Most laser-based aerosol mass spectrometers rely on a single ultraviolet laser to both ablate and ionize the aerosol particle. This technique produces complex and fragmented mass spectra, especially for organic compounds. The approach presented here achieves a more robust and quantitative analysis using a CO2 laser to evaporate the aerosol particle and a vacuum ultraviolet laser to ionize the vapor plume. Vacuum ultraviolet laser ionization produces little fragmentation in the mass spectra, making the identification of an aerosol particle's constituents more straightforward. An analysis of simple, three-component mixtures of aniline, benzyl alcohol, and m-nitrotoluene shows that the technique also provides a quantitative analysis for all the components of the mixture. Furthermore, the detection of predominantly parent ion signal from anthracene particles demonstrates the utility of the technique in the analysis of lower vapor pressure, solid-phase aerosols. Finally, we discuss the potential and limitations of this technique in analyzing organic atmospheric aerosols.

A new ion deposition apparatus was designed and constructed in our laboratory. Our research objectives were to investigate interactions of biomolecules with hydrophilic and hydrophobic surfaces and to carry out exploratory experiments aimed at highly selective deposition of spatially defined and uniquely selected biological molecules on surfaces. The apparatus includes a high-transmission electrospray ion source, a quadrupole mass filter, a bending quadrupole that deflects the ion beam and prevents neutral molecules originating in the ion source from impacting the surface, an ultrahigh vacuum (UHV) chamber for ion deposition by soft landing, and a vacuum lock system for introducing surfaces into the UHV chamber without breaking vacuum. Ex situ analysis of surfaces following soft landing of mass-selected peptide ions was performed using 15 keV Ga+ time-of-flight secondary ion mass spectrometry and grazing incidence infrared reflection-absorption spectroscopy. It is shown that these two techniques are highly complementary methods for characterization of surfaces prepared with a range of doses of mass-selected biomolecular ions. We also demonstrated that soft landing of peptide ions on surfaces can be utilized for controlled preparation of peptide films of known coverage for fundamental studies of matrix effects in SIMS.

Laser-initiated decomposition of single aerosol particles in the source region of a time-of-flight spectrometer, followed by vacuum ultraviolet laser ionization and mass analysis of the reaction products, reveals the identity of species formed in the early stages of condensed-phase decomposition in nitromethane and nitromethane−diethylamine mixtures. In pure nitromethane, the initial stages of decomposition are characterized by unimolecular processes. At near-threshold energies, the cleavage of the C−N bond to form CH3 and NO2 dominates the dynamics, and at higher energies we find evidence of rearrangement to methylnitrite, followed by the formation of CH3O and NO. In nitromethane seeded with 0.1% diethylamine, an additional channel is observed which produces ionic intermediates. Detection of protonated diethylamine and the nitromethane aci-anion confirm that an acid−base reaction is involved in this case. We also detect the radical anion, NO2 -, whose production is correlated with an increase in methyl radical yield compared to the results for pure nitromethane. This correlation shows that NO2 - is directly implicated in nitromethane decomposition. Furthermore, deuterium labeling studies indicate a hydrogen atom transfer reaction between the nitromethane aci-anion and neutral nitromethane that produces the weakly bound nitromethane anion, the precursor to CH3 and NO2 -. These data support and extend a sensitization mechanism proposed by Gruzdkov and Gupta [Gruzdkov, Y. A.; Gupta, Y. M. J. Phys. Chem. A 1998, 102, 2322] and discriminate against others.

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 CO2 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.