WHAT HAPPENS TO SULFURIC ACID-AMMONIA CLUSTERS INSIDE THE CI-API-TOF?
M. PASSANANTI, M.P. RISSANEN, C. YAN, X.C. HE, J. KANGASLUOMA, F. BIANCHI, H.
JUNNINEN, M. EHN and H. VEHKAMÄKI
Department of Physics, University of Helsinki, Helsinki, FIN-00014, Finland.
Keywords: CI-API-TOF, CLUSTER, SULFURIC ACID, AMMONIA.
INTRODUCTION
Chemical Ionization Atmospheric Pressure interface Time Of Flight mass spectrometer (CI-APi-TOF) has
revolutionized the study of atmospheric new particle formation. This instrument is able to detect small
clusters which are involved in the first stages of new particle formation, even at environmental low
concentration. It can give the elemental composition of clusters and an estimate of their concentration, if
the instrument is carefully calibrated. Moreover, the development of highly selective chemical ionization
techniques (e.g. using nitrate as reagent ion) can allow the selective ionization of interested clusters,
eliminating the interferences from all other molecules and clusters present in the atmosphere (Jokinen et
al., 2012). Although the use of the CI-APi-TOF is exponentially grown in the last years, a systematic
study on the fate of atmospheric clusters inside the instrument has not been carried out until now. It has
been reported that clusters can undergo transformation inside the instrument, in particular the charging
process, the low pressure and also energetic collisions with neutral molecules can affect the molecular
composition of clusters (Kürten et al., 2014). This hypothesis is also supported by computational studies,
indeed, it has been shown that theoretical models predict a higher cluster concentration than the measured
concentration (Olenius et al., 2013). This discrepancy has been attributed to the cluster fragmentation
process inside the instrument which is not taken into account.
Sulfuric acid is a key species in new particle formation, and together with ammonia and amines, it seems
to be involved in many nucleation processes (Vehkamäki and Riipinen, 2012). In this study we
investigated the fate of sulfuric acid and sulfuric acid-ammonia clusters inside the CI-APi-TOF to
determine the factors and the conditions that lead to transformation of clusters inside the instrument.
METHODS
The experiments were carried out in the laboratory and sulfuric acid (H2SO4) clusters were produced by a
gas saturator. The sulfuric acid-ammonia clusters were generated by mixing an airflow containing sulfuric
acid vapors and an ammonia (NH3) flow (from an NH3 standard gas tank) in a small quartz tube. The
experiments were carried out at different relative humidity (RH) and different sulfuric acid and ammonia
concentrations. The clusters were then injected into the CI-APi-TOF. The initial tuning of the instrument
was a standard tuning to optimize sensibility and resolution. After observing a stable signal of sulfuric
acid and sulfuric acid-ammonia clusters, the tuning was changed to evaluate the effect of applied electric
field on the clusters signal.
A schematic representation of the CI-APi-TOF is shown in Figure 1. Clusters enter in the chemical
ionization (CI) chamber where nitrate ions and its cluster with nitric acid react with sulfuric acid (and its
clusters) through the following proton transfer reaction:
H2SO4 + (HNO3)n ∙ NO3- → HNO3HSO4- + (HNO3)n
(0 ≤ n ≤ 2)
The ions are then guided inside the atmospheric pressure interface (APi) through a series of three vacuum
chambers before arriving to the time-of-flight mass spectrometer. The pressure decreases between
successive chambers until arriving to 10-6 mbar in the mass spectrometer. In the first two chambers (SSQ
and BSQ) the ions are guided through quadrupoles (Quad1 and Quad2), while in the last chamber (PB)
several lenses focus the ions. In total, 27 voltages and 2 radio frequencies are applied to APi-TOF, most of
which can be changed to optimize the results as a function of the specific application of the instrument. In
this work we evaluated the effects of the voltages applied to SSB and BSQ chambers without changing the
radio frequencies.
CONCLUSIONS
In this study we demonstrate that the tuning of the instrument can significantly affect the sensibility of the
instrument as well as the fragmentation and evaporation of clusters. The fragmentation seems to happen
mainly in the first chamber (SSQ) where the pressure is relatively high (~2 mbar). Here, the charged
clusters are accelerated by an electric field and can produce energetic collisions with neutral molecules. As
expected, we observed a more significant fragmentation when high voltages are applied to Quad1 and
Quad2, moreover the voltage difference between the SSQ and BSQ chamber seems to have an important
impact on cluster fragmentation.
This preliminary study allows to better understand the fate of clusters inside the CI-APi-TOF and it could
be useful to correctly determinate the concentration and composition of sulfuric acid-ammonia clusters in
the atmosphere. Moreover, it lays the bases for developing a theoretical model to describe and predict the
fate of clusters inside the CI-APi-TOF.
Figure 1. Schematic representation of a section of the CI-APi-TOF and possible fate of atmospheric
clusters inside the instrument.
ACKNOWLEDGEMENTS
This work was supported by the European Research Council project 692891-DAMOCLES.
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