Soft or hard ionization of molecules in helium nanodroplets? An electron impact investigation of alcohols and ethers

Shengfu Yang,Andrew M Ellis,Scott M Brereton,Martyn D Wheeler

doi:10.1039/b511628g

Shengfu Yang, Andrew M Ellis + Show 2 more

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https://doi.org/10.1039/b511628g

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Abstract

Electron impact (70 eV) mass spectra of a series of C1-C6 alcohols encased in large superfluid liquid helium nanodroplets (approximately 60,000 helium atoms) have been recorded. The presence of helium alters the fragmentation patterns when compared with the gas phase, with some ion product channels being more strongly affected than others, most notably cleavage of the C(alpha)-H bond in the parent ion to form the corresponding oxonium ion. Parent ion intensities are also enhanced by the helium, but only for the two cyclic alcohols studied, cyclopentanol and cyclohexanol, is this effect large enough to transform the parent ion from a minor product (in the gas phase) into the most abundant ion in the helium droplet experiments. To demonstrate that these findings are not unique to alcohols, we have also investigated several ethers. The results obtained for both alcohols and ethers are difficult to explain solely by rapid cooling of the excited parent ions by the surrounding superfluid helium, although this undoubtedly takes place. A second factor also seems to be involved, a cage effect which favors hydrogen atom loss over other fragmentation channels. The set of molecules explored in this work suggest that electron impact ionization of doped helium nanodroplets does not provide a sufficiently large softening effect to be useful in analytical mass spectrometry.

Highlights

Fragmentation of molecular ions is commonplace in electron impact (EI) mass spectrometry
The helium droplet spectra were restricted to electron impact energies of 70 eV, since this is the typical impact energy employed in analytical EI mass spectrometry
The above results clearly show that electron impact ionization of helium droplets doped with alcohols and ethers gives different ion fragmentation patterns when compared with gas phase samples

Summary

Introduction

Fragmentation of molecular ions is commonplace in electron impact (EI) mass spectrometry. Alternative techniques have been developed for ionizing molecules in mass spectrometry, such as chemical ionization, matrix-assisted laser desorption ionization (MALDI) and electrospray These techniques tend to produce ions with much less fragmentation than electron impact ionization and are often referred to as ‘soft’ ionization methods. A softer form of electron impact ionization is potentially possible by attaching the analyte molecule to, or embedding the molecule within, a weakly bound cluster in the gas phase. When this cluster is ionized by EI, additional bodies are present to help absorb and remove any excess energy and if sufficiently effective this might reduce fragmentation of the analyte ion. The surrounding argon is capable of removing some of this excess energy from the parent ion, e.g. by evaporative loss of argon atoms, for many ion channels this process is too slow to be significant and so fragmentation of the organic ion is still observed and the ionization process cannot be described as soft

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