Fragmentation analysis of gas-phase oxidation products of α-pinene using high resolution collision-induced dissociation mass spectrometry

Abstract:

Forests emit large quantities of monoterpenes, especially a-pinene, into the atmosphere. α-Pinene can be converted to highly oxidized multifunctional organic compounds by ozone (O₃), hydroxyl radicals, and NO₃ radicals. The resulting oxidation products are significantly involved in secondary organic aerosols (SOA) formation. It is important for atmospheric chemistry to identify and quantify the oxidation products of α-pinene. In this study, we investigated the structure of the oxidation products of α-pinene and its homologues by using an atmospheric pressure negative corona discharge ionization mass spectrometry (APCDI-MS) and collision-induced dissociation (CID) method.

To understand the relationship between the functional group in the oxidation products and its specific fragmentation pathway, CID experiments were performed on the ion peaks derived from known compounds, e.g., three standards of α-pinene oxidation products (pinonic acid C₁₀H₁₆O₃, pinic acid C₉H₁₄O₄, pinolic acid C₁₀H₁₈O₃), oxidation products of α-pinene homologues (myrtenal C₁₀H₁₄O, myrtenol C₁₀H₁₆O) and peroxides. CID results of these compounds suggested that functional group of oxidation products and deprotonated site in a given oxidation product can be estimated from observing which neutral species are dissociated. Considering CID experiments of actual oxidation products of α-pinene, focus on deprotonated ion ([C₉H₁₄O₃-H]^-) at m/z 169.0867, neutral losses of CO₂, HCOOH, H₂O, CO and C₂H₂O were observed from precursor ions. These results indicate that C₉H₁₄O₃ has several structural isomers, each of which has multiple functional groups such as carboxyl, hydroxyl, aldehyde, and acetyl groups.

These analysis methods can be applied to other monoterpenes and are useful for biogenic VOC oxidation products analysis.