Characterization of benzene polycarboxylic acids and polar nitroaromatic compounds in atmospheric aerosols using UPLC-MS/MS
Introduction
The impact of atmospheric fine particulate matter (PM2.5, particles diameter ≤2.5 μm in aerodynamic diameter) in urban areas on human health, visibility degradation, and intensified global climate change has prompted an increasing scientific focus on identifying the components and sources of PM2.5 that are most directly responsible for such effects [1], [2], [3]. Depending on the chemical composition, some particles may be more dangerous than others. Many of those contaminants are organic carbon (OC) compounds emitted from multiple primary (e.g., fossil fuel and biomass combustions) and secondary sources that can evolve due to aging processes [4], [5], [6], [7]. Thus, considerable interest has been focused on understanding the sources and formation of the OC. Most studies on the OC fraction of atmospheric PM have been dedicated to qualitative and quantitative aspects of the nonpolar or hydrocarbon fraction such as polycyclic aromatic hydrocarbons (PAHs) and their oxygenated and nitrated derivatives [4,8]. However, to fully support source apportionment efforts, the identification of the organic composition of atmospheric aerosols needs to be expanded beyond measurement of nonpolar organic tracers. Therefore, a particularly active research area has been focussed on the characterization of water-soluble organic carbon (WSOC), which is a highly complex fraction of OC composed of a multitude of individual oxygenated compounds [5,[9], [10], [11]. Studies on WSOC speciation have identified several groups of organic species, i.e., mono-, di- and poly-carboxylic acids with additional functional groups, such as hydroxy-, methoxy- and/or nitro-groups [12], [13], [14], [15].
Among these different polar subgroups of WSOC, benzene polycarboxylic acids (BCAs) and nitroaromatic compounds (NACs) with the NO2 and OH functional groups attached to an aromatic ring have gained considerable attention [5,7,11,[16], [17], [18], [19], [20], [21], [22]. Benzene polycarboxylic acids and related polar compounds have been reported as indicators of secondary organic aerosol (SOA) from aromatic precursor gasses [6,7,14,17]. Direct emissions from vehicular exhaust, biomass burning and cooking have also been suggested as their sources [4,5,[23], [24], [25], [26]. Moreover, benzene polycarboxylates are an important family of ligands capable of bonding with transition metals such as copper (II), iron (II), manganese (II) and zinc (II), thereby mediating the bioavailability of metals [1,7,27]. Thus, quantitative abundance measurements of benzene polycarboxylic acids may provide necessary data to assist the assessment of toxicological effects posed by metals.
Another prominent group of WSOC, NACs like nitrophenols (NPs), nitrocatechols (NCs) and nitrosalicylic acids, have gained much attention due to their relatively high atmospheric concentrations (especially in urban environments) associated with optically active brown carbon (BrC) [7,11,[28], [29], [30], [31], [32] and their potentially adverse effects on human health and the environment [33], [34], [35]. These organic compounds are emitted into the atmosphere either from primary sources including biomass burning, coal combustion and vehicular exhausts, or formed in the atmosphere as a result of nitration reaction of precursor aromatic compounds [22,36]. Some nitroaromatic compounds, such as 4-nitrocatechol and 4-methyl-5-nitrocatechol, were linked to the biomass emissions as a result of secondary transformation of phenol and cresol in presence of NOx and OH radicals at the combustion flame [29,37]. More recently, Ikemori et al. suggested that 3,5-dinitrosalicylic acid and 4-nitrophthalic acid could serve as new tracers for SOA since their concentrations were highly correlated with other SOA tracers (such as phthalic and 4-methylphthalic acids) [38]. In this context, the improved molecular characteristic and distribution of WSOC constituents is critical important because it allows to gain insights into aerosol sources and the underlying mechanisms of SOA formation and transformation [2,5,39]. This emphasis the importance of developing the most effective and sensitive methods for the determination of polar compounds in airborne particulate matter.
