Determination of Benzenes in Atmosphere by Multi-capillary Column Enrichment Portable Photoionization TOF-MS

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 requireme