On-line analysis of organic components in fine and ultrafine particles by photoionization aerosol mass spectrometry.

A new method, photoionization aerosol mass spectrometry (PIAMS), is described for real-time analysis of organic components in airborne particles below approximately 300 nm in diameter. Particles are focused through an aerodynamic lens assembly into the mass spectrometer where they are collected on a probe in the source region. After a sufficient amount of sample has been collected, the probe is irradiated with a pulsed infrared laser beam to vaporize organic components, which are then softly ionized with coherent vacuum ultraviolet radiation at 118 nm (10.5 eV). Since the photon energy is close to the ionization energies of most organic compounds, fragmentation is minimized. Both aliphatic and aromatic compounds of atmospheric relevance are detected and quantified in the low- to midpicogram range. The photoionization signal intensity increases linearly with the amount of material sampled and is independent of particle size. The fragmentation induced by laser desorption is greater than that observed with thermal vaporization, suggesting that the internal energy imparted by the former is greater. Although some molecular fragmentation is observed, mass spectra from common sources of ambient organic aerosol are distinguishable and consistent with previous off-line measurements by gas chromatography/mass spectrometry. These results illustrate the potential of PIAMS for molecular characterization of organic aerosols in ambient and smog chamber measurements.