Characterization of Polycyclic Aromatic Compounds in Commercial

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Environmental Measurements Methods

Characterization of polycyclic aromatic compounds in commercial pavement sealcoat products for enhanced source apportionment David T Bowman, Karl J. Jobst, Paul A. Helm, Sonya Kleywegt, and Miriam L. Diamond Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06779 • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 13, 2019

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Characterization of polycyclic aromatic compounds in commercial pavement sealcoat products for enhanced source apportionment David T. Bowman1†, Karl J. Jobst2,3, Paul A. Helm2,4, Sonya Kleywegt2, Miriam L. Diamond1,4* 1Department

of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, Ontario, Canada M5S

3B1 2Ministry of Environment, Conservation and Parks, 125 Resources Road, Toronto, Ontario, Canada M9P 3V6 3Department of Chemistry and Chemical Biology, McMaster University, 1280 Main St.W., Hamilton, Ontario, Canada L8S 4M1 4School of the Environment, University of Toronto, 33 Willcocks St., Toronto, Ontario, M5S 3E8

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*Corresponding author: Email: [email protected]; Phone: (+1) 416-978-1586.

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† Current address: SepSolve Analytical, 826 King Street North, Waterloo, Ontario, Canada, N2J 4G8

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Abstract:

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Coal tar-based sealcoat (CTSC) products are an urban source of polycyclic aromatic compounds

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(PACs) to the environment. However, efforts to assess the environmental fate and impacts of

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CTSC-derived PACs are hindered by the ubiquity of (routinely monitored) PACs released from

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other environmental sources. To advance source identification of CTSC-derived PACs, we use

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comprehensive two-dimensional gas chromatography-high resolution mass spectrometry

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(GC×GC/HRMS) to characterize the major and minor components of CTSC products in

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comparison to those in other sources of PACs, viz., asphalt-based sealcoat products, diesel

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particulate, diesel fuel, used motor oil and roofing shingles. GC×GC/HRMS analyses of CTSC

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products led to the confident assignment of compounds with 88 unique elemental compositions,

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which includes a set of 240 individual PACs. Visualization of the resulting profiles using Kendrick

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mass defect plots and hierarchical cluster analysis highlighted compositional differences between

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the sources. Profiles of alkylated PAHs, and heteroatomic (N,O,S) PACs enabled greater

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specificity in source differentiation. Isomers of specific polycyclic aromatic nitrogen heterocycles


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(PANHs) were diagnostic for coal tar-derived PAC sources. The compounds identified and

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methods used for this identification are anticipated to aid in future efforts on risk assessment and

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source apportionment of PACs in environmental matrices.

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Keywords:

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PAHs; GCxGC; comprehensive two-dimensional gas chromatography; environmental forensics; coal tar; Kendrick mass defect

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Graphical Abstract

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40 41



42 43 44

INTRODUCTION

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lots in North America to improve the appearance and to purportedly increase the longevity of

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asphalt surfaces. The two most common products are: (a) coal tar-based sealcoats (CTSC), and (b)

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asphalt-based sealcoats (ASC). Polycyclic aromatic hydrocarbon (PAH) concentrations (defined

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as sum of concentrations of 16 EPA PAHs) are approximately 1000 times higher in CTSC

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(~66,000 mg kg-1) than in ASC (~50 mg kg-1).1 CTSC products are suspected to be a major source

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of PAH contamination in urban and suburban areas2–5 and pose potential risks to aquatic life6–10

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and human health1,3,11. Consequently, CTSC products have been banned in some cities, counties,

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and states in the United States.1

Pavement sealcoats are black, viscous liquids that are applied to driveways and parking

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Polycyclic aromatic compounds (PACs) are a diverse collection of compounds which

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includes PAHs, alkylated-PAHs, and sulfur-, nitrogen-, and oxygen-containing polycyclic

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aromatic heterocycles (PASHs, PANHs, PAOHs). In addition, PACs with heteroatom-containing

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functional groups (such as quinones, nitro-PAHs, and hydroxy-PAHs) can also be present in

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complex PAC mixtures.12 Source apportionment models used to determine the relative

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contribution of PAHs from CTSC products to lake sediment are often limited to the 16 priority

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PAHs.2,3,5,13 Alkylated-PAHs and heteroatomic PACs can provide a higher degree of specificity

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among PAH sources14–17 and are also toxicologically relevant since they are known to exhibit

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mutagenic properties, in some cases exceeding the mutagenicity of benzo[a]pyrene18–23.

