A Bipolar Chemical Ionization Mass Spectrometer for the Detection of Aerosol Precursors

Markus Leiminger, Tobias Reinecke, MARKUS MUELLER, Tobias Fügenschuh, Zsolt Dányi, Alfons Jordan, Ionicon Analytik GmbH, Austria

     Abstract Number: 346
     Working Group: Instrumentation and Methods

Abstract
Volatile organic and inorganic compounds (VOC, VIC) react in the gas phase with ozone, nitrate or hydroxyl radicals, creating reaction products of major relevance for atmospheric and indoor chemistry with implications for climate and health. An established technique to study these compounds is chemical ionization mass spectrometry (CIMS). However, since there is no universal reagent ion covering all compounds of interest, combining ionization techniques is necessary.

Here we present the first dual-polarity PTR3 CI-TOF (IONICON Analytik, Austria), considerably extending the number of possible reagent ions and thus the application range. This instrument reaches sensitivities of 30.000 cps/ppbV at a typical mass resolution of >10.000 m/dm. The tripole reaction chamber is operated at around 75 mbar and allows for precise control of the reaction conditions by adjusting the reduced field strength (E/N) from field-free up to 120 Td (1 Td = 10−17 Vcm−2). Bipolar electronics enable electrical switching between positive and negative polarity, extending the range of positive reagent ions (i.e. H₃O⁺, NO⁺, NH₄⁺) to negative ions like I^- and CH₃COO^-.

To characterize this dual-polarity PTR3 CI-TOF, we have conducted a series of oxidation experiments with relevant and well-studied VOC precursors. In this work, we compare the chemical response to secondary organic aerosol (SOA) produced from the oxidation of limonene with ozone as characterized by different reagent ions. The bipolar electronics offer switching speeds between ions of the same polarity of down to 1 s and between polarities within 10 s. This allows quasi-simultaneous tracking of experiments with all three reagent ions. Preliminary results demonstrate the advantages of I^- and CH₃COO^- for detecting highly oxygenated molecules (HOMs) compared to the benefit of detecting precursor VOCs in positive mode with H₃O⁺.