10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Fragmentation of Ionised Atmospheric Clusters inside a Mass Spectrometer
Monica Passananti, EVGENI ZAPADINSKY, Juha Kangasluoma, Nanna Myllys, Michel Attoui, Hanna Vehkamäki, University of Helsinki
Abstract Number: 726 Working Group: Aerosol Physics
Abstract The development of Mass Spectrometers (MS) as the Atmospheric Pressure interface Time Of Flight (APi-TOF) and the Chemical Ionisation APi-TOF (CI-APi-TOF) has revolutionised the study of new atmospheric aerosol particle formation. These instruments are able to detect molecules and small clusters, which are involved in the first stages of new particle formation, even at environmental low concentration. However, clusters’ binding energies are much weaker compared to molecules; hence they can undergo transformations (fragmentation and/or evaporation) inside a MS easier than molecules. Correct accounting of collision induced cluster fragmentation (CICF) is of vital importance for retrieving the initial cluster distribution obtained in the experiments. Therefore, we decide to carry out a systematic study on the fate of clusters inside the APi-TOF and to develop a model to predict the fragmentation of clusters inside the mass spectrometer.
To investigate the fate of clusters inside the APi-TOF we combined it with a high-resolution Differential Mobility Analyser (DMA). The DMA allows us to measure the clusters size and separate them based on their size. Injecting into the APi-TOF only a mono-mobile size ions distribution it is easier to understand the fate of clusters inside the instrument because only one kind of clusters is studied at a time. We analysed sulphuric acid clusters produced by ElectroSpray Ionisation (ESI) and we focused our study on sulphuric acid trimer fragmentation inside the APi. This latter is made by three vacuum chambers (SSQ, BSQ and PB) where an electric field is applied to guide the ions through the interface. We evaluated the effects of the voltages applied to SSB, BSQ and PB chambers without changing the radio frequencies.
We developed a model to describe the CICF inside the APi. In this model, the charged clusters move through the APi under applied constant and uniform electrical field (defined by the tuning of the instrument). We consider each cluster individually and its trajectory is simulated as a random process. The clusters can be affected by energy transfer at collisions with residual carrier gas molecules and following fragmentation. The probabilities governing the energy transfer and the cluster fragmentation are based on level densities of the clusters and the products of fragmentation calculated by quantum chemical methods. After simulating the trajectory (and the fate) of a big enough number of clusters we calculate the proportion of the fragmented clusters.
The results are very promising, we have observed by the experiments and we confirm it with the simulations that the clusters are mainly fragmented at the interface between the first (SSQ) and second (BSQ) chambers. In this area, the pressure is low enough for the charged clusters to be considerably accelerated under electric field and the same time is still high enough for the clusters to experience collisions with the carrier gas molecules. The collisions induce transferring of the cluster translational energy to rotational and vibrational ones leading to clusters’ fragmentation in some cases. We have observed good agreement between the experiments and the model. The development of this model and its validation by laboratory experiments is crucial to correctly interpret the experimental data obtained by APi-TOF instruments for simulating the atmospheric processes. It advances both understanding of the process of the new atmospheric aerosol particles formation and developing of the instruments.