Laser-induced acoustic desorption/Fourier transform ion cyclotron resonance mass spectrometry and its application to peptide, hydrocarbon polymer and petroleum analysis
The analytical utility of the combination of laser-induced acoustic desorption (LIAD) with gas-phase ion-molecule reactions in a Fourier-Transform ion cyclotron resonance mass spectrometer (FT-ICR) was examined for the characterization of thermally labile and/or nonvolatile molecules, such as peptides, saturated hydrocarbon polymers and petroleum. LIAD is used to desorb these analytes into the gas phase as intact neutral molecules. The neutral molecules are ionized either by chemical ionization (CI) or electron ionization (EI) to produce the desired molecular or pseudomolecular ion, or, if desired, fragment ions. ^ The LIAD/CpCo+· CI method yield pseudomolecular ions for each individual alkane oligomer for saturated hydrocarbon polymers, such as polyethylene. The mass spectra allow for easy interpretation of the polymer's molecular weight distribution. No fragmentation was observed. ^ Both the LIAD/CpCo+· CI and LIAD/EI method were employed in the analysis of two petroleum distillates (MW ∼ 350, MW ∼ 500). LIAD coupled to low (10 eV) and high (70 eV) energy EI yielded mass spectra with extensive fragmentation for both of the petroleum distillates studied. However, the LIAD/CpCo+· CI method revealed hundreds of compound without fragmentation and enabled facile interpretation of the molecular weight distributions. ^ The LIAD/CpCo+· CI method also offered an alternative way to obtain sequence information on peptides. The method yields various informative fragments ions, such as b, y and a type ions. Lastly, the LIAD/CI method was evaluated in determining thermochemical data, such as bond dissociation energies (BDE). The BDE of the O-H and S-H bonds in the side chain of the amino acids tyrosine and cysteine, respectively, were successfully determined. ^
Major Professor: Hilkka I. Kenttamaa, Purdue University.
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