Molecular Depth Profiling of Histamine in Ice Using a Buckminsterfullerene Probe

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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...

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confidence: 99%

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

Cheng

Kozole

Hengstebeck

et al. 2007

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“…These are the gold [40] and bismuth [41] cluster guns, and the C60ϩ gun developed by Vickerman and workers [42,43]. The enhanced sputtering by the latter polyatomic primary ion has been explicitly demonstrated: using a quartz-crystal microbalance it was estimated that, on average, each impact of C60 ions on a frozen sample sputtered Ϸ2400 water molecules [44].…”

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confidence: 99%

Higher sensitivity secondary ion mass spectrometry of biological molecules for high resolution, chemically specific imaging

McDonnell

Heeren

Lange

et al. 2006

To expand the role of high spatial resolution secondary ion mass spectrometry (SIMS) in biological studies, numerous developments have been reported in recent years for enhancing the molecular ion yield of high mass molecules. These include both surface modification, including matrix-enhanced SIMS and metal-assisted SIMS, and polyatomic primary ions. Using rat brain tissue sections and a bismuth primary ion gun able to produce atomic and polyatomic primary ions, we report here how the sensitivity enhancements provided by these developments are additive. Combined surface modification and polyatomic primary ions provided Ϸ15.8 times more signal than using atomic primary ions on the raw sample, whereas surface modification and polyatomic primary ions yield Ϸ3.8 and Ϸ8.4 times more signal. This higher sensitivity is used to generate chemically specific images of higher mass biomolecules using a single molecular ion

“…For example, these sources can be focused onto the sample with a probe size about 1 micron, allowing greatly improved molecule-specific imaging experiments. The high secondary ion yield associated with the cluster/solid interaction also allows for molecular depth profiling studies without the accompanying damage accumulation normally associated with atomic bombardment [7][8][9][10][11][12][13][14][15][16][17].…”

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confidence: 99%

Energetic ion bombardment of Ag surfaces by C₆₀⁺ and Ga⁺ projectiles

Sun

Szakal

Winograd

et al. 2005

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The ion bombardment-induced release of particles from a metal surface is investigated using energetic fullerene cluster ions as projectiles. The total sputter yield as well as partial yields of neutral and charged monomers and clusters leaving the surface are measured and compared with corresponding data obtained with atomic projectile ions of similar impact kinetic energy. It is found that all yields are enhanced by about one order of magnitude under bombardment with the C 60 ϩ cluster projectiles compared with Ga ϩ ions. In contrast, the electronic excitation processes determining the secondary ion formation probability are unaffected. The kinetic energy spectra of sputtered particles exhibit characteristic differences which reflect the largely different nature of the sputtering process for both types of projectiles. In particular, it is found that under C 60 ϩ impact (1) the energy spectrum of sputtered atoms peaks at significantly lower kinetic energies than for Ga ϩ bombardment and (2) the velocity spectra of monomers and dimers are virtually identical, a finding which is in pronounced contrast to all published data obtained for atomic projectiles. T he elucidation of the mechanism of interaction of energetic polyatomic or cluster ions with surfaces is of current interest since these projectiles are now being extensively employed as desorption probes in secondary ion mass spectrometry (SIMS) experiments [1,2]. Because of their propensity to produce higher molecular ion signals than corresponding atomic ions and the emergence of commercially available C 60 ϩ [3,4] and Au 3 ϩ [5, 6] ion guns, applications have expanded dramatically. For example, these sources can be focused onto the sample with a probe size about 1 micron, allowing greatly improved molecule-specific imaging experiments. The high secondary ion yield associated with the cluster/solid interaction also allows for molecular depth profiling studies without the accompanying damage accumulation normally associated with atomic bombardment [7][8][9][10][11][12][13][14][15][16][17].The mechanisms associated with the observed secondary ion yield enhancement are not yet clear, although details are emerging quickly. A basic question that must be resolved is to determine whether the yield enhancement occurs as a consequence of increased ionization efficiency, or is due to more effective sputtering in the neutral desorption channel. There is mixed information about this point. For In and Ag surfaces, for example, it has been shown that the enhanced secondary ion yield under SF m ϩ bombardment (m ϭ 1-5) largely arises from the enhanced ionization efficiency attributable to implanted F atoms [18]. For organic systems, however, very high removal rates of neutral species have been reported [2], obviating the need to invoke enhancement of the secondary ionization probability. Most recently, yields and velocity distributions were measured for In and In 2 sputtered from In surfaces bombarded with Au ϩ , Au 2 ϩ , and Au 3 ϩ [19]. Although these experiments do not ad...