Synthesis of Four Monoterpene-Derived Organosulfates and Their

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Synthesis of Four Monoterpene-derived Organosulfates and their Quantification in Atmospheric Aerosol Samples Yuchen Wang, Jingyun Ren, X. H. Hilda Huang, Rongbiao Tong, and Jian Zhen Yu Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 26 May 2017 Downloaded from http://pubs.acs.org on May 27, 2017

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Environmental Science & Technology

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Synthesis of Four Monoterpene-derived Organosulfates and their Quantification in Atmospheric Aerosol Samples

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Yuchen Wang,† Jingyun Ren,‡ X. H. Hilda Huang,§ Rongbiao Tong,*,‡ Jian Zhen Yu*,†,‡ † Environmental Science Programs, Hong Kong University of Science & Technology, Hong Kong, China

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§

Institute of Environment, Hong Kong University of Science & Technology, Hong Kong,

China

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‡ Department of Chemistry, Hong Kong University of Science & Technology, Hong Kong, China *Corresponding Authors Phone: (852) 2358 7389, Fax: (852) 2358 1594, Email: [email protected], [email protected] Abstract

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Monoterpenes, a major class of biogenic volatile organic compounds, are known to produce

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oxidation products that further react with sulfate to form organosulfates. The accurate

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quantification of monoterpene-derived organosulfates (OSs) is necessary for quantifying this

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controllable aerosol source; however, it has been hampered by a lack of authentic standards.

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Here we report a unified synthesis strategy starting from the respective monoterpene through

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Upjohn dihydroxylation or Sharpless asymmetric dihydroxylation followed by mono-sulfation

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with sulfur trioxide-pyridine complex. We demonstrate the successful synthesis of four

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monoterpene-derived OS compounds, including -pinene OS, β-pinene OS, limonene OS and

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limonaketone OS. Quantification of OSs is commonly achieved using liquid chromatography-

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mass spectrometry (LC-MS) by either monitoring the [M-H]- ion or through multiple reaction

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monitoring (MRM) of mass transitions between the [M-H]- and m/z 97 ions. Comparison

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between the synthesized standards and previously adopted quantification surrogates reveals

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that camphor-10-sulfonic acid is a better quantification surrogate using [M-H]- as the

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quantification ion while the highly compound-specific nature of MRM quantification makes it

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difficult to choose a suitable surrogate. Both could be rationalized in accordance to their

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respective MS quantification mechanisms. The in-house availability of the authentic standards

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enables us to discover that β-pinene OS, due to the sulfate group at the primary carbon, partially

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degrades to a dehydrogenated OS compound during LC/MS analysis and a hydroperoxy OS

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over a prolonged storage period (> 5 month), and forms a regioisomer through intermolecular

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isomerization. Limonene OS was positively identified for the first time in ambient samples and

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found to be more abundant than -/-pinene OS in the Pearl River Delta, China. This work

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ACS Paragon Plus Environment


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highlights the critical importance of having authentic standards in advancing our understanding

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of the interactions between biogenic VOC emissions and anthropogenic sulfur pollution.

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TOC graph

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1. Introduction

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Biogenic volatile organic compounds (BVOCs), such as isoprene, monoterpenes,

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sesquiterpenes, contribute significantly to the global secondary organic aerosol (SOA) budget1,

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2

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promoted in the presence of high anthropogenic emissions3-5. One such a pathway is the

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interaction of BVOC oxidation products with sulfate particles leading to formation of

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organosulfates (OSs) compounds. Such OSs derived from BVOC precursors have been

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observed in both chamber-generated6-14 and field-collected aerosol samples15-20. These BVOC-

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derived OSs represent controllable biogenic SOA 21, as their formation would be diminished

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with the reduction in sulfate. Hence, this portion of SOA mass needs to be considered when

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assessing effectiveness of SO2 reduction measures on particulate matter (PM) pollution level.

. Recent research has shown that the conversion of BVOCs to SOA can be significantly

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Quantification of key biogenic OSs is a necessary step in assessing their contribution to the

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effects of PM on climate and public health and the extent of controllable biogenic SOA; however,

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a lack of commercially available OS standards presents a significant obstacle. The OS

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concentrations reported in the majority of studies in the literature are only estimates obtained

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through using one or more surrogate compounds that share chromatographic and mass

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spectrometric behaviors as close as possible to the target analytes (Table S1) in the liquid

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chromatography-mass spectrometric (LC/MS) analysis under negative electrospray ionization

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(ESI-) mode. A few alkyl OSs, including octyl sulfate 22-24, ethylhexyl sulfate 25, and camphor

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sulfonic acid 13, 26-28 are among the commonly adopted surrogates in quantification of BVOC-

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derived OSs (Table S1). The LC/MS instrument response factors of two structurally-similar

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aromatic OSs differed by a factor of 4.337 while the response factor of 2-methyltetrol sulfate 2


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esters34 is a factor of 2.25 lower than that of sodium propyl sulfate used as a surrogate standard18.

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It remains unknown how close the less polar alkyl sulfate surrogates represent the LC/MS

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response by the BVOC-derived OSs or whether the sulfonic acid functional group (-SO3)

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imparts similar ESI efficiency to the sulfate group (-OSO3-). As such, the degree of uncertainty

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for the concentration estimates of the BVOC-derived OSs is unknown. It is clear that lack of

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authentic standards has become a bottleneck in achieving accurate quantitation of individual

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OSs.

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There have been a small number of synthetic efforts, creating authentic quantification

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standards of OSs derived from isoprene oxidation products of first and later generations, such

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as glycolic acid sulfate29-32, lactic acid sulfate29,30,32,33, hydroxyactone sulfate30,31,33, 2-

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methyltetrol sulfate32-34, 2-methylglyceric acid sulfate32. Two studies reported synthesis of two

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monoterpene-derived OSs, β-pinene OS35 and limonene OS36, however, their purity information

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was not available from the papers.

