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.

                          

 
 

 

Ozonolysis of Oleic Acid Particles                  

At left is a series of PERCI mass spectra monitoring the heterogeneous reaction between ozone gas and oleic acid particles.  The four anticipated major products of oleic acid ozonolysis, nonaldehyde (141), nonanoic acid (157), 9-oxononanoic acid (171) and azelaic acid (187), are detected as their [M-H]- ions and clearly identifiable from the PERCI mass spectrum. This experiment paints a clear picture of the evolution of the heterogeneous chemical reaction taking place on the particle as the oleic acid peak at 281 m/z decreases and molecular product peaks at 141, 157, 171, and 187 m/z increase with increasing O3 exposure.  

Typical MS ionization methods adapted for heterogeneous reaction studies generally suffer from limitations inherent fragmentation, which can mask intermediates and products. The disadvantages of “hard” ionization schemes are especially pronounced when attempting to analyze complex reaction branches pervade heterogeneous chemistry. In contrast to “hard” ionization schemes, PERCI-MS entails “soft” ionization in conjunction with real-time, on-line measurement of intact heterogeneous reaction products. In order to illustrate the advantages of PERCI-MS, the figure at left compares (a) PERCI and (b) electron impact mass spectra of the ozonolysis products of oleic acid. The EI spectrum was taken with direct-injection probe mass spectrometry using EI energy of 70 eV. The four anticipated major products are more clearly identifiable from the PERCI mass spectrum.

In addition to the 4 major products identified, previously unobserved products and intermediates have been detected for this system using PERCI-AMS. A close up of the higher mass region between 280-340 m/z shows the presence of minor products, having molecular weights greater than that of oleic acid, which allude to the occurrence of a complex series of products, intermediates, and mechanisms involved in this system. These product ions have been assigned as higher order oxygenates, such as trioxalanes and peroxides, in accord with the Criegee mechanism for the ozonolysis of alkenes. Detailed descriptions of these findings appear in Zahardis et al, 2005 (see below).

References

  • Zahardis, J.; LaFranchi, B. W.; Petrucci, G. A., Photoelectron Resonance Capture Ionization-Aerosol Mass Spectrometry of the Ozonolysis Products of Oleic Acid Particles: Direct Measure of Higher Molecular Weight Oxygenates. Journal of Geophysical Research 2005, 110, D08307, doi:10.1029/2004JD005336.

  • 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.