IR Vaporization Mass Spectrometry of Aerosol Particles with Ionic Solutions: The Problem of Ion-Ion Recombination

摘要

Rapid laser-induced thermal ablation of aerosol particles can result in the ejection of desolvated ions into the gas phase. In an effort to understand this phenomenon, we have carried out studies of ethylene glycol droplets containing 10~(-6) and 10~(-1) M RbCl. A high-power infrared CO_2 laser pulse vaporized the aerosol particles, and the resulting ions were detected by time-of-flight mass spectrometry and by total current measurements. The absolute number of ions in the particle was varied by controlling the size and the concentration of the ions in the particles. The results show that, at low concentrations, the ion signal is directly proportional to the ion concentration, whereas the signal saturates above 10~(-4) M RbCl. By comparing the TOF signal to the total current measured on the ion repeller plate, we were able to determine that ion-ion recombination is the major cause of this signal saturation. The ion collection efficiency of the TOF mass spectrometer at low ion concentration, where ion-ion recombination is not important, was found to be 5%. An ion-ion recombination model, developed by Langevin and Thomson, was found to accurately reproduce the measured ion collection efficiencies for various particle sizes, as well as ion concentrations ranging over 4 orders of magnitude.