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In this study, we describe a unified approach for synthesis of monoterpene-derived OSs

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of high purity (Fig. 1), which enables us to discover one of the synthesized OSs partially

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degrades to four OSs, underlining the importance of having available authentic standards. By

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comparing the LC/MS responses of authentic OS standards and those of the common surrogates,

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we provide some guidelines for selecting surrogates in LC/MS analysis for OS compounds

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when standards are not commercially available.

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2. Experimental

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All commercial compounds used in synthesis and analyses are of higher than 95% purity.

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The list of chemicals and their sources are provided in Sections 1 and 2 in the Supplementary

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Information (SI). The operation conditions of instruments (NMR and LC/MS) used in this work

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are also included in SI.

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Synthesis. A unified strategy (Fig. 2) was developed to prepare three monoterpene-

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derived organosulfates from the respective monoterpenes (i.e., α-pinene, β-pinene, and

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limonene). Specifically, monoterpenes underwent diastereoselective dihydroxylation to

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produce the respective diols, which were then treated with the commercially available sulfur

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trioxide-pyridine complex (SO3·py) to afford the O-monosulfate products 41,42 (3a, 3b, 3c and

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3c’ in Fig. 2) in 51-94% yield. Two dihydroxylation methods, Upjohn dihydroxylation and

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Sharpless asymmetric dihydroxylation, were explored (Section 4 in SI). In Upjohn

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dihydroxylation, a catalytic amount of osmium tetraoxide38 and stoichiometric N-methyl

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morphine N-oxide (NMO) as the terminal oxidant were mixed with monoterpenes to provide 3



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the corresponding diols. The steric hindrance from the bicyclic structure of /-pinene led to

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formation of the diol as a single diastereomer (i.e., 2a and 2b, respectively, in Fig. 2), while the

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pseudo-equatorial isopropenyl group of limonene has a poor stereocontrol on Upjohn

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dihydroxylation and led to formation of a mixture of two diastereomeric diols (2c and 2c’, dr

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2:1)39. Sharpless asymmetric dihydroxylation40 was then employed for limonene using AD

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mix-β to provide the diol 2c with excellent diastereo- and enantioselectivity (dr 20:1).

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Oxidative cleavage of the double bond of O-monosulfates 3c and 3c’ by ozonolysis

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provided new O-monosulfates 3d and 3d’ with 73% yield. All the synthesized standards are

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stored in clear round flask without special shading from light at 4 oC in a freezer.

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Structural characterization of the synthesized diols and OSs. The intermediate diol

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products and the OSs were characterized by 1H NMR (400 MHz),

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Distortionless Enhancement by Polarization Transfer (DEPT)-135, and heteronuclear single

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quantum coherence spectroscopy (HSQC). The NMR spectra are included in Figs. S3-S9 and

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the chemical shift values are listed in Section 8 in SI. The introduction of the sulfate group

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results in a downfield shift in both 1H- and 13C-NMR due to the deshielding effect of sulfate.

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For example, proton resonance at 3.46 ppm of -pinene diol is shifted to 3.94 ppm in 1H-NMR

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of -pinene OS (Fig. S7a) and the corresponding carbon signal at 70.6 ppm is found at 75.7

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ppm in

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negative carbon signal at 75.7 ppm suggests it is derived from a methylene carbon. This NMR

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analysis concludes that the sulfation reaction of the -pinene diol occur regioselectively at the

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primary alcohol. Meanwhile, the HSQC (H-C one bond correlation) spectral analysis (Fig. S7d)

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shows the protons resonating at 3.94 ppm is bonded to the carbon of 75.7 ppm, which reinforces

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our assignments of the NMR signals to the respective sulfated products. The other

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organosulfates derived from-pinene, limonene, and limonaketone exhibits similar chemical

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shift changes in 1H-NMR and 13C-NMR as -pinene OS. For example, we proposed similar

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deshielding effects of the sulfate on chemical shifts of the limonene OS derived from Sharpless

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asymmetric dihydroxylation and sulfation. The proton resonance at 3.36 ppm of limonene diol

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(2c) is shifted to 4.11 ppm upon sulfation (3c) (Fig. S8a), and the corresponding carbon signal

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at 75.1 ppm (2c) is found after sulfation at 83.9 ppm in 3c (Fig. S8b). The minor isomer 3c’

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was proposed to be arisen from the minor isomer in the Sharpless asymmetric dihydroxylation.

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The molecular masses of the O-monosulfates were confirmed with MALDI-MS analysis by

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directly infusing the product solutions (dissolved in methanol) into the mass spectrometer.

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Mass scans were performed in positive ionization mode using 2,5-dihydroxybenzoic acid as

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C-NMR (100 MHz),

C-NMR (Fig. S7b). In DEPT-135 of the synthesized -pinene OS (Fig. S7c), the

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the matrix and the results are shown in Table 1.

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Determination of purity. The purity of all the synthesized OSs were determined to be

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higher than 99% with NMR spectra. -pinene OS, limonene OS and limonaketone OS did not

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show any degradation/decomposition when stored in solid form over two years; however,

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degradation of -pinene OS was observed. We determined the purity of -pinene OS using 1H

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NMR analysis of a mixture of the degraded -pinene OS sample (41.2 mg) with 15.3 mg

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dichloroacetic acid in CD3OD solvent as an internal standard. The relative integral values of

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the characteristic chemical shift peaks were used to determine the purity of -pinene OS, and

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we observed 19% of degradation product at five months after -pinene OS was synthesized

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(Section 3 in SI).

