SIMS2015 Session CI1-MoA: Cluster Ions

One possible approach is to use a beam of massive gas cluster ions for sputtering since the energy per atom can be varied over a large range from a few eV to a several hundred eV per atom. Originally these massive gas clusters employed were huge aggregates consisting of several thousand Ar atoms combined by Van der Waals forces [2]. However, using Ar as primary species the chemical yield enhancement effect of oxygen and caesium, being classically applied in SIMS depth profiling is lost. We therefore operated our gas cluster ion source with oxygen gas in order to generate massive oxygen clusters with a typical cluster size of 2000 O atoms per cluster.

In this study we used a beam of large oxygen cluster at different energies to sputter a set of inorganic and organic samples. We will compare the results with classical low energy O2 as well as Ar cluster sputtering. The results will be discussed with respect to secondary ion yield and depth resolution.

[1] D. Kouzminov, A. Merkulov, E. Arevaloa and H.-J. Grossmann

Surf. Interface Anal. 45 (2013) 345 – 347

[2] N. Toyoda, J. Matsuo, T. Aoki, I.Yamada, D.B. Fenner

Nucl. Instr. And Meth. In Phys Res. B 190 (2002) 860-864

Large gas cluster ion beams are often cited for their superior performance in molecular depth profiling. This characteristic is in general attributed to the cluster dissociation upon impact with the surface, at which its total primary energy is partitioned over each constituent atom in the cluster. Consequently, large cluster projectiles are intuitively expected to reduce the primary ion penetration depth, the collisional mixing, and hence to improve the depth resolution. Indeed for organic materials and devices the superiority of Ar_n⁺ cluster beams has been extensively documented, demonstrating improved depth resolution at low Ev/atom. Although one could apply the same intuitive reasoning for inorganic systems, there is so far a lack of experimental work proving that energy partitioning upon cluster impact is the only relevant phenomenon to be considered or whether the total energy of the cluster is a (more) relevant parameter as well. Moreover in order to promote high sensitivity through oxidation assisted ionization, one would be inclined to consider O_n⁺ gas cluster above Ar_n⁺ cluster beams.

In recent years, ToF-SIMS utilizes cluster ions as primary projectiles has been widely used for organic analysis. In particular, Argon gas cluster ion beam (Ar-GCIB) has been used not only as an analytical probe but also as an etching probe for depth profiling. Ar-GCIB SIMS is now widely used for the analysis of synthetic polymers and organic devices; however, there are some serious issues with its application for biological analysis.

Biological samples are crude and contain a wide variety of biomolecules. Thus, molecular ion signals, not fragment ion signals, have to be detected to determine the chemical composition. The secondary molecular ion yield is still insufficient for imaging mass spectrometry with high spatial resolution. In addition, matrix effects as well as sensitivity and detection limits can be severe for quantitative analysis.

In this study, we performed a fundamental investigation using several model samples containing organic substances. For the model samples of polymers, polystyrene and polyethylene glycol were used; as well, several kinds of organic functional materials used for OLEDs were selected as model samples for organic devices; in addition, several amino acids, lipids, peptides and compounds of these substances were prepared as samples of biological materials.

To investigate the relationship between the secondary ion yield and spatial resolution, mass images of model patterned samples were obtained. The results indicated that for almost all biomolecules, the signal intensity of the secondary ions, not beam diameter, determined the spatial resolution of the mass images. To evaluate the matrix effects in biological samples, compounds of biomolecules were measured next, and the standard curves were obtained from the secondary ion signals of the acquired mass spectra. As a result, it was suggested that the matrix effects strongly appear between two organic molecules with large difference of secondary ion yields. Moreover, the results obtained with Ar-GCIB SIMS were compared to those with Bi cluster SIMS and related techniques.

The detailed results will be shown, and the possibilities and the limitation of biological applications using Ar-GCIB SIMS will be discussed.

Argon clusters are increasingly important in the surface analysis of soft materials, polymer, organic coatings, biomaterials and bio-related surfaces[1]. There is a pressing need for reference data to allow sputter depth-profiling using clusters and the quantification of depth in SIMS and XPS.

We have used a quartz crystal microbalance to measure the total sputter yield for argon cluster ions in a number of materials important in studies of biomaterials and diagnostic devices, including polymethyl methacrylate, collagen, hydroxyapatite, borosilicate glass, soda lime glass, silicon dioxide and the native oxides on titanium and stainless steel[2]. These data fit a simple semi-empirical equation[3] very well, so that the total sputter yield can now be estimated for any of them for the entire range of cluster ion energy typical in XPS or SIMS. Very recently, for sputter measurements on soft materials we have developed a new low-frequency resonator mass sensor that extends our measurement capability to a wide range of polymeric and organic materials.

On the basis of our total sputter yield measurements, we discuss three useful ‘figures-of-merit’ for choosing the optimum cluster ion energy to use in depth profiling organic/inorganic samples.

[1] TOF-SIMS with Argon Gas Cluster Ion Beams: A Comparison with C60+, S Rabbani, A M Barber, J S Fletcher, N P Lockyer, and J C Vickerman, Anal. Chem. 83 (2011) 3793–3800.

