Laser desorption/ionization of ultrafine aerosol particles

Abstract

On-line analysis of ultrafine aerosol particles in the 12 to 150 nm size range is performed by laser desorption/ionization. Particles are size selected with a differential mobility analyzer and then sent into a linear time-of-flight mass spectrometer where they are ablated with a 248 nm laser pulse. Ultrafine particles exhibit two key laser desorption ionization characteristics. First, although the positive ion mass spectra of ultrafine particles are similar to those of large (micron-size) particles, the negative ion spectra are dominated by free electrons rather than atomic or molecular negative ions. Second, the positive ion signal intensities of ultrafine particles decrease only slightly with decreasing particle size. Relative to the total mass, small particles give much higher ion yields than large particles. These characteristics are discussed in relationship to the mechanism of ion formation and to potential analytical applications.