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Heteroatomic PACs are also expected to exhibit greater mobility and bioavailability in the

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environment than PAHs due to their increased solubility in water, which in turn increases the risk

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of exposure to aquatic biota and potentially human exposure through groundwater

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contamination.24 To date, few studies have reported on the occurrences of alkylated-PAHs and


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heteroatomic PACs present in CTSC and ASC products.10,25,26 This reflects the paucity of

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commercially available reference standards and the limitations of targeted analytical

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methodologies, often involving extensive sample clean-up and detection by selected ion

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monitoring. This knowledge gap can be addressed through the development of sensitive, non-

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targeted analytical methodologies that enable the simultaneous analysis of PAHs, alkyl-PAHs, as

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well as heteroatomic-PACs and related compounds, whose presence in CTSC products and the

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environment may not be anticipated beforehand.

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Comprehensive two-dimensional gas chromatography coupled to mass spectrometry

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(GC×GC/MS) is a non-targeted analytical technique that offers unparalleled chromatographic

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resolution and peak capacity.27–30 Novel GC×GC column combinations have been explored to aid

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in the speciation of (heterocyclic) PACs from sample matrix components.31,32 Profiles generated

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by GC×GC/MS, sometimes referred to as a ‘chemical fingerprint’, have proven to be vital for

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source identification31,33 and environmental forensics investigations34,35. GC×GC/MS has been

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successfully employed for the characterization of PACs (PAHs, alkyl-PAHs, and heteroatomic

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PACs) in coal tar and coal tar-derived samples.12,36–41 Gallacher et al.12 present a prime example,

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where they identified 1505 PACs in creosote, including several PASHs and PANHs.

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The first step to identify an (unknown) contaminant is the determination of the number and

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type of its constituent elements, viz. its elemental composition. Mass spectrometry (MS) is ideally

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suited for this task, provided the measurements are performed with sufficient mass accuracy and

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resolution. However, high resolution MS (HRMS) experiments performed on environmental

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samples generate enormous datasets, yielding the detection of hundreds to thousands of unique

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elemental compositions. Kendrick mass defect analyses can greatly assist in the interpretation of

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high resolution mass spectra, and can distill complex datasets into plots which rapidly distinguish



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compounds based on heteroatom content, double bond equivalency, and degree of alkylation.42,43

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Tian et al.44 exploited this approach to identify 232 PANHs (aza-arenes) in coal tar and creosote

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contaminated soils.

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The goal of this study was to identify unique chemical markers in CTSC and ASC products

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(prior to pavement surface application) to enhance future source apportionment studies and to more

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reliably identify contamination originating from pavement sealcoat products in complex urban

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environments. Herein, GC×GC/HRMS was used in conjunction with Kendrick mass defect

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filtering and hierarchical cluster analysis to identify significant compositional differences between

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CTSC and ASC products as well as other relevant PAC sources to the urban environment (e.g.,

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diesel particulate matter, diesel fuel, used motor oil, roofing shingles, and rail road ties). Diverse

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collections of (alkylated) PAHs, PANHs, PASHs, and PAOHs were identified in the

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environmental source samples and enabled greater specificity for source identification. The CTSC

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products contained a unique set of PANHs which could distinguish coal tar-related sources from

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other PAC sources. The implications of these results to source apportionment are discussed.

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MATERIALS AND METHODS

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Chemicals and Source Samples.

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Dichloromethane (DCM, distilled in glass) and toluene (distilled in glass) were purchased

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from Fisher Chemical (Fair Lawn, NJ, USA) and Caledon Laboratories (Georgetown, Ontario,

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Canada), respectively. Pavement sealcoats products, consisting of two batches each of CTSC and

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ASC, were obtained from a major industry provider in the province of Ontario and are

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representative of materials used in the southern Ontario region. The major ingredients of each

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product are listed in the SI.


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Other representative PAC source samples in this study included: National Institute of

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Standards and Technology (NIST, Gaithersburg, MD) standard reference materials (SRM 1975,

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diesel particulate extract; 1597a, complex mixture of PAHs from coal tar), a sample of diesel fuel,

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a commonly used formulation of used motor oil (10W-30) from a transport truck, asphalt shingles,

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pot hole repair product from a local building center, coal tar from a transport spill in the region,

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and a cutting of a rail way tie from a local track, all obtained from southern Ontario, Canada. These

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additional samples were chosen to represent other potential urban sources which may contribute

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PAC loadings to the environment.

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Sample Preparation.