[2] Depth profiling organic/inorganic interfaces by argon gas cluster ion beams: sputter yield data for biomaterials, in‐vitro diagnostic and implant applications, P J Cumpson, J F Portoles, A J Barlow, N Sano, M Birch, Surf. and Interface Anal. 45 (2013) 1859-1868.

[3] Accurate argon cluster-ion sputter yields: Measured yields and effect of the sputter threshold in practical depth-profiling by x-ray photoelectron spectroscopy and secondary ion mass spectrometry, P J Cumpson, J F Portoles, A J Barlow, N Sano

J. Appl. Phys 114 (2013) 124313

Relative ionization efficiencies in mixtures of peptides, lipids and other species have been measured to quantify matrix effects in ionization by massive cluster impact (MCI). The results delineate the extent to which bioimaging using MCI, and possibly other cluster impact approaches, can be expected to produce quantitative information concerning the spatial distribution of biomolecules. Systematic variation of the chemistry and concentration of various matrix species may help illuminate the ionization process, which is currently a mystery for all forms of cluster impact.

Massive gas cluster ion beams are finding increased application in ToF-SIMS analysis due to their improved molecular sputtering characteristics compared to other sources [1-3]. Previously we reported on the development and application of a water GCIB source producing (H₂O)_n where n<10,000 [4]. We demonstrated the increased secondary ion yield that this source produced relative to argon clusters of similar size (n) and energy E [4,5]. In this paper we extend this study to include a range of E/n values, comparing the secondary ion yields obtained from biomolecules using water, water-doped argon or pure argon clusters [6]. We observe different behaviour for each beam composition suggesting different mechanisms of ionisation. Whereas under Ar_n bombardment secondary ion yield falls below E/n~10 eV, under water-containing beams the yields increase sharply and peak at E/n~3 eV. The steady state secondary ion yields for the biomolecular analytes we have studied are increased by 2-3 orders of magnitude relative to the yields obtained from C₆₀ and Ar_n beams under static conditions. This demonstrates the cumulative advantages of applying novel cluster ion beams in experimental configurations which facilitate practical high dose analysis. We have performed a study using D₂O containing clusters to further probe the ionisation mechanism, demonstrating that deuterium (or hydrogen) from the D₂O- (or H₂O-) containing primary ion beams is incorporated into the secondary ions. The level of incorporation is dependent on the E/n of the ion beam and mirrors the overall secondary ion yield trends.

[1] J.S. Fletcher, J.C. Vickerman and N. Winograd, Current Opinion in Chemical Biology (2011) 15, 733–740

[2] I.S. Gilmore, J. Vac. Sci. Technol A (2013) 31, 050819

[3] C. Bich, D.Touboul and A. Brunelle, Mass Spectrom. Rev.(2014) 33, 442-451

[4] S. Sheraz (née Rabbani), A. Barber, J. S. Fletcher, N. P. Lockyer, J. C. Vickerman, Anal. Chem. (2013), 85, 5654 – 5658.

[5] S. Sheraz (née Rabbani), A. Barber, I. Berrueta Razo, J.S. Fletcher, N.P. Lockyerand J.C. Vickerman. Surf. Interface Anal., (2014) 46, 51–53

[6] S. Sheraz, I.Berrueta Razo, T.P. Kohn, N.P. Lockyer, and J.C. Vickerman Anal. Chem. (2015) 87, 2367–2374

One of the most interesting problem in the mass spectrometry is how can we analyze the large biomolecules without complicated sample preparation. Some primary ion beam projectiles enable the detection of protein molecular ions with molecular masses exceeding several thousands daltons so far. One is a massive water droplet which is reported by K. Hiraoka’s group.[1] And then, the detection of some molecular ions of protein using Ar cluster ion [2] and neutral SO₂ cluster [3] beams were reported. They are very similar in the point of using large cluster, but very different the energy region of the primary beam. In the case of water droplet, which is highly charged and thus has large kinetic energy, Hiraoka suggests that a coherent collision in the picosecond time scale excites electron then ionize. On the other hand, a neutral SO₂ cluster has very low energy about 0.8 Ev/molecule. In this case, M. Dürr suggests that the SO₂ cluster works as just like solvent of intact, transiently. However, detailed mechanisms of soft-sputtering and ionization of large biomolecules remain unknown. To elucidate the mechanism of protonation via the energetic large clusters, we have investigated the secondary ion emission from organic thin film bombarded by water, methanol and methane cluster ions, which has an intermediate energy (1~10 Ev/molecule) between the energy of the massive water droplet and the SO₂ cluster beams.

In the SIMS spectrum of aspartic acid thin film bombarded by the (D₂O)_n⁺ (n=500~1500) ion beam at 5 Kv, a proton (hydrogen or deuterium) attached intact ion appeared. It was already reported that the protonated intact ion intensity showed several tens-fold enhancement using water[4, 5] and methanol[5] cluster ion projectile, compared to Ar-GCIB. The spectrum indicated that the intact ions are attached protons during the water cluster bombardment, but the attached protons not always originated from the projectile. This result indicates that the proton exchange reaction in several picoseconds at the cluster-surface collision should dominate the intact ion formation.

