Surface analysis technique selection

The purpose of this section is to show, by example, how the concerns of technique selection, potential problems, data acquisition and analysis have been applied for several different corrosion problems and techniques. Examples of fundamental research work and industrial problem solving have been included to show the range of applicability of the techniques. In most cases, more than one technique was used to solve the problem. Frequently, a surface analysis technique was used in combination with one or more other types of analysis method. These examples are not comprehensive it is hoped that sufficient references have been supplied to enable the reader to find other work of relevant interest. 

In Sect. 1.2.1 of the present chapter, we describe the most important cluster sources successfully used today. Section 1.2.2 introduces experimental techniques for mass-selecting single cluster sizes from the distribution generated by the cluster sources. In the gas phase as well as for clusters on surfaces, the densities are extremely low, thus only highly sensitive methods can be used for the characterization of the chemical and catalytic properties of the model systems. Some of the most commonly used techniques employed in gas-phase experiments are presented and discussed in Sect. 1.2.3, surface analysis techniques for cluster studies are presented in Sect. 1.2.4. 

The relatively slow rate of hydrocarbon fuel cell oxidations prompted an intensive examination of the adsorption characteristics of organic reactants in the 1960s. Because of the low potential for the development of hydrocarbon fuel cells, such studies have largely subsided today and no modern surface analysis techniques have been applied to characterize intermediates. Conventional adsorption studies of carbonaceous species have been reviewed repeatedly (7, 9-12, 100 -, therefore, we summarize here only some essential adsorption features for fuel cell and selective electrocatalytic oxidations. 

It was demonstrated with surface analysis techniques that the polymer surface is modified selectively with different laser irradiation wavelengths. The two laser energy regimes, above and below the threshold for laser ablation, reveal pronounced differences. For both irradiation wavelengths (248 and 308 nm) the polymer surface modification is solely chemical after treatment with fluences below the threshold. Each irradiation wavelength leads to a surface oxidation, as shown with the contact angle and XPS measurements. The oxidation is a result of the radical pathway of photodecomposition of the triazene chromophore. 

The first method can be applied with about 70 elements and has a detecting sensitivity in the ppm range. The activation analysis is very sensitive but also very selective. For all chemical elements, however, mass spectrometry can be applied. With samples of a few micrograms, the detecting sensitivity lies in the ppb range [103-105]. Very high detecting sensitivities can be achieved by the use of modem surface analysis techniques, some of which were discussed in Chapter 3 Section... 

It is also common for pol3rmeric compoimds to form surface regions with compositions different from the bulk material, by selective diffusion of components. This process is termed blooming when the surface component is solid, and bleeding if it is liquid. Sulfur and fatty acid blooms can inhibit adhesion in rubber laminates (3). Laser desorption mass spectroscopy has been employed to identify surface species on vulcanized rubber (4). X-ray scattering methods for the study of polymer surfaces and interfaces have been reviewed (5). Other surface analysis techniques commonly used with polymers include attenuated total reflectance (6-8), electron microprobe (9), Auger electron spectroscopy (10), x-ray photoelectron spectroscopy (11), and scanning probe microscopic methods (12). Overviews on polymer surface analysis have been published (13,14). 

The chemical, stmctural, and electronic characteristics of surfaces and interfaces are usually different from those of the bulkphase(s). Thus, methods to be used for the analysis of surfaces must be selective in response to the surface or interfacial region relative to the bulk. Surfaces and interfaces are most commonly explored using techniques based on the interaction of photons, electrons, or ions with the surface or using a force such as electric field or van der Waals attraction. These excitations generate a response involving the production of photons, electrons, ions or the alteration of a force that is then sensed in the analysis. 

Laser desorption methods (such as LD-ITMS) are indicated as cost-saving real-time techniques for the near future. In a single laser shot, the LDI technique coupled with Fourier-transform mass spectrometry (FTMS) can provide detailed chemical information on the polymeric molecular structure, and is a tool for direct determination of additives and contaminants in polymers. This offers new analytical capabilities to solve problems in research, development, engineering, production, technical support, competitor product analysis, and defect analysis. Laser desorption techniques are limited to surface analysis and do not allow quantitation, but exhibit superior analyte selectivity. 

Contamination can be present not only as a surface deposit or a surface feature but can also be located within the bulk of a manufactured part. The selection of an appropriate series of analytical techniques, applied to failure, defect, and contamination analysis projects, is influenced by the location of the contamination or defect and the optical properties of the manufactured component. Microscopic analysis of opaque parts is limited to surface analysis... 

Coupled columns packed with different stationary phases can be used to optimize the analysis time (71, 75). In this approach the different columns are connected in a series or in parallel. liie sample mixture is first fractioned on a relatively short column. Subsequently the fractions of the partially separated mixture are separated on other columns containing the same or other stationary phases in order to obtain the individual components. Columns differing in length (number of theoretical plates), adsorptive strength or phase ratio (magnitude of specific surface area), and selectivity (nature of the stationary phase) can be employed, whereas, the eluent composition remains unchanged. Identification of the individual sample components via coupled column technique requires a careful optimization of each column and precise control of each switching step. 

No single surface-analysis tool can solve every problem facing the surface scientist. For this reason, the inclusion of two or more techniques in one system is useful. If the researcher expects to attack certain problems, he can select a combination of techniques such that the strengths of one will overlap the weak-... 

Another class of techniques monitors surface vibration frequencies. High-resolution electron energy loss spectroscopy (HREELS) measures the inelastic scattering of low energy ( 5eV) electrons from surfaces. It is sensitive to the vibrational excitation of adsorbed atoms and molecules as well as surface phonons. This is particularly useful for chemisorption systems, allowing the identification of surface species. Application of normal mode analysis and selection rules can determine the point symmetry of the adsorption sites./24/ Infrarred reflectance-adsorption spectroscopy (IRRAS) is also used to study surface systems, although it is not intrinsically surface sensitive. IRRAS is less sensitive than HREELS but has much higher resolution. 

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