Fragmentation at and above surfaces in SIMS : effects of biomolecular yield enhancing surface modifications

Matrix-enhanced SIMS and metal-assisted SIMS are successfully employed to increase the organic ion yield in SIMS. In this study we compare kinetic energy distributions obtained for the SIMS, ME-SIMS and MetA-SIMS sputtering of molecular ions. In comparison to the SIMS kinetic energy distributions, those obtained for ME-SIMS display larger energy deficits, indicative of entrainment of analyte ions by matrix molecules or collisions taking place above the sample surface. In the case of MetA-SIMS high energy broadening of the distributions is observed, resulting from the high stopping power of the gold used. A selection of substituted benzylpyridinium salts is used to investigate the effect of internal energy reduction in ME-SIMS. Kinetic energy distributions were used to separate the daughter ions formed in the sample region from those resulting from unimolecular decay on nanosecond timescales, in the first tens of micrometers above the sample surface. The longer-timescale decay was monitored by changing the energy acceptance window of the mass spectrometer used. From the decay rate constants internal energies of the precursor ions decaying on nanosecond timescales were determined using RRKM theory. Within the framework of the precursor model the results indicate an extension of the collision cascade over a wider area than in SIMS.

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