[1] K. Hiraoka et al, J. Mass. Spectrom. Soc. Jpn. 58, 175 (2010).

[2] K. Mochiji, et. Al, Rapid Commun. Mass Spectrom. 23, 648 (2009).

[3] C. R. Gebhardt, et al, Angew. Chem. Int. Ed. 48, 4162 (2009).

[4] S. Rabbani, et al, Anal. Chem. 85, 5654–5658 (2013) and 87, 2367-2374 (2015).

[5] K. Moritani, et al, Data presented at the 19th International Conference on Secondary Ion Mass Spectrometry SIMS-19, Jeju, Korea, (2013)

Surface analysis techniques cover a wide range of different surface sensitive probes, however, the chemical information obtained is still limited. This is especially true when it comes to the investigation of larger molecules on surfaces, which are of growing interest for surface functionalization. Among standard surface analysis techniques, secondary ion mass spectrometry is widely used for the investigation of surface adsorbates due to its high surface sensitivity and its capability to detect molecular fragments which give information on the chemical entity adsorbed on the surface. A major disadvantage, the high degree of fragmentation during ion bombardment, has been significantly reduced when using molecular clusters as primary ions but still limits its application in various fields.

More recently, we have shown that desorption/ionization induced by neutral clusters (DINeC) is a soft and matrix-free ion source for mass spectrometry of biomolecules [1,2]. DINeC employs molecular clusters of 10³ to 10⁴ SO₂ molecules; the clusters do not only provide the energy necessary for desorption but, due to the high dipole moment of SO₂, also serve as a transient matrix in which the desorbing molecule is dissolved during the desorption process. As a consequence, desorption takes place at comparably low cluster energies (< 1 eV/molecule) and desorption takes place without fragmentation of the desorbing molecules [1].

In this contribution, we demonstrate that DINeC can give additional information on surface-adsorbed molecules when compared to standard surface analysis techniques. Using electrospray ion beam deposition as a soft deposition method [3], a variety of model systems were prepared and analyzed by means of DINeC-MS. Among others, intact angiotensin II molecules individually adsorbed on gold substrates could be analyzed down to a coverage of 10^-13 mol/cm²; a linear relationship between signal intensity and surface coverage was established over three orders of magnitude. Depending on substrate material and substrate preparation, the molecules were either physisorbed and thus the intact molecules were desorbed by means of DINeC. On the other hand, when a covalent bond was established between molecule and substrate, molecular fragments were desorbed indicating the functional group of the molecule involved in the covalent bond. Reactions of surface-adsorbed molecules such as peptide cleavage and H/D exchange on the surface could be monitored in real time.

[1] Gebhardt et al., , Angew. Chem. Int. Ed. 48, 4162 (2009).

[2] Baur et al., Rapid Comm. Mass Spectrom. 28, 290 (2014).

[3] Rauschenbach et al., ACS Nano 10, 2901 (2009).

The development of cluster primary ion sources such as Au_n⁺, Bi_n⁺, SF₅⁺, C₆₀⁺, and Ar_n⁺ has been an exciting advancement in SIMS analysis.Relative to atomic primary ion sources, cluster ion sources provide higher secondary ion yields. Furthermore, C₆₀⁺, and Ar_n⁺ impart significantly less chemical damage to the sample thus enabling molecular depth profiling. This paper will highlight the application of ToF-SIMS to study commercial materials such as contact lenses and paints.

In the first application, ToF-SIMS was used to investigate the surface composition of two commercial contact lenses. Lens material I is composed of 2-hydroxy-ethyl methacrylate (HEMA) and glycerol methacrylate. Lens material II is composed of 2-hydroxy-ethyl methacrylate (HEMA) and 2-methacryloxyethyl phosphorylcholine cross-linked with ethyleneglycol dimethacrylate. The addition of the phosphorylcholine improves biocompatibility by mimicking the polar lipids found in cell membranes; this helps maintain hydration and decrease protein deposition. The ToF-SIMS data confirm the presence of the 2-methacryloxyethyl phosphorylcholine component on the surface of Lens material II. ToF-SIMS was also used to characterize a HEMA based contact lens which had been worn for about 2 weeks. The analysis reveals the presence N-containing species, fatty acids, phosphorylcholine, and diallyldimethyl ammonium from a disinfecting solution. Ar gas cluster ion beam (Ar GCIB) depth profiling shows the N-containing species, the fatty acid, and the diallyldimethyl ammonium species decrease with sputter time and thus appear to be concentrated at the surface region.

In the second application, a combination of O₂ and Ar GCIB depth profiling was used to study the pigment volume concentration (PVC) for different interior paints. Paints with low PVC contain more binder than pigment and therefore appear glossier. In contrast, paints with high PVC will have more pigment than binder and appear flat. In this study, an O₂⁺ sputter beam was used to profile into the bulk of dried paint film. Ar GCIB sputtering was then used to remove the damaged material. ToF-SIMS analysis of the crater bottom reveals differences the paint composition.

In summary, ToF-SIMS depth profiling using the Ar GCIB is a value tool for evaluating the composition of a wide range of organic and polymeric materials.