Electron impact ionization mass spectrometry of aliphatic alcohol clusters in helium nanodroplets

Electron impact ionization mass spectrometry has been applied for the first time to a range of molecular clusters inside helium nanodroplets. The clusters chosen for investigation are clusters of aliphatic alcohols. Fragmentation channels for the cluster cations in helium droplets differ significantly from previous findings in the gas phase. In addition to formation of abundant (ROH)nH+ ions, as already reported in gas phase studies, there are also important contributions from (ROH)n+ and (ROH)n−1RO+. The observation of substantial quantities of parent cluster ions, (ROH)n+, has not been reported previously and shows that helium nanodroplets, in combination with electron impact ionization, form a potentially useful source of metastable ions in the gas phase. The survival of these intact alcohol cluster ions is attributed to rapid evaporative cooling of the ions by the helium before they can react to produce (ROH)n−1H+ + RO. The major enhancement of the (ROH)n−1RO+ fragment channel when compared with gas phase cluster studies is attributed to a cage effect by the surrounding helium atoms. This favours the loss of a hydrogen atom by α-cleavage over the loss of a larger hydrocarbon fragment, although both channels are observed.