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The sample preparation methods used in this study were based upon previous work

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reported by Titaley et al.10 Briefly, the pavement sealcoat and pothole repair products were painted

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onto glass slides and allowed to dry at ambient temperature in the dark for 36 hours. The solid

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material was then scraped off the glass slide, and transferred to a pre-cleaned amber glass container

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with a Teflon-lined lid. A standard mixture containing phenanthrene-d10, chrysene-d12, and

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perylene-d12 (dissolved in dichloromethane) was spiked into the samples. Approximately 100 mg

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of the dried pavement sealcoat product (ASC and CTSC), railroad ties, pothole filler, and asphalt

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shingles was extracted using sonication with 15 mL of DCM for 20 minutes (triplicate extraction).

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The resulting DCM extracts were then centrifuged at 2800 rpm for 30 minutes and the supernatant

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was transferred to a pre-cleaned amber glass container with a Teflon-lined lid. When necessary,

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DCM extracts were diluted prior to analysis. Diesel fuel, used motor oil (10W-30), and the NIST

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standards (diesel particulate and coal tar) were diluted with DCM. Pyrene-d10 (final concentration:

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20 ppb in toluene) was added as an injection standard to all samples prior to analysis. The percent



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recoveries of phenanthrene-d10, chrysene-d12, and perylene-d12 were 81% ± 7, 85% ± 8, 88 % ± 5,

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respectively. Samples were analyzed by GC×GC/HRMS in triplicate. Method blanks were

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extracted alongside samples and PACs were not detected in the blanks.

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Instrumentation.

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GC×GC/HRMS analysis was performed using an Agilent 7890 B gas chromatograph

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(Agilent Technologies, Santa Clara, CA, USA) equipped with a Zoex ZX2 thermal modulator

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(Zoex, Houston, TX, USA) and coupled to a Waters Xevo G2-XS quadrupole time-of-flight mass

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spectrometer (Waters Corp., Milford, MA, USA). The first-dimension column was a DB–5

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(60 m × 0.25 mm × 0.25 μm film thickness, Agilent) followed by a Restek Siltek deactivated

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guard column (1 m × 0.15 mm, Restek, Bellenfonte, PA) in the modulator loop. The second-

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dimension column, Rtx-17sil MS (1 m × 0.15 mm × 0.15 μm film, Restek), was placed in the

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secondary oven. The secondary column was then connected to a Restek Siltek deactivated guard

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column 1 m × 0.15 mm, which was inserted into the transfer line. Columns were connected using

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SilTite μ-Union Connectors (Trajan Scientific and Medical, Victoria, Australia). Additional

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GC×GC parameters and descriptions of method optimization are included in the SI. The mass

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spectrometer was operated in the positive mode under atmospheric pressure chemical ionization

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(APCI) conditions. Mass spectra were collected between 50–1200 amu with a resolving power

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>20,000 (full width-half maximum) and an acquisition rate of 30 Hz. APCI source conditions are

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described in the SI. Data processing was performed using GC Image HRMS R2.5 (Zoex) and

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MassLynx 4.2 (Waters Corp.).

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Atmospheric pressure chemical ionization (APCI) of PACs.


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APCI is a soft ionization technique, involving charge exchange between analyte molecules

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M and N2•+, resulting in the formation of radical cations M•+. Unsubstituted and alkylated PAHs

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form abundant molecular ions (see Figure S1 of the SI) and protonated adducts [M + H]+ (typically

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10 to 30% of M•+) due to residual H2O. Collisional cooling minimizes fragmentation and ensures

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the molecular ions are preserved. This is apparent in the case of 2-ethylanthracene (Figure S1e of

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the SI), which displayed significantly less fragmentation under APCI than EI. Minor oxygen

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adducts [M + O]•+ were also formed (relative abundance less than 1% of the molecular ion) by

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reactions between M•+ and trace O2.45–47 Fortunately, the [M + O]•+ ions resulting from ion-

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molecule reactions could be distinguished from genuine PAOH compounds. For example, the

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selected ion chromatogram (SIC) of m/z 218.072 ± 0.003 (Figure S2) displays two clusters of ions

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identified as C16H10O•+: (i) the [M + O]•+ ion of fluoranthene and pyrene, and (ii) the [M]•+ ion of

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several benzonaphthofurans. Although the two isobars have similar first dimension retention times,

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the potential artefact is efficiently separated from the genuine PAOH due to the high resolving

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power of GC×GC.

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Kendrick mass defect analysis and compound class assignment.

