Article pubs.acs.org/est
Polycyclic Aromatic Hydrocarbons (PAHs) in Ambient Aerosols from Beijing: Characterization of Low Volatile PAHs by Positive-Ion Atmospheric Pressure Photoionization (APPI) Coupled with Fourier Transform Ion Cyclotron Resonance Bin Jiang, Yongmei Liang,* Chunming Xu, Jingyi Zhang, Miao Hu, and Quan Shi* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China S Supporting Information *
ABSTRACT: Aromatic fractions derived from aerosol samples were characterized by gas chromatography and mass spectrometry (GC-MS), high temperature simulated distillation (SIMDIS), and positive-ion atmospheric pressure photoionization (APPI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), respectively. It was found that about 27 wt % compounds in aromatic fractions could not be eluted from a GC column and some large molecule PAHs were neglected in GC-MS analysis. APPI FT-ICR MS was proven to be a powerful approach for characterizing the molecular composition of aromatics, especially for the large molecular species. An aromatic sample from Beijing urban aerosol was successfully characterized by APPI FT-ICR MS. Results showed that most abundant aromatic compounds in PM2.5 (particles with aerodynamic diameter ≤2.5 μm) were highly condensed hydrocarbons with 4−8 aromatic rings and their homologues with very short alkyl chains. Furthermore, heteroatomcontaining hydrocarbons were found as the significant components of the aromatic fractions: O1, O2, N1, and S1 class species with 10−28 DBEs (double bond equivalents) and 14−38 carbon numbers were identified by APPI FT-ICR MS. The heteroatom PAHs had similar DBEs and carbon number distribution as regular PAHs.
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INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are a large group of organic compounds with two or more fused aromatic rings. PAHs in atmosphere originate mainly from anthropogenic processes, particularly from incomplete combustion of organic fuels.1−3 The larger PAHs (5−7 rings) are found mainly in fine particulate matter (PM) especially PM2.5, a number of them are known to be strong mutagens and potential human carcinogens and have been classified by the International Agency for Research on Cancer (IARC) as probable (2A) or possible (2B) human carcinogens.4−6 Benzo[a]pyrene (BaP), for example, is often used as an indicator of human PAH exposure due to its high carcinogenic potency and its presence in the environment.7,8 Furthermore, PAHs are produced at trace levels in combustion processes and have the potential to serve as “molecular markers” or tracers for specific ambient aerosol sources.9,10 Therefore, PAHs are a major concern among the toxic organic pollutants in the environment.11 The concentrations, distributions, and source apportionment of PAHs in ambient aerosols from different regions have been analyzed by chromatographic techniques in many previous studies.12−22 The total concentrations of the 16 EPA (the United States Environmental Protection Agency) priority PAHs in ambient aerosols generally are between 1 and 300 ng/m3, accounting for only less than 0.1% of the PM2.5 mass.13,16,17,20,22 Most PAHs are not taken into account, © 2014 American Chemical Society
some of them have not been identified yet, and some PAHs with high molecular weight and/or high condensation cannot be eluted from the gas chromatographic column at all. It is important to get a deep understanding of these compounds, if we want to provide an objective and comprehensive evaluation of the environmental impacts of PAHs in atmospheric aerosols. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with ultrahigh resolution and mass accuracy allows distinct assignment of a unique elemental composition for each mass spectrum peak. It has become a powerful tool for the characterization of complex organic compounds at the molecular level.23,24 FT-ICR, as well other ultrahigh-resolution mass spectrometries has been widely used to analyze petroleum,23 coal tar,25 bio-oil,26 humic-like substances27,28 and natural organic matter (NOM) in aerosols.29−35 In addition, atmospheric pressure photoionization (APPI) is suitable for the ionization of nonpolar and polar compounds.36−38 In several recent studies, FT-ICR MS coupled with APPI has been demonstrated to be a very powerful tool for the analysis of petroleum.39,40 Received: Revised: Accepted: Published: 4716
November 27, 2013 March 17, 2014 April 4, 2014 April 4, 2014 dx.doi.org/10.1021/es405295p | Environ. Sci. Technol. 2014, 48, 4716−4723
Environmental Science & Technology
Article
Figure 1. Separation flowchart of PM2.5 DCM extracts.
concentrated to 2 mL by a rotary evaporator and placed on the top of a glass column which was prepacked with 5 g of silica gel that had been wetted by nC6. The silica gel was activated at 120 °C for 5 h before use. Column chromatographic separation was performed. Saturates, aromatics, and resins were obtained by a sequential solvent elution using nC6, DCM/nC6 (2:1, v/v) and DCM/methanol (4:1,v/v), respectively. The solvent in each fraction was removed by a vacuum rotary evaporator. The solvent-free saturates, aromatics, resins, and asphaltenes were collected and weighed. GC-MS Analysis. An Agilent7890A GC equipped with a 5975C MS detector was used to analyze saturates and aromatics. The mass spectrometer was equipped with an electron impact (EI) source at 70 eV ionization energy. The carrier gas was helium at 1 mL/min, and the ion source temperature was maintained at 250 °C. An HP-5 MS column (60 m × 0.25 mm × 0.25 μm) was used for GC-MS analysis. The GC oven for saturates was held at 50 °C for 1 min, increased to 120 °C at 20 °C/min, and then increased to 250 °C at 4 °C/min, and increased to 310 °C at 3 °C/min, and then held constant at 310 °C for 30 min. The GC oven for aromatics was held at 50 °C for 1 min, increased to 120 °C at 15 °C/min, and then increased to 300 °C at 3 °C/min, and then held constant at 300 °C for 35 min. GC-MS was run in full scan and select ion monitoring (SIM) mode separately. The 16 priority PAHs were quantified in the SIM mode. An internal standard of 4-Terphenyl-D14 was added to all samples. The limit of quantification (LOQ) for individual PAHs was between 0.13 and 3.33 pg/m3. The R2 of the calibration curve for the 16 PAHs ranged from 0.994 to 0.999. High Temperature Simulated Distillation. A high temperature simulated distillation instrument modified on an Agilent 6890N GC by AC Ltd. (Netherland) equipped with an HP-1 capillary column (5 m × 0.53 mm × 0.09 μm) was used to perform simulated distillation analysis of aromatics. Helium was used as the carrier gas with a flow rate of 19 mL/min. The column temperature was held at 40 °C for 1 min, increased to 430 °C at a rate of 10 °C/min, and then held isothermal for 5 min. A flame ionization detector (FID) was used at 430 °C. The hydrogen and air flow rates were 35 mL/min and 350 mL/ min, respectively. The initial temperature of PTV inlet was 100 °C, increased to 430 °C at 15 °C/min, and then held constant at 430 °C for 22 min. The injection volume was 1 μL. APPI FT-ICR MS Analysis. The aromatics was dissolved in toluene at a concentration of 1 mg/mL and then directly
In this study, APPI FT-ICR MS has been utilized to characterize aromatic fractions derived from aerosol samples in Beijing. The main objective is to reveal the composition of large molecular aromatic compounds.
