Deployment and evaluation of an NH₄⁺/H₃O⁺ reagent-ion switching chemical ionization mass spectrometer for the detection of reduced and oxygenated gas-phase organic compounds

Reactive organic carbon (ROC) is diverse in its speciation, functionalization, and volatility, with varying implications for ozone production and secondary organic aerosol formation and growth. Chemical ionization mass spectrometry (CIMS) approaches can provide in situ ROC observations and the CIMS reagent-ion controls the detectable ROC species. To expand the range of detectable ROC, we describe a method for switching between the reagent-ions NH4+ and H3O+ in a Vocus chemical ionization time-of-flight mass spectrometer (Vocus-CI-ToFMS). We describe optimization of ion-molecule reactor conditions for both reagent-ions, at the same temperature, and compare the ability of NH4+ and H3O+ to detect a variety of volatile organic compounds (VOCs), semi-volatile, and intermediate volatility organic compounds (S/IVOCs) including oxygenates and organic sulfur compounds. Sensitivities are comparable to other similar instruments (up to ~5 count s-1 pptv-1) with detection limits on the order of 1–10 s of pptv. We deploy NH4+/H3O+ reagent-ion switching in a rural pine forest in central Colorado, US, and report a method for characterizing and filtering periods of hysteresis following each reagent-ion switch. We use our ambient measurements to compare the capabilities of NH4+ and H3O+ in the same instrument, without interferences from variation in instrument and inlet designs. We find that H3O+ optimally detects reduced ROC species with high volatility, while NH4+ improves detection of functionalized ROC compounds, including organic nitrates and oxygenated S/IVOCs that are readily fragmented by H3O+.