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A composite mass spectrum was generated for each sample using the GC Image software

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by creating a ‘blob’ spectrum (mass spectral bin size = 0.003 Da) for the entire chromatogram

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(chromatographic regions with column bleed excluded). Kendrick mass defect plots were

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employed to visualize the composite mass spectra and to aid in elemental composition

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assignment.43,48 Briefly, the mass of each ion (M) was converted to its Kendrick mass (KM):

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KM = MIUPAC × (14/14.01565)


Equation 1


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Note that the mass measurements were obtained using the IUPAC (International Union of Pure

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and Applied Chemistry) mass scale (i.e. C = 12 amu) and that the factor in equation 1

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(14/14.01565) was derived from the nominal and exact masses of methylene (CH2). The Kendrick

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mass defect (KMD) was obtained by subtracting the Kendrick mass (KM) from the nominal

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Kendrick mass (NKM), which may be approximated by rounding the KM to the nearest integer as

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KMD = NKM – KM.

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Kendrick mass defect plots were constructed by graphing MIUPAC vs. KMD (x-axis vs. y-

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axis) in Microsoft Excel 2010. Elemental compositions were assigned with a standard elemental

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composition calculator using the following criteria: C0-100, N0-3, O0-3, S0-3. All assignments

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were within 5 ppm of the theoretical values. Compounds were also sorted into heteroatom classes

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(using custom macros) to construct heteroatom class histograms. For example, compounds

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consisting of carbon and hydrocarbon (i.e. PAHs) were categorized in the hydrocarbon (HC) class.

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Compounds consisting of carbon, hydrocarbon, and sulfur were categorized in the sulfur (S) class.

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The classification system by Schymanski et al. for the identification of unknowns was used to

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categorize PACs identified in this study.49 The levels are as follows: Level 1: confirmed structure

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(by retention times and mass spectrum of reference standard); Level 2: probable structure (by

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retention index values50–53 and accurate mass); Level 3: tentative structure (by logical elution

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orders of unknowns relative to known compounds; e.g. isomers elute in bands, and ‘roof-tile’

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elution pattern of alkylated isomers); and Level 4: molecular formula assignment (by accurate

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mass).

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Statistical Analysis.


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Two-dimensional hierarchical cluster analysis (2D-HCA) was performed using

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Metaboanalyst 4.0.54 This technique was applied to a dataset containing peak areas (normalized

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to internal standard) of the 34-EPA PAHs, their alkylated homologues and various parent and

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alkylated heteroatomic PACs that were present in the CTSC and ASC products. The list of

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monitored PACs is included Table S1 of the Supporting Information document. In order to reduce

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the number of analytes included in 2D-HCA plot, the alkylated homologues were reported as a

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total value by summing the peak areas of all of the isomers. Non-detects were replaced with one

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half of the lowest value in the dataset. The relative standard deviations of the peak areas of each

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profiled PAC were < 25%. The dataset was subjected to log transformation and pareto scaling

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prior to 2D-HCA.

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RESULTS AND DISCUSSION

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Characterization of PAC profiles in pavement sealcoat products by GC×GC/HRMS.

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Total ion current (TIC) chromatograms obtained from selected CTSC and ASC products

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are displayed in Figure 1. The 16 priority PAHs are highlighted on the TIC chromatograms with

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red boxes. A visual examination of the TICs reveals that the two CTSC products are less complex

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than the two ASC products. The former consisted predominantly of unsubstituted PAHs as well as

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their alkylated homologues. The two CTSC products were enriched in: (i) three to four ring PAHs

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(including, phenanthrene, anthracene, fluoranthene, pyrene, benz[a]anthracene, chrysene), which

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is consistent with previous studies of CTSCs3,10,25; (ii) five and six ring PAHs (e.g.

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benzo[b]fluoranthene, benzo[e]pyrene, benzo[a]pyrene, benzo[g,h,i]perylene, indeno[1,2,3-

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c,d]pyrene, dibenzo[a,h]anthracene); and (iii) several parent PAHs which are not routinely

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monitored in PAC studies and are excluded from the 16 priority PAH and the expanded 34-PAH



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lists, including cyclopenta[def]phenanthrene, cyclopenta[ghi]perylene and indeno[2,1,7-

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cde]pyrene. Indeno[2,1,7-cde]pyrene has been identified as a major component of a highly

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mutagenic fraction of a sediment extract.55 Alkylated PAHs, including isomers of methyl-

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phenanthrene/anthracene, methyl-pyrene/fluoranthene, methyl-chrysene/benzo[a]anthracene, and

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methyl-benzo[a]pyrene, were also detected and their relative abundance followed a distribution

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consistent with the pyrogenic origin of coal tar: C0 >> C1 > C2 > C3 > C4.56 Consistent with our

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results, alkylated PAHs have also been identified in CTSCs2,10 as well as coal tar38,40 and

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creosote12. The presence of alkylated PAH in CTSC products is of concern because the methylated

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derivatives of chrysene, benzo[a]anthracene, and benzo[a]pyrene exhibit greater toxicity

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compared to the parent compound.22,57 Brack and Schirmer55 identified methylated-chrysene

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isomers (along with heteroatomic compounds, such as dinapthofurans) as major CYP1A-inducers

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in contaminated sediment.

