Surface-Specific Chemical Analysis

Chemical analysis may he applied to a material in hulk usually to determine if it has met product specifications. Chemical analysis can also he conducted on individual phases in a material, deposits on a surface, or wear particles. Most of the chemical analysis techniques are used to identify or quantify elements, ions, or functional groups, ft is also very useful in many cases to identify and quantify compounds. 

Specific gravity ig.loss Specific surface area Chemical analysis (%) insol. Si02 A1203 Fe203 CaO MgO S03... 

Apart from these four main techniques there are many other techniques but their applicability is often limited to some elements. MAS-NMR technique turns out to be very useful for determining if non zero nuclear spin elements are incorporated into the lattice because of its sensitivity to local environment symmetry. Also in favorable cases techniques as Mossbauer, diffuse reflectance UV-Vis and ESR spectroscopies are very helpful while high resolution electron micrographs and simulated imaging are of great interest (38) with in addition the possibility of specific chemical analysis at nanometer scale by EDX-STEM. At last XPS may be informative to determine if surface and bulk compositions are identical or not, although the technique by definition is only sensitive to the first top layers (1 to 2 nm) while zeolitic features concern the whole material. Some examples will be given below. 

In addition to data obtained using the spectral mode of analysis, it is often important to know the location of a particular chemical group or compound on the sample surface. Such information is achieved by static SIMS chemical mapping—a procedure in which a specific chemical functionality on the material is imaged, providing information as to its lateral distribution on the surface. 

Alkaline earth oxides (AEO = MgO, CaO, and SrO) doped with 5 mol% Nd203 have been synthesised either by evaporation of nitrate solutions and decomposition, or by sol-gel method. The samples have been characterised by chemical analysis, specific surface area measurement, XRD, CO2-TPD, and FTIR spectroscopy. Their catalytic properties in propane oxidative dehydrogenation have been studied. According to detailed XRD analyses, solid solution formation took place, leading to structural defects which were agglomerated or dispersed, their relative amounts depending on the preparation procedure and on the alkaline-earth ion size match with Nd3+. Relationships between catalyst synthesis conditions, lattice defects, basicity of the solids and catalytic performance are discussed. 

Chemical force microscopy (CFM) [26] is a progression from the physicochemical based detection of LFM to specific chemical detection. CFM performs the nanoscale chemical analysis of the sample, through the measurement of forces related to specific chemical interaction between a chemically functionalised tip (e.g., with carbon nanotubes or oligonucleotides) and a surface that is chemically functionalised with complementary (or non-complementary) chemical species, e.g., complementary oligonucleotides. 

Although not capable of the micrometer-sized lateral resolutions available with the aforementioned techniques, the surface spectroscopy, electron spectroscopy for chemical analysis (ESCA), also deserves mention. The ESCA experiment involves the use of X-rays rather than electrons to eject core electrons (photoelectrons), and it has comparable surface specificity and sensitivity to that of Auger electron spectroscopy (AES) (25, 26, 29). The principal advantage of ESCA relative to AES is that small... 

Then we discuss the principles involved in the measurements of surface-specific physical quantities. Since each of the many techniques of surface analysis is sensitive to a few particular aspects of the surface (such as relative atomic positions, electronic levels, chemical composition, binding energies and vibration frequencies), we classify these techniques according to the surface characteristic that they are most sensitive to. 

The specific surface areas were determined by means of nitrogen adsorption and the metallic surface areas by using adsorption of 3-methylthiophen in liquid phase (ref. 9). The bulk composition of each sample was determined by chemical analysis and expressed by the atomic ratios Al/Ni and M/Ni. The catalysts were observed by transmission electron microscopy (JE0L 200 CX-TEM) and analysed either globally or at point level with a lateral resolution of 1.5 nm by means of a STEM (VG - HB 501) connected to an energy - dispersive X-ray analyser (EDAX). 

The XPS mechanism, which can be used for quantitative and qualitative chemical analysis of surfaces, is based on the photoelectric effect. A monochromatic soft Mg or Al anode X-ray source is used to irradiate the surface. The absorbed X-rays ionize die core shell, and in response, the atom creates a photoelectron that is transported to the surface and escapes. The ionization potential of a photoelectron that must be overcome to escape into vacuum is the binding energy (BE) plus the work function of the material. The emitted photoelectrons have a remaining kinetic energy (KE), which is measured by using an electron analyzer. Individual elements can be identified on the basis of their BE. The resulting XP spectrum is a characteristic set of peaks for a specific element, with BE as the abscissa and counts per unit time as... 

The specific surface areas, Sg, of the carbides were determined chromatographically from the thermal desorption of nitrogen. The phase composition of the samples was checked by X-ray and electron diffraction analyses. X-ray analysis was carried out using an URS-55a X-ray unit with CuKa (Ni-filtered) radiation. Electron diffraction analysis was performed using an EG-100A diffractometer unit. The chemical composition of the carbides samples was determined by chemical analysis. The results obtained are summarized in Table 16.1. 

This review gives a brief summary of the "types of chemically modified electrodes, their fabrication, and some examples of their uses. One especially promising area of application is that of selective chemical analysis. In general, the approach used is to attach to the electrode surface electrochemically reactive molecules which have electrocatalytic activity toward specific substrates or analytes. In addition, the incorporation of biochemical systems should greatly extend the usefulness of these devices for analytical purposes. 

Consideration of Surface Analysis Concerns. The researchers in this study used a wide range of surface and other tools, taking appropriate advantage of the strengths of the various methods. Previous work had shown that AES and XPS could be used to study chromate films without unreasonable problems and provided a basis for the current study. XPS was used to obtain specific chemical information while AES was used whenever spatial resolution and electron imaging were desired. RBS and electron microprobe work was used to analyze composition structures of thicker layers. 

In addition to the Drosophila sex pheromone CHCs mentioned above, it is well known that CHCs are used by ants for various chemical communications nestmate recognition, caste discrimination, etc. (see related chapters). In various ant species, chemical analysis of the body surface materials suggested that the colony-specific blends of a multi-component CHC mixture act as the nestmate-discriminative pheromone (Bonavita-Cougourdan et al., 1987 Yamaoka, 1990 Howard, 1993 Vander Meer and Morel, 1998 Howard and Blomquist, 2005). In a Japanese carpenter ant, Camponotus japonicus, the CHC pheromone, consisting of 18 CHC components of 20-40 carbons, is used as a chemical cue for nestmate and non-nestmate discrimination (Yamaoka, 1990). Because of the antennation behavior for inspecting encountered ants, the CHC-sensitive sensillum was expected to be discovered on the antenna. 

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