Evaluating the performance of five different chemical ionization techniques for detecting gaseous oxygenated organic species
Authors: Riva Matthieu , Rantala Pekka , Krechmer Jordan E. , Peräkylä Otso , Zhang Yanjun , Heikkinen Liine , Garmash Olga , Yan Chao , Kulmala Markku , Worsnop Douglas , Ehn Mikael 
Published: April 2019

Abstract. The impact of aerosols on climate and air quality remains poorly understood
due to multiple factors. One of the current limitations is the incomplete
understanding of the contribution of oxygenated products, generated from the
gas-phase oxidation of volatile organic compounds (VOCs), to aerosol
formation. Indeed, atmospheric gaseous chemical processes yield thousands of
(highly) oxygenated species, spanning a wide range of chemical formulas,
functional groups and, consequently, volatilities. While recent mass
spectrometric developments have allowed extensive on-line detection of a
myriad of oxygenated organic species, playing a central role in atmospheric
chemistry, the detailed quantification and characterization of this diverse
group of compounds remains extremely challenging. To address this challenge,
we evaluated the capability of current state-of-the-art mass spectrometers
equipped with different chemical ionization sources to detect the oxidation
products formed from α-Pinene ozonolysis under various conditions.
Five different mass spectrometers were deployed simultaneously for a chamber
study. Two chemical ionization atmospheric pressure interface time-of-flight
mass spectrometers (CI-APi-TOF) with nitrate and amine reagent ion
chemistries and an iodide chemical ionization time-of-flight mass
spectrometer (TOF-CIMS) were used. Additionally, a proton transfer reaction
time-of-flight mass spectrometer (PTR-TOF 8000) and a new “vocus” PTR-TOF
were also deployed. In the current study, we compared around 1000 different
compounds between each of the five instruments, with the aim of determining
which oxygenated VOCs (OVOCs) the different methods were sensitive to and
identifying regions where two or more instruments were able to detect
species with similar molecular formulae. We utilized a large variability in
conditions (including different VOCs, ozone, NOx and OH scavenger
concentrations) in our newly constructed atmospheric simulation chamber for
a comprehensive correlation analysis between all instruments. This analysis,
combined with estimated concentrations for identified molecules in each
instrument, yielded both expected and surprising results. As anticipated
based on earlier studies, the PTR instruments were the only ones able to
measure the precursor VOC, the iodide TOF-CIMS efficiently detected many
semi-volatile organic compounds (SVOCs) with three to five oxygen atoms, and the
nitrate CI-APi-TOF was mainly sensitive to highly oxygenated organic (O > 5) molecules (HOMs). In addition, the vocus showed good
agreement with the iodide TOF-CIMS for the SVOC, including a range of
organonitrates. The amine CI-APi-TOF agreed well with the nitrate CI-APi-TOF
for HOM dimers. However, the loadings in our experiments caused the amine
reagent ion to be considerably depleted, causing nonlinear responses for
monomers. This study explores and highlights both benefits and limitations
of currently available chemical ionization mass spectrometry instrumentation
for characterizing the wide variety of OVOCs in the atmosphere. While
specifically shown for the case of α-Pinene ozonolysis, we expect
our general findings to also be valid for a wide range of other VOC–oxidant
systems. As discussed in this study, no single instrument configuration can
be deemed better or worse than the others, as the optimal instrument for a
particular study ultimately depends on the specific target of the study.