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LC/MS analysis. Two types of LC/MS systems were used for OSs quantification. One is

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a Dionex Ultimate 3000 HPLC system coupled to an ion-trap mass spectrometer (amaZon SL,

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Bruker Daltonics). The ESI source was operated in negative ionization mode under the

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following conditions: nebulizer pressure at 25 psi, dry gas flow at 10.0 L min-1, source voltage

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at 3.5 kV and temperature at 320 oC. The m/z range was 70 to 600 Da. LC separation was

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carried out on an Acquity UPLC HSS T3 column (2.1 mm×100 mm, 1.8 µm particle size;

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Waters, Milford, MA, USA) with a VanGuard column (HSS T3, 1.8 µm) at a flow rate of 0.3

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mL/min. The mobile phase consisted of water (eluent A) and methanol (eluent B), each

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containing 0.1% acetic acid. The gradient elution program was as follows: eluent B initially

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was set at 1% for 2.7 min, increased to 54% in 15.2 min and held for 1 min, then increased to

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90% in 7.5 min and held for 0.2 min, finally decreased to 1% in 1.8 min and held for 9.3 min.

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The injection volume was 5 µL. Calibration curves on this LC/MS were based on the [M-H]-

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molecular ions in the extracted ion chromatogram (EIC) (abbreviated as EIC quantification

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thereafter) and the concentrations ranged from 10 to 1000 ng/ mL (average R2>0.997). D17-

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octyl sulfate was used as internal standard.

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The second type of LC/MS system is Agilent LC-QTRAP mass spectrometers with a

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TurboIonSpray ionization source operated in ESI- mode. Two such LC/MS systems were used,

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one equipped with a QTRAP4000 mass spectrometer and the second equipped with a

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QTRAP4500 mass spectrometer. The MS was operated in multiple-reaction monitoring (MRM)

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mode for OS quantification. The MS parameters for MRM quantification were individually

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optimized for the mass transition between the [M-H]- ion and m/z 97 fragment ion for each OS

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or m/z 80 fragment ion for camphor sulfonic acid (Table S3). The dwell time for each transition

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was set to 100 ms. The optimal MS parameters for the three monoterpene OSs were similar

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and the set of parameters obtained with -pinene OS was later adopted for the MRM

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quantification of -pinene and limonene OSs. Chromatographic separation was performed on

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an Acquity UPLC HSS T3 column (2.1 mm×100 mm, 1.8 µm particle size; Waters, Milford,

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MA, USA) with Van Guard column (HSS T3, 1.8 µm) at a flow rate of 0.2 mL/min. The mobile

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phase and gradient elution program were the same as those used on the LC/ion trap MS system.

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Example MRM chromatograms for a standard mixture and two ambient samples are shown in

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Fig. 3.

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Ambient particulate matter samples. Ambient aerosol samples analyzed in this study

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were collected in Hong Kong and in Guangdong province, both are part of the Pearl River

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Delta (PRD) in southern China. The sampling dates were synchronized to follow the same 1-

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in-6 day schedule in November and December 2010. The sites include one suburban and one

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roadside in Hong Kong, and four sites in Guangdong covering suburban, urban, industrial and

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residential areas45. PM2.5 samples were collected onto pre-baked 20×25 cm quartz fiber filters

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using high-volume (HV) aerosol samplers (Tisch Environmental, Cleves, OH). The sampled

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filters were stored in a freezer at –18 ºC until analysis.

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An aliquot of 25 cm2 was removed from each HV filter and extracted with high purity

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methanol (LiChrosolv LC grade, 99.9%; Merck Millipore) in a sonication bath three times

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using 3, 2, and 1 mL in sequence, each extraction for 30 min. Extracts were filtered with a pre-

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rinsed polytetrafluoroethylene (PTFE) syringe filter (Bulk Acrodisc CR 13 mm, pore size: 0.25

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µm; Pall Life Sciences), combined and reduced in volume to near dryness under high-purity

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nitrogen, followed by reconstitution in 100 L 1:1 water: methanol (v/v) containing 200 ppb

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D17-octyl sulfate.

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3. Results and Discussion

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Synthesis of organosulfate standards. Two previous studies reported the synthesis of β-

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pinene OS35 and limonene OS36 using monoterpene oxide (i.e., β-pinene oxide and limonene

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oxide) as the starting materials. This synthetic approach usually resulted in a mixture of OS

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regioisomers due to the poor regioselective opening of the epoxide with the sulfate nucleophile

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(i.e., structures bp-1 to bp-4 in Fig. 1). In the synthesis of β-pinene OS, Iinuma et al35 did not

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attempt to separate the OS mixture products, while in the synthesis of limonene OS36 ion

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exchange hydrophilic column used to isolate the limonene OSs from other side products was

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not able to separate the isomeric mixture of the limonene OSs. Different from the previous

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work, we have developed an efficient and unified route that employs commercially available

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and cheap monoterpenes as starting materials. Upjohn dihydroxylation of the monoterpenes as

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the first step in the synthesis can typically produce desired diols (2a, 2b in Fig. 2) with excellent

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chemo- and diastereoselectivity without other byproducts. For example, for -pinene OS and

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freshly synthesized -pinene OS using Upjohn dihydroxylation and sulfation, only one peak

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was observed in its HPLC chromatogram (Fig. 4a), and the NMR spectra (Figs. S6 and S7)

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confirm the structures of 3a and 3b in Fig. 2. However, diastereomers are produced when the

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substrate has poor stereocontrol group (steric hindrance), as in the case of limonene. Two

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diastereomers in a 5:2 ratio, which were determined by LC/MS (Fig. 3a & Fig. S10b), were

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formed from Upjohn dihydroxylation of limonene and subsequent sulfation,39. To achieve

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improved stereoselectivity, we then carried out Sharpless asymmetric dihydroxylation on

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limonene using AD mix-β,40 in which the bulky chiral ligand [(DHQD)2-PHAL] bound to OsO4

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effectively differentiates the two faces of the alkene (absolute configuration) (Fig. S2), thereby

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leading to the production of the diol 2c with excellent diastereoselectivity (1H-NMR cannot

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detect the minor isomer, 2c’ in Fig. 2). Upon sulfation, the diastereomeric ratio of limonene

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OS (3c: 3c’) was determined to be 20:1 by LC/MS (Fig. S10a).

