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., (BaF+)-Ba-138-F-19, Gd-157(+), and (BaOH+)-Ba-138-O-18) enter the reaction cell (RC), (3) shifting (BaF+)-Ba-138-F-19 to a new mass (BaF)-Ba-138-F-19((NH3)-N-14)(3)(+) by reacting with NH3 in RC to avoid the interfering ions (i.e., Gd-157(+) and (BaOH+)-Ba-138-O-18), and (4) passing interference-free (BaF)-Ba-138-F-19((NH3)-N-14)(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 + Ba-138(+) -> (BaF)-Ba-157 + 3NH(3) (BaF)-Ba-208(NH3)(3)(+), while the reaction pathway of dominant Gd-157(+) in RC proposed to Gd-157(+) + NH3 -> (GdNH+)-Gd-157-N-14-H-1 nNH(3) -> (GdN1)-Gd-157-N-14 H ((NH3)n+)-N-14-H-1 (n 0-5). Under the optimized setting of tandem MS (Q1 -> Q2 = 157 -> 208) and RC reaction gas flow rate (NH3/He = 10:90, 8.0 mL min(-1)), the background equivalent concentrations (BECs) and limits of detection (LODs) were 0.021 and 0.022 mu g mL(-1), respectively. The calibration curve was linear in the range between 0.1 and 10,mu g 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 mu g g(-1). The tea infusions contained F between 23.5 and 85.4 pig mu 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 pig mu g-1 for filtrates 1, 2, 3, and 4, respectively.