Development of an Analytical Method Based on Temperature Controlled Solid-Liquid Extraction Using an Ionic Liquid as Solid Solvent.

At the present paper, an analytical method based on temperature controlled solid-liquid extraction (TC-SLE) utilizing a synthesized ionic liquid, (N-butylpyridinium hexafluorophosphate, [BPy]PF6), as solid solvent and phenanthroline (PT) as an extractant was developed to determine micro levels of Fe2+ in tea by PT spectrophotometry. TC-SLE was carried out in two continuous steps: Fe2+ can be completely extracted by PT-[BPy]PF6 or back-extracted at 80 °C and the two phases were separated automatically by cooling to room temperature. Fe2+, after back-extraction, needs 2 mol/L HNO3 as stripping agent and the whole process was determined by PT spectrophotometry at room temperature. The extracted species was neutral Fe(PT)mCl2 (m = 1) according to slope analysis in the Fe2+-[BPy]PF6-PT TC-SLE system. The calibration curve was Y = 0.20856X − 0.000775 (correlation coefficient = 0.99991). The linear calibration range was 0.10–4.50 μg/mL and the limit of detection for Fe2+ is 7.0 × 10−2 μg/mL. In this method, the contents of Fe2+ in Tieguanyin tea were determined with RSDs (n = 5) 3.05% and recoveries in range of 90.6%–108.6%.