Mass spectrometer influences molecular structure

Memory effects in Electrospray ionisation

30 September 2009
Researchers of the FOM Institute for Plasma Physics Rijnhuizen have shown that the solvent used in ElectroSpray Ionisation (ESI), a commonly used ionisation method in mass spectrometry, can influence the structure of relatively simple molecules.

ESI transfers molecules from solution into the gas phase, while at the same time ionising them, so that they can be analysed in a mass spectrometer. Details of the ESI-process are, however, not well understood. It is commonly assumed that ESI-conditions do not influence the molecular structure of the resulting ionised molecule. The current study shows, that this is not always the case. Results were published in the September 25 issue of the Journal of the American Chemical Society (JACS).

ESI creates a fine mist of a solution of the sample under study, using a steel capillary at high voltage. The small, charged droplets that develop gradually evaporate, leaving the bare, ionised molecules in the gas phase. The assumption is, that the ionised molecules assume their lowest energy state in the gas phase, irrespective of the structure they had in solution.

In their JACS-article, researchers Jeffrey Steill and Jos Oomens report on an ESI-study of the deprotonated form of para-hydroxybenzoic acid (p-HBA). IR spectroscopy on the resulting anions (using the free electron laser FELIX) indicates that the starting conditions of the ESI-process – in this case, the type of solvent used – determines the site of deprotonation.

Further experiments have shown that p-HBA is not the only molecule where the ESI-conditions can influence the resulting molecular structure. Some related molecular anions show the same behaviour. Hence, counter to what has been generally assumed, even for small molecules such as those investigated here, ESI can play a decisive role in the structure formed in the gas phase.

Two isomeres of the molecule p-HBA. Both can develop during the ESI-process, even though in the gas phase, one of the isomeres has a lower energy state than the other.

Influence of a solvent on molecular structure. Comparison of experimental and calculated infrared spectra, showing that two different structures are being formed, depending on the type of solvent used in ESI.

Press release (in Dutch)

http://www.fom.nl/live/nieuws/artikel.pag?objectnumber=102010