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To achieve regioselective monosulfation and minimize the possible disulfation, one

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equivalent of sulfating reagent (SO3·py) 41, 42 was used. Gratifyingly, such regioselectivity was

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successfully achieved as SO3·py predominately reacted with the less sterically hindered

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hydroxyl group (i.e., the primary alcohol in 2b and the secondary alcohols in 2a and 2c). The

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minor regioisomeric OS products were formed, as evidenced by the detection of four limonene

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OS peaks when we injected the sulfation products of limonene after Upjohn dihydroxylation

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prior to flash chromatography purification. The minor regioisomers were removed in the

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subsequent purification step by flash chromatography on silica gel. This two-step method can

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be applied to synthesize OSs from other carbon-carbon double bond-containing monoterpenes.

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We note that the minor regioisomers, although not isolated in the current synthesis work, could

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be formed in the atmosphere and worth future efforts in creating their pure standards in order

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to quantify their atmospheric importance.

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Two peaks appeared in the HPLC chromatograms of the synthesized standards for the OSs

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derived from limonene and limonaketone (Fig. 3a). These two peaks arose from diastereomers.

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Diastereomers have been reported to be separated on reverse-phase columns46. For -pinene

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OS and freshly synthesized -pinene OS, only one peak was observed in their HPLC

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chromatogram, which might be attributed to the excellent diastereoselectivity achieved due to

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the presence of the bridgehead carbon.

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MS/MS spectra of the synthesized standards: The MS2 spectra for the synthesized OSs

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are shown in Figs. S13-S16. Fragments detected in the MS2 spectra are explained in Schemes

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S1-S4. The synthesized OSs all show HSO4- at m/z 97 as the dominant fragment ion. Limonene

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OS produces a noticeable fragment at m/z 135 (Fig. S13), resulting from loss of H2O and •OSO3

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(Scheme S1). -pinene OS shows two relatively abundant fragments at m/z 169 and 151 (Fig.

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S14), corresponding to loss of •SO3 and a further loss of H2O, respectively (Scheme S2). -

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pinene OS also has a prominent fragment ion •SO3- (m/z 80) (Fig. S15) resulting from the

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sulfate function group (Scheme S3). Surratt et al14 reported MS2 spectra of two structural

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isomers each for - and -pinene OSs produced in a chamber study (i.e., aP_1/3 and aP_2/4

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derived from -pinene and bP_1/3 and bP_2/4 derived from -pinene in Fig. 1). The MS2

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spectra of our synthesized OS are mostly consistent with their result and divergences were

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minor. For -pinene OS and -pinene OS, we did not attempt to synthesize the other structural

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isomers (i.e., aP_2/3/4 and bP_2/3), as they were most likely minor products in our synthesis

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strategy and were not targeted for further isolation if some small amounts were produced.

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Limonaketone OS produces multiple low-intensity fragment ions, including m/z at 171,

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153, 152, 137, 135, and 93 (Fig. S16 and Scheme S4). The fragment characteristics recorded

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in this work could be used to assist identification of these OSs in ambient samples, especially

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for researchers who do not have access to the standards.

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Comparison of LC/MS responses between authentic standards and surrogates. Octyl

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sulfate 22-24 and camphor sulfonic acid 13,26-28 are commonly employed as surrogates to quantify

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monoterpene-derived OSs because of the similar structures and possessing the same carbon

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numbers. 2-ethylhexyl sulfate

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surrogates. We established calibration curves for these OSs previously used as quantification

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surrogates and for the newly synthesized OSs under both EIC and MRM quantification modes.

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Slopes of the calibration curves on the basis of both ppm/ppb and molar concentrations are

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summarized in Table 2. The molar slopes provide comparison of MS response per molecular

255

basis.

25

and dodecyl sulfate

23

are also used in some studies as

The EIC molar slope of the calibration curves ranged from the low value of 0.046

256 257

0.014 for limonaketone OS to the high value of 0.295

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differing by a factor of 6.4 between the two. The two C8 alkyl OSs differ in their MS response

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by a factor of 2.3, with the branched C8 OS (i.e., 2-ethylhexyl sulfate) giving a higher response.

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These results indicate that the MS response of the [M-H]- ion is not dominated by the sulfate

8

0.070 for 2-ethylhexyl sulfate,


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functional group, instead, the remaining carbon chain structure significantly affects the MS

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response, likely through the ESI process.

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Under the EIC quantification mode, octyl sulfate, which has a similar carbon number (but

264

a different carbon skeleton) to the synthetic standards, has a MS response 42% larger than -

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pinene OS, 50% larger than -pinene OS, 65% larger than limonene OS, and 171% larger than

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limonaketone OS. On the other hand, the molar slopes of camphor sulfonic acid-pinene OS,

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-pinene OS, limonene OS are similar, not statistically different at the 95% confidence level

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as evaluated using t-test. Camphor sulfonic acid has a similar carbon skeleton as the three

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monoterpenes, although it possesses a different polar functional group tail from those of OSs.

