Determination of Sodium Monofluoroacetate in Dairy Powders by Ion Chromatography-Triple Quadrupole Mass Spectrometry

Sodium monofluoroacetate is a rodenticide with high acutetoxicity. Although banned for use in China, sodium monofluoroacetate is still commonly used in New Zealand, Australia, Israel and the USA. Fluoroacetate also exists naturally in many botanical species worldwide. The obtainability and stability of sodium monofluoroacetate and its proximity to agricultural products could potentially lead to the accidental or intentional contamination of food. In this work, a simple and sensitive method of ion chromatography triple quadrupole mass spectrometry (IC-MS/MS) was developed for the determination of sodium monofluoroacetate in dairy powders and infant formulas. The main factors including sample pretreatment and instrumental conditions of ion chromatography-triple quadrupole mass spectrometry were optimized. The sodium monofluoroacetate in dairy powder sample was ultrasonically extracted with aqueous solution containing 3% perchloric acid, and then the extract was centrifuged to remove the impurities, such as protein and lipid. The monofluoroacetic acid in the mixture was extracted by methyl tert-butyl ether (MTBE) under pH 0.5-1.0. After the MTBE was removed by N₂, the monofluoroacetic acid in the residues was dissolved into 0.1% ammonia solution. The ion chromatographic analysis was separated on a Dionex IonPac AS 19 analytic column (2 mm×250 mm×7.5 μm) with gradient elution of using eluent of KOH solution electrolytically generated from on-line eluent generation cartridge. An in-line suppressor was used to remove potassium ion from the eluent before the eluent flow entered the mass spectrometer. A triple quadrupole mass spectrometer, equipped with electrospray ionization (ESI) in negative ion mode was used to detect monofluoroacetate anion in multiple reaction monitoring (MRM) mode. A ¹³C₂-labeled monofluoroacetate anion internal standard was used to correct for any matrix effects. The results showed that the linear range of the sodium monofluoroacetate was 0.3-1300 μg/L with the correlation coefficients greater than 0.999. The limit of detection (LOD) and the limit of quantification (LOQ) of sodium monofluoroacetate were 0.2 μg/kg and 0.6 μg/kg, respectively. The recoveries were 89.7%-104%, and the relative standard deviations were 0.50%-11%. The method is simple, sensitive and accurate, and has been applied to the measurement of sodium monofluoroacetate in dairy powders and infant formulas, and also can be applied to the detection of target compound in other poisoning food samples.