Bipolar FUSION PTR-TOF Mass Spectrometer: Advantages of Multiple Reagent Ions to Characterize Oxidation and Secondary Organic Aerosol Formation

Aerosol particles significantly impact Earth’s climate, air quality, and human health. Secondary Organic aerosols (SOA) present a major fraction of the particulate mass in the troposphere. Due to the involved processes ranging from molecular to particle scales, SOA remains a complex topic with a continuing need of method and instrument development. 

Proton-transfer-reaction mass spectrometry (PTR-MS) is a well established technique for the characterization of SOA and its precursors. More advanced instruments like the recently introduced FUSION PTR-TOF feature positive selective reagent ions like H₃O⁺ for quantitative measurements of the widest range of organic compounds or NH₄⁺ for soft adduct ionization that enables the detection of highly oxidized compounds. Complemented by NO⁺ and O₂⁺  ionization mode, this instrument covers the detection of the vast majority of organic and inorganic compounds. However, the measurement of certain inorganic compounds like SO₂, H₂SO₄, other small inorganic acids and organic acids still poses a challenge utilizing only positive reagent ions.

To close this gap, the FUSION PTR-TOF was upgraded with bipolar electronics, enabling operation in negative ion mode using negative reagent ions such as CO₃^- which provides enhanced selectivity for acids and volatile inorganic compounds, including SO₂, HNO₃, H₂SO₄, and halogenated compounds.

In this work we focus on limonene, a monoterpene emitted by plants and widely used in consumer products. It is of particular interest due to its high aerosol yield and structural features like endocyclic and exocyclic double bonds, which influence its oxidation pathways. To study limonene oxidation and its contribution to SOA formation on a molecular level, a laminar flow oxidation reactor was set up. This reactor allows for a controlled oxidation of limonene with oxidants like OH or ozone with residence times of up to 15 min that is sufficient for SOA formation. Limonene and its volatile oxidation products were monitored in real-time with a FUSION PTR-TOF and the particle phase was measured with a CHARON particle inlet for a direct detection of SOA constituents. Based on these measurements we will highlight the benefits and limitations of complementary ionization modes of the new Bipolar FUSION PTR-TOF. CO₃^- proves to be highly selective to the formed acids and effectively captures products like pinonic acid with virtually no fragmentation significantly simplifying data interpretation. Sequentially ionizing with H₃O⁺, NH₄⁺, NO⁺, and CO₃^- primary reagent ion modes allows for capturing the complete picture of the formation process from precursor to SOA.