Ammonium CI-Orbitrap: a tool for characterizing the reactivity of oxygenated organic molecules

Abstract. Oxygenated organic molecules (OOMs) play an important role in the formation of atmospheric aerosols. Due to various analytical challenges in measuring organic vapors, uncertainties remain in the formation and fate of OOMs. The chemical ionization Orbitrap mass spectrometer (CI-Orbitrap) has recently been shown to be a powerful technique able to accurately identify gaseous organic compounds due to its great mass resolving power. Here we present the ammonium ion (NH₄⁺) based CI-Orbitrap as a technique capable of measuring a wide range of gaseous OOMs. The performance of the CI-(NH₄⁺)-Orbitrap was compared with that of state-of-the-art mass spectrometers, including a nitrate ion (NO₃⁻) based CI coupled to an atmospheric pressure interfaced to long time-of-flight mass spectrometer (APi-LTOF), a new generation of proton transfer reaction-TOF mass spectrometer (PTR3-TOF), and an iodide (I⁻) based CI-TOF mass spectrometer equipped with a Filter Inlet for Gases and AEROsols (FIGAERO-CIMS). The instruments were deployed simultaneously in the Cosmic Leaving OUtdoors Droplets (CLOUD) chamber at the European Organization for Nuclear Research (CERN) during the CLOUD14 campaign in 2019. Products generated from α-pinene ozonolysis across multiple experimental conditions were simultaneously measured by the mass spectrometers. NH₄⁺-Orbitrap was able to identify the widest range of OOMs (i.e., O ≥ 2), from low oxidized species to highly oxygenated volatile organic compounds (HOM). Excellent agreements were found between the NH₄⁺-Orbitrap and the NO₃⁻-LTOF for characterizing HOMs and with the PTR3-TOF for the less oxidized monomeric species. A semi-quantitative information was retrieved for OOMs measured by NH₄⁺-Orbitrap using calibration factors derived from this side-by-side comparison. As other mass spectrometry techniques used during this campaign, the detection sensitivity of NH₄⁺-Orbitrap to OOMs is greatly affected by relative humidity, which may be related to changes in ionization efficiency and/or multiphase chemistry. Overall, this study shows that NH₄⁺ ion-based chemistry associated with the high mass resolving power of the Orbitrap mass analyzer can measure almost all-inclusive compounds. As a result, it is now possible to cover the entire range of compounds, which can lead to a better understanding of the oxidation processes.