The formation of oligomeric molecules, an important step in secondary organic aerosol production, is reported. Aerosols were produced by the reaction of alpha-pinene and ozone in the presence of acid seed aerosol and characterized by exact mass measurements and tandem mass spectrometry. Oligomeric products between 200 and 900 u were detected with both electrospray ionization and matrix-assisted laser desorption ionization. The exact masses and dissociation products of these ions were consistent with various combinations of the known primary products of this reaction ("monomers") with and/or without the expected acid-catalyzed decomposition products of the monomers. Oligomers as large as tetramers were detected. Both aldol condensations and gem-diol reactions are suggested as possible pathways for oligomer formation. Exact mass measurements also revealed reaction products that cannot be explained by simple oligomerization of monomers and monomer decomposition products, suggesting the existence of complex reaction channels. Chemical reactions leading to oligomer formation provide a reasonable answer to a difficult problem associated with secondary organic aerosol production in the atmosphere. It is unlikely that monomers alone play an important role in the formation and growth of nuclei in the atmosphere as their Kelvin vapor pressures are too high for them to significantly partition into the particle phase. Polymerization provides a mechanism by which partitioning to the particle phase becomes favored.

The properties and relative ionization efficiencies of a series of polystyrenes (PS) with hydroxyl, hydrogen, tertiary amine, and quaternary amine end-functionalities were examined by matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS). The hydrogen and hydroxyl functionalized PS ionized via attachment of a single Ag ϩ cation, as expected. However, tertiary amine PS oligomers were found in (M Ϫ H) ϩ and (M ϩ H) ϩ forms, in addition to M ϩ Ag ϩ , while the quaternary amine PS oligomers only ionized to the M ϩ form. Analysis of tertiary amine PS spectra revealed varying ratios of the three ionic forms depending on the oligomer length, pointing to a change in ionization efficiency. When the bulk samples were compared, the quaternary amine end-functionalized PS gave a ten-fold higher ionization efficiency over all others studied, likely because of the preexisting charge on the functionality. Samples with hydroxyl and hydrogen functionalities had similar ionization efficiencies, with the tertiary amine slightly higher, depending on the molecular weight. Changes in molecular weight affected the relative ionization efficiencies in varying fashion depending on the end functionality, though average molecular weight measurements were largely unaffected by end-functionality. Quantification of end-functionalized polystyrenes with different ionization efficiencies was found to be possible if due care was taken. (J Am Soc Mass Spectrom 2003, 14, 648 -657)

A low molecular weight predominantly polyolefin copolymer of isobutylene and para methylstyrene (IMS) was studied using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. Average composition information derived from the spectra was skewed to higher para methylstyrene (pMS) content as compared to that obtained using multiple NMR techniques, and drifted towards lower pMS incorporation at higher oligomer lengths. Although both observations were initially attributed in total to an inability to ionize the isobutylene component, comparison with subsequent field desorption (FD) mass spectrometry results gave similar values to that obtained via MALDI, even though FD ionizes oligomers not detected by MALDI. Instead, the compositional drift observed with MALDI roughly mirrored the mass distribution, and was determined to arise from a mass bias effect in oligomer ionization and detection. Composition with respect to oligomer mass was found to be relatively constant, although similarly higher in pMS content. Comparison of experimental peaks with a Bernoullian statistical model revealed severe overrepresentation of higher pMS composition oligomers with regard to the calculated distribution. This discrepancy is attributed to preferential ionization of oligomers with greater pMS content, and likely results in the observed difference between MALDI and NMR compositions.

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Low molecular weight ethylene/carbon monoxide (E/CO) copolymers were characterized by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. Because of the nominal mass overlap between ethylene and carbon monoxide monomers, the normal copolymer oligomer distribution cannot be observed by time-of-flight (TOF) mass analysis, and no composition or microstructure information can be obtained. Chemical reduction of the carbon monoxide to a hydroxyl permits observation of an ECO copolymer distribution by increasing the mass of each oligomer in proportion to the carbon monoxide content. Unfortunately, the poor ionization efficiency of the fully reduced copolymer precludes quantitative composition and microstructure measurements. MALDI in combination with Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry permits oligomers having the same total number of monomers but different amounts of ethylene and carbon monoxide to be mass resolved. The discrete oligomer distribution obtained by MALDI-FT-ICR allows E/CO composition and microstructure to be determined. A first-order Markovian model was fit to the experimental data. Oligomers were found to have an alternating sequence of single carbon monoxide monomers between longer lengths of ethylene monomers.