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238 239

Figure 1 – GCxGC/HRMS total ion current (TIC) chromatograms of a typical (a) coal tar-based

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and (b) asphalt-based sealcoat product. The 16 priority PAHs are labelled with red boxes. The


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colour scheme of the chromatogram represents the signal intensity: the minimum and maximum

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intensity values are visualized by the colour transition from white (min.) to blue to green to

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yellow to red (max.).

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The ASC products consisted of a more diverse set of organic compounds, as witnessed by

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the complex TIC displayed in Figure 1b. The alkyl PAHs were relatively more abundant than the

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parent PAHs in the ASC products and displayed a bell-shaped distribution with the maxima

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observed at C2, C3, or C4, depending on the double bond equivalency (DBE) of the homologous

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series. This observation is consistent with the petrogenic origin of ASC products.56 PAHs with

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DBEs of 4 to 17 were detected in the two ASC products at significantly lower concentrations

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compared to the PAH levels in the two CTSC products.

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GC×GC/HRMS experiments yielded a large quantity of information and the assignment of

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elemental compositions and structures to (unknown) peaks is time-consuming. In order to

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efficiently probe the GC×GC datasets, composite mass spectra were generated from the TIC

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chromatograms and elemental compositions were assigned based on accurate mass. A total of 88

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and 337 unique elemental compositions were assigned in the CTSC and ASC products,

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respectively. As with any complex hydrocarbon mixture, numerous isobars for a given integer

258

mass were detected: for example, five unique elemental compositions shared a nominal mass of

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272 amu (see Figure S3). Differentiation of these compounds requires both the enhanced

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separation afforded by GC×GC and accurate mass measurements provided by HRMS. For

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example, C18H24S+ (m/z 272.1597) and C19H28O+ (m/z 272.2138) co-elute in both the first and

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second dimensions, but they are readily distinguished by the 54 mDa difference in their exact

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masses (Figure S4). On the other hand, isobars C21H20+ and C18H24S+ differ in mass by only 3.4



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mDa and thus cannot be easily differentiated by mass spectrometry alone. Fortunately, such isobars

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can be identified if they are separated in the time domain by GC×GC prior to detection.48,58 This

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is indeed the case for isobars C21H20+ and C18H24S+, as evident from Figure S5.

267 268

Figure 2 - Kendrick mass defect (KMD) plots of (a) a coal tar sealcoat product, and (b) an

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asphalt sealcoat product.

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Kendrick mass defect plots were used to provide an overview of the identified PAC

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elemental compositions in selected ASC and CTSC samples (see Figure 2). The elemental

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compositions displayed differ by carbon number (x-axis), degree of saturation (y-axis), and

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chemical class (color). The ASC product contained a wider range of (alkylated) PACs. Chemical

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class histograms were also generated from the composite mass spectra and showed the relative

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contribution of each compound class (C, N, O, S) for each sample (see Table 1). As expected, the


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hydrocarbon class was the most abundant in both of the ASC and CTSC samples. The S class was

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the second most abundant class in the ASC products, followed by the N class, and the O class. The

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heteroatoms classes were more relatively abundant in ACS than CTSC products. However, in

280

contrast to the ASC products, CTSC products possessed higher contributions of the N class relative

281

to the S class. These results suggest that the relative proportions of PACs from the heteroatom

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compound classes may be used to estimate the contributions of ASC and CTSC to environmental

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samples contaminated with PACs.

284 285 286 287 288

Table 1: The relative compound class contributions obtained from the coal tar sealcoat (CTSC) and asphalt sealcoat (ASC) extracts. The abundance of each chemical class was normalized to the total ion current. Sample

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CTSC 1 CTSC 2 ASC 1 ASC 2

Hydrocarbon Oxygen Class Class (HC) (O) 94.1 1.8 94.0 2.2 74.3 0.8 85.8 0.5

Sulfur Class (S) 1.5 1.3 22.3 13

Nitrogen Class (N) 2.6 2.5 2.6 0.7

290 291

Identification of heteroatomic PACs in pavement sealcoat products.

