Fragmentation of benzylpyridinium “thermometer” ions and its effect on the accuracy of internal energy calibration

Abstract

Electrospray ionization mass spectrometry (ESI-MS) is a powerful analytical method to study biomolecules and noncovalent complexes. The prerequisite for their intact observation is soft ionization. In ESI, the internal energy of ions is primarily influenced by collisional activation in the source. The survival yield method is frequently used to probe the energy deposition in ions during the electrospray process. In the present work, we investigate the fragmentation pathways of para-substituted benzylpyridinium ions, the most widely used "thermometer ions" in the survival yield method. In addition to the C-N bond cleavage, alternative fragmentation channels were found for the compounds studied. We consider these pathways to result from intramolecular rearrangements. The effect of these additional fragments on the accuracy of the internal energy calibration is estimated for both collision-cell and in-source collision-induced dissociation (CID). Altogether, results presented suggest that a correction of the energy scale is necessary for the method based on benzylpyridinium ions to precisely quantify ion internal energies. (J Am Soc Mass Spectrom 2010, 21, 172-177) © 2010 American Society for Mass Spectrometry E lectrospray ionization mass spectrometry (ESI-MS) [1] is widely used to characterize various species, from small organic compounds [2] to large supramolecular assemblies of biopolymers [3]. In the ESI source, small charged droplets containing dissolved analyte are produced at atmospheric pressure. As the droplets migrate along a voltage gradient to the low-pressure region of the mass spectrometer, solvent evaporates, releasing unsolvated ions. Ions produced in the ion source undergo collisions with ambient gas molecules and thereby accumulate internal energy. Such collisional activation often results in fragmentation and/or rearrangement of ions [4,5]. The effect is more pronounced for weak interactions, such as those involved in noncovalent complexes, which are widely studied by ESI-MS [5]. In addition, for instruments with restricted MS/MS capabilities, so-called in-source CID is frequently the only way to obtain structural information on the parent ions. In-source CID strongly depends on instrument parameter settings and experimental conditions, thus resulting in poor reproducibility of the MS/MS spectra [5]. Internal energy deposition in ions generally affects the mass spectra. It is therefore important to control the ion internal energy for many applications, such as optimizing the molecular ion abundance, structure determination, differentiation of isomers, MS n experiments, and in the study of non-ovalent complexes [5].The survival yield method was introduced to calibrate the internal energy distribution of ions after collisional activation [6,7]. In this method, compounds with a simple and well known dissociation pattern, so-called thermometer ions, are used to probe the energy uptake due to the activation process. The survival yield is the ratio of the parent ion intensity to the sum of parent and f...

Internal Energy Deposition for Low Energy, Femtosecond Laser Vaporization and Nanospray Post-ionization Mass Spectrometry using Thermometer Ions

“…3). The fragments observed in these experiments have been observed previously [34,36,37] and presumably result from reaction pathways other than the simple loss of a neutral pyridine including intramolecular rearrangements [37] and rearrangement to the tropylium cation [28,38,39]. The peak at m/z 80.1 is assigned to [pyridine+H + ] + .…”

Gas-Phase Chemistry of Benzyl Cations in Dissociation ofN-Benzylammonium andN-Benzyliminium Ions Studied by Mass Spectrometry

“…In electrospray ionization mass spectrometry (ESI-MS), fragmentation of benzylated cations prefers to generate benzyl cations [7,8,[20][21][22][23][24][25][26][27][28][29][30], though a few exceptions have been reported that tropylium ion can be co-produced in the fragmentation of benzylpyridinium ions [18,31,32]. Besides the formation of benzyl cations, other interesting ions resulting from hydride transfer [24,25] or electrophilic aromatic substitution reactions [20][21][22] via the nonconventional ion/neutral complex (INC) can also be generated in the dissociation of benzylated cations.…”

What Happens to Hydrophobic Interactions during Transfer from the Solution to the Gas Phase? The Case of Electrospray-Based Soft Ionization Methods

“…This "in-source collision-induced activation" can be very strong and will lead under certain circumstances to pronounced dissociation. Even covalent bonds (~40-60 kcal mol -1 [50,51]) can be disrupted, not to appeal of relatively weak supramolecular forces, such as dispersion interactions. Low signal intensities of assembled RuCD complexes along with strong signals of fragment ions might therefore be attributed to the processes during ionization and transfer into the mass spectrometer.…”