Elina Kalenius and Pirjo Vainiotalo
Department of Chemistry, University of Joensuu, PO Box 111, 80100 Joensuu, Finland. E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

During the last few decades, solution and solid state techniques have been utilised to obtain information about the properties of supramolecular host–guest complexes. Mass spectrometric analysis of these fragile non-covalent complexes has been focused on the determination of the molecular mass of the interacting molecules and the analysis has concentrated on the characterisation of covalent compounds. Since the invention of the soft ionisation techniques [namely ESI (electospray ionisation) and MALDI (matrix-assisted laser desorption/ionisation)] and their development for mass spectrometry (MS) instruments, the area and way that MS analysis is used have greatly changed and expanded.

In particular, ESI has attained a steady position for the analysis of biomolecules, their non-covalent complexes and other rather fragile systems, which were earlier impossible to study by mass spectrometric methods. Today, MS can be employed not only for molecular weight identification purposes but also for sophisticated analyses on versatile properties of compounds. In the area of supramolecular chemistry, MS studies are becoming more and more general, although MS utilisation is still quite limited.

The main advantages of ESI-MS in the case of supramolecular complexes are its softness, sensitivity, specificity, speed and versatility. Fragile supra­molecular complexes can undergo versatile analysis with low time and material consumption. Soft ionisation techniques allow even remarkably weak non-covalent complexes to be transferred to the gas phase from a variety of solvents as intact complexes with minimal sample consumption, and as a consequence their intrinsic properties can be studied without the interference of solvent. Properties such as stoichiometry of the self-assembly, relative binding affinities, the stability of the complexes and their gas-phase properties can be revealed. In fact, mass spectrometry can be used as a complete gas-phase “laboratory” covering the most important properties of non-covalent complexes in the absence of solvation interactions. Consequently, this also enables one to determine the importance of solvation interactions present in the solution. Moreover, MS analysis provides information about the gas-phase properties of the complexes, which is rather difficult or even impossible to obtain by other methods, in particular, for applications, in which supramolecular complexes are utilised in a gas-phase environment, this information is clearly vital.

In this article we wish to highlight a few possibilities to exploit mass spectrometric analysis in supramolecular chemistry. We will give examples on competition experiments, dissociative experiments and ion–molecule reactions. Example spectra given here have been obtained by using a very sophisticated ESI-MS instrument, namely ESI-FT ICR MS (Fourier transform ion cyclotron resonance mass spectrometer). However, it must be emphasised that applications of these selected experiments also exist for other instrumental setups.

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