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. Chemical ionization massspectrometry (CIMS) instruments routinely detect hundreds of oxidized organic compoundsin the atmosphere. A major limitation of these instruments is the uncertaintyin their sensitivity to many of the detected ions. We describe thedevelopment of a new high-resolution time-of-flight chemical ionization massspectrometer that operates in one of two ionization modes: using eitherammonium ion ligand-switching reactions such as for NH4+ CIMS orproton transfer reactions such as for proton-transfer-reaction massspectrometer (PTR-MS). Switching between the modes can be done within 2 min.The NH4+ CIMS mode of the new instrument has sensitivities of upto 67 000 dcps ppbv−1 (duty-cycle-corrected ion counts per second perpart per billion by volume) and detection limits between 1 and 60 pptv at2σ for a 1 s integration time for numerous oxygenated volatileorganic compounds. We present a mass spectrometric voltage scanning procedurebased on collision-induced dissociation that allows us to determine thestability of ammonium-organic ions detected by the NH4+ CIMS instrument.Using this procedure, we can effectively constrain the sensitivity of theammonia chemical ionization mass spectrometer to a wide range of detectedoxidized volatile organic compounds for which no calibration standards exist.We demonstrate the application of this procedure by quantifying thecomposition of secondary organic aerosols in a series of laboratoryexperiments.

atmospheric measurement techniques

Using collision-induced dissociation to constrain sensitivity of ammonia chemical ionization mass spectrometry (NH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;&amp;lt;sup&amp;gt;+&amp;lt;/sup&amp;gt; CIMS) to oxygenated volatile organic compounds