Secondary ion mass spectrometry (SIMS)

15 ToF-SIMS Study of Organic Materials in Cultural Heritage: Identification and Chemical Imaging Vincent Mazel and Pascale Richardin

Secondary ion mass spectrometry (SIMS) is a widespread analytical technique for the study of surfaces in materials science. Mostly used for elemental analyses and depth profiling, it is particularly relevant for many different fields of research including cultural heritage studies. Reviews of its use for the study of ancient glasses or metal artefacts already exist in the literature [Spoto 2000, Darque-Ceretti and Aucouturier 2004, Dowsett and Adriaens 2004, Adriens and Dowsett 2006, Anderle et al. 2006, McPhail 2006], but as only elemental information is obtained, these studies are limited to inorganic materials. Nevertheless, the introduction of time-of-flight (ToF) analysers for SIMS analyses at the beginning of the 1980’s, as well as the recent development of liquid ion sources delivering cluster projectiles now permit the analysis of organic materials with high sensitivity and selectivity. Moreover, thanks to its excellent lateral resolution (in the order of micrometers), and its minimal sample preparation, ToF-SIMS has become the reference technique for chemical imaging by mass spectrometry. Beside numerous applications for biological samples [Belu et al. 2003, Brunelle et al. 2005], studies of organic materials from cultural heritage artefacts have been developing since the beginning of the twenty-first century. In this review we show, through some selected examples, the different aspects and possibilities of ToF-SIMS analyses. First, because ToF-SIMS is not yet a classical technique for cultural heritage studies, we will begin with a brief overview of the physical principle, the instrumentation and sample preparation. The aim is not to present a complete description of the technique, since this can be found elsewhere [Belu et al. 2003, Sodhi 2004], but to give the reader an idea of the specificity and applicability of ToF-SIMS analyses.

15.1 Physical Principle, Instrumentation and Sample Preparation 15.1.1 Physical Principle SIMS involves bombarding a material surface with a primary ion beam, with a typical energy in the keV range. Ion impacts on the surface induce a so-called ‘collision cascade’ sputtering process, where the energy of the primary ions is transferred to the surface through nuclear collisions [Brunelle et al. 2005]. Part of this energy allows desorption of neutral molecules, positive or negative ions and atoms from the surface when the energy is sufficient to overcome the binding energy [Sodhi 2004]. Close to the impact point, the transferred energy is higher than the bond energy of the molecules, leading to extensive fragmentation of molecules and to desorption of atoms or small fragments. In contrast, far from the collision site, this energy decreases, leading to desorption of larger molecular fragments and even of complete molecular species [Belu et al. 2003]. The entities desorbed that are charged (positively or negatively) are called secondary ions (Figure 15.1).

Figure 15.1 Collision cascade and secondary ion production during ToF-SIMS analysis

In some publications, ToF-SIMS is also called Static-SIMS (S-SIMS) because ToFSIMS analyses are performed in static conditions. In this mode, the damage of the surface is very low. Indeed, the primary ion flux is set so that the collisions sites do not overlap with each other. A limit of 10 13 atoms cm-2 is generally allowed [Belu et al. 2003]. Under these conditions, secondary ions are emitted from the very extreme surface, i.e. from the first few nanometers, and therefore ToF-SIMS is truly a technique for the study of the ultrasurface. The secondary ion yield depends both on the energy and the nature of the primary ions. It has been demonstrated that the use of clusters instead of monoatomic ions improves the secondary yield. Classical primary ions, like Ga + and Cs+, are increasingly replaced by Au3+, Bi3+ or C60+ which has permitted great progress in organic ToF-SIMS studies [Kollmer 2004, Touboul et al. 2004, Winograd 2005]. 15.1.2 Instrumentation 15.1.2.1 Apparatus Figure 15.2 shows the schematic representation of a typical ToF-SIMS device. All the system is placed under high vacuum (typically 10 -7 torr) to avoid interactions between ions and air molecules. Primary ions are produced by a liquid metal ion gun and then focused on the sample to a spot with a typical size of less than 1 mm. After they impinge the surface, secondary ions are extracted and analysed by the ToF analyser. To synchronize the ToF analyser, the primary ion beam must be in pulsed mode. One of the specificities of ToF-SIMS is the possibility to switch easily from positive to negative ion mode by reversing the extraction potential. The mass calibration of spectra is internal, using low mass ions (H+, H2+, H3+, C+, CH+, CH2+ and CH3+ ion peaks in positive ion mode and H-, C-, CH-, CH2-, C2-, C3-, and C4- in negative ion mode) that are always present in spectra. 15.1.2.2 Charge Neutralization Organic materials are insulator materials and they can accumulate charge on their surface during the analysis. This charge build-up can lead to signal suppression. Neutralization of the surface during experiments is achieved using low energy electrons that are flooded to the surface, by a so-called ‘flood gun’ in pulsed mode.

15.1.2.3 Imaging As the primary ion beam can be focused to less than 1 mm, ToF-SIMS is well suited to chemical imaging. For this purpose, the beam is raster by electrostatic fields all over the surface, and a spectrum is recorded for each point. This allows the distribution of a specific ion all over the analysed surface to be mapped, and also to access a mass spectrum characteristic of an area by summing the spectra of each pixel of this region. Only a few minutes are usually necessary to obtain a 256 x 256 pixels image of an area of generally 500 x 500 mm2 or less.

Figure 15.2 Schematic representation of a ToF-SIMS apparatus

15.1.3 Sample Preparation SIMS is one of the only mass spectrometric techniques that allow solid samples to be analysed without any extraction of compounds or matrix addition. Generally, no specific preparation technique is required, and solid samples can directly be analysed if they are small enough to be fixed on the sample holder. In most cases, this means that the sample size must not be more than 1 cm. In practice, some precautions are necessary. The first arises directly from the physical principle of the technique. As only the first few nanometres of the surface are analysed, ToF-SIMS is greatly affected by surface pollution. A prolonged contact of the sample with the atmosphere can lead to disturbing

pollution. Poly(dimethylsiloxane)s are classical pollutants which can be detected in numerous studies [Sodhi 2004]. As a consequence, fresh prepared surfaces are recommended for ToF-SIMS analyses. The second requirement is related to chemical imaging applications. Very flat samples are required to avoid problems of depth of field. Even if this is common to every imaging technique, it is in this case coupled with surface pollution problems. Sample preparation must then lead to flat surfaces without surface pollution. For cultural heritage samples, imaging techniques are mostly used for the study of cross-sections. To prepare flat surfaces, two methods are described in the literature. Keune et al. [2005] recommended dry-polishing the surface of the sample embedded in resin with fine polishing cloths. Other studies describe the use of microtomy [Saito et al. 2008] or ultramicrotomy [Mazel et al. 2006] to cut the surface. This allows very flat surfaces to be obtained and avoids contact with possible contaminants from polishing cloths. 15.1.4 Mass Spectra Interpretation ToF-SIMS mass spectra are generally obtained for m/z ratio from1 to 1000 or 2000. However, due to a high fragmentation process, ToF-SIMS mass spectra are generally very complex and the fragmentation/ionization processes are very different compared with other mass spectrometric techniques. The main peaks are generally low mass fragments (m/z