Cationization and fragmentation of molecular ions sputtered from polyethylene glycol under gas cluster bombardment: An analysis by MS and MS/MS.

In secondary ion mass spectrometry (SIMS), the detection of large organic molecules is accomplished using cluster ion bombardment. Ion formation often proceeds via cationization, through the attachment of (alkali) metal ions to the molecule. To study this process and its efficiency, the emission of secondary ions sputtered from polyethylene glycol (PEG 3000) mixed with Na trifluoroacetic acid (NaTFA) was investigated for 10 keV A r 2000 + cluster irradiation by MS and MS/MS. Apart from cationized molecular ions [M n +Na] + (with repeat units n of up to ∼80), fragment species were found corresponding to the loss of a water moiety or of two H atoms, respectively. In addition, an abundant flux of doubly-charged molecular ions, [M n +2Na] 2+ was observed in the mass spectra. Quite surprisingly, even triply-charged ions [M n +3Na] 3+ were detected. The ratio of [M n +2Na] 2+ to [M n +Na] + increases with increasing size of the molecules and reaches unit for n ∼70. The yield of [M n +Na] + ions around m / z 3000 depends strongly on the molecular composition ratio of NaTFA/PEG in the samples and can be enhanced by a factor of 50 by increasing the Na content. In MS/MS operation, [M n +Na] + and [M n +2Na] 2+ ions were subjected to collisions with Ar atoms; for sufficiently high collision energies, dissociation leads to characteristic fragment species. Collisions of [M n +2Na] 2+ assert the existence of doubly-charged ions in the sputtered flux. Collision cross sections were found to amount to ∼(0.5–1)×10 −15  cm 2 for center-of-mass collision energies of ∼3 eV. [ABSTRACT FROM AUTHOR]