Characterization of an improved PTR3 mass spectrometer for the detection of highly oxidized aerosol precursors

The recently introduced PTR3-TOF mass spectrometer (Proton Transfer Reaction Time-Of-Flight) allows for a direct and quantitative detection of volatile organic compounds (VOC) and their oxidation products. With a design of the inlet system and the ionization chamber that allows analyte transfer with virtually no wall interactions, organics ranging from volatile to extremely low volatility (ELVOC) can be measured, even at ambient temperature. In addition, PTR3 has recently shown to detect and quantify RO₂ radicals. Unlike the traditional PTR-MS ionization technique, the PTR3 is operated at an elevated reaction pressure of 50 to 80 mbar while reaction kinetics are precisely defined via radial electric fields emitted from a tripole ion guide. With this setup, outstanding sensitivities of more than 30,000 cps/ppbV are achieved.

Herein, we present an improved version of a PTR3-TOF instrument. The inlet comprises three cylindrically arranged ion sources allowing for fast electrical switching between a set of reagent ions including H₃O⁺, NO⁺, O₂⁺ and NH₄⁺. The tripole geometry is aerodynamically improved to further reduce surface interactions. Extraction of analyte ions from the PTR3 ionization chamber and subsequent transfer to the TOF mass analyzer is now enhanced by an ion booster in series to a hexapole ion guide. This setup enables a precise control of extraction energies to reduce unwanted collision induced fragmentation and at the same time efficiently transmits ions of a broad m/z range. Analyte ions are analyzed with a high-resolution Time-Of-Flight mass spectrometer achieving mass resolving powers of typically 13,000 to 15,000.

We have characterized the performance of this optimized PTR3-TOF instrument using pure chemical compounds of intermediate to low volatility, including carboxylic acids and peroxides. Hereby, the effects of PTR3 reaction conditions and ion extraction settings are studied. Monoterpene ozonolysis experiments demonstrate the performance in detecting aerosol precursors from intermediate to extremely low volatility. These new insights in gas phase chemistry are further combined with particle phase measurements conducted with a CHARON PTR-MS to emphasize the analytical capabilities of the PTR3.