Method Development for the Determination of Total Fluorine in Foods by Tandem Inductively Coupled Plasma Mass Spectrometry with a Mass-Shift Strategy

A method for total F determination in food and tea samples based on a mass-shift strategy using tandem inductively coupled plasma quadrupole mass spectrometry (ICP–MS–MS) was developed. This method consists of four steps: (1) conversion of the hardly ionized F atoms to BaF + via ICP, (2) use of the first quadrupole (Q1, set at 157) to ensure only the m / z 157 ions (i.e., 138 Ba 19 F +, 157 Gd +, and 138 Ba 18 OH + ) enter the reaction cell (RC), (3) shifting 138 Ba 19 F + to a new mass 138 Ba 19 F( 14 NH 3 ) 3 + by reacting with NH 3 in RC to avoid the interfering ions (i.e., 157 Gd + and 138 Ba 18 OH + ), and (4) passing interference-free 138 Ba 19 F( 14 NH 3 ) 3 + to the second quadrupole (Q2, set at 208) for detection by the MS detector. The mass-shift process of the target F (in ICP and RC) expected to follow the path: F + 138 Ba + → 157 BaF + + 3NH 3 → 208 BaF(NH 3 ) 3 +, while the reaction pathway of dominant 157 Gd + in RC proposed to 157 Gd + + NH 3 → 157 Gd 14 N 1 H + + n NH 3 → 157 Gd 14 N 1 H( 14 N 1 H 3 ) n + ( n = 0–5). Under the optimized setting of tandem MS (Q1 → Q2 = 157 → 208) and RC reaction gas flow rate (NH 3 /He = 10:90, 8.0 mL min –1 ), the background equivalent concentrations (BECs) and limits of detection (LODs) were 0.021 and 0.022 μg mL –1, respectively. The calibration curve was linear in the range between 0.1 and 10 μg mL –1, with a correlation coefficient of R 2 = 0.9999. The results obtained for 14 different food-related standard reference materials (SRMs) were in good agreement with the certified values on a 95% confidence level. The proposed method was then employed to evaluate the F contents of 13 branded tea samples. The total F concentrations ranged from 39.2 to 93.2 μg g –1 . The tea infusions contained F between 23.5 and 85.4 μg g –1, with an extraction efficiency of 56.0–91.6%, and the water-soluble F contents of a Pu’er brick tea were 58.7, 21.4, 3.82, and 1.41 μg g –1 for filtrates 1, 2, 3, and 4, respectively.