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[AAS07-P25] Functional group analysis of gas-phase oxidation products of α-pinene using HR-CID-MS: The effects of oxidants
Keywords:Monoterpene, Oxidation reaction, Functional group, HR-CID-MS
Introduction
Forests emit large quantities of monoterpenes (C10H16), especially α-pinene, into the atmosphere. It can be converted to multifunctional highly oxygenated molecules (HOM) by ozone (O3), hydroxyl radicals, and NO3 radicals. The resulting oxidation products are significantly involved in secondary organic aerosols (SOA) formation that affect the Earth's radiation balance [Ehn et al., Nature, 2014]. In addition, structural analysis of oxidation products of monoterpene is important in atmospheric chemistry because SOA production efficiency strongly depends on functional groups of oxidation products [Atkinson et al., Atmos. Environ., 1995].
There have been many reports on the estimated structures of the oxidation products of a-pinene using mass spectrometry, but most of these analyses have used "nominal mass". Therefore, it has been difficult to distinguish oxidation products with the same nominal mass but different elemental compositions (e.g., C9H14O3 and C10H18O2 with nominal mass 170), the detailed analysis has hardly been done.
In this study, we investigated the functional groups of the unknown oxidation products of a-pinene by using an atmospheric pressure corona discharge ionization mass spectrometry (APCDI-MS) which is similar to atmospheric pressure chemical ionization mass spectrometry (APCI-MS) [Sekimoto et al., Eur. Phys. J. D., 2010] and high-resolution collision-induced dissociation (HR-CID) method.
Results and Discussion
In this study, standard products of a-pinene oxidation products were measured before the analysis of actual oxidation products for investigating the relationship between functional groups and dissociated neutral species from oxidation products by CID. As an example, the results of a CID experiment for pinonic acid are shown in Figure 1. When the deprotonated molecules of pinonic acid were dissociated, the most dominant CO2 loss, medium intensity for the neutral loss of C2H2O and minor neutral loss of H2O were detected. These losses were derived from the respective functional groups, CO2 loss corresponding to the carboxyl group, H2O loss to the hydroxy group within the carboxyl group, and C2H2O loss corresponding to the acetyl group. Other standards were measured under the same conditions, and a relationship was found between the functional group and the neutral loss (Figure 2).
Based on Figure 2, the functional groups of the actual unknown oxidation products of a-pinene were analyzed qualitatively. In this presentation, the discussion focuses on [C10H16O4-H]- at m/z 199 under the different oxidation conditions. The losses of neutral species suggesting the presence of functional groups and their percentages were different under the three oxidation conditions (Figure 3). The loss of C2H2O, suggesting the presence of acetyl groups, was detected under conditions where O3 was the oxidant, however, it was not detected under the other conditions. On the other hand, the loss of H2O2 was not detected, suggesting the presence of peroxy groups under this condition. It was found that O3 is likely to form oxidation products with acetyl groups and unlikely to form oxidation products with peroxy groups.
Forests emit large quantities of monoterpenes (C10H16), especially α-pinene, into the atmosphere. It can be converted to multifunctional highly oxygenated molecules (HOM) by ozone (O3), hydroxyl radicals, and NO3 radicals. The resulting oxidation products are significantly involved in secondary organic aerosols (SOA) formation that affect the Earth's radiation balance [Ehn et al., Nature, 2014]. In addition, structural analysis of oxidation products of monoterpene is important in atmospheric chemistry because SOA production efficiency strongly depends on functional groups of oxidation products [Atkinson et al., Atmos. Environ., 1995].
There have been many reports on the estimated structures of the oxidation products of a-pinene using mass spectrometry, but most of these analyses have used "nominal mass". Therefore, it has been difficult to distinguish oxidation products with the same nominal mass but different elemental compositions (e.g., C9H14O3 and C10H18O2 with nominal mass 170), the detailed analysis has hardly been done.
In this study, we investigated the functional groups of the unknown oxidation products of a-pinene by using an atmospheric pressure corona discharge ionization mass spectrometry (APCDI-MS) which is similar to atmospheric pressure chemical ionization mass spectrometry (APCI-MS) [Sekimoto et al., Eur. Phys. J. D., 2010] and high-resolution collision-induced dissociation (HR-CID) method.
Results and Discussion
In this study, standard products of a-pinene oxidation products were measured before the analysis of actual oxidation products for investigating the relationship between functional groups and dissociated neutral species from oxidation products by CID. As an example, the results of a CID experiment for pinonic acid are shown in Figure 1. When the deprotonated molecules of pinonic acid were dissociated, the most dominant CO2 loss, medium intensity for the neutral loss of C2H2O and minor neutral loss of H2O were detected. These losses were derived from the respective functional groups, CO2 loss corresponding to the carboxyl group, H2O loss to the hydroxy group within the carboxyl group, and C2H2O loss corresponding to the acetyl group. Other standards were measured under the same conditions, and a relationship was found between the functional group and the neutral loss (Figure 2).
Based on Figure 2, the functional groups of the actual unknown oxidation products of a-pinene were analyzed qualitatively. In this presentation, the discussion focuses on [C10H16O4-H]- at m/z 199 under the different oxidation conditions. The losses of neutral species suggesting the presence of functional groups and their percentages were different under the three oxidation conditions (Figure 3). The loss of C2H2O, suggesting the presence of acetyl groups, was detected under conditions where O3 was the oxidant, however, it was not detected under the other conditions. On the other hand, the loss of H2O2 was not detected, suggesting the presence of peroxy groups under this condition. It was found that O3 is likely to form oxidation products with acetyl groups and unlikely to form oxidation products with peroxy groups.