Analysis of synthetic hydrocarbon and biogenic aerosol polymers with mass spectrometry
Mass spectrometry (MS) seems an ideal technique for determining the molecular weight (MW) distribution of polymers. Since the advent of matrix-assisted laser desorption coupled to a time-of-flight spectrometer, MS has been ubiquitous as an analytical tool for polymers. However, some polymer classes, such as saturated aliphatic polymers (e.g. polyethylene, polypropylene, polyisobutylene, etc.), have proven difficult to analyze owing either to their resistance to ionization and/or their propensity to decompose upon ionization, with subsequent loss of molecular weight information. ^ The Kenttämaa group has developed a technique called laser-induced acoustic desorption, which utilizes a laser pulse to produce an ultrasonic wave that volatilizes a polymer placed on the side of a metal foil opposite that of the laser strike. This affects the desorption of low-energy neutral molecules into the mass spectrometer which can then undergo chemical ionization by awaiting ions in the trap of a Fourier-transform ion cyclotron mass spectrometer. The use of cyclopentadienyl cobalt radical cation as the chemical ionization reagent ion enables the preservation of each oligomer’s MW information upon ionization. We have shown that this technique has a uniform efficiency as a function of the oligomer’s chain length, allowing for the easy interpretation of mass spectra. This technique has been effective for polyethylenes with number average MWs up to 650. It has also shown promise for branched polymers, polymers with functional end-groups, aromatic polymers, and model petroleum distillate compounds (especially the isoparaffinic fraction which are inaccessible by any other MS technique). In nearly all cases, the MW information of the neutral parent molecule was preserved. ^ In addition to this project, the study of atmospheric aerosols has been pursued. Atmospheric aerosols are presently poorly understood in terms of their sources, formation, and climate forcings. They have multiple effects on climate change and respiratory health. Secondary organic aerosols (SOA) have undergone an explosion in research activity, due to the recent discovery of oligomeric compounds resulting from the heterogeneous reactions of volatile organic compound (VOC) oxidation products. Previously, the SOA was thought to arise entirely simple gas-particle partitioning of VOCs. In this work, Desorption Electrospray Ionization (DESI) MS was used to study the oligomerization chemistry of the biogenic volatile organic compound, methacrolein. This system was found to undergo acid-catalyzed reactions, including hetero-Diels-Alder reaction, oxidation, reduction/oxidation, oxidative dehydrogenation, and hydrolysis. Samples of cloud water were also analyzed for using the DESI technique and were found to contain high MW species and organosulfates.^
Paul B. Shepson, Purdue University, R. Graham Cooks, Purdue University.
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