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The similar MS responses indicate that the carbon skeleton plays a dominating role in affecting

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the MS response of [M-H]- ions. As for limonaketone OS, the additional ketone group lowers

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its MS response by ~60% in comparison with camphor sulfonic acid and the three monoterpene

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OSs, also indicating the carbon chain structure is an influential factor. However, we could not

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rule out the possibility that the high water content in mobile phase (water: methanol at 84:26)

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at the eluting time of limonaketone OS could have a significant factor contributing to its lower

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MS response. The results, although based on a limited number of authentic OSs, suggest that

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carbon chain structural similarity is an important selection criterion for surrogates in

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quantifying OSs without available standards. Specifically for monoterpene-derived OS,

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camphor sulfonic acid is a better quantification surrogate than alkyl OSs in LC/MS analysis

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using [M-H]- as the quantification ion. For OSs with additional polar function groups, use of

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surrogates not matching the polarity would significantly bias quantification.

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LC/MS quantification under MRM mode 24, 28-30 is also used in recent research to quantity

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ambient OSs. Quantification by MRM usually provides improved sensitivity, due to lower

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baseline that can be attained. In current work, the detection limit under the EIC mode was found

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to be higher than the MRM mode by 2 to 10 fold (Tables S2 and S4).

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Under MRM mode, a series of MS parameters, among them declustering potential (DP)

287

and collision energy (CE) being more critical, are varied to achieve maximum fragmentation

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ion intensity for the target mass transition. The optimal MS conditions are inherently

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compound-specific and dependent on the type of mass transition as well. For the same mass

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transition of [M-H]-/97, the optimal CE values for 2-ethylhexyl and octyl OSs were ~10 v

291

more positive than those for the monoterpene OSs (Table S3). When the MS parameters

292

optimized for octyl OS were used for MRM quantification, the MS response varied from 0.040

293

to 0.365 among the OSs, differing by a factor up to ~9. More specifically, the MS responses of

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the synthesized OS were only 15-40% of that for octyl OS (Table 2). This means the

295

concentrations of monoterpene OSs would be underestimated by 2.5-6 times if octyl OS were

296

used as the quantification surrogate. We also determined the MS responses of the OS

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compounds under the MS conditions optimized for -pinene OS. The molar slope of octyl OS

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decreased from 0.270 to 0.110 while that of -pinene increased from 0.040 to 0.144 when the

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MS parameters were switched from those optimized for octyl OS to those for -pinene OS

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(Table 2). The nature of MRM quantification made it difficult to select a suitable surrogate for

301

quantification of OSs without standard, as demonstrated for the monoterpene OSs.

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The MS response for camphor sulfonic acid was 0.377 under its optimized MRM

303

conditions for mass transition of [M-H]-/80. The monitored fragmentation ion for camphor

304

sulfonic acid was different from those of OSs. This makes camphor sulfonic acid not a good

305

surrogate candidate under MRM mode for OSs.

306

Observation of degradation of -pinene OS. -pinene OS, limonene OS and

307

limonaketone OS showed neither hydrolysis nor any other side reactions in solid state over two

308

years, as monitored by NMR analysis. However, -pinene OS degradation was clearly revealed

309

in our LC/MS analysis of the synthesized standard. Four degradation products containing the

310

sulfate functional group were observed.

311

In the LC/MS chromatogram of the newly synthesized -pinene OS standard (Fig. 4),

312

three peaks were detected, one being the expected -pinene OS and two having a [M-H]- at m/z

313

231. The m/z 231 peaks are postulated to be formed after loss of a H2O moiety from the -

314

pinene OS (Compounds 8 and 8’ in Fig. 4). The MS2 spectrum of peak 8 shows a prominent

315

fragment ion at m/z 163 in addition to ions at m/z 80 and 97 that signify the presence of –SO4

316

group (Fig. S17). The m/z 163 ion could be rationalized as a result of fragmentation of the [M-

317

H]- ion through retro-Diels-Alder reaction47. This dehydrated -pinene OS was not present in

318

the NMR spectrum, therefore it is more likely formed during the LC/MS analysis in the

319

presence of 0.1% acetic acid in the mobile phase. The hypothesis of acid facilitating the

320

dehydration finds supporting evidence in the LC/MS chromatogram of a -pinene OS standard

321

prepared in 1% HCl (Fig. 4b), in which the presence of the dehydrated OSs is significantly

322

enhanced. 13C-NMR analysis of the -pinene OS standard mixed with 1% HCl confirmed the

323

presence of C=C bond (characteristic chemical shift at 125 and 133 ppm). Fig. 4b also reveals

324

a second -pinene OS with [M-H]- at m/z 249 (peak b1’) that was not present in Fig. 4a. We

325

propose the second -pinene OS could be the regioisomer of the synthesized -pinene OS. The

326

detailed rational and lines of evidence are presented in Section 12 in SI. 10


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327

Over the course of our study spanning ~20 months, we injected mixtures of the four

328

synthesized OSs in different batches of LC/MS analysis. In the LC/MS chromatograms of the

329

standard mixtures prepared from synthesized compounds ~5-20 months beyond their synthesis,

330

two -pinene OS (m/z 249) and one additional OS with [M-H]- at m/z 263 were recorded while

331

the m/z 231 OS disappeared (Fig. 4c). When the standard is over 20 months old, the first -

332

pinene OS peak (peak 3b) disappeared while the m/z 263 peak remained (Fig. 4d).

333

The accurate mass data for the m/z 263 by-product by MALDI was determined to be

334

263.0595 ([M+2Na]+ at m/z 309.0379), corresponding to an elemental formula of C10H15SO6-.

335

The product ion mass spectrum for the [M-H]- ion at m/z 263 using a collision energy of 30 eV

336

shows a strong fragment ion at m/z 97 (i.e., [HSO4]- bisulfate ion), the typical fingerprint for

337

OS. The other structurally informative ion is the m/z 231 fragment ion, corresponding to the

338

loss of O2 from the [M-H]- ion. The MS data appear to suggest that the two extra O atoms

339

beyond those in the –OSO3 group could be associated with either –OOH or two –OH groups.

