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Benzenes of wide sources can cause serious air pollution. In this study, a novel enrichment sampling device based on multi-capillary column was developed and coupled to portable photoionization time-of-flight mass spectrometer (PI-TOF-MS) to highly and sensitively detect benzenes in ambient air. The enrichment sampling device was composed of seven identical HP-PLOT Q capillary columns (0.10 m×0.53 mm×40 μm) coated with polystyrene-divinylbenzene, which could achieve efficient adsorption of benzenes at room temperature. The homebuilt portable PI-TOF-MS has a volume of (0.47×0.35×0.38) m3, a weight of 35 kg, and a resolution of better than 1 000 (full width at half-maximum, FWHM) at m/z 92. Benzene, toluene and p-xylene gas diluted by nitrogen were used to optimize the operating parameters of the whole system. The optimal sampling flow rate was selected as 500 mL/min, the enrichment time was 4 min, the thermal desorption temperature was 160 ℃, and the analytical time for single sample was less than 10 min. Compared with direct sampling, the signal intensities of benzene, toluene and p-xylene were enhanced by 130, 220 and 255 folds under the optimal conditions, respectively. The linearity, limit of quantitation (LOQ) and stability of the system were evaluated. As a result, the linear concentration range of 6.96-1 741.05 μg/m3 for benzene, 8.21-2 053.55 μg/m3 for toluene, and 9.46-2 366.05 μg/m3 for p-xylene were obtained with the correlation coefficients (R2) of 0.993 8, 0.999 7 and 0.999 1, respectively. The limits of quantification (LOQs, S/N=10) were 0.78, 0.63 and 0.76 μg/m3, respectively. The relative standard deviations of the signal strengths for the three compounds were less than 6% during seven analytical cycles. Finally, the developed multi-capillary column enrichment portable PI-TOF-MS was used for comparative analysis of indoor and outdoor air in the laboratory. As a consequence, toluene and p-xylene were detected in both indoor and outdoor air of the laboratory. The indoor concentrations of the two compounds were 2.6 and 23.3 μg/m3, respectively, which were much lower than the legal limit concentrations. Due to better ventilation, the outdoor concentrations of toluene and p-xylene were 0.5 and 16.4 μg/m3, respectively. In addition, ethanol, methanthiol, acetone, etc. were also detected in indoor air of the laboratory, which may come from human exhalations or chemical reagent emissions. The results showed that the instrument can meet the requirements of trace benzenes detection in ambient air, and has broad application prospects in the field of on-site environmental analysis. |