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Table S2 summarizes the number of PASHs that were identified in the CTSC and ASC

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products. The ASC products were enriched in a larger collection of PASHs (see Table 1 and Figure

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3a), due to the increased sulfur content of petroleum asphalt.17 Abundant PASHs include the parent

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and alkylated homologues of dibenzothiophene (DBE = 9), phenanthrothiophenes (DBE = 11),

296

and benzonaphthothiophene (DBE = 12). A number of high molecular weight PASHs with DBE

297

values of 14 and 15 were also identified in both of the ASC products (Level 449). The two

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commercial CTSC products contained a suite of PASHs with DBE values of 9 to 14 (see Figure



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3b). Abundant PASHs in the CTSC samples included dibenzothiophene (Level 1, DBE = 9),

300

phenanthro[4,5-bcd]thiophene (Level 2, DBE = 11), and isomers of benzonaphthothiophene

301

(Level 2, DBE = 12). Trace amounts of several compounds tentatively identified as isomers of

302

benzo[2,3]phenanthro[4,5-bcd]thiophene (Level 4, DBE = 14) were also detected in the CTSC

303

products.

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Figure 3 - Double bond equivalency vs carbon number plots for the sulfur class (a: ASC, b: CTSC), nitrogen class (c: ASC, d: CTSC), and oxygen class (e: ASC, f: CTSC). The size of the circles are proportional to the relative abundance of each elemental composition.

309



310

The CTSC samples contained a collection of PANHs with DBEs of 9 to 14 (see Figure 3d

311

and Table S3). Carbazole was the most abundant compound identified in the N chemical class

312

(Level 1)49. Benzo[def]carbazole was also tentatively identified in the samples by RI and accurate

313

mass (Level 2). Benzo[lmn]phenanthridine (Level 2) and indeno[1,2,3-ij]isoquinoline (Level 2),

314

as well as four additional structural isomers (Level 3), were also tentatively identified by RIs and

315

logical elution orders in both of the CTSC samples. Lastly, benz[c]acridine and benz[a]acridine

316

were identified in the two CTSC samples (Level 2). The aza-arenes were not as frequently detected

317

in the ASC products in comparison to the CTSC products (Figure 3c). Carbazole and alkylated

318

(carbon numbers: C9 to C20; Levels 2 and 3) homologues were detected in one of the ASC products,

319

displaying a petrogenic distribution. In addition, unsubstituted (Level 2) and alkylated (Level 3)

320

benzocarbazole homologues (petrogenic distribution) were identified in the ASC products. Nitro-

321

PAHs were not detected in the ASC or CTSC products tested, but they have been detected by

322

others in CTSCs using more extensive clean-up procedures.10

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Table S4 lists the PAOHs that were identified in the pavement sealcoat samples.

324

Dibenzofuran (C12H8O, DBE = 9) was identified in the two CTSC samples (Level 1), along with

325

C1- and C2-dibenzofuran homologues (Level 3). Several (alkylated) benzonaphthofuran and

326

pyrenofuran homologues were also identified in both of the CTSC samples (Levels 2/3). The ASC

327

products contained PAOHs with similar DBE values as the CTSC products, but the alkylated

328

homologues were present in higher relative proportions in the ASCs. In addition, a suite of

329

compounds tentatively identified as C0-dinapthofuran (C20H12O), and C1-dinapthofuran (C21H14O)

330

were detected in the ASC products based on accurate mass (Level 4). Dinaphthofurans have been

331

identified as major CYP1A-inducers in contaminated sediment.55 Hydroxyl-PAHs were not


Page 18 of 34

Page 19 of 34


332

detected in any of the CTSC or ASC sealcoat samples, but trimethylsilylation derivatization could

333

be used to improve their detection.12,59

334

335 336

Figure 4 - Two-dimensional hierarchical cluster analysis with heat map plot of various

337

environmental PAH sources. Red and blue colors represent the most intense and least intense



338

relative abundance values (log scale), respectively. The relative abundances in the heat map plot

339

are presented as an average of 3 replicates.