340

We subsequently carried out NMR analysis of the mixture of -pinene OS and its

341

degradation product. Both 13C NMR and 1H NMR spectra show the presence of C=C bond in

342

the degradation product. In the 1H NMR spectrum, the resonance with chemical shift at 5.5

343

ppm is an indication of alkene double bond (CDCl3; 400 MHz; see Fig. S11). In the 13C NMR,

344

chemical shifts at 125.1 ppm and 134.3 ppm are typical resonances of SP2-hybrized carbons

345

(CD3Cl; 400 MHz; see Fig. S12). The 13C NMR spectrum shows two signals resonating in the

346

range of 60-90 ppm, suggesting two different oxygen-bonded carbons.

347

Two possible structures in line with the NMR evidence are proposed in Fig. 5 (Structure

348

5 and 5’). The observation of loss of two O atoms in the MS2 spectrum (Fig. S18) appears to

349

favor the hydroperoxy structure (i.e., Structure 5). The close RT (24.1 min) in MRM

350

chromatogram of this degradation product to that of -pinene OS (22.9 and 24.1 min) does not

351

support the diol structure (Structure 5’) either. The similar polarity of the -pinene OS and its

352

degradation product was also confirmed by the observation that they could not be separated by

353

silica gel thin-layer chromatography. If the degradation product were a diol, one would expect

354

earlier elution than the -pinene OS on the reverse phase column, as is the case with

355

limonaketone OS (RT = 14.2 min) in comparison with limonene OS (RT = 22.2, 24.5 min). The

356

extra polarity introduced by one carbonyl group in limonaketone OS shifts its retention time to

357

much shorter time in comparison with the limonene OS. The presence of another hydroxyl

358

group would similarly increase polarity and decrease the retention time.48 We used MOPAC

359

2016 to calculate the dipole moments of the OS compounds. The dipole moments of limonene

11



Page 12 of 28

360

OS and limonaketone OS are 1.466 Debye and 3.205 Debye while those of -pinene OS,

361

Structures 5 and 5’ are 1.511 Debye, 1.663 Debye, and 3.387 Debye, respectively.

362

indicated by the dipole moment, the replacement of hydroxyl group with hydroperoxyl group

363

would maintain polarity and the retention time. In summary, the cumulative evidence supports

364

the identification of the degradation product as Structure 5. The definitive identification has

365

yet to be achieved with a purified sample.

49-51

As

366

We propose in Fig. 6 the possible mechanisms for the formation of OS degradation

367

products (compounds 3b’, 8/8’, and 5 corresponding to m/z at 249, 231, and 263, respectively)

368

from the synthesized -pinene OS (compound 3b). The isomerization from compound 3b to

369

3b’ might take place over time and can be facilitated by acid through cyclic sulfate intermediate

370

that could be opened by external water nucleophile (path a). Dehydration of the tertiary alcohol

371

of compound 3b (possibly via cyclic sulfate) leads to compound 8 (path b). The subsequent

372

allylic oxidation of compound 8 with oxygen via ene-type reaction52 forms compound 5.

373

Similar reaction process was previously proposed to take place in acetonitrile (CH3CN)

374

solvent.53,54 The generation of compound 8’ might be arisen from skeletal rearrangement of the

375

cyclic sulfate intermediate (four-membered ring expansion) followed by β-hydrogen

376

elimination (path c).

377

To rationalize the intrigue fact that -pinene OS can undergo various degradation while

378

other monoterpene OSs do not, we speculate on the basis of comprehensive molecular

379

structural analysis that the degradation uniquely observed with -pinene OS is a result of

380

generation of the unstable cyclic sulfate intermediate via intramolecular SN2 substitution of the

381

tertiary alcohol with the sulfate anion (Fig. 6). On contrast, other monoterpene OSs (i.e., -

382

pinene OS, limonene OS, and limonaketone OS) are not able to form such cyclic sulfate

383

because the sulfate anion and the neighboring alcohol (serving as the potential leaving group)

384

are on the ring that prevent the backside attach of sulfate anion on leaving alcohol (SN2

385

mechanism). We expect similar degradation could occur to other terminal carbon OSs and

386

caution is warranted for their quantification in future studies.

387

For the archived ambient samples collected in 2010, the LC/MS chromatograms show the

388

presence of -pinene OS peak 3b’ (no peak 3b) and the compound 5, consistent with the

389

observations with the synthesized -pinene OS. The abundance of compound 5 has a moderate

390

positive correlation with the -pinene OS (peak 3b & 3b’) (R2 = 0.48, n = 68, Fig. S19). Iinuma

391

et al. 13 also reported the detection of an OS compound having its [M-H]- at m/z 263 in indoor

392

aerosol chamber experiments of gas-phase ozonolysis of -pinene in the presence of neutral or

12


Page 13 of 28


393

acidic sulfate particles and in ambient aerosol samples collected at a forest site in northeastern

394

Bavaria, Germany. The RT of this OS was close to the m/z 249 OS on a reverse phase column,

395

which appears to suggest that the m/z 263 OS likely was the same as the one in our work. They

396

suggested that the m/z 263 OS was the ring opening product of myrtenol with carbonyl group

397

and aldehyde group (i.e., Structure 9 in Fig. 5). However, our NMR analysis of the degradation

398

OS product did not show peaks with chemical shifts in the range of 170 ppm to 200 ppm,

399

negating the possibility of a carbonyl group as shown in Structure 9.

400

In the recent ambient samples collected in October and November of 2016, no m/z 263

401

peak was detected. This result suggests that the peroxy -pinene OS was formed during storage.

402

Two m/z 249 -pinene OS peaks were present in all the recent ambient samples, and the average

403

relative abundance of the two is 1.9:1. The two m/z 213 OS peaks were also present in these

404

recent samples, consistent with the degradation behaviors seen for the synthesized -pinene

405

OS. However, we could not exclude the possibility of formation of the m/z 213 OS in the

406

atmosphere.

