Mass spectrometry of liquid aniline aerosol particles by IR/UV laser irradiation

The first results are reported from a new single-particle two-color laser t ime-of-flight mass spectrometer, incorporating a combination of infrared (C O2) and UV (excimer) laser irradiation. This combination of lasers has the capability to effectively separate the desorption or evaporation step from the ionization step, thereby greatly improving the analytical capabilities of such an instrument. The results on Liquid aerosols, such as aniline, sho w that prior evaporation of the aerosol particle with the IR laser increase s the ion signal produced by the excimer laser by more than 2 orders of mag nitude. In the case of nitrobenzene aerosols, the excimer laser alone produ ces no ions, while a very large signal is observed when the aerosol is firs t irradiated with the CO2 laser. A simple model, based on the Coulomb explo sion of the ionized aerosol, is used to estimate the number of ions generat ed by the excimer laser (similar to 10(5) ions). Experimental evidence base d on the observed time delay of protonated aniline parent ions indicates th at the laser irradiation of the liquid aerosol results in a stable neutral plasma which separates into positive and negative charges only after a 100- 500-ns delay.