340 341

Source Identification

342

PAC profiles of the CTSC and ASC products, as well as other environmental sources of

343

PACs that we analyzed (viz. asphalt shingles, rail road ties, coal tar, used motor oil, diesel fuel and

344

diesel particulate matter), were compared by 2D-HCA (Figure 4). Cluster analysis along the x-axis

345

of the heatmap revealed relationships between the source samples based on relative peak areas of

346

the profiled analytes. The samples separated into two main groups: petrogenic (ASC, asphalt

347

shingles, motor oil, diesel) and pyrogenic sources (CTSC, coal tar, rail road ties, and diesel

348

particulate matter). As expected, this differentiation was primarily driven by the greater relative

349

abundance of alkylated PAHs and alkylated heterocyclic PACs in the petrogenic sources. As

350

discussed above, PAC mixtures with a petrogenic origin contain alkylated homologues which

351

follow a bell-shaped distribution, while the pyrogenic samples are enriched in unsubstituted PAHs.

352

This phenomenon is well known and is often exploited in environmental forensic studies to

353

identify samples of petrogenic origin.56

354

The petrogenic source samples were further separated into two additional clusters: (i)

355

asphalt-derived samples (n = 3: ASC products and shingles), and (ii) a cluster containing diesel

356

fuel (n = 1) and used motor oil (n = 1). The diesel fuel and used motor oil samples were

357

distinguished from the asphalt samples because they did not contain any PANHs, or high molecular

358

weight PAOHs (i.e. pyrenofuran, dinaphthofuran). Diesel was distinguished from used motor oil

359

since it contained a lighter fraction of PAHs; the heaviest PAHs detected in diesel and used motor

360

oil possessed DBE values of 13 and 17, respectively.


Page 20 of 34

Page 21 of 34


361

The pyrogenic samples were differentiated into two clusters: coal tar-derived samples

362

(n=5) and diesel particulate (n=1, NIST 1975). The diesel particulate sample was characterized by:

363

(i) a relatively high abundance of PAOHs, including dibenzofurans, benzonaphthofuran, and

364

naphthoxanthene as well as many other O-heterocycles not detected in the other samples

365

considered in this study (the O class represented 16% of total intensity of diesel particulate); (ii) a

366

low relative abundance of some high molecular weight PAHs; and (iii) very low levels of PASHs

367

compared to the coal tar-related samples, which is consistent with the findings of Wilson et al.60

368

In addition, the coal tar-related sources contained a more diverse collection of PANHs than diesel

369

particulate, as well as the other environmental sources included in this study (see Fig. 3 and S6).

370

Selected PANHs were detected exclusively in the coal tar-related samples, such as isomers

371

of benzoacridine (Level 2), methyl-benzoacridine (Level 3), and 4H-Naphtho[1,2,3,4-

372

def]carbazole (Level 4). Isomers of benzocarbazole were detected in a number of the PAC source

373

(diesel particulate, and the coal tar- and asphalt-related samples); however, a comparison of the

374

isomer profiles (Figure 5) revealed significant differences between the source samples.

375

Benzocarbazoles have previously been identified in creosote12 and coal tar41. Three isomers of

376

benzocarbazole (benzo[a]carbazole, isomer ‘a’; benzo[b]carbazole, isomer ‘b’; benzo[c]carbazole,

377

isomer ‘c’; mass spectra and retention times of aforementioned analytes are found in Figure S7)

378

were consistently detected in the coal tar-derived samples, but were absent in the diesel particulate

379

matter sample. While the three isomers were detected in all of the asphalt-related source samples,

380

the isomers were found in differing relative proportions than the coal tar-related samples. It is clear

381

from the extracted ion chromatograms (Figure 5) that benzo[b]carbazole was found in lower

382

proportions relatively to the other isomers in the asphalt-related source samples. The compounds

383

tentatively identified as isomers of methyl-benzocarbazole (Level 3) provided another



384

distinguishing feature, since they were enriched in the asphalt samples and were absent or near

385

detection limits in the coal tar-related samples. Further exploration of PAC isomer profiles is

386

expected to generate more diagnostic PAC source profiles and potentially enable greater

387

specificity of PAC source identification.

388 389

390 391 392 393 394 395

Figure 5 - Isomer profiles of benzocarbazole (C16H11N) and methyl-benzocarbazole (C17H13N) in (a) CTSC product 1, (b) CTSC product 2, (c) Coal tar (NIST 1597a), (d) Coal tar, (e) Rail road ties, (f) diesel particulate, (g) ASC product 1, (h) ASC product 2, and (i) asphalt shingles. Note: isomers ‘a’, ‘b’, and ‘c’ were identified as benzo[a]carbazole, benzo[b]carbazole, and benzo[c]carbazole, respectively, based on retention index values and accurate mass measurements.

396 397 398

Environmental Significance and Implications for Future Studies.