407

The observation of -pinene OS degradation indicates attention is needed in

408

characterizing the stability of OSs during storage. Degradation products induced during storage,

409

if mistaken as atmospheric degradation product, could mislead interpretation of atmospheric

410

oxidation mechanism.

411

Ambient abundance of monoterpene-derived OS in ambient samples. Fig. 3 shows

412

reconstructed MRM ion chromatograms for m/z 249, 251, and 263 for a standard mixture and

413

two ambient sample examples, one for an archived sample and one for a recent sample. The

414

synthesized OS standards were all positively identified in the ambient samples, confirmed by

415

matching the retention times, [M-H]- ions, and characteristic fragments with those of the

416

synthetic standards. However, the number of -pinene and limonene OS isomers detected differ

417

in the recent and the archived samples, as noted in previous section. In the MRM chromatogram

418

of m/z 249 ion that generate m/z 97 fragment ions (Figs. 3b & 3c), additional peaks were present

419

besides those associated with -/-pinene and limonene. They are likely OSs products derived

420

from other monoterpenes or the regioisomers of the synthesized OSs. Surratt et al.14 presented

421

an m/z 249 EIC chromatogram of an ambient sample collected in southeast US and five peaks

422

were visually detectable, with four of them identified to be -/-pinene OS.

423

The four target monoterpene-derived OSs were quantified for ambient PM2.5 samples

424

collected in November and December in 2010 at six locations in the Pearl River Delta. The

425

concentration statistics are summarized in Table 3 and in the form of box plots in Fig. S24. The

13



426

average concentration of individual OSs was at the level of ~ 0.1-1 ng/m3, with limonene OS

427

and limonaketone OS being more abundant than -/-pinene OS. The maximum

428

concentrations were up to a few ng/m3 for individual OSs except for -pinene OS,

429

concentrations of which was for certain underestimated due to its degradation.

430

The spatial gradient of OSs and sulfate among the three of the Guangdong sites was

431

largely indiscernible despite their distinct site characteristics ranging from a park surrounding

432

to an urban center (Fig. S24). Lack of spatial variation was also observed between the two sites

433

in Hong Kong, one roadside and one suburban site surrounded by vegetation (Fig. S24).

434

However, there was a considerable difference between sites in Guangdong and Hong Kong in

435

that sulfate concentration in Guangdong was approximately twice that in Hong Kong (18.7 vs

436

9.7 g/m3). In comparison, the spatial gradient in the biogenic OSs was larger than that of

437

sulfate; with the quantities of the monoterpene OSs in Guangdong sites exceed those in Hong

438

Kong by ~2.5-5 fold. This result likely suggests nonlinear chemistry in the formation of OSs

439

and the need for accurate quantification of OSs in order to evaluate how OS formation would

440

respond to reduction in sulfate.

441

A few studies in the literature reported the concentration of monoterpene OS (m/z 249)

442

using quantification surrogates. Kristensen and Glasius26 detected two m/z 249 OS peaks and

443

reported an average concentration of 0.04 ng/m3 for 17 samples collected in a forest site in

444

Denmark using camphor sulfonic acid as the quantification surrogate under EIC mode. Ma et

445

al.24 also reported two m/z 249 OS peaks in samples collected in Shanghai and estimated the

446

seasonal average concentration from 0.087 ng/m3 in spring to 0.366 ng/m3 in winter using octyl

447

sulfate as the quantification surrogate under MRM mode. Iinuma et al. 35 synthesized -pinene

448

OS standard and used it for quantification. They reported a -pinene OS concentration of 23

449

ng/m3 in one single sample from a forest site of Germany. No other m/z 249 OS peaks were

450

recorded in the m/z 249 EIC chromatogram shown in their paper. They did not report the purity

451

of their synthesized standard, nor was degradation reported. It is clear more refined

452

quantification is needed to evaluate whether the large difference is due to different sources of

453

the standard or a result of genuine differences in the ambient environments.

454

We are the first to report identification of limonene and limonaketone OSs in ambient

455

samples. Limonene OS was more abundant than -/-pinene OS in our samples, with an

456

average concentration of 1.14 and 0.36 ng/m3 in Guangdong and Hong Kong, respectively.

457

The concentration of limonaketone OS was on average 0.97 ng/m3 in Guangdong and 0.38

458

ng/m3 in Hong Kong. The highest detected concentration was 3.83 ng/m3 at the suburban site

14


Page 14 of 28

Page 15 of 28


459

on 5 Nov. 2010, coinciding with the highest sulfate concentration (38.0 g/m3) among all the

460

samples. Considering all the samples (n = 55), the limonaketone OS had a moderate correlation

461

with sulfate (r = 0.59) while the correlation of limonene OS with sulfate was significantly

462

weaker (r = 0.29). This possibly reflects that limonaketone OS is a later generation product

463

derived from limonene oxidation. The abundance ratio of the limonaketone OS to the limonene

464

OS was on average 0.88 in Guangdong while 1.5 in Hong Kong, consistent with the formation

465

sequence and Hong Kong is downwind of Guangdong in the months of October and November.

466

The number of monoterpene OS peaks having [M-H]- at m/z 249 detected in this work

467

was more than any other studies in the literature that rely on LC/MS for quantification. This

468

outcome is largely attributable to the availability of authentic standards, which permits

469

optimization of detection sensitivity under MRM quantification mode and thereby enhanced

470

detectability. Creating more OS standards of high purity, including those minor regioisomers

471

not isolated in the current work, is suggested and envisaged invaluable in probing the OS

472

formation mechanisms in the atmosphere.

