Elsevier

Science of The Total Environment

Volume 668, 10 June 2019, Pages 917-924
Science of The Total Environment

Molecular characterization of water-soluble organic compounds in PM2.5 using ultrahigh resolution mass spectrometry

https://doi.org/10.1016/j.scitotenv.2019.03.031Get rights and content

Highlights

  • Elemental composition characteristics of water-soluble organic compounds in PM2.5.

  • Estimation of WSOC partitioning behaviors by stir bar sorptive extraction.

  • Multifunctional losses from biogenic SOA molecular observed by MS/MS analysis.

Abstract

Water-soluble organic compounds (WSOCs) are a complex mixture of organic components with a variety of chemicals structures that may have significant impacts on the formation process and health hazards of atmospheric fine particles. In this study, the molecular characteristics of WSOCs in PM2.5 were investigated using ultrahigh resolution mass spectrometry. In total 7389 compounds in PM2.5 water extracts were identified, including CHO±, CHOS±, CHON±, CHONS±, CH+, CHS+, CHN+ and CHNS+ species. CHO± and CHON± were the major components in PM2.5 water extracts. S-containing compounds detected in both ionization modes were observed with distinct molecular characteristics. Selective partitioning of WSOCs between PM2.5 water extracts and polydimethylsiloxane (PDMS, log DPDMS = 0.51–3.87) coating phases was found, which was significantly correlated with molecular characteristic (i.e. double-bond equivalent, number of O and H atoms, O/C ratios, and aromaticity equivalent). The extent of accumulation for negatively charged compounds was generally lower, which related to the existence of polar functional groups, such as hydroxyl, carboxyl, nitrate, and sulfate, as observed by MS/MS fragmentation analysis.

Introduction

Fine particulate matter with an aerodynamic diameter <2.5 μm (PM2.5) is an atmospheric pollutant of great concern in many cities across the world (Huang et al., 2014), and it contains a wide variety of components including metal oxides, inorganic salts, elemental carbon and organic compounds. Water-soluble organic compounds (WSOCs) are a critical component in particulate matter in significant proportions, which have impacts on the particle hygroscopicity and activity of cloud condensation nuclei (Asa-Awuku et al., 2010; Facchini et al., 1999; Jing et al., 2016; Sun et al., 2011). They may also have influence on human health, with potential biological effects from reactive oxygen and nitrogen species (Perring et al., 2013). WSOCs originate from a diversity of primary emission sources and secondary production. Anthropogenic and biogenic volatile organic compounds could be oxidized by ozone, the OH radical or the NO3 radical to form semivolatile secondary organic aerosols (Kondo et al., 2007; Putman et al., 2012; Zhang et al., 2012).

WSOC components are quite complex, making structural elucidation of individual compounds challenging (Altieri et al., 2009; Mazzoleni et al., 2010; Wang et al., 2010; Wozniak et al., 2008). Ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has the inherent advantage of high resolving power (i.e., mass accuracy ≤1 part-per-million), which allows study of the molecular characteristics of chemicals in complex organic mixtures (Nozière et al., 2015). Recent studies have use FT-ICR MS for elemental composition characterization of dissolved organic matter (Herzsprung et al., 2017; Reemtsma et al., 2008; Zhang et al., 2019), disinfection byproducts (Kellerman et al., 2014; Lavonen et al., 2013; Zhang et al., 2012a, Zhang et al., 2012b), and aerosol phase humic-like substances (HULIS) (Hao et al., 2017; Lin et al., 2012b). Although a large quantity of WSOC molecular formulas have been observed, additional information on chemical structural features is scarce, being mainly based on bulk characterization approaches such as nuclear magnetic resonance and infrared spectroscopy analysis (Sun et al., 2011). Tandem mass spectrometry monitors the neutral loss of precursor ions by fragmentation methods such as collision-induced dissociation, in which information on specific functional groups and molecular structures can be obtained (Lin et al., 2012a). Carboxyl groups were observed as the abundant fragments of CHO species in a HULIS fraction isolated from aerosol samples, and precursor ions with high O/S ratios and sulfate fragments were found as organosulfate compounds (Lin et al., 2012b).

Polydimethylsiloxane (PDMS)-coated stir bar sorptive extraction (SBSE) is a solid phase microextraction sampling technique, which measures the partitioning of neutral polar and nonpolar chemicals between water and organic phases. PDMS-water distribution ratios (DPDMS) are the parameter for quantitative description of the distribution in equilibrium of all forms of WSOC in water and organic phases at certain pH conditions. It is known to correlate linearly with the octanol-water partition coefficient (Kow, a physical-chemical parameter determined by neutral or polar functional groups) (LeClair et al., 2012), and is thus an appropriate descriptor of molecular structure characteristics. It was also used as a biomimetic approach for prioritizing water-soluble substances with bioaccumulative potentials, which helped in understanding the source of acute toxicity in complex aquatic environments. For instance, polar neutral and basic (O+, NO+, and SO+) species were found to be more hydrophobic than naphthenic acids in oil sands process-affected water, and some of these compounds may contribute to the toxicity in the samples (Difilippo and Eganhouse, 2010; Surratt et al., 2007).

In this study, the characteristics of the molecular structure of water-soluble organic components in PM2.5 samples were investigated using FT-ICR ultrahigh resolution mass spectrometry, both in the negative and positive ion mode. Composition profiles of WSOCs were examined by accurate measurement of ion monoisotopic mass and elemental assignment. The hydrophobicity of WSOC analogues was estimated by a PDMS-water distribution test, with further structure information elucidated by MS/MS fragmentation analysis, in order to reveal the influence of crucial functional groups to the measured physical-chemical property.

Section snippets

Chemicals and materials

Glass-encapsulated magnetic stir bars (Twisters) were purchased from Gerstel (10 mm in length, surface-coated with a 1.0 mm film of PDMS, Mülheim a der Ruhr, Germany). Partitioning model compound standards (atrazine, propazine, prometryn, procaine, propranolol, warfarin, ibuprofen, and diclofenac) were obtained from Sigma-Aldrich (St. Louis, MO). Acetic acid was from Dikma (Richmond Hill, Canada) and Merck (Darmstadt, Germany), respectively. The purity of all chemicals was 95% or higher unless

Elemental composition profiles of WSOCs

Overall, 2508 species were detected with assigned elemental combinations in the negative ionization mode in PM2.5 water extracts, and 4881 species were found in the positive ionization mode. The identified elemental formulas could be classified into twelve major compound species, including CHO±, CHOS±, CHON±, CHONS±, CH+, CHS+, CHN+ and CHNS+. CHONS± compounds refer to compounds that were detected both in the ESI+ and ESI- modes containing carbon, hydrogen, oxygen nitrogen, and sulfur elements.

Conclusion

In this study, elemental compositions and molecular characteristics of >7300 WSOC compounds were investigated. Bioconcentration potentials of both positively and negatively charged compounds in the ESI source were found by estimating the distribution ratios between PM2.5 water extracts and stir-bar PDMS coating phases. The results from the MS/MS fragmentation analysis could be complementary for the assessment of contributions from diverse chemical structures to physical-chemical property such

Acknowledgements

We thank the National Natural Science Foundation of China (91843301, 21622705, 91743101, 21577151, and 21461142001) and the Youth Innovation Promotion Association CAS projects for joint financial support.

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