Probing Interfacial Chemistry of Single Droplets with Field-Induced Droplet Ionization Mass Spectrometry: Physical Adsorption of Polycyclic Aromatic Hydrocarbons and Ozonolysis of Oleic Acid and Related Compounds

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Abstract
The recently developed technique of field-induced droplet ionization (FIDI) is applied to study interfacial chemistry of a single droplet. In a new variation of the FIDI method, 1−2-mm-diameter droplets hang from a capillary and undergo heterogeneous reactions between solution-phase analytes and gas-phase species. Following a specified reaction time, the application of a high electric field induces FIDI in the droplet, generating fine jets of highly charged progeny droplets that are characterized by mass spectrometry. Sampling over a range of delay times following exposure of the droplet to gas-phase reactants, the spectra yield the temporal variation of reactant and product concentrations. We illustrate the technique with three examples: the adsorption of the polycyclic aromatic hydrocarbon naphthalene into a water−methanol droplet, the ozonolysis of oleic acid, and localization of the carbon−carbon double bond within a lysophosphatidic acid. Gas-phase naphthalene reacts with 80% methanol−20% water droplets containing 100 μM silver nitrate. Positive ion mass spectra show increasing concentrations of silver ion−naphthalene adducts as exposure times increase. To examine the ozonolysis of organic molecules, gas-phase ozone generated by a mercury pencil-style lamp reacts with either 10 μM oleic acid or 100 μM oleoyl-l-α-lysophosphatidic acid (LPA; 18:1). Negative ion spectra from the ozonolysis of oleic acid show azelaic acid and 9-oxononanoic acid as the principle reaction products. Ozonolysis products from LPA (18:1) unambiguously demonstrate the double bond position in the original phosopholipid.