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Description:
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Secondary ion mass spectrometry (SIMS ) is a surface analysis technique capable of providing isotopic and molecular information . SIMS uses keV projectiles to impinge upon a sample resulting in secondary ion emission from nanometric dimensions . It is well documented that secondary ion emission is enhanced using cluster projectiles compared to atomic projectiles . Previous studies of enhanced secondary ion yields with cluster projectiles have led to the present study dealing with the scope of C60 as a projectile for SIMS . The secondary ion yields (i .e . , the number of secondary ions detected per projectile impact ) from impacts of 10 -26 keV C24H12+ , C60+ , gramicidin S+ and C60F40+ projectiles were examined to compare the effectiveness of the projectiles . The [M -H] - secondary ion yields from several organic samples varied inversely with the molecular weight . Multiple ion emission decreases monotonically as a function of the number of secondary ions emitted per impact and varies with impact energy such that higher energies produce more multiple ion emission . The emission of CN - from biological samples as a function of carbon -based projectile characteristics was examined to explore the possibility of using CN - as a molecular identifier . CN - emission was found to be the product of both direct and recombination /rearrangement emission . Re -emitted projectile atoms in the form F - were found under C60F40+ bombardment . Two forms of re -emitted F - were found : One form in which F atoms retained a portion of the initial kinetic energy , and a second in which the F atoms deposited most of the initial kinetic energy into the surface before being ejected . The [M -H] - secondary ion yield of gramicidin S was increased ~ 15 times by embedding the analyte in a matrix of sinapic acid . These results show the optimum carbon based projectile for a given sample is dependent upon the signal to be monitored from the surface . The results also show CN - has potential as a molecular identifier . Additionally , the detection of re -emitted F - confirms prior predictions of re -emitted projectile atoms . |