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EXPERIMENTAL SECTION Sample Collection and Extraction. The sampling was carried out on the roof of a 5-story building on the campus of China University of Petroleum (CUP) in Beijing. The building is about 18 m above ground and the campus is located in a northern suburb of Beijing that is primarily a residential and commercial area without any major industrial sources. A highvolume aerosol sampler (TH-1000 series, Tianhong Corp., China) was used to collect particles less than 2.5 μm in aerodynamic diameter (PM2.5) on prebaked microquartz fiber filters. Twenty ambient samples were collected from April 4 to 23, 2013 and each sample was collected for 24 h at a flow rate of 1.05 m3/min. The filters were weighed before and after sample collection for PM2.5 mass. One field blank sample was taken following the same procedure without airflow. The filter samples were stored at −20 °C prior to analysis. Each of the four aerosol samples were mixed and then Soxhlet extracted twice for 24 h with dichloromethane (DCM). The two extracts were mixed and concentrated to 2 mL by vacuum rotary evaporation at 30 °C. At the end, all of the concentrated extracts were mixed together. DCM was removed by rotary evaporation and the extracts were dried in a vacuum desiccator. The final extracts were sealed in a glass tube and stored at −20 °C in a freezer for analysis. One field blank sample was also prepared for positive-ion APPI FT-ICR MS. Solvent Precipitation and Chromatographic Separation. A modified separation method, which was commonly used in the petroleum community, was used to separate the extracts into four fractions, namely saturates, aromatics, resins, and asphaltenes (SARA).41,42 Separation flowchart of PM2.5 DCM extracts is shown in Figure 1. Briefly, a total of 20−30 mg of DCM extracts were dissolved in 30 mL of n-hexane (nC6), followed by ultrasonic treatment for 10 min to promote the dissolution. The solution was stabilized for 12 h in dark for asphaltenes precipitation, and then filtered by packed absorbent cotton. The precipitate was dissolved in DCM to obtain asphaltenes. Silica gel (80−100 mesh, Qingdao Marine Chemical Factory) and absorbent cotton were extracted by Soxhlet extraction with DCM for 24 h. The nC6 filtrate was 4717
dx.doi.org/10.1021/es405295p | Environ. Sci. Technol. 2014, 48, 4716−4723
Environmental Science & Technology
Article
injected with a syringe pump at a flow rate of 180 μL/h. Toluene works well as the dopant because it can increase sensitivity by promotion of proton-transfer and charge exchange reactions.43 The APPI source was manufactured by Syagen (Syagen Technology Inc., Tustin, CA, U.S.A.) and comprised a heated capillary needle and a krypton UV lamp with ionization energy of 10.6 eV. The APPI source was purchased from Bruker Daltonics, and the analysis was performed in the positive ionization mode. The temperature for nebulizing gas and drying gas was 350 and 200 °C, respectively; and the flow rate of nebulizing and drying gas was 1.2 and 2.0 L/min, respectively. Ion source potentials were set as follows: capillary: 3.0 kV, spray shield: 2.5 kV. A Bruker apex-ultra FT-ICR mass spectrometer equipped with a 9.4 T superconducting magnet was used for MS analysis. Capillary column end voltage was 320 V. The skimmer voltage was set to 30 V. Ions were accumulated for 0.001 s in a hexapole with 2.4 V DC voltage and 300 Vp-p RF amplitude. The quadruple mass was set to 120 Da. The instrument was used in RF only mode. An argon-filled hexapole collision cell was operated at 5 MHz and 300 Vp-p RF amplitude, in which ions accumulated for 0.2 s. The extraction period for ions from the hexapole to the ICR cell was set to 0.9 ms. The RF excitation was attenuated at 11 dB and used to excite ions over the range of 115−600 Da. A 4 M data set was acquired. 128 FT-ICR transients were coadded for the final mass spectrum to enhance the signal-to-noise ratio and dynamic range. Field blank filters were processed and analyzed following the same procedure to eliminate the possible contamination. Mass spectra were internally calibrated from extended homologous alkylation series (molecular ion of aromatic hydrocarbons and thiophenes) of high relative abundance in a heavy oil mixture within the mass range of 150−500 Da and recalibrated with the hydrocarbon mass series from the mass spectra of the aromatics, which exhibited relative high abundant positive-ion APPI mass spectrum. The typical mass resolving power (m/Δm50%, in which Δm50% is the magnitude mass spectral peak full width at half-maximum peak height) >410 000 at m/z 302 with
