Detection of weakly bound clusters in incipiently sooting flames via ion seeded dilution and collision charging for (APi-TOF) mass spectrometry analysis

Abstract

This study introduces an atmospheric pressure chemical ionization method that relies on low-energy thermal collisions (i.e., <0.05 eV) of aerosolized analytes with bipolar ions pre-seeded in a sample dilution flow and allows for the detection of weakly bound molecular clusters. Herein, the potential of the method is explored in the context of soot inception by performing mass spectrometric analysis of a laminar premixed flame of ethylene and air whose products are sampled through a tiny orifice and quickly diluted in nitrogen pre-flowed through a Kr85 based neutralizer to generate the bipolar ions. Analyses were performed with an Atmospheric Pressure Interface Time-of-Flight (APi-TOF, Tofwerk AG) Mass Spectrometer whose high sensitivity, mass accuracy, and resolution (over 4000) allowed for the discrimination of the flame products from the pre-seeded ions. Since ionization of neutrals occurs by either ion attachment or charge exchange following ion collision, the identification of the origin of each peak in the measured mass spectra is not-trivial. Nevertheless, the results provide valuable information on the overall elemental composition of the neutral flame products ionized in either polarity. Results show that the clustering of hydrocarbons lighter than 400 Da and having a C/H ratio between 2 and 3 leads to soot inception in the flame. The dehydrogenation of the flame products, expected to occur as they are convected in the flame, is observed only for measurements in positive polarity because of a higher probability of soot nuclei and precursors to get a positive rather than a negative charge.