Surface-sensitive analytical methods

Summary. The development of in-situ scanning tunneling microscopy (STM) has opened new avenues of research in electrochemical surface science. By itself, this nanometer-scale structural tool cannot be regarded as a panacea for the many problems that confront researchers in the interfacial sciences. However, when employed in tandem with other surface-sensitive analytical methods, even exceedingly complex processes can be investigated. Two cases are presented here that showcase the power of in-situ STM coupled with combined electrochemical UHV techniques. 

The understanding of the interactions which take place between a material surface and the components of the biological system is an important requirement of biomaterial development. The uppermost atomic layers of a biomaterial, which present characteristic chemical structural parameters and physical properties, define the contact surface. An important contribution to biomaterial development is, therefore, made by surface-sensitive analytical methods [71] which allow the surface modifications to the biomaterial to be proved. 

X-ray photoelectron spectroscopy (XPS) is a more surface-sensitive analytical method which suppHes information not only about the type and amount of elements present but also about their oxidation state and chemical surroundings. Applying this method informational depths of approximately 10 nm can be achieved, which means about 50 atomic layers. In secondary ion mass spectroscopy (SIMS) primary ions interact with the polymer surface and the mass spectra of the formed secondary ions are obtained which give information about the chemical composition of the outermost atomic layers (approximately 1 nm in thickness). 

Surface analysis is the study of the chemistry, crystal structure, and morphology of surfaces, using a combination of various surface sensitive analytical methods. Surface sensitivity can mean different things for different applications however, a good working definition would be the uppermost 0.5 to 3 nanometers (nm) of a surface, or two to ten layers of atoms. Because surfaces are often modified with films in the tens to hundreds of nanometers range, the uppermost 100 nm can be considered the surface for some applications. Any surface thicker than 100 nm is typically considered bulk material. 

The strategy adopted in this chapter is to describe the type of information which can be obtained from a variety of analytical techniques. These techniques have been applied to a set of different industrial catalyst samples. The discussion will focus on only the qualitative results which were common for all samples. It is beyond the scope of this chapter to make a detailed analysis of the significant quantitive discrepancies found with different catalysts. It should be stressed that although such differences between the catalysts were large for bulk-sensitive properties, they were generally less pronounced in the properties of the outer (geometric) surfaces which are probed by surface-sensitive analytical methods, e.g., electron spectroscopy. 

Kolpin DW, Goolsby DA, Thurman EM. 1995. Pesticides in near-surface aquifers An assessment using highly sensitive analytical methods and tritium. J Environ Qual 24 1125-1132. 

As more sensitive analytical methods for pesticides are developed, greater care must be taken to avoid sample contamination and misidentification of residues. For example, in pesticide leaching or field dissipation studies, small amounts of surface soil coming in contact with soil core or soil pore water samples taken from further below the ground surface can sometimes lead to wildly inaccurate analytical results. This is probably the cause of isolated, high-level detections of pesticides in the lower part of the vadose zone or in groundwater in samples taken soon after application when other data (weather, soil permeability determinations and other pesticide or tracer analytical results) imply that such results are highly improbable. 

Trace impurities in noble metal nanoclusters, used for the fabrication of highly oriented arrays on crystalline bacterial surface layers on a substrate for future nanoelectronic applications, can influence the material properties.25 Reliable and sensitive analytical methods are required for fast multi-element determination of trace contaminants in small amounts of high purity platinum or palladium nanoclusters, because the physical, electrical and chemical properties of nanoelectronic arrays (thin layered systems or bulk) can be influenced by impurities due to contamination during device production25 The results of impurities in platinum or palladium nanoclusters measured directly by LA-ICP-MS are compared in Figure 9.5. As a quantification procedure, the isotope dilution technique in solution based calibration was developed as discussed in Chapter 6. 

The basic information in the study of sorption processes is the quantity of substances on the interfaces. In order to measure the sorbed quantity accurately, very sensitive analytical methods have to be applied because the typical amount of particles (atoms, ions, and molecules) on the interfaces is about I0-5 mol/m2. In the case of monolayer sorption, the sorbed quantity is within this range. As the sorbed quantity is defined as the difference between quantities of a given substance in the solution and/or in the solid before and after sorption processes (surface excess concentration, Chapter 1, Section 1.3.1), all methods suitable for the analysis of solid and liquid phases can be applied here, too. These methods have been discussed in Sections 4.1 and 4.2. In addition, radioisotopic tracer method can also be applied for the accurate measurement of the sorbed quantities. On the basis of the radiation properties of the available isotopes, gamma and beta spectroscopy can be used as an analytical method. Alpha spectroscopy may also be used, if needed however, it necessitates more complicated techniques and sample preparation due to the significant absorption of alpha radiation. The sensitivity of radioisotopic labeling depends on the half-life of the isotopes. With isotopes having medium half-time (days-years), 10 14-10-10 mol can be measured easily. 

Dyno has contributed to the development of a method, named the Bell method, for the quantitative determination of the formaldehyde emission from a panel surface ( 5). A glass flask or bell having a plane flange is placed on the surface to be measured. A tight sealing between the flange of the bell and the panel surface is very important. The air can be kept in circulation by means of a membrane pump, pumping about 2 liters per minute in a closed loop, which also contains a gas burette. After a predetermined time the formaldehyde concentration of the air in the gas burette is determined by a sensitive analytical method. 

Due to a very low capacity of the glass surface, traditional analytical tools cannot be used for adequate assessment of fidelity of the light-directed synthesis. A highly sensitive analytical method to characterize quality of peptides synthesized by this technology was developed [43]. This involves derivatization of the surface with an acid-cleavable Knorr linker (Fig. 16 1) followed by the attachment of a C-terminal chromophoric tag, Lys(Dabs) (Fig. 16 2). 

Surface Chemical Composition. The atomic composition of catalyst surfaces plays a decisive role for the catalyst properties. Electron and ion spectroscopies (48) are surface-sensitive analytical tools, which provide information on the atomic composition within the topmost atomic layers. The information depth, that is, the number of atomic layers contributing to the measured signal, depends on the method used. Concentration profiles can be obtained by sputter etching of the surface by ion bombardment. The application of these particle spectroscopies requires ultrahigh vacuum (UHV) conditions. 

Testing of engineered biomaterial/sensor surfaces requires appropriate and highly sensitive analytical methods. In the case of biosensor surfaces, the system can be... 

Chemical analysis of the medium may alternatively be used to follow the corrosion rate if the metal ions are not trapped on the surface but dissolve in the corrosive medium. If sensitive analytical methods are applied [atomic absorption spectroscopy (AAS) or ion coupled plasma (ICP)], an extremely small amount of ions (ppb-range) will be detected and therefore very small corrosion rates are measurable (Bendicho, 1994 Botha, 1998 Muller et al., 1990 Seo, 1995 Telegdi et al., 1994). This method is used in medical applications and the food industry. Strongly localized attack is not detected. 

ToF-SIMS is a siirface-sensitive analytical method that uses a pulsed ion beam to remove molecules from the very outermost surface of the sample. The secondary ions are removed from the uppermost monolayers on the surface and then accelerated into a flight tube their mass is determined by measuring the exact time at which they reach the detector. Three operational modes are available when using ToF-SIMS, namely surface spectrometry, surface imaging, and depth profiling. 

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