Organic depth profiling of a binary system: the compositional effect on secondary ion yield and a model for charge transfer during secondary ion emission.

In recent years, it has been demonstrated that cluster ion beams may be used to sputter some materials, particularly organic materials, without the significant accumulation of damage. It is therefore possible to use cluster ion beam sputtering in conjunction with a surface analytical technique, such as SIMS, to obtain depth profiles and three-dimensional images of the distribution of organic species in the near-surface region. For SIMS organic depth profiling to be useful as an analytical tool, it is important that it is able to measure physically meaningful quantities, such as the local concentration of a species within a blend. In this paper, we investigate a model system of a miscible binary mixture of codeine and poly(lactide). We show that there is a strong surface enrichment of poly(lactide), which provides a reference signal and permits the direct comparison of different samples in terms of secondary ion yield behavior. We demonstrate that it is possible to relate secondary ion intensities to local concentrations for a binary system and that there is a direct correspondence between the yield enhancement of one component and the yield suppression of the other. The dependence of secondary ion yield on composition is described using a model of the kinetically limited transfer of charge between secondary ions and secondary neutrals. Application of the model to pure materials under the assumption that only highly fragmented secondary ions are initially produced and interact with unfragmented secondary neutrals leads to the prediction that high molecular mass quasi-molecular ions have intensities proportional to the square of the total secondary ion yield. This relationship has been independently observed in other work (Seah, M. P. Surf. Interface Anal. 2007, 39, 634.).

Quantitative XPS depth profiling of codeine loaded poly(l-lactic acid) films using a coronene ion sputter source.

The controlled release of active pharmaceutical ingredients from polymers over prolonged periods of time is vital for the function of drug eluting stents and other drug loaded delivery devices. Characterisation of the drug distribution in polymers allows the in vitro and in vivo performance to be rationalised. We present the first X-ray photoelectron spectroscopy (XPS) depth profiling study of such a drug eluting stent system for which we employ a novel coronene ion sputter source. The rationale for this is to ascertain quantitative atomic concentration data through the thickness of flat films containing codeine and poly(l-lactic acid) (PLA) as a model of a drug loaded polymer device. A range of films of thickness of up to 96 nm are spun cast from chloroform onto Piranha cleaned silicon wafers. Ellipsometry of the films is undertaken prior to depth profiling to determine the total film thickness and provide a measure of the relative loading of drug within the PLA matrix through spectroscopic analysis. Progressive XPS analysis of the bottom of the sputter crater with sputter time indicated codeine to be depleted from the surface and segregated to the bulk of the polymer films by comparison with a uniform distribution calculated from the bulk loading. This serves to illustrate that surface depletion of drug occurs, which poses important implications for drug loaded polymer delivery systems.