Manipulation of Ion Conversion in Dichloromethane-Enhanced Vacuum Ultraviolet Photoionization Mass Spectrometry of Oxygenated Volatile Organic Compounds

The ion conversion processes in CH2Cl2-enhancedvacuum ultraviolet photoionization of oxygenated volatile organiccompounds (OVOCs) have been systematically studied by regulating thepressure, humidity, and reaction time in the ionization source ofa time-of-flight mass spectrometer. As the ionization source pressureincreased from 100 to 1100 Pa, the main characteristic ions changedfrom CH2Cl+ to CH2Cl+(H2O), CH2OH+, and C2H4OH+ and then to the hydrated hydronium ions H3O+(H2O) (n) (n = 1, 2, 3). The total ion current (TIC) almost remainedunchanged even if the humidity increased from 44 to 3120 ppmv, indicatinginterconversion between ions through ion-molecule reactions. The intensityof protonated methanol/ethanol (sample S) ion was almost linearlycorrelated with the intensity of H3O+(H2O) (n) , which pointed to the protontransfer reaction (PTR) mechanism. The reaction time was regulatedby the electric field strength in the ionization region. The intensityvariation trends of different ions with the reaction time indicatedthat a series of step-by-step ion-molecule reactions occurred in theionization source, i.e., the primary ion CH2Cl+ reacted with H2O and converted to the intermediate productions CH2OH+ and C2H4OH+, which then further reacted with H2O and led tothe production of H3O+, and finally, the protonatedsample ion SH+ was obtained through PTR with H3O+, as the ion-molecule reactions progressed. This studyprovides valuable insights into understanding the formation mechanismof some unexpected intermediate product ions and hydrated hydroniumions in dopant-enhanced VUV photoionization and also helps to optimizeexperimental conditions to enhance the sensitivity of OVOCs.

Automated summary

The ion conversion processes in CH2Cl2-enhanced vacuum ultraviolet photoionization of oxygenated volatile organic compounds (OVOCs) were studied by regulating pressure, humidity, and reaction time in the ionization source. The main characteristic ions changed with varying pressure, and interconversion between ions through ion-molecule reactions was observed with increasing humidity. The intensity of protonated methanol/ethanol ion was linearly correlated with the intensity of hydrated hydronium ions, indicating a proton transfer reaction mechanism. The reaction time was regulated by the electric field strength, and step-by-step ion-molecule reactions were observed. This study provides insights into the formation mechanism of intermediate product ions and hydrated hydronium ions in dopant-enhanced VUV photoionization and helps optimize experimental conditions for enhancing the sensitivity of OVOCs.