Synthesis of Four Monoterpene-Derived Organosulfates and Their Quantification in Atmospheric Aerosol Samples

Abstract

Monoterpenes, a major class of biogenic volatile organic compounds, are known to produce oxidation products that further react with sulfate to form organosulfates. The accurate quantification of monoterpene-derived organosulfates (OSs) is necessary for quantifying this controllable aerosol source; however, it has been hampered by a lack of authentic standards. Here we report a unified synthesis strategy starting from the respective monoterpene through Upjohn dihydroxylation or Sharpless asymmetric dihydroxylation followed by monosulfation with the sulfur trioxide-pyridine complex. We demonstrate the successful synthesis of four monoterpene-derived OS compounds, including α-pinene OS, β-pinene OS, limonene OS, and limonaketone OS. Quantification of OSs is commonly achieved using liquid chromatography-mass spectrometry (LC-MS) by either monitoring the [M-H] ion or through multiple reaction monitoring (MRM) of mass transitions between the [M-H] and m/z 97 ions. Comparison between the synthesized standards and previously adopted quantification surrogates reveals that camphor-10-sulfonic acid is a better quantification surrogate using [M-H] as the quantification ion, while the highly compound-specific nature of MRM quantification makes it difficult to choose a suitable surrogate. Both could be rationalized in accordance to their respective MS quantification mechanisms. The in-house availability of the authentic standards enables us to discover that β-pinene OS, due to the sulfate group at the primary carbon, partially degrades to a dehydrogenated OS compound during LC/MS analysis and a hydroperoxy OS over a prolonged storage period (>5 month) and forms a regioisomer through intermolecular isomerization. Limonene OS was positively identified for the first time in ambient samples and found to be more abundant than α-/β-pinene OS in the Pearl River Delta, China. This work highlights the critical importance of having authentic standards in advancing our understanding of the interactions between biogenic VOC emissions and anthropogenic sulfur pollution.

Quantification of known and unknown terpenoid organosulfates in PM10 using untargeted LC–HRMS/MS: contrasting summertime rural Germany and the North China Plain

“…Such an enhanced calibration function would result in an underestimation of the actual OSs concentration in ambient particulate matter (PM) samples. Consistent with our results, Wang et al (2017b) also found a much larger response of octyl sulfate compared with other authentic OSs and surrogate compounds in ESI-MS. Therefore, it seems likely that previous studies that applied this compound as a calibration proxy significantly underestimated the ambient OS concentrations.…”

“…Taking four additional isomeric MT-OSs from bpinene and limonene into account, they found an average concentration of 0.51 ng m À3 for MT-OSs with the MF C 10 H 18 O 5 S. However, we note that the b-pinene-derived OS alone accounted for 0.4 ng m À3 in that study, and thus, was ,10 times larger compared with all other MT-OSs. In contrast, we found an average concentration of 0.01 ng m À3 for 2-hydroxy-3-carene OS (m/z 249.0806, RT ¼ 5.9 min) and only one further isomeric OS derived from a-pinene (RT ¼ 6.7 min), which together gave an average concentration of 0.09 ng m À3 ( (Wang et al 2017b), which thus indicated the local MT-OS formation at the NCP site. Moreover, as discussed below, several other MT-OSs were detected with much larger concentrations, which were never quantified in previous studies.…”

“…measured by nuclear magnetic resonance; 1 H-and 13 C-NMR) was obtained for the synthesised OS compounds. The detailed synthesis procedure has been described previously by Wang et al (2017b). Furthermore, the NMR spectra will also be discussed in a forthcoming publication (Wang et al 2019, in preparation).…”

“…For future studies, we recommend to avoid using octyl sulfate as a surrogate for terpenederived OS quantification. Instead, as suggested by others Wang et al 2017b), camphor-10-sulfonic acid should rather be used as the surrogate standard of choice if no other authentic OSs are available.…”

Molecular Characterization of Organosulfates in Highly Polluted Atmosphere Using Ultra‐High‐Resolution Mass Spectrometry

“…Lactic acid sulfate (LAS, 15%) and glycolic acid sulfate (GAS, 8%) were synthesized as described by Olson et al (2011). Four monoterpene‐derived OS standards (sodium salts, >95%) were synthesized as detailed in Wang, Ren, et al (2017).…”