Quantification of C₂₋₃ organosulfates in atmospheric aerosols : method comparison and evaluation

Abstract

Organosulfates (OSs) are prominent components of secondary organic aerosols (SOA) and have far-reaching implications for the environment. Among the various OS species, C_2-3 organosulfates (C_2-3OSs) have emerged as a crucial small-size subset with high reactivity that contribute significantly to not only the overall abundance of OSs but also the new SOA formation. Considering their important role as oxidation products derived from isoprene, precise quantification data of C_2-3OSs allows for refined modeling of isoprene chemistry, enabling a more accurate perception of its broader implications in atmospheric processes. Despite the importance of airborne C_2-3OSs, few unambiguous measurements of these compounds have been conducted, resulting in a critical data gap that needs to be addressed for a comprehensive understanding of the relevant atmospheric chemistry. Existing data are often obtained through offline liquid chromatography/mass spectrometry (LC/MS) methods that vary in instrument configurations among different laboratories, particularly in the separation techniques employed. This variability can lead to uncertainties when comparing results.

In this work, we evaluated the efficacy of two previously adopted LC methods – reverse-phase liquid chromatography (RPLC) and hydrophilic interaction liquid chromatography (HILIC) – coupled with ESI(-)-Orbitrap MS, in characterizing and quantifying PM_2.5-bound C_2-3OSs. Our results demonstrated that HILIC outperformed RPLC in retentive capacities and peak resolving abilities, generating more reliable results closer to the true concentrations. In contrast, RPLC failed to provide satisfactory outcomes, neither qualitatively nor quantitatively, due to its poor retention of small polar analytes. The results from RPLC were significantly underestimated, which was attributed to the prevalent matrix effect (ME) occurring in the gradient-front and the lack of adequate internal standards (ISs) for compensation. The presence of abundant bisulfate ion eluting in proximity to the analytes was speculated as the dominant endogenous suppressor attenuating the signal. Surprisingly, our supplementary experiment with triple-quadrupole Qtrap MS showed that multiple reaction monitoring (MRM) mode produced even more negatively biased results than the high-resolution Orbitrap system, indicating its higher susceptibility to ME. False-positive signals resulting from insufficient mass resolution were also identified.

Overall, this work serves as a case study highlighting the need for atmospheric chemistry analysts to take note of the potential challenges and consider adopting appropriate analytical methods to ensure accurate and reliable results in their research.

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Jianzhen YU (Supervisor)