Characteristics of positive and negative secondary ions emitted from Au3+ and Au400+4 impacts


E. A. Schweikert, Texas A&M University, College Station, TX 77843, USA.



The current limitation for SIMS analyses is insufficient secondary ion yields, due in part to the inefficiency of traditional primary ions. Massive gold clusters are shown to be a route to significant gains in secondary ion yields relative to other commonly used projectiles. At an impact energy of 520 keV, Au400+4 is capable of generating an average of greater than ten secondary ions per projectile, with some impact events generating >100 secondary ions. The capability of this projectile for signal enhancement is further displayed through the observation of up to seven deprotonated molecular ions from a single impact on a neat target of the model pentapeptide leu-enkephalin. Positive and negative spectra of leu-enkephalin reveal two distinct emission regimes responsible for the emission of either intact molecular ions with low internal energies or small fragment species. The internal energy distribution for this projectile is measured using a series of benzylpyridinium salts and compared with the small polyatomic projectile Au3+ at 110 keV as well as distributions previously reported for electrospray ionization and fast atom bombardment. These results show that Au400+4 offers high secondary ion yields not only for small fragment ions, e.g. CN, typically observed in SIMS analyses, but also for characteristic molecular ions. For the leu-enkephalin example, the yields for each of these species are greater than unity. Copyright © 2012 John Wiley & Sons, Ltd.