Multiple Personalities of Gaseous Ions Generated under Mass Spectrometric Conditions
For mass spectrometric investigations, neutral molecules are converted to gaseous ions. Product-ion mass spectra are then recorded by determining the mass-to-charge ratios and intensities of fragment ions generated by activating a mass-selected ion. Such data are compiled as libraries and used for identification of compounds by Artificial Intelligence (AI) methods. Moreover, textbooks provide rules for identification of compounds by interpreting fragmentation spectra. Generally, most interpretation attempts start by presuming a specific structure for the precursor ion. However, recent advances in ion-mobility and other methods demonstrate that an ensemble of tautomeric ions protonated, or deprotonated, at different loci are produced upon ionization of polyfunctional molecules. Moreover, thermodynamically less favored tautomers are often found to be the predominant forms in the ion ensemble. Several models have been proposed to rationalize such observations. One model proposed for electrospray ionization stipulates that protons are relocated within the last solvation shell of the desolvation process. Aprotic solvents are said to cause kinetic trapping and prevent transformation to the stable form. However, a survey using many ionization methods and model compounds demonstrates the nature of vapors in the ion source, and not the spray solvent, is the key factor that determines the tautomer ratio. Because fragmentation spectra of individual tautomers are often different from each other, the spectra recorded without separating the isomeric mixtures are composites. Although large collections of spectra are available as libraries, the time has come to for us query of the quality of these compilations.
Hosted by Professor Gene Hall
~Coffee/tea will be served prior to the lecture~