473 474

Supporting Information

475

Detailed synthesis procedures and LC/MS analytical conditions, NMR spectra, and MS2

476

spectra of the synthesized organosulfate standards. This material is available free of charge via

477

the Internet at http://pubs.acs.org.

478 479 480

Acknowledgements We thank AB Sciex (Hong Kong) for providing us the QTRAP4000 mass spectrometer and

481 482

funding from the Hong Kong University of Science and Technology (SBI15SC04).

483 484

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Table 1. Organosulfate compounds synthesized in this study Exact mass by MALDI-MS

249.08

295.0587

295.0561

C10H17O5S-

249.08

295.0587

295.0577

C10H17O5S-

249.08

295.0587

295.0611

5-acetyl-2-hydroxy-2C9H15O6Smethylcyclohexyl sulfate

251.06

297.0379

297.0411

IUPAC name

formula

[M-H]-

-pinene OS

2-hydroxy-2,6,6-trimethylbicyclo[3.1.1]heptan3-yl sulfate

C10H17O5S-

-pinene OS

(2-hydroxy-6,6-dimethylbicyclo[3.1.1]heptan-2yl)methyl sulfate

Limonene OS

2-hydroxy-2-methyl-5(prop-1-en-2yl)cyclohexyl sulfate

Limonaketone OS

21

sodium adduct [M+2Na]＋

Common Name

Structure



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Table 2. Slopes of standards and surrogates in this study Name

2-ethylhexyl sulfate

Octyl sulfate

Camphor sulfonic acid

-pinene OS

-pinene OS

Limonene OS

Limonaketone OS

Dodecyl sulfate

209

209

231

249

249

249

251

265

16.7 0.220 0.036

16.4, 17.4 0.197 0.016

15.8, 17.8 0.199 0.027

0.116

0.034

0.360

26.6 0.119

0.089

0.079

0.080

0.046

0.014

0.136

0.045

Structure

EIC

MRM

Quan ion [M-H](m/z) RT (min) Slope (ppm) Molar Slope (×10-2) RT (min) Slope (ppb), #1a Molar Slope (×10-2)

#1a #2b

0.617

22.6 0.146

0.295

0.070 28.1

0.612

0.030

0.293 0.015 0.365

22.0 0.262 0.038

0.176

13.1 0.035

0.126

0.076

0.015

0.018

27.7 0.229

19.0

0.020

0.870

0.110 0.009 0.270

0.377 /

0.014

23.2

0.285

0.359

0.083

0.123

0.144 0.033 0.040

0.006

22.9, 24.1 0.224

0.065

0.090 0.026 0.095

0.011

8.5

22.2, 24.5 0.252

0.037

0.101 0.015 0.110

14.2

/

0.206c

/

0.246 0.082c 0.063

/ /

0.616

a Slopes obtained under MS parameters optimized using -pinene (except for limonaketone, see note c), expressed as the average and standard deviation of multiple calibrations over a period of 10 months. See Table S3 for specific parameter values. b Slopes obtained under MS parameters optimized using octyl sulfate. See Table S3 for specific parameter values. c Slope obtained under MS parameters optimized using limonaketone OS.

22


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Table 3. Concentration statistics of organosulfates quantified in this work Species -pinene OS (m/z 249) -pinene OS (m/z 249) Limonene OS (m/z 249) Sum of three monoterpene OSs Limonaketone OS (m/z 251) Ratio of limonene and Limonaketone OS

Concentration in Guangdong sites n=36 a (ng/m3) median max avg 0.41 0.27 2.78 0.19 0.19 0.46 1.14 0.94 3.22 1.74 1.47 6.28

Concentrations in Hong Kong n=19 b (ng/m3) avg median max 0.08 0.08 0.17 0.06 0.05 0.12 0.36 0.25 1.60 0.50 0.45 1.86

0.97

0.54

3.83

0.38

0.37

0.78

0.88

0.48

3.8

1.5

1.1

4.7

a

The samples were collected at four sites in Guangdong in November and December 2010. The average and maximum inorganic sulfate are 18.7 and 38.0 g/m3, respectively. b The samples were collected at two sites in Hong Kong in November and December 2010. The average and maximum inorganic sulfate are 9.7 and 17.6 g/m3, respectively.

23



Figure 1. Chemical structures of organosulfuates derived from -pinene, -pinene, limonene and limonaketone. The structures in solid-line boxes were synthesized in this work. bP_4 was a transformation product of bP_1 observed after a prolonged storage period.

24


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Figure 2. Reaction strategy for synthesis of monoterpene derived organosulfates

25



Figure 3. Reconstructed MRM ion chromatograms: (a) a standard mixture containing eight organosulfates at 500 ng/mL (b) an archived ambient aerosol sample (NS 20101205), and (c) a recent ambient aerosol sample (UST 20161020 am). The peak labels correspond to the structure labels (if available) in Figs. 2 and 6.

26


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Figure 4. Reconstructed MRM ion chromatograms for m/z 231, 249, and 263. (a) A newly synthesized -pinene OS standard, (b) A newly synthesized -pinene OS standard prepared in 1% HCl, (c) a standard mixture of -pinene OS, -pinene OS, and limonene OS prepared from synthesized standards more than 5 months old, and (d) a standard mixture of -pinene OS, -pinene OS, and limonene OS prepared from synthesized standards more than 20 months old. The peak labels correspond to the structure labels (if available) in Figs. 2 and 6.

27



Figure 5. Three possible structures of the degradation OS products of -pinene OS with [MH]- at m/z 263. Structure 5 is the most likely structure conforming LC/MS and NMR evidence. Structure 5’ conforms with NMR data but not LC/MS data. Structure 9 was proposed by Iinuma et al. (2007)13.

Figure 6. Proposed transformation and degradation pathways and products of the synthesized -pinene OS (compound 3b).

28


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Synthesis of Four Monoterpene-Derived Organosulfates and Their

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