| Chemical ionization (CI) using organometallic cations coupled with laser-induced acoustic desorption (LIAD) can be used to ionize underivatized polyisobutylene oligomers (PIB) without fragmentation in the Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. CI reagents such as cyclopentadienylcobalt (CpCo+), cyclopentadienylcobalt carbonyl (CpCo(CO)+ ) and aquachloromanganese (ClMn(H2O)+) cations also ionize small aliphatic hydrocarbons, including 2,2,4,-trimethylpentene, 2,2,4,4-tetramethylpentane and 2,2,4,4,6,6,8-hexamethylnonane (PIB 226). CI agents except CpCo+ produce pseudomolecular ions with no fragmentation of the hydrocarbon. CpCo+ generally reacts by addition followed by the loss of H2 and CH4, whereas CpCo(CO)+ and ClMn(H2O)+ react largely by addition followed by the loss of the weakly bound CO and H2O ligands respectively.;All the reagent cations reacted with PIB 400, a saturated and underivatized PIB sample with MW distribution from between 250 Da and 600 Da. CpCo + reacted by adduct-ion formation and adduct-ion formation followed by single and double dehydrogenation, whereas ClMn(H2O) + reacted purely by H2O displacement. Also, upon reaction with PIB550, a PIB sample with > 60 % exo-olefin content, CpCo+ produced adduct ions and adduct ions minus H2 and CH4 as expected, while ClMn(H2O)+ reacts exclusively by H2O displacement. Both CI methods produce mass spectra with a low MW bias. The use of higher laser powers in LIAD/ClMn(H2O) + analysis of PIB550 resulted in an increase in the MW distribution by three more oligomers compared to lower laser powers. |