399

PAH-contamination in sediments is a world-wide problem. Knowledge of sources of the

400

contamination is needed to reduce levels, however, efforts to distinguish between different PAH

401

sources are complicated by the ubiquity of the 16 priority PAHs that are typically measured.61 The

402

analytical methods developed here can be used to trace sources of PAHs based on the relative


Page 22 of 34

Page 23 of 34


403

abundances of (alkylated) PAHs and heteroatomic (N, O, S) PACs present in pavement sealcoat

404

products and numerous other environmental sources.

405

Analyses of the composite mass spectrum obtained from the CTSC products led to the

406

assignment of 240 PACs with 88 unique elemental compositions. The profiling of this diverse

407

collection of compounds significantly improved the ability to differentiate PAC source samples.

408

Notably, petrogenic and pyrogenic sources could be easily distinguished based on relative

409

contributions from alkylated PAHs and PASHs. It was also shown that PANHs, such as

410

benzocarbazoles, were very useful for the distinction of CTSC products from other pyrogenic and

411

petrogenic sources. These results follow from a previous study by Mahler et al.26, in which they

412

analyzed run off from ASC- and CTSC-coated parking lots and reported that elevated

413

concentrations of five low molecular weight PANHs (quinoline, isoquinoline, acridine,

414

phenanthridine, carbazole) in run-off were associated with the use of CTSCs. High molecular

415

weight PANHs have also recently been identified as abundant PACs in sediment samples

416

associated with coal tar contamination44, further supporting their applicability as markers in PAC

417

source apportionment studies.

418

The improved characterization of PACs by GC×GC/HRMS is expected to aid in the risk

419

assessment of pavement sealcoat products. Current risk assessment methods for the evaluation of

420

PACs are based on the chemical analysis of the 16 priority PAHs and, consequently, overlook

421

other toxicologically relevant PACs. Larsson et al.24 demonstrated that the concentrations of the

422

16 priority PAHs could only account for a small proportion of the bio-assay-determined toxicity

423

of remediated soil samples. GC×GC/HRMS analyses in this study revealed the presence of a

424

number of PACs in the pavement sealcoat products the toxicity of which is known to equal or

425

exceed the toxicity of benzo[a]pyrene. For example, both of the ASC and CTSC products



426

contained PASHs (e.g. phenanthro[3,4-b]thiophene, benzo[2,3]phenanthro[4,5-bcd]thiophene)

427

and alkylated PAHs which have been reported to be toxic in previous studies.18,20,22,57 Furthermore,

428

a number of polar PACs (such as PANHs or PAOHs) were identified in both of the sealcoat

429

products. The polar fractions (containing heteroatomic PACs) of both ASC and CTSC products

430

have been shown to be acutely toxic by the embryonic zebrafish (Danio rerio) development

431

toxicity test.10 Polar PACs possess higher water solubilities than their PAH counterparts and,

432

therefore, such compounds pose a risk in leaching events.24 Historically, ASC products have been

433

deemed as a safer alternative to CTSCs since they contain significantly lower concentrations of

434

the 16 priority PAHs4,62, are not mutagenic10, and have been reported to pose fewer human and

435

biological health concerns1. However, due the abundance of individual alkylated-PAHs and

436

heteroatomic-PACs in the mixture, there is a critical need to better understand the persistence,

437

toxicity, and environmental fate and behavior of PACs present in such products, as the use of ASCs

438

is expected to increase in the future.

439

While the method developed and presented here has enabled the identification of promising

440

markers of CTSC contamination, we note that there may well be additional source-diagnostic

441

PACs whose polarity inhibits analysis using GCxGC, and may instead be discovered through

442

derivatization or by LC-MS. The characterization of polar PACs, such as phenols, would be

443

beneficial since such compounds have been noted to be of toxicological relevance and, due to their

444

enhanced water solubilities, are more bioavailable than their PAH-analogues.12

445 446

Supporting Information


Page 24 of 34

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447

SI contains additional details concerning chemical analysis and products analyzed. Figures

448

S1-S6 show ion chromatographs and details of compounds analyzed, and Tables S1- S4 list

449

compounds identified.

450 451

Acknowledgements:

452

Funding was provided by the Government of Ontario (Grant #2304). Such support does not

453

indicate endorsement by the Government of Ontario of the contents of this contribution. The

454

authors are grateful to Eric J. Reiner and Jack Cochran for enlightening discussions on GC×GC.

455

References:

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Characterization of Polycyclic Aromatic Compounds in Commercial

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