A process model to predict the faith of clusters in CI-APi-TOF mass spectrometers

Recent technological developments have made it possible to detect the elemental composition of even the smallest airborne clusters at low ambient concentrations through chemical ionization atmospheric pressure interface time-of-flight mass spectrometry (CI-APi-TOF-MS). The charging process and collisions inside the instrument will however affect the molecular composition of the original sample clusters. A process model is therefore needed to connect the mass spectra to the original molecular clusters in the measured sample.

 

Our research has been focused on developing this process model for clusters involving highly oxygenated organic molecules (HOM) with molecular formula C₁₀H₁₆O₈. This elemental composition corresponds to one of the most common mass peaks observed in experiments on ozone-initiated autoxidation of α-pinene.

 

For the model, two situations inside the mass spectrometer have to be taken into account: the chemical ionization and collisions with carrier gas inside the atmospheric pressure interface. Recently, a model predicting the fragmentation of clusters as a result of collisions in APi has been developed by Zapadinsky et al. (2018). We used this model to perform numerical simulations of fragmentation in the APi on our selected clusters. The results show that fragmentation is highly unlikely for the considered clusters, provided their bonding energy is large enough to allow formation in the atmosphere in the first place.

 

Current research is focused on developing a model for the charging process inside CI-APi-TOF MS. Configurational sampling is combined with high quality quantum chemistry calculations to derive the binding energies and evaporation rates of the studied clusters. This model should eventually connect the ionized clusters after charging to the original distribution of neutral clusters in the measured sample.