FAB mass spectrometry of Au25(SR)18 nanoparticles

The molecular ion of the nanoparticle Au-25(SCH2-CH2Ph)(18) (A(25)(SR)(18)) is observed at 7394 Da in fast atom bombardment (FAB, Xe atoms) ionization mass spectrometry using a 3-nitrobenzyl alcohol matrix. A distinctive pattern of positive fragment ions is evident in the mass interval 5225-7394 Da, where peaks are seen for successive mass losses equivalent to R2S entities. Because the Au-25(SCH2CH2Ph)(18) nanoparticle structure is crystallographically known to consist of a centered Au-13 icosahedral core surrounded by six Au-2(SR)(3) semirings, the R2S loses are proposed to represent serial rearrangements and decompositions of the semiring structures. Mass losses equivalent to R2S2 and R-2 entities also appear at the lower end of this mass interval. The most intense spectral peak, at m/z = 5246 Da, is assigned to the fragment Au25S10, from which all of the CH2CH2Ph organic units have been cleaved but from which no gold atoms have been lost. A different pattern of fragmentation is observed at lower masses, producing ions corresponding to serial losses of one gold atom and varied numbers of sulfur atoms, which continues down to a Au9S2 fragment FAB mass spectra of the Au nanoparticle are much easier to interpret than laser desorption/ionization spectra, but they show more. extensive fragmentation than do electrospray and low laser pulse intensity MALDI spectra. The loss of R2S fragmentation in FAB is distinctive and unlike that seen in the other ionization modes. The FAB spectrum for the nanoparticle Au-25(S(CH2)(9)CH3)(18) is also reported; its fragmentation parallels that for Au-25(SCH2CH2Ph)(18), implying that this nanoparticle has the same surprising stellated (staples) structure.