Improving the sensitivity of on-line chemical ionization orbitrap mass spectrometry for detecting low-concentration atmospheric organic vapors

As precursors of organic aerosols, organic vapors in the atmosphere have a wide range of concentrations and complicated compositions. Online measurements of atmospheric organic vapors are fundamental to understanding their interactions with aerosols. The chemical ionization Orbitrap mass spectrometry (CI-Orbitrap) is a powerful tool for resolving organic vapors with an ultra-high mass resolution. However, when measuring trace vapors in the atmosphere, this advantage in the resolving power is partially masked by the non-linear sensitivity of CI-Orbitrap to trace species. In this study, we improve the sensitivity of a nitrate CI-Orbitrap to low-concentration organic vapors by excluding reagent ions from the measurement and addressing the challenge of quantification that arises with the exclusion of reagent ions. We introduce a workflow that includes CI-Orbitrap measurement with its mass range switched between a full mass range and a reagent-ion-excluded mass range, and data inversion with corrections for sensitivity and transmission. Using this workflow, the signal of trace vapors detected by the nitrate CI-Orbitrap can be increased by up to 20 times, and the concentration at which the sensitivity reduces by 50% can be reduced to 1 × 103 #/cm3. Using the nitrate CI-Orbitrap with mass range switching, we measured oxygenated organic molecules (OOMs) in urban Beijing. Enabled by the improved sensitivity, trace OOMs (especially those with high masses and oxidation states, e.g., OOM dimers) are better resolved, and the homologous and oxygen-addition characteristics in OOM formulas at high masses are uncovered.