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

Abstract

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.

Nanodomain analysis with cluster‐SIMS: application to the characterization of macromolecular brush architecture

“…A custom‐built secondary ion mass spectrometer equipped with a gold‐liquid metal ion source was used in this study. The instrument has been described elsewhere . Briefly, the massive Au clusters, Au 400 4+ , were mass‐selected by a Wien filter installed on a 120‐kV platform and pulsed across a 500‐µm aperture to reduce the Au 400 4+ beam to a sequence of individual cluster ions each isolated in time and space .…”

“…The instrument has been described elsewhere. [5,12] Briefly, the massive Au clusters, Au 400 4+ , were mass-selected by a Wien filter installed on a 120-kV platform and pulsed across a 500-μm aperture to reduce the Au 400 4+ beam to a sequence of individual cluster ions each isolated in time and space. [12,13] The single Au 400 4+ hit a À10 kV biased target, resulting in a total kinetic energy of 130 qkeV.…”

“…A key feature of SIMS using massive projectiles is the abundant emission of secondary ions, SIs . For the case of Leu‐enkephalin, the impact of one gold nanoparticle, specifically Au 400 4+ at 520 keV, causes on average ~13 SIs versus ~0.8 SIs generated by the impact of one Au 3+ at 50 keV . The difference is more pronounced for the molecular ion where 1.7 ions are detected per 520‐keV Au 400 4+ impact, while less than 10 −5 ions are detected per 50‐keV Au 3 + impact .…”

“…[1][2][3][4] For the case of Leu-enkephalin, the impact of one gold nanoparticle, specifically Au 400 4+ at 520 keV, causes on average~13 SIs versus~0.8 SIs generated by the impact of one Au 3+ at 50 keV. [5] The difference is more pronounced for the molecular ion where 1.7 ions are detected per 520-keV Au 400 4+ impact, while less than 10 À5 ions are detected per 50-keV Au 3 + impact. [6] At the level of a single Au 400 4+ impact, the ejecta originate from molecules co-localized within a surface volume of 10-20 nm in diameter and up to 10 nm in depth.…”

Mass Spectrometry of Nanoparticles is Different

“…One projectile impact at a velocity of a few tens of km/s, or energies≥keV per projectile atom, can generate notable SI emission [18,19]. The critical criterion for projectile selection is the probability of emitting analyte-specific ions.…”