Direct detection and quantitation of He@C60 by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry.

In this paper, we report negative ion microelectrospray Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry of C60 samples containing approximately 1% 3He@C60 or 4He@C60. Resolving He@C60- and 4He@C60- from C60 containing 3 or 4 13C instead of 12C atoms is technically challenging, because the target species are present in low relative abundance and are very close in mass. Nevertheless, we achieve baseline resolution of 3He@C60- from 13C3(12C57-) and 4He@C60- from 13C4(12C56-) in single-scan mass spectra obtained in broadband mode without preisolation of the ions of interest. The results constitute the first direct mass spectrometric observation of endohedral helium in a fullerene sample at this (low) level of incorporation. The results also demonstrate the feasibility of determining the extent of He incorporation from the FT-ICR mass spectral peak heights. The present measurements are in agreement with those obtained by the pyrolysis method [1-3]. Although limited in sensitivity, the mass spectral method is faster and easier than pyrolysis.