Ultra-trace determination of sodium fluoroacetate (1080) as monofluoroacetate in milk and milk powder by GC-MS/MS and LC-MS/MS

Abstract

A sensitive analytical method based on derivatization with 3-nitroaniline is established for the trace determination of sodium fluoroacetate (1080) in milk and milk powder. The method reported here utilizes protein precipitation with tungstic acid, solid phase extraction (SPE) clean-up in pass-through mode with a mixed-mode anionic exchange (MAX) cartridge and N,N′-dicyclohexylcarbodiimide (DCC) mediated derivatization with 3-nitroaniline (3-NA). The 2-fluoro-N-(3-nitrophenyl)acetamide (MFA–3NA) derivative can be determined with both gas chromatography tandem mass spectrometry (GC-MS/MS) and liquid chromatography tandem mass spectrometry (LC-MS/MS). In our study, negative chemical ionization (NCI) GC-MS/MS and electrospray ionization (ESI) LC-MS/MS in negative ionization mode were used. The quantification of MFA–3NA was accomplished by using matrix-matched calibration with stable isotope labeled sodium monofluoroacetate (¹³C₂D₂–MFA) as an internal standard (I.S.). The method was validated with MFA fortified milk samples at 0.1, 0.2 and 0.4 μg L⁻¹ and milk powder samples at 0.5, 1 and 2 μg kg⁻¹. In GC-MS/MS, the LOQs for milk and milk powder samples were 0.0042 μg L⁻¹ and 0.0085 μg kg⁻¹, respectively. The RSDs at all spike levels for milk and milk powder samples were less than 6% with recoveries ranging from 90 to 105%. In LC-MS/MS, the LOQs for milk and milk powder samples were 0.00033 μg L⁻¹ and 0.0088 μg kg⁻¹, respectively. The RSDs at all spike levels for milk and milk powder samples were less than 13% with recoveries ranging from 90 to 112%. The proposed method has been applied to detect 1080 as monofluoroacetate (MFA) in milk and milk powder samples purchased from the local market.