Quantification of gas phase hydrogen peroxide and methyl peroxide in ambient air: Using atmospheric pressure chemical ionization mass spectrometry with O 2 − , and O 2 − (CO 2 ) reagent ions

Abstract Instrumentation and ion chemistry are described for the measurement of hydrogen peroxide (H 2 O 2 ) and methyl peroxide (CH 3 OOH) in ambient air by chemical ionization mass spectrometry (CIMS). A CIMS, designed and certified for aircraft deployment, was used in this work. The reagent gas was ultrapure air containing 400 ppm CO 2 . The resultant reagent ions, O 2 − and O 2 − (CO 2 ), form cluster ions with CH 3 OOH and H 2 O 2 , respectively, and are monitored at 80  m / z [O 2 − (CH 3 OOH)] and 110  m / z [O 2 − (CO 2 )(H 2 O 2 )]. The CIMS instrument was periodically calibrated using gas-phase standards generated from aqueous solutions. A Carulite-200 ® catalyst was used to remove peroxides from ambient air to blank the system to account for variations in the ambient air matrix. H 2 O 2 also forms a stable cluster ion with O 2 − though its calibration behavior was more complex than that for O 2 − (CO 2 ) in ambient air. The instrument was deployed on the National Center for Atmospheric Research Gulfstream-V aircraft. Representative measurements from the May-June 2012 Deep Convective Clouds and Chemistry experiment spanning altitudes between 0 and 13 km over the south-central midwest and southeastern United States are shown. Laboratory experiments, airborne experiments and theoretical molecular modeling approaches were utilized to identify and select reagent ions and to understand the ion-molecule reactions for the formation of peroxide-ion clusters in the ambient air matrix under tropospheric conditions. The analytical viability of a particular ion-molecule adduct was supported by ab-initio molecular orbital calculations.