Microanalysis small particles

Trace analysis refers to analysis for a minor component in a homogenous mixture. The definition of the term trace analysis changes from time-to-time due to the rapid development of instrumentation or the use of sophisticated monitoring and measuring devices. By the current definition of the term trace component proposed by the International Union for Pure and Applied Chemistry, the limit above which the term trace analysis can be used is 100 ppm (100pgg ). Microanalysis is a special case of trace analysis and is concerned with the analysis of a single small particle or a minor constituent in a heterogenous mixture. Based on the analyte concentration in the sample to be examined, analytical methods and techniques are classified as below (see Table 2). The choice of analytical technique for a... 

SIMS is a very surface-sensitive technique because the emitted particles originate from the uppermost one or two monolayers. The dimensions of the collision cascade are rather small and the particles are emitted within an area of a few nanometers diameter. Hence, SIMS can be used for microanalysis with very high lateral resolution (50 nm to 1 pm), provided such finely focused primary ion beams can be formed. Furthermore, SIMS is destructive in nature because particles are removed from the surface. This can be used to erode the solid in a controlled manner to obtain information on the in-depth distribution of elements.109 This dynamic SIMS mode is widely applied to analyze thin films, layer structures, and dopant profiles. To receive chemical information on the original undamaged surface, the primary ion dose density must be kept low enough (<1013 cm-2) to prevent a surface area from being hit more than once. This so-called static SIMS mode is used widely for the characterization of molecular surfaces (see Figure 3.10). 

As can be seen, the chances that a material appears inhomogeneous with respect to a trace component are greater with a lower concentration of this component and greater particle size of the material [35, 50]. Therefore, CRM producers should always state the minimum mass of material that should be taken for analysis. Modem instmmental methods now allow analysis of very small samples however, one should keep in mind that errors due to inhomogeneity can dramatically increase with a decrease in the effective mass being analyzed, and that CRMs intended for microanalysis are scarce or nonexistent [42, 43]. 

The introduction in catalysis of bimetallic formulations created an important area of application of microanalysis in transmission electron microscopy. In particular, with selective hydrogenation and postcombustion catalysts, where the metallic particle sizes are several nanometres, the STEM can be used to determine the composition particle by particle and thus confirm the success of the preparation. Figure 9.16 shows the analysis of individual particles in a bimetallic preparation. It is easy to detect the existence of genuinely bimetallic particles and others containing only platinum. It should, however, be noted that this analysis, obtained on a few nanometer sized particles, concerns only a very small quantity of the catalyst (in the present case approximately 10" g of metal ). As we have noted, it is dangerous to extrapolate only one result of this type to the solid as a whole. A statistical analysis of the response of a very large number of particles, in addition to a preliminary study of the chemical composition at different scales, can be used to confirm that this case indeed concerns two groups of particles. 

Dendritic particle obtained by tapping of the powdered deposit obtained with a deposition pulse of 3 ms is shown in Fig. 3.13a. Copper dendrites are constmcted from comcob-like elements. Microanalysis of comcob-like elements revealed that they are composed of small agglomerates of copper grains (Fig. 3.13b). 

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. 

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