Traditionally, the chemical analysis of organic compounds in PM is performed using gas chromatography/mass spectrometry (GC-MS) analysis. However, the high polarity and low levels of non-volatile compounds (e.g., acidic organic compounds) preclude direct GC-MS analysis, making their identification and quantification a challenge. As a result, much less information is available on polar oxygenated species presented in WSOC fraction of OC [40]. Therefore, modern liquid chromatographic and electrophoretic techniques, such as high pressure liquid chromatography (HPLC) [14,15,[18], [19], [20],26,29] or capillary electrophoresis (CE) [24,41,42], especially coupled to MS have gained interest in the analysis of polar compounds in airborne particulate matter. For example, Kitanovski et al. [19] established a protocol for accurate quantification of nitroaromatic compounds in complex OC fraction of PM using LC-MS/MS. In recent years, ultrahigh pressure liquid chromatography-triple quadrupole MS (UPLC-QqQ MS) has shown a tremendous growth due to its greater efficiency and resolution, speed, simplicity, and economy compared to more conventional chromatographic techniques [43].
This study describes an efficient and reliable UPLC-QqQ MS (hereafter denoted by UPLC-MS/MS) method that is sensitive for the simultaneous quantitative determination of polar organic compounds distributed among three classes of compounds: (i) benzene polycarboxylic acids; (ii) nitroaromatic acids and (iii) nitrophenols in atmospheric aerosols. The different steps of the analytical process including sample preparation have been optimized and quality parameters were established. This study also emphasises on identifying of potential compounds that may be used as tracers for specific sources. To address this, PM2.5 from engine emissions (diesel and gasoline-powered), and Urban Dust and Diesel PM Standard Reference Materials (SRM 1649b and SRM 1650b, respectively) were analyzed to assess the abundance and characteristics of target analytes in these samples. Further, urban PM2.5 samples with different dominant emission sources (heavy-duty traffic and biomass burning) were studied to investigate the presence of any potential tracers in these samples.
Section snippets
Chemicals and standards
All analytical standards used in this study were >97% purity and purchased from Sigma-Aldrich (Oakville, ON, Canada). 3-methyl-5-nitrocatechol was purchased from Santa Cruz Biotechnology Inc. (Dallas, TX, USA). Chemical structures of all analytes are shown in Fig. 1. Isotopically labeled internal standards (98 atom%D) namely terephthalic-d4 (tPh-d4), 3-nitrobenzoic-d4 (3NB-d4) and 4-nitrophenol-d4 (4NP-d4) acids, and isotopically labeled surrogate (phthalic-d4 acid; Ph-d4) were purchased from
Optimization of UPLC-ESI-MS/MS parameters
Twenty eight analytes distributed among three classes of target compounds including (i) ten benzene polycarboxylic acids (BCAs); (ii) ten nitroaromatic acids (NAAs), and (iii) eight nitrophenols (NPs) were selected for this study (Table 2; Fig. 1). In order to achieve the chromatographic separation of all target compounds two reversed phase C18 columns, namely Zorbax Poroshell 120 SP-AQ and Acquity UPLC HSS T3 were tested. Under conditions described in Section 2.2, the Zorbax Poroshell column
Conclusions
An efficient and reliable UPLC-ESI-MS/MS method was developed for the simultaneous quantitative determination of 28 target analytes distributed among 3 classes of compounds: (i) 10 benzene polycarboxylic acids; (ii) 10 nitroaromatic acids and (iii) 8 nitrophenols in atmospheric particulate matter. All target analytes were eluted within a total time of 12 min which makes this method suitable for the routine analysis of atmospheric particulate matter samples. The MRM acquisition mode offered high
CRediT authorship contribution statement
Mahmoud M. Yassine: Methodology, Investigation, Validation, Visualization, Writing - original draft, Writing - review & editing. Michal Suski: Validation. Ewa Dabek-Zlotorzynska: Conceptualization, Methodology, Validation, Project administration, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported as part of the National Air Pollution Surveillance (NAPS) program under the Climate Change Air Pollution (CCAP) within Environment and Climate Change Canada (ECCC). The authors would like to thank Debbie Rosenblatt and Greg Rideout (Emissions Research and Measurement Section) for providing transportation emitted particulate matter samples.
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