Quantitation of ultra-trace nitrated polycyclic aromatic hydrocarbons isomers in water by online solid-phase extraction coupled-liquid chromatography-mass spectrometry

An online solid-phase extraction (SPE)-coupled liquid chromatography-mass spectrometry (LC-MS) method was established for the determination of 10 nitrated polycyclic aromatic hydrocarbons (nitro-PAHs) in water. Water samples were mixed with methanol to generate 40% methanol solutions (v/v), and filtered by 0.45 μm membrane. The filtration with polytetrafluoroethylene(PTFE) membrane got higher recovery rates than nylon membrane, especially for 4-ring and 5-ring nitro-PAHs. 2.5 mL solution was directly injected into online SPE flow path to allow for online purification and enrichment of target analytes in the SPE column. The nitro-PAHs eluted from the SPE column were automatically transferred to the analytical flow path by a well-designed valve-switching system. With the optimization of LC and MS condition, ten nitro-PAH isomers was separated and detected from each other by LC-MS/MS with negative atmospheric pressure chemical ionization (APCI). It was firstly found that nitro-PAHs could produce strong [M-H]− precursor ions in the primary MS besides [M+e]− and [M+15]−. In the secondary MS, the precursor ions mainly lose NO neutral molecule (30 Daltons) to produce daughter ions. The online SPE and LC-MS analysis process was completed in 15.5 min. The linear correlation coefficients of 10 nitro-PAH standard curves were higher than 0.99. The detection limits of nitro-PAHs were about 1.2~22.2 ng/L (S/N=3). The intra-day and inter-day reproducibility (RSD, n=6) were 1.6%~8.4% and 5.3%~16.9%, respectively. The recoveries of 10, 40 and 200 ng/L in tap water were 71.7%~106.4%, 79.7%~100.9% and 73.0%~105.5%, with the corresponding RSD of 2.4%~10.5%, 2.1%~8.6% and 2.7%~6.2%, respectively.