Is proton cationization promoted by polyatomic primary ion bombardment during time‐of‐flight secondary ion mass spectrometry analysis of frozen aqueous solutions?

The sputtering properties of two representative cluster ion beams in secondary ion mass spectrometry (SIMS), C 60 ϩ and Au 3 ϩ , have been directly compared. Organic thin films consisting of trehalose and dipalmitoylphosphatidylcholine (DPPC) are employed as prototypical targets. The strategy is to make direct comparison of the response of a molecular solid to each type of the bombarding cluster by overlapping the two ion beams onto the same area of the sample surface. The ion beams alternately erode the sample while keeping the same projectile for spectral acquisition. The results from these experiments are important to further optimize the use of cluster projectiles for SIMS molecular depth profiling experiments. For example, Au 3 ϩ bombardment is found to induce more chemical damage as well as Au implantation when compared with C 60 ϩ . Moreover, C 60 ϩ is found to be able to remove the damage and the implanted Au effectively. Discussions are also presented on strategies of enhancing sensitivity for imaging applications with cluster SIMS. (J Am Soc Mass Spectrom 2007, 18, 406 -412) © 2007 American Society for Mass Spectrometry E nergetic cluster ion bombardment and secondary ion mass spectrometry (SIMS) experiments have opened new applications in molecular depth profiling and 3D chemical imaging [1][2][3]. With this approach, it has been reported for many systems, that cluster projectiles remove large amounts of material from molecular solids without the damage accumulation associated with atomic projectiles [4 -12]. Moreover, erosion of the material occurs without significant interlayer mixing and/or the formation of topographical features, allowing depth resolution of 10 to 30 nm to be achieved [8 -10]. These attributes have compelled a majority of SIMS workers to quickly adopt this new technology.Although many different cluster projectiles have been examined, practical considerations have led to wide adaptation of liquid metal ion sources consisting of Au or Bi projectiles (Au 3 ϩ and Bi 3 ϩ , respectively) [13][14][15][16], and gas ion sources consisting of SF 5 ϩ or C 60 ϩ projectiles [6,17,18]. The physics of the ion/solid interaction is likely to be quite different between the carbon and metal-type sources since at 20 keV incident energy, each carbon atom carries 333 eV of kinetic energy while each metal atom in a trimer carries 6667 eV of kinetic energy. Moreover, there is a large mass variation between the atoms comprising these cluster species.The imaging capability of the two types of sources is quite different. The liquid metal ion gun (LMIG) technology yields very bright ion beams with a probe size of less than 100 nm in diameter [19]. The gas ion source requires apertures to define the beam size, sacrificing beam current. Recently, a C 60 source has been introduced [20] that achieves a submicron probe size with enough current to acquire images in a reasonable amount of time. In general, however, higher lateral resolution is achieved with the LMIG design if there is enough sensitivity in the mass...

mentioning

confidence: 90%

Direct comparison of Au₃⁺ and C₆₀⁺ cluster projectiles in SIMS molecular depth profiling

Cheng

Kozole

Hengstebeck

et al. 2007

“…Even the sputtering of diamond is enhanced by H 2 O injection. Finally, the water ice present upon sample freezing of biological materials for SIMS analysis is also believed to be an efficient source of protons for cationization [25][26][27]. Therefore, flooding the analyzed surface with H 2 O gas appears to be a promising approach to enhance the molecular ion yields in organic SIMS.…”

mentioning

confidence: 99%

Organic secondary ion mass spectrometry: Signal enhancement by water vapor injection

Mouhib

Delcorte

Poleunis

et al. 2010

The enhancement of the static secondary ion mass spectrometry (SIMS) signals resulting from the injection, closely to the sample surface, of H 2 O vapor at relatively high-pressure, was investigated for a set of organic materials. While the ion signals are generally improved with increasing H 2 O pressure upon 12 keV Ga ϩ bombardment, a specific enhancement of the protonated ion intensity is clearly demonstrated in each case. For instance, the presence of H 2 O vapor induces an enhancement by one order of magnitude of the [M ϩ H] ϩ static SIMS intensity for the antioxidant Irgafos 168 and a ϳ1.5-fold increase for polymers such as poly(vinyl pyrrolidone).

“…With a high abundance, and a gas-phase basicity of 660 kJ/mol, analyzing cells and tissue in a frozen hydrated form would ensure an abundant amount of matrix molecules that could be an important source of protons, without having to disrupt the native chemistry of the sample. Recent work from thisögroupösupportsöthisöpropositionö [43],öandöitöisöan area of ongoing interest.…”

Section: Discussionmentioning

confidence: 99%

Suppression and enhancement of secondary ion formation due to the chemical environment in static-secondary ion mass spectrometry

Jones

Lockyer

Kordys

et al. 2007

Self Cite

Through analyzing mixtures of compounds of known gas-phase basicities, the importance of this property on the secondary ions emitted from a surface under primary ion bombardment is investigated. The aim is to obtain a greater understanding of the ionization mechanisms that occur in secondary ion mass spectrometry (SIMS). The commonly used matrix assisted laser desorption/ionization (MALDI) matrix 2,4,6-trihydroxyacetophenone (THAP) and a range of low molecular weight biomolecules were used to investigate whether analyte/matrix suppression effects that have been observed in analogous MALDI experiments were also present in static-SIMS. The outcome of the experiments demonstrates that strong suppression of the quasi-molecular signal of one molecule in a mixture can occur due to the presence of the other, with the gas-phase basicity of the compounds being a good indicator of the secondary ions detected. It is also demonstrated that the suppression of the quasi-molecular ion signal of a compound in a two-component mixture can be minimized by the inclusion of a third compound of suitable gas-phase basicity. (J Am Soc Mass Spectrom 2007, 18, 1559 -1567