Understanding the Temporal Evolution of Isoprene Organosulfates Using a Novel Hydrophilic Interaction Chromatography...

Organosulfates (OSs) form a significant component of secondary organic aerosol (SOA) in the atmosphere. Previously, their measurement using reversed-phase liquid chromatography-high-resolution mass spectrometry (RPLC-HRMS) has led to poor retention and significant matrix effects. Hydrophilic interaction liquid chromatography (HILIC) provides improved retention of polar OSs, such as those formed from isoprene oxidation, and reduced matrix effects. Studies employing HILIC predominantly use gradient elution to separate OSs with varying polarities. HILIC, however, is very sensitive to changes in the water layer within the column, which facilitates separation, meaning the column needs to be thoroughly equilibrated before each injection, significantly increasing analysis time, cost, and solvent waste. The work presented here describes the development of an isocratic HILIC method to analyze OSs in complex ambient PM_2.5 samples. This new method significantly reduces solvent usage and analysis time, which is vital for the analysis of large quantities of samples. The method was first validated with synthesized isoprene-derived OS tracers (iOS) and commercially available standards, then applied in the iOS analysis of filter samples collected during the summertime in Manchester, England. Eight filter samples were taken each day, allowing the temporal evolution of iOS tracers to be elucidated. For 336 ambient samples analyzed in this work, the adoption of the isocratic method resulted in a total time saving of 61 h and reduced organic solvent usage by 1.68 L. The targeted analysis of iOS was quantified using authentic standards. Five 2-methyltetrol OS isomers were quantified (C₅H₁₂O₇S; MTOS_1-4, 6) averaging 0.36, 0.11, 1.25, 2.12, and 0.17 ng m^-3, respectively. The average concentration of total 2-methyltetrol OS was 3.99 ng m^-3, and 2-methylglyceric acid OS (C₄H₈O₇S: MGOS) was 1.07 ng m^-3.