Aerosol particle mass spectrometry with low photon energy laser ionization

The problem of excessive ion fragmentation observed in the mass spectra of organic aerosols is addressed by varying the photon energy used to ionize the vaporized particles. Oleic acid aerosols are vaporized by either an IR CO2 laser or by impaction on a heater and ionized by pulsed laser radiation produced by either third harmonic generation in Xe (118 nm; 10.48 eV) or by resonance difference frequency mixing in Kr (142 nm; 8.75 eV). The use of the lower energy laser light greatly reduced the fragmentation of the oleic acid ions. However, spectra of oleic acid particles taken at several different CO2 laser powers showed that the internal energy of molecules in the vapor plume vary in time so that the appearance of the mass spectrum is a function of the delay time between CO2 and VUV laser pulses. Hot molecules produced in the early stages of vaporization generated large amounts of fragment ions, whereas the mass spectra of colder molecules consisted predominantly of parent ions. Vaporization with the heater on the other hand, shows a more uniform heating of the particle, with very little ion fragmentation. The ionization energy (8.6 ± 0.1 eV) and the fragment ion appearance energy (9.0 ± 0.1 eV) of oleic acid were determined by photoelectron photoion coincidence spectrometry.