Ion energization and thermalization in electrospray-ionization mass spectrometry

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Electrospray-ionization (ESI) mass spectrometry is among the most powerful and versatile methods of modern analytical chemistry. To a good deal, its utility results from the gentle nature of the ESI process, which imparts only relatively small amounts of energy into the analyte molecules. In this way, it permits the transfer of even weakly bound ions, such as biomolecules or organometallics, into the gas phase without their dissociation. Despite its prime importance, the problem of energy deposition during the ESI process and the subsequent ion-transfer stage remains incompletely understood. This lack of knowledge reflects the difficulty of measuring the energy transfer into the analyte molecules. The most versatile approach makes use of so-called thermometer ions, which fragment in a well-defined manner if their internal energies exceed their dissociation energies. Correlating the observed fractions of remaining intact thermometer ions with the dissociation energies affords a measure of the energy taken up by the ions. Until recently, the by far most common thermometer ions have been substituted benzylpyridinium ions ArCH2(NC5H4)+, which dissociate by the loss of pyridine and formation of benzyl cations ArCH2+. However, the rather high dissociation energies of these ions imply that they undergo only very little fragmentation under typical ESI conditions, thus preventing an accurate assessment of the energy uptake of the ions. To meet this problem, we have developed para-substituted benzhydrylpyridinium ions Ar2CH(NC5H4)+, which have significantly lower dissociation energies. These new thermometer ions have proven very well suited for characterizing the energetic conditions of ESI process. In the proposed work, we first seek to develop new thermometer ions with even lower dissociation energies. We will then apply these new as well as the established thermometer ions to probe ion energization and thermalization in an ESI time-of-flight mass spectrometer. Besides examining the effect of different solvents and different ESI conditions, we will also probe the influence of the ion-transfer optics. In addition, we will compare standard ESI with the variants nanospray and cryo-spray ionization. The results of this work promise to advance both our fundamental understanding of mass spectrometry and its practical applicability.