Determination of four classes of 34 chlorinated persistent organic pollutants in seawater by solid-phase extraction and gas chromatography-electrostatic field orbitrap high resolution mass spectrometry

Ocean acts as a “sink” for pollutants in the natural environment. Consequently, issues focused on marine pollution from terrestrial origin is attracting increasing attention. Persistent organic pollutants (POPs), including organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), short-chain chlorinated paraffins (SCCPs), and dechlorane plus isomers (DPs), are serious hazards for both the environment and humans. These POPs have been widely detected in the marine environment and are typically present at trace levels; however, separating and determining individual contaminants require large amounts of sampling and time. Establishing an accurate analytical method for determining typical POPs is critical for studying their environmental behavior and associated ecological risks to the marine environment. In this study, we developed a method based on solid-phase extraction (SPE) combined with gas chromatography-electrostatic field orbitrap high resolution mass spectrometry (GC-Orbitrap-HRMS) for determining 34 chlorinated POPs in seawater, including 25 OCPs, six PCB congeners, SCCPs and two DPs. The chromatographic conditions and MS parameters were optimized, and the effects of the extraction solvent and purification method were systematically studied. Dichloromethane exhibited satisfactory extraction efficiencies during the liquid-liquid extraction (LLE) of seawater samples, with recoveries of 73.1%-120.5% for OCPs, 87.2%-101.7% for PCBs, 105.5% for SCCPs, and 74.9%-78.6% for DPs, respectively. Purification using a SPE column with 500 mg of Florisil was adopted, and 9∶1 (v/v) n-hexane/acetone was confirmed as the eluent with recoveries between 68.2% and 122.8% for all the 34 chlorinated POPs. A DB-5MS (15 m×0.25 mm×0.10 μm) capillary chromatographic column was used to separate the target compounds, with an electron ionization (EI) source used to detect OCPs and PCBs, whereas SCCPs and DPs were determined in negative chemical ionization (NCI) source. All target compounds were analyzed in full-scan mode. An internal standard quantification method was used for OCPs and SCCPs while isotope dilution quantification was used for PCBs and DPs. The severe interference observed during the detection of chlorinated POPs in the mixture of co-extracted substances was completely eliminated following the purification. The 34 target chlorinated POPs exhibited good linearities in their corresponding ranges, with correlation coefficients (R2) exceeding 0.9. The method demonstrated low detection limits under the optimized conditions, with values of 0.009-0.061 ng/L for the 25 OCPs, 0.006-0.016 ng/L for the six PCBs, 2.78 ng/L for the SCCPs, and 0.021-0.023 ng/L for the two DPs, with lower limits of determination of 0.06-0.24, 0.02-0.06, 11.12, and 0.08-0.09 ng/L, respectively. Accuracy and precision were validated by the recoveries of samples spiked at low, medium, and high levels, which ranged between 70.6% and 128.9%. Relative standard deviations (n=6) were determined to be 0.2%-19.2%. These results highlight the suitability of the developed method for analyzing trace amounts of chlorinated POPs in seawater. The method is characterized by simple sample pretreatment, high sensitivity, fast analytical throughput, cost-effectiveness, and good stability for trace-level detection; hence, it is suitable for the rapid and accurate analysis of typical chlorinated POPs in seawater. This method is expected to play a significant role in marine environmental monitoring and the emergency surveillance of seawater pollution. The developed method was applied to seawater samples collected from Bohai, which revealed that the highest detection frequency (90%) was recorded for the SCCPs, while α-hexachlorocyclohexane (α-HCH) was only detected in 30% of the samples. All other OCPs were below the detection limit. PCB-52 was the only PCB congener detected in the seawater samples. The SCCPs were detected in much higher concentrations than the other POPs, with the highest value of 130.6 ng/L recorded. Consequently, particular attention must be paid to SCCPs.