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., 138Ba19F+, 157Gd+, and 138Ba18OH+) enter the reaction cell (RC), (3) shifting 138Ba19F+ to a new mass 138Ba19F(14NH3)3+ by reacting with NH3 in RC to avoid the interfering ions (i.e., 157Gd+ and 138Ba18OH+), and (4) passing interference-free 138Ba19F(14NH3)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 + 138Ba+ → 157BaF+ + 3NH3 → 208BaF(NH3)3+, while the reaction pathway of dominant 157Gd+ in RC proposed to 157Gd+ + NH3 → 157Gd14N1H+ + nNH3 → 157Gd14N1H(14N1H3)n+ (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 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 R2 = 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.