Spatial location, compositional analysis

Compositional analysis involves the determination of three quantities. The most fundamental of these is the elemental identity of surface species, i.e., the atomic number of each species. It also is desirable to know, however, the chemical identities of these species. For example, is CO adsorbed as a molecule or is it dissociated into separate C and 0 complexes with the substrate. Finally, it is necessary to determine the approximate spatial location of the various chemical species. Are they "on top" an otherwise undisturbed substrate Do they reconstruct the substrate or diffuse into it, e.g., along grain boundaries Or perhaps they form localized islands or even macroscopic segregated phases at various positions across the surface. An important trend in modern compositional analysis is the increasing demand for spatial resolution laterally across the surface on a scale (d 0.1 u m = 10 A) comparable to the dimensions of modern integrated circuits (10-12). Compositional analysis is by far the most extensively used form of surface analysis and is the subject of most of the papers in this symposium as well as of numerous reviews in the literature (5-9., 13, 14). 

Whilst the qualitative analysis of filler dispersion in polymer composites poses its own difficulties, quantitative evalnation of mixing in these systems creates further challenges. Firstly, to establish the spatial location or size distribution of the additive, a statistically representative number of particles must be examined, preferably from various fields of view within the specimen. Providing there is sufficient contrast between the phases, as is discussed later, automatic image analysis techniques can be applied to rapidly assimilate and process data. Secondly, additive particles frequently have an irregular geometry and may also be exposed in a two-dimensional array at sections other than their mid-point, (i.e., only the tips of the particles may be on view). Thirdly, there is the question of how to define mixing and express this numerically. 

One vital method that can be applied to refractory sections is that of microanalysis in which small areas can be analyzed chemically after excitations with the incident electron beam. Microanalysis is normally used on polished sections while being examined by scanning electron microscopy. Analysis can be used to determine or confirm the composition of specific grains, particles, crystals, or bonds. By integrating the composition of grains or areas with spatial locations, reactions, diffusion profiles, and attack mechanisms can be evaluated particularly at interfaces. 

For the most part, sediments are also stratigraphically uniform, showing only a few percentage variation in lithologic composition. Cores from Mountain Lake, which consistently show up-core decreases in carbonate content (to about 60% that at depth), are the only exception. A number of shallow-water cores that contain a thin veneer of organic-rich sediments overlying silt and sand were also excluded from analysis. In most locations the spatial boundary between organic-rich profundal-type sediments and littoral deposits of coarse detritus or massive silt was clearly defined. 

If the purpose of sampling is the detailed description of the composition of an object, the character of the internal correlation has to be investigated. The methods of autocorrelation and/or semivariogram analysis, as described in Sections 6.6 and 4.4.2, may be useful for clarification of the internal spatial and/or temporal relationships existing within the parent population to be sampled. Geostatistical methods, e.g. kriging, enable undistorted estimation of the composition of unsampled locations in the area of investigation. 

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