Decomposition of Clusters of Oxygenated Compounds with NO3– by Applying Voltage Scanning to Chemical Ionization Mass Spectrometry in Steady-State Experiments

Abstract

Oxygenated volatile organic compounds (OVOCs) contribute to atmospheric secondary organic aerosols. To better constrain OVOC distributions, e.g., from the oxidation of phenolics, voltage scanning was applied for the targeted destruction of product nitrate (NO₃^–) clusters in a chemical ionization mass spectrometer. Herein, the voltage difference at which half of the clusters remain (dV₅₀) represents their bond strength. This study identified the type and relative bond strength of adducts for product distributions that can be observed for hours in our steady-state chamber (SAPHIR*). An unexpected increase was observed in voltage scanning curves of clusters containing nitrated phenols [e.g., C₇H₇NO₃(NO₃^–)], which was attributed to the declustering of double-analyte clusters [e.g., C₁₄H₁₄N₂O₆(NO₃^–)] at small voltage differences. Double-analyte clusters were distinguished from accretion product clusters [e.g., C₁₂H_(10,12)O_x(NO₃^–)] by their significantly lower intermolecular forces. Misidentifying C₁₄H₁₄N₂O₆ as accretion products could lead to an overestimation of its contribution to particle mass. In addition, the higher bonding strength in C₆H_(6,8)O_4–9(NO₃^–) compared to that in H₂SO₄(NO₃^–) indicates maximum sensitivities of C₆H_(6,8)O_4–9 at the collision limit. We could elucidate the relative acidity of the analytes to HNO₃. This study highlights additional dimensions gained from voltage scanning and suggests performing it to clarify the product distribution in complex urban air in the presence of nitrated phenols.