Capturing Elusive Volatile Organics in Atmospheric Particles via Dual-Mode Solid-Phase Microextraction Coupled with Gas Chromatography-Orbitrap Mass Spectrometry.

Molecular-level characterization of organic aerosol (OA) is critical for understanding its sources and atmospheric evolution, yet remains analytically challenging due to its broad volatility distribution and complex chemical composition. In this study, we develop a dual-mode solid-phase microextraction (DM-SPME) approach coupled with gas chromatography-orbitrap mass spectrometry (GC-Orbitrap MS) that enables direct, high-sensitivity detection of OA in complex ambient samples. By integrating headspace and direct immersion extraction, DM-SPME improves the detection of higher-volatility compounds and expands the accessible volatility range, spanning pure saturation mass concentration from 10-4 to 106 μg m-3. Application to particulate samples from pristine marine and forest air to polluted urban air through nontargeted analysis shows that DM-SPME detects ∼50% more compounds and ∼40% higher measured concentrations than conventional solvent extraction, with pronounced enhancement for polycyclic aromatic hydrocarbons, ketones, aldehydes, and aliphatic hydrocarbons. Despite these differences, intersite variability in OA numbers and concentrations was consistently captured by both methods, supporting the robustness of DM-SPME for comparative analysis. Volatility-oxidation analysis further reveals molecular differences across sites, with urban samples exhibiting a greater abundance of more oxidized and nitrogen-containing compounds, whereas background samples exhibit broader volatility distributions and relatively lower oxidation states. This work provides an analytical strategy for improving molecular-level analysis of OA and advancing understanding of its composition and atmospheric processes.