Differentiation of Isomeric Hydrocarbons by Using [ClMn(H2O)]+ Chemical Ionization and Collision-Activated Dissociation in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

Chemical ionization (CI) using the [ClMn(H2O)](+) reagent ion combined with collision-activated dissociation is demonstrated to allow the differentiation of isomeric alkanes (C8H18 and C9H2O) and alkenes (C8H16) in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The [ClMn(H2O)](+) ion reacts with each isomeric hydrocarbon molecule to produce a water replacement product [hence providing molecular weight (MW) information], with no accompanying fragmentation. Differentiation of the isomers is achieved on the basis of the different fragmentation patterns observed upon sustained off-resonance irradiation collision-activated dissociation (SORI-CAD) of the isolated water replacement products. Upon activation, the weak bond of the hydrocarbon to ClMn+ is broken. ClMn+ is the major fragment ion for linear alkanes. However, for branched alkanes and all alkenes, the leaving ClMn+ abstracts a weakly bound hydride, an alkyl anion, or an alkene from the hydrocarbon to generate diagnostic carbocation fragment ions. Because the experiment involves isolation of the ionized analytes prior to structural characterization (MS/MS), it is well-suited for direct analysis of hydrocarbon mixtures.