A headspace sorptive extraction method with magnetic mesoporous titanium dioxide@covalent organic frameworks composite coating for selective determination of trace polychlorinated biphenyls in soils

It's crucial to develop some simple, selective and rapid extraction pretreatments for detection of trace polychlorinated biphenyls (PCBs) residues in soils. Herein, a new magnetic mesoporous titanium dioxide@covalent organic frameworks (Fe3O4@mTiO2@COFs) composite coating was fabricated and adsorbed onto a Nd-Fe-B magnet for headspace sorptive extraction (HSSE) of seven indicative PCBs (PCB28, PCB52, PCB101, PCB118, PCB138, PCB153 and PCB180) in soils. The coating was assembled layer by layer using hydrothermal method. After being extracted by the magnetic stir bar, the adsorbed PCBs can be desorbed in thermal desorption unit (TDU) and measured by gas chromatography-mass spectrometry (GCMS). Under the optimal conditions, the limit of detection (S/N = 3) was measured less than 0.06 ng/g and the linear range was from 0.01 to 100 ng/g. The new coating exhibited good extraction capacity (its saturated adsorption amount was found to be 35 mg/g), selectivity and can be reused for at least 80 times with a recovery above 90%. The HSSE assay showed the following distinctive advantages: Firstly, the outer COFs coating has a uniform size of cavities (about 3.9 nm), thus PCBs (less than 2 nm) can freely pass through them, while the larger macromolecules can be effectively excluded. Secondly, the inner flower-like mTiO2 coating has good affinity towards chlorine-containing organic compounds, which further improves the selectivity for PCBs. Thirdly, both mTiO2 and COFs have large specific surface area. The synergistic effects of the composite coating make the HSSE assay exhibit high capacity, selectivity and prominent sensitivity towards PCBs detection coupling with TDGCMS. Finally, it is an environmentally friendly pretreatment method that does not require any organic solvents.