Exploratory analysis of a sooting premixed flame via on-line high resolution (APi–TOF) mass spectrometry

Abstract

By taking advantage of recent advances in High-Resolution Atmospheric Pressure intake Time of Flight (APi–TOF) Mass Spectrometry (MS), the chemical analysis of naturally charged flame-generated soot nuclei and precursors is explored using a well-characterized dilution sampling approach. Measurements were performed for mass-to-charge ratio up to 2000 Thomson, bridging the gap between the gas phase and the particle phase. The flame products were sampled at several heights above the burner (HAB) in the soot inception zone of the flame, quickly diluted in nitrogen and directly transported to the APi–TOF inlet. The investigated sooting premixed flame has been the object of multiple studies over the years and the present results complement existing literature data. The analyses of flame products naturally carrying charge of either polarity revealed the chemical and polarity-dependent complexity of the nucleation and chemi-ionization processes. The measured high-resolution mass spectra include peaks attributed to (hydrocarbon) molecules/clusters containing oxygen and nitrogen atoms and suggest that collision charging of flame pyrolysis products likely involves protonation/deprotonation of neutral materials. Results clearly show the change of the overall composition of the charged flame products at different HABs. Patterns in the mass spectra under different conditions were investigated to discriminate between collision charging, chemical reaction and physical clustering (i.e., coagulation and condensation) growth mechanisms. A comparison of the results with those obtained with High-resolution Differential Mobility Analysis (HR-DMA) in a recent study allowed for a more quantitative determination of the ion number concentrations.