Abstract. Chemical ionization mass
spectrometry (CIMS) instruments routinely detect hundreds of oxidized organic compounds
in the atmosphere. A major limitation of these instruments is the uncertainty
in their sensitivity to many of the detected ions. We describe the
development of a new high-resolution time-of-flight chemical ionization mass
spectrometer that operates in one of two ionization modes: using either
ammonium ion ligand-switching reactions such as for NH4+ CIMS or
proton transfer reactions such as for proton-transfer-reaction mass
spectrometer (PTR-MS). Switching between the modes can be done within 2 min.
The NH4+ CIMS mode of the new instrument has sensitivities of up
to 67 000 dcps ppbv−1 (duty-cycle-corrected ion counts per second per
part per billion by volume) and detection limits between 1 and 60 pptv at
2σ for a 1 s integration time for numerous oxygenated volatile
organic compounds. We present a mass spectrometric voltage scanning procedure
based on collision-induced dissociation that allows us to determine the
stability of ammonium-organic ions detected by the NH4+ CIMS instrument.
Using this procedure, we can effectively constrain the sensitivity of the
ammonia chemical ionization mass spectrometer to a wide range of detected
oxidized volatile organic compounds for which no calibration standards exist.
We demonstrate the application of this procedure by quantifying the
composition of secondary organic aerosols in a series of laboratory
experiments.
Structural characterization of abscisic acid and related metabolites by chemical ionization mass spectrometry.
