PERCI Aerosol Mass Spectrometry

The PERCI Aerosol Mass Spectrometer (AMS) consists of 4 main components: 1) Particle inlet; 2.) Particle Vaporization source; 3) PERCI source; and, 4) Time-of-Flight MS. The particle inlet is used to concentrate aerosol particles (in the air sample) with respect to the gas phase while decreasing the sample pressure from atmospheric pressure to about 10-6 mbar. Under given operating conditions, the particle inlet may also be used to deliver only particles of one given diameter to the analytical zone of the instrument. After the particles are focused, they enter the vaporization and ionization region. The particles are thermally vaporized and the molecular components ionized for mass analysis by the reflectron time-of-flight mass spectrometer. The minimal molecular fragmentation caused during ionization allows for easy identification of the components of even the most complex organic particles. The adaptation of PERCI to AMS has been applied to the heterogeneous reaction of gas phase ozone with oleic acid particles and has provided new insights into this well-studied model system.

                          

 

Vaporization and PERCI Source

 

 

 

                          

A schematic of the PERCI source is shown in Figure 1. Briefly, it consists of a low energy (sub-mJ) pulsed (10 Hz), tunable (235 – 300 nm) ultraviolet laser (Opotek Inc., Carlsbad, CA) gently focused onto the surface of a pure aluminum photocathode. The pulsed laser generates a short (7 ns) burst of photoelectrons with a measured quantum efficiency of 6 x 10-4 photoelectrons/incident photon. The photoelectron energy is nominally equal to the difference between the incident photon energy and the photocathode metal work function. Photoelectron flux densities as high as 1021 cm-2 s-1 have been obtained, resulting in sensitivities more than an order of magnitude greater than conventional photoionization methods, commonly used for particle mass spectrometry. Additional details on the PERCI source and a demonstration of its ionization capabilities for gas phase organics are described here.

In order to extend PERCI to particle analysis, efficient particle vaporization is required prior to ionization. In an earlier set-up, particle vaporization is achieved thermally by placing a resistively heated filament in the particle beam path (as shown above). By aligning the photocathode in close proximity to the filament, the molecular vapors from the particle can be efficiently ionized. Two modes of vaporization can be used: analysis can be done in real time as the particles impact the hot filament or, to improve signal to noise, after collecting for a short time on the cool filament. The firing of the laser is not synchronized with the particle arrival and subsequent vaporization.

Presently, we are adopting an IR laser for this task. IR lasers are common vaporization sources in soft ionization schemes (e.g. two-color mass spectrometry) in laboratory instruments. The main advantages of IR laser vaporization for PERCI-MS are (a) shorter vaporization time, which yields a more concentrated vapor plume that closely matches the temporal evolution of the photoelectron generation pulse, and (b) low IR photon energy, which minimizes molecular fragmentation of organic analytes.  

References

  • LaFranchi, B. W.; Zahardis, J.; Petrucci, G. A., Photoelectron resonance capture ionization mass spectrometry: A soft ionization source for mass spectrometry of particle-phase organic compounds. Rapid Communications in Mass Spectrometry 2004, 18, 2517-2521, doi:10.1002/rcm.1653.

  • LaFranchi, B. W.; Petrucci, G. A., Photoelectron resonance capture ionization (PERCI): a novel technique for the soft-ionization of organic compounds. Journal of the American Society for Mass Spectrometry 2004, 15, (3), 424-430, doi:10.1016/j.jasms.2003.11.015.