On the detection of oxidized and highly oxidized organic molecules by CHARON PTR-MS via ammonium adduct ionization

Oxidized and highly oxidized organic molecules are important target analytes in atmospheric air samples. In recent years, several chemical ionization mass spectrometry (CIMS) methods have been developed for detecting these target analytes in real time and at ultra-trace levels. One of these CIMS techniques is proton-transfer-reaction mass spectrometry (PTR-MS), which, in combination with the so-called CHARON inlet, measures oxidized and highly oxidized organic molecules in the atmosphere in the gaseous and particulate state. PTR-MS typically uses hydronium ions (H₃O⁺) as reagent ions for detecting organic analytes in their protonated form, [A+H⁺]. H₃O⁺ ions react with all oxidized organics at unit efficiency, meaning that PTR-MS universally detects these target analytes, with little dependency of the signal response on their oxidation state. A drawback of PTR-MS operation in the H₃O⁺ mode is that oxidized functional groups are often ejected upon protonation.

Herein, we present the results obtained when a CHARON PTR-MS analyzer was operated with ammonium (NH₄⁺) ions as CI reagent ions. We studied the instrumental response to a set of oxidized and highly oxidized compounds including hydroxy, carboxy and peroxy functional groups. We found that fragmentation was greatly suppressed, with ammonium adducts, [A+NH₄]⁺, being the main analyte ions formed. The ionization efficiency ranged from 10 to 80% of the collisional limit, meaning that the NH₄⁺ mode is less quantitative than the H₃O⁺ mode. The performance and advantages of ammonium adduct ionization are demonstrated on two application examples: i) secondary organic aerosol generated in the laboratory from the ozonolysis of limonene, with a particular focus on the detection of peroxides and dimers, and (ii) ambient organic aerosol in Innsbruck, Austria, which was characterized at the molecular level at single digit ng m^-³ mass concentrations.