We are presently measuring these compounds using a variety of techniques. One such technique is a negative ion proton transfer reaction time-of-flight (NI-PTR-ToF) mass spectrometer. This instrument uses negative-mode chemical ionization to ionize and subsequently analyze compounds with acidic functionalities. The NI-PTR technique allows limits of detection that are compatible with the low concentrations of acidic compounds found in the atmosphere.
A framework for assessing potential reagent ions will be explored and based on the known, calculated, or estimated thermochemical properties of reagent ions. The following criterion will determine an appropriate reagent ion:
Achieve stoichiometric conversion of a neutral to an ion. To achieve the lowest limit of detection, all substrates must be ionized within the drift tube. Ion/molecule collision rates should be maximized while ensuring that the equilibrium in the collision complex strongly favors charge transfer.
Ionization without fragmentation. Ionization would maintain structure and allow further characterization using selective ion/molecule reactions.
Robustness. A reagent ion should ionize entire classes of compounds, such as carboxylic acids. Additionally, due to the ubiquity of water in real sample, the ionization scheme must either be resilient to water clustering or already account for clustering.
This research is supported by NSF award #0803016 and NOAA-EPP award #NA06OAR4810187.