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)NH3)3(+) by reacting with NH3 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)NH3)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(+) + 3NH3 → (208)BaF(NH3)3(+), while the reaction pathway of dominant (157)Gd(+) in RC proposed to (157)Gd(+) + NH3 → (157)Gd(14)N(1)H(+) + nNH3 → (157)Gd(14)N(1)H((14)N(1)H3)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 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.