The sputter profiling of inorganic materials during X-ray Photoelectron Spectroscopy (XPS) is a well established technique to investigate quantitative chemical composition variations with depth. Until recently XPS sputter depth profiling of organic materials has suffered from several seemingly intractable problems, not least the significant ion induced surface damage. Secondary Ion Mass Spectroscopy (SIMS) of organic materials using noble gas or liquid metal ion sources suffered related problems. These problems have largely been overcome for the SIMS analysis of a wide range of organic materials by the introduction of cluster ion sources such as SF₅ and C₆₀. Advantages of cluster sources include an increase in secondary ion yield over conventional mono-atomic sources and a reduction in beam induced damage of the surface.

Following on from these innovations in SIMS, cluster ion sources have been utilised for XPS sputter profiling of organics. These sources have been shown to significantly reduce ion induced surface damage of organic materials as measured by XPS thus making the XPS sputter profiling of organic materials feasible.

In this study we investigate the sputter profiling of several model polymer films using a new polyaromatic hydrocarbon (PAH) (coronene) cluster ion source. Initial results demonstrate that yield volumes per incident ion are dependent on the nature of the polymer, with approximate values of: 90 nm³ for poly(lactic-co-glycolic acid) (PLGA); 117 nm³ for polyacrylic acid (PAA); and 142 nm³ for polylactic acid (PLA). Other polymers, such as polystyrene (PS), do not appear to be sputtered by cluster sources under the currently reported conditions. Early results also indicate that various experimental parameters, such as incident beam energy, affect the sputter yield and amount of ion beam induced surface damage.

Clearly there is a need to investigate the various parameters which may influence ion yields and damage with the aim of optimising sputtering conditions for a range of organic materials. The work presented here attempts to elucidate the effects of variables such as ion incident energy; ion beam angle of incidence; sample temperature and ion mass on the sputtering performance of the aforementioned PAH ion source on several thin polymer films.

However, ToF-SIMS is not yet routinely used to solve biological problems because the data interpretation is complicated and the fundamental cluster ion/sample interaction is not fully understood. In this work, we combined ToF-SIMS with other complementary surface characterization techniques (i.e., XPS, AFM, SEM, etc.) to supplement the complex ToF-SIMS 2D and 3D datasets obtained from biological samples. For this purpose, human HeLa cells were seeded on silicon substrates and prepared according to different protocols (fixation with paraformaldehyde, cryofixation, etc.) for UHV analysis. The HeLa cells were also treated with bromine labeled small interfering RNA (siRNA) and with bromodeoxyuridine (BrdU) to provide a target for ToF-SIMS and XPS analysis. A series of ToF-SIMS experiments dealing with the different ion species (Bi₁⁺, Bi₃⁺, and C₆₀⁺), and various beam parameters (single beam, dual beam, different energies and fluences) was designed to obtain a reproducible and reliable depth profile of single HeLa cells when analyzed by the above listed techniques. XPS along with principal component analysis, for the first time has been used to image and provide quantitative elemental information about individual cells. Our results also show that AFM and SEM provide a better understanding of the effect of topography on the ToF-SIMS data.

Secondary ion mass spectrometry (SIMS) has seen improvements in spatial and depth resolution that now approach the tens of nanometer limit for even polymeric and biological samples. Laser desorption-based strategies such as matrix-assisted laser desorption ionization (MALDI) and laser desorption postionization mass spectrometry (LDPI-MS) [1,2] display spatial resolution that barely approaches one micron. Depth resolution in laser desorption methods is even worse at tens of microns and typically requires new sample sections to be prepared for each depth. Nevertheless, MALDI-MS and LDPI-MS remain attractive because of the more detailed chemical information they often provide, especially when applied to chemically complex samples and those with higher molecular weight species. Ultrashort pulse laser irradiation is known to cause minimal laser induced damage when interacting with soft biological materials. Data will be presented evaluating the prospects for employing ultrashort laser pulses for depth profiling in mass spectrometric imaging. Ongoing experiments utilizing single photon ionization of laser desorbed neutrals via vacuum ultraviolet radiation will also be presented for detection and selectivity that are complementary to MALDI-MS. These strategies will be applied to the analysis of molecular analytes within intact bacterial biofilms grown on biomaterials surfaces.

[1] L. Hanley and R. Zimmermann, Anal. Chem., to be published June 2009.

[2] G.L. Gasper, R. Carlson, A. Akhmetov, J.F. Moore, and L. Hanley, Proteom. 8 (2008) 3816.

Soft materials, such as organic or biological materials are of interest since last decade, because of their structural, functional and environmental flexibility. However, very few techniques are available for soft material analysis, because energetic probes destroy structure and change chemical states of soft materials during the analysis. SIMS analysis and molecular depth profiling of soft materials with polyatomic and cluster ions have been demonstrated recently. The multiple collisions and high-density energy deposition of these ions on solid surfaces induce enhancement of sputtering and secondary ion yields, as well reduced residual surface damage compared with other techniques.

We have demonstrated that molecular depth profiling with large Ar cluster ions is possible for poly-carbonate (PC) and poly-styrene (PS), which is difficult to analyze with C₆₀ ion beam. These results indicate that extreme low energy beam is necessary for molecular depth profiling. In case of large Ar cluster beam, the kinetic energy of a few eV/atom, which is comparable to the bonding energy of molecules, is realized. In addition to this, no Ar remains on the surface, because of its low binding energy. Therefore, Large Ar cluster ion beam irradiation rarely leads to damage accumulation on the surface of the polymers, and these characteristics as etching beam are also suitable for other depth profiling techniques. The surface chemical states of the polymers were measured with X-ray photoelectron spectrometry (XPS) before and after etching. The chemical states of the poly m ethyl methacry late (PMMA) sample etched with Ar atomic ion beams differed significantly from those of the unirradiated sample, whereas the chemical states were maintained even after etching with large Ar cluster ion beams. According to the detail analysis of C1s and O1s spectra, atomic composition and chemical state are very close to the ideal values.

Atomistic mechanism of energetic cluster impacts and prospect for this technique will be discussed in conjunction with possible applications.

We show that these samples possess an overlayer of almost pure poly(lactide), which fortuitously allows the direct comparison of different samples in terms of secondary ion yield behaviour. By comparing between data obtained from samples with different concentrations, it is found that secondary ion intensities do not scale linearly with composition. However, it is possible to relate secondary ion intensities to local concentrations for a binary system. The dependence of secondary ion yield on composition is described in terms of a model based on the kinetically limited transfer of charge between secondary ions and secondary neutrals. In this case, secondary ions from codeine, which contain a basic amine group, have an enhanced yield when dilute in poly(lactide). The enhancement diminishes with increasing concentration. The suppression in secondary ion yield for ions arising from poly(lactide) has the same functional form as the enhancement for secondary ions from codeine. The model is found to describe the data very well and has some wider implications for the interpretation of SIMS data from organic systems.

[1] A.G. Shard, F.M. Green, P.J. Brewer, M.P. Seah, I.S. Gilmore, Journal of Physical Chemistry B 112 (2008) 2596-2605.