Surface modification using a commercial triple quadrupole mass spectrometer.

We report instrumental modifications to a commercial mass spectrometer that allow surface modification experiments to be performed using low-energy (electronvolt range) mass-selected ion beams. The design of the detector housing allows placement of the surface on the ion optical axis and some distance beyond the off-axis detector. Manipulation of the potentials applied to the final lens, detector housing, conversion dynode, and electron multiplier allow the ions to pass through the detector housing and impinge upon the surface without loss of the normal mode of detector operation. Ex situ analysis of the modified surface is performed using a home-built multisector mass spectrometer. The ability to modify organic thin films is demonstrated by a number of soft landing and surface modification experiments including (i) soft landing of (CH3)2SiNCS+ ions formed from trimethylsilyl isothiocyanate upon a fluorinated self-assembled monolayer (F-SAM) surface, (ii) soft landing and dissociative soft landing of the pseudomolecular cation of triphenylpyrylium tetrafluoroborate, viz. the triphenylpyrylium cation, upon an F-SAM surface, (iii) dissociative soft landing of 35ClCH2(CH3)2SiOSi(CH3)2+ formed from 1,3-bis(chloromethyl)disiloxane upon an F-SAM surface, (iv) surface passivation by reaction of the trimethylsilyl cation, Si(CH3)3+, with a hydroxyl-terminated self-assembled monolayer (OH-SAM), and (v) transhalogenation by reaction of CCl3+ (m/z 119) with an F-SAM surface.

Fluorinated methyl cation (CF(3)(+), CFH(2)(+))-alkyl nitrile cluster ions.

Ion-molecule reactions of CF(3)(+) or CFH(2)(+) with an acetonitrile-butyronitrile mixture yield product cluster ions in which the two neutral molecules are associated with the cation. The structures of the ion-molecule product ions were investigated by collision-induced dissociation in a multi-quadrupole mass spectrometer. The cluster ions fragment by loss of one neutral alkyl nitrile. The abundance of the fragment ions shows an unexpected inverse correlation with the proton affinity of the alkyl nitrile. This result is inconsistent with the formation of a simple cation-bound dimeric cluster containing two alkyl nitrile molecules and a fluorinated methyl cation; instead, a pair of non-symmetrical dimeric complexes, separated by a large internal barrier, is indicated. This result is analogous to that observed for the putative methyl cation-bound heterodimer of acetonitrile and butyronitrile. Collision energy dependence studies and ab initio calculations suggest that the dimeric complexes are formed in potential energy wells located in an approximately symmetrical potential energy surface. Quasi-equilibrium theory calculations were used in order to obtain additional insights into the experimental data.