Electrospray ionization from aprotic solvents
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Final Report Abstract
Electrospray ionization (ESI) transfers analyte ions from solution into the gas phase, where they can be analyzed by mass spectrometry, thus giving rise to an exceptionally powerful analytical method. The ESI process depends strongly on the properties of the applied solvent. So far, chiefly protic solvents have been considered, but the interest in ESI from aprotic solvents, e.g., for the analysis of reactive organometallics, has been growing continuously. Nevertheless, the physicochemical foundations of the ESI process from aprotic solvents remain poorly understood, which renders the correct interpretation of the resulting ESI mass spectra difficult. The present project aimed at improving this unsatisfactory situation by a careful characterization of the ESI process from aprotic solvents. By highlighting possible differences between the behavior in aprotic and protic solvents, the obtained results should also contribute to an improved understanding of the ESI process from protic solvents. The project first determined the mass-dependent relative ion transmission of the employed mass spectrometer in order to correct for instrumental artifacts, which otherwise would compromise the following experiments. In these experiments, equimolar mixtures of model electrolytes A+X− (A+ = Li+, Cs+, NEt4+, NBu4+; X− = Br−, BF4−, ClO4−, BPh4−) in CH3CN, tetrahydrofuran (THF), H2O, and CH3OH were subjected to ESI. In all cases, the observed relative ESI activities correlated with the hydrophobicity of the analyte ions. This trend had previously been observed for protic solvents, in which large differences in the solvation of hydrophobic and hydrophilic ions result in their uneven partitioning in the nanodroplets produced during the ESI process, thus explaining their deviating ESI activities. As the present results demonstrate, the situation is similar in aprotic solvents. At the same time, the relative ESI activities are also influenced by ion pairing. Smaller counter-ions preferentially interact with small, less hydrophobic analyte ions and thereby further reduce their ESI activities. In contrast, larger counter-ions with a lower tendency toward ion pairing do not strongly differentiate between different analyte ions and, thus, exert a leveling effect on their ESI activities. Moreover, addditives that can bind to the counter-ions were also found to modulate the ESI activities of the analyte ions in an indirect manner. Next, it was investigated to which extent the concentration of the analyte ions increases during the ESI process from aprotic solvents and, thus, can shift aggregation equilibria. Employing Grignard reagents RMgCl in THF as model systems, these were found to afford almost exclusively trinuclear anionic complexes upon negative-ion mode ESI, whereas they formed mononuclear species in solution. From this difference, it could be deduced that the effective concentration increased by a factor of ≥ 40 in this case. Quantum-chemical calculations suggested that the trinuclear anionic complexes produced by ESI adopted open-cubic geometries and thereby strongly resembled structural motifs of Grignard reagents known from the solid state. This result implies that the ESI process does not generate arbitrary higher aggregates, but those closely related to the condensed-phase chemistry of the analyte. Moreover, the successful observation of intact organomagnesium anions demonstrated that even the most reactive and sensitive analytes are amenable to ESI mass spectrometry. Finally, efforts were undertaken to determine the internal energies of ions produced by ESI and by cryospray-ionization (CSI), a variant of the former, which actively cools the ion source to suppress the unwanted decomposition of thermolabile analytes. In the course of these studies, CSI mass spectrometry was applied to the analysis of solutions of MeMgCl in THF. This system had been analyzed previously with this method and was considered one of the prime examples for its superior performance. However, the present studies proved the assignment in the literature to be erroneous and the ions in question to result from inadvertent degradation reactions. To improve the CSI technique further, measures for cooling the sample solution and the inlet line into the ion source were implemented. With these measures in place, the stage is now set for comparing the internal energies imparted to the analyte ions by the ESI process on the one hand and by the CSI process on the other. First results indicate that ions produced by conventional ESI have effective energies in the range of 80 – 90 kJ mol^−1.
Publications
- Int. J. Mass Spectrom. 2013, 354/355, 219-228. “Counter-ion and solvent effects in electrospray ionization of solutions of alkali metal and quaternary ammonium salts”
K. Koszinowski, F. Lissy
(See online at https://doi.org/10.1016/j.ijms.2013.06.023) - J. Mass Spectrom. 2015, 50, 1393-1395. “Which cations form upon CSI or ESI of solutions of Grignard reagents?“
C. Schnegelsberg, T. D. Blümke, K. Koszinowski
(See online at https://doi.org/10.1002/jms.3710)
