In-trap headspace liquid-phase microextraction for highly sensitive UV-Vis spectrophotometric determination of...

The main approach to headspace liquid-phase microextraction (HS-LPME) is to hold the extraction phase at the tip of a microsyringe needle located in the headspace above the sample solution in a hermetically sealed vial. The use of passive HS-LPME mode often results in low extraction efficiency, for example, for semi-volatile compounds. Achieving extraction equilibrium sometimes requires a long extraction time. These disadvantages are greatly increased when extracting from sample volumes larger than 5-10 mL. The potential of the dynamic HS-LPME mode, which uses effective displacement of the volatile analyte by a gas flow and absorption by a microvolume of extractant to improve extraction kinetics and completeness of extraction, remains underutilized. We present a new hybrid method combining spectrophotometry and a new dynamic variant of HS-LPME termed in-trap HS-LPME. The donor phase and the microvolume of the extraction phase are placed in two different hermetically sealed vessels connected by a tube. To intensify the mass transfer of volatile compounds, air or an inert gas is passed through both phases in series. The potential of the method was evaluated by determining the bromide content of water samples. Bromide in the donor phase was converted to volatile Br₂ by oxidation with 25 mM KBrO₃ in 0.3 M H₂SO₄. Bromine was displaced from 80 mL of sample solution by air or inert gas flow and absorbed by a 1 % KI solution or 0.5 mM phenol red solution with pH 5.5. For the procedure using KI, the calibration plot for the bromide determination was linear in the range of 10-250 μg L^-1 (l = 0.1 cm, volume of acceptor phase 500 μL). For the second procedure, the calibration plot was linear in the range from 3 to 50 μg L^-1 (l = 1.0 cm, volume of acceptor phase 250 μL) with detection limit of 1 μg L^-1. When the sample volume was changed from 5 to 180 mL, the time required for the complete conversion of bromide to bromine and its displacement from the solution increased from 3 to 30 min. The extraction percentage exceeded 50 % and was two orders of magnitude higher than in passive mode for volumes greater than 10 mL. Selectivity was improved compared to the known phenol red method with respect to hydrocarbonate, chloride, and ammonium ions. The developed method was successfully applied to the determination of bromide in natural waters.