefficient of r = 0.9993 in urine calibrated with internal standard. The recoveries at four spiking levels (5, 10, 50 and 500 ng mL−1 in urine) were 87.2%–107% with relative standard deviations ranged between 4.3%–8.8%.