Ph.D Thesis

Ph.D StudentBarzilai Iris
SubjectDirect Multiphoton Ionization Analysis of Organic Compounds
in Ambient Air
DepartmentDepartment of Chemistry
Supervisor PROF. Israel Schechter


The goal of our research is to develop new analytical methods, based on multiphoton ionization (MPI) processes, for on-line screening of hazardous chemicals in ambient air. Two detection schemes of the ionization products served in this study: fast-conductivity (FC) and time-resolved ion-induced voltages.

MPI-FC was applied in three studies:

1. On-line pollution analysis. Polycyclic Aromatic hydrocarbons (PAH) polluted aerosols were on-line sampled by means of renewable water microdroplets. Useful calibration curves were obtained. Two droplet contamination regimes were clearly observed. These have been argued to be associated with either a volume uniform (i.e., a bulk type) or a surface-favored contamination.

2. Determination of aqueous solubility and surface adsorption of PAH.  The measurements were conducted on aqueuos microdroplets, as in the previous study. We found that at the onset of aqueous equilibrium solubility, XE, a sharp increase in the detected photocharges is recorded. We show that this is directly related to enhanced surface adsorption of a particular PAH material. Applicability of the method for studying the surface excess and adsorption is specified.

3. Fast-filter sampling and analysis of PAH aerosols. We propose a new method for fast screening of the PAH contaminated filters. We show that the porous morphology of glass fiber filters allows direct application of the MPI-FC technique. It has been found that the MPI-FC data represent a surface analysis, rather than a bulk integration.

Time-resolved ion-induced voltages method served to develop a sniffer, which is sensitive to organic compounds in ambient air. The MPI method was coupled to laser-induced fluorescence (LIF). A good approximation of the ion's mass was obtained by the MPI-generated time-resolved voltage signal. For fluorescent molecules, the information about the mass, together with the fluorescence spectrum and fluorescence decay curve, provide a unique, 3-dimensional dataset, which is sufficient for both identification and quantification of PAH, either individually or in simple mixtures. Thus, the proposed experimental setup is suggested for selective analysis of airborne PAH.