Compositional maps

Fig. 4. Scanning electron micrograph of 5-p.m diameter Zn powder. Neck formation from localized melting is caused by high-velocity interparticle coUisions. Similar micrographs and elemental composition maps (by Auger electron spectroscopy) of mixed metal coUisions have also been made.

Electron Probe Microanalysis, EPMA, as performed in an electron microprobe combines EDS and WDX to give quantitative compositional analysis in the reflection mode from solid surfaces together with the morphological imaging of SEM. The spatial resolution is restricted by the interaction volume below the surface, varying from about 0.2 pm to 5 pm. Flat samples are needed for the best quantitative accuracy. Compositional mapping over a 100 x 100 micron area can be done in 15 minutes for major components Z> 11), several hours for minor components, and about 10 hours for trace elements. 

Figure 6 (a) Quantitative compositional map of the distribution of zinc at the grain... 

D. E. Newbury, C. E. Fiori, R. B. Marinenko, R. L. Myklebust, C. R. Swyt, and D. S. Bright. Compositional Mapping with the Electron Probe Microanalyzer. Anal. Ghent. 1990, 62, Parti, 1159A PartII, 1245A. 

Figure 6.8. Compositional map made with an early model of the scanning electron microprobc. The pictures show the surface segregation of Ni. Cu and Sn dissolved in steel as minor constituents the two latter constituents enriched at the surface cause hot shortness (embrittlement at high temperatures), and this study was the first to demonstrate clearly the cause (Melford I960).

Various ion-optical tricks have to be used to compensate for the spread of energies of the extracted ions, which limit mass resolution unless corrected for. In the latest version of the atom probe (Cerezo et at. 1988), spatial as well as compositional information is gathered. The hole in the imaging screen is dispensed with and it is replaced by a position-sensitive screen that measures at each point on the screen the time of flight, and thus a compositional map with extremely high (virtually atomic) resolution is attained. Extremely sophisticated computer control is needed to obtain valid results. 

Modern spectrometers only require electron beam currents in the range 0.1 lOnA and hence probe sizes of 20-200 nm may be readily achieved with thermionic sources and 5-15 nm with a FEG. Spatially resolved compositional information on heterogeneous samples may be obtained by means of the Scanning Auger Microprobe (SAM), which provides compositional maps of a surface by forming an image from the Auger electrons emitted by a particular element. 

Technique Depth Depth resolution Depth profile Insulators Vacuum Composition maps Beam diameter... 

The already critical need for molecular-scale compositional mapping will increase as more complex structures are assembled. Currently, electron microscopy, scanning probe microscopy (SPM) and fluorescence resonance energy transfer (FRET) are the only methods that routinely provide nanometer resolution. 

Holt and Muenker (1972) at Esso carried out small-scale spiUs of LNG on ambient water and developed compositional maps delineating areas... 

The left vertical line is the Thom isomorphism, and the right one is given by the restriction maps. From the dehnition of the Thom isomorphism, we know that the composite map... 

Emulsilication through phase inversion is based on a change in the surfactant spontaneous curvature induced by temperature. This concept can be generalized considering any parameter influencing the spontaneous curvature of a surfactant, for example, salinity, pH, presence of a cosurfactant, and nature of the oil. The concept of inversion has often been reported in the literature by means of a formulation-composition map. In the following, we shall sum up this empirical approach which can be useful for formulators. 

Rgure 1.6. (a) Schematic formulation-composition map. SAD is the surfactant affinity difference it is positive for a lipophilic surfactant and negative for a hydrophilic one. The gray zones are abnormal, (b) Schematic representation of the proposed mechanism for a transitional inversion, (c) Schematic representation of the proposed mechanism for a catastrophic phase inversion. 

In the TLC of the toxins (lower part of Figure 5), Rf values are not presented because they show small variations and are potentially misleading. However, the relative elution sequence has proven to be highly reproducible, such that the two-dimensional composition maps produced by this method can be interpreted with a high level of confidence. 

Chemical and morphological information can be combined by scanning lines or areas of the sample with the appropriate detection system. Compositional mapping of the studied region of the sample is then obtained. 

Fig. 5.20 (a) and (b) are POSAP composition maps of Ga and As at an interface of a GalnAs/InP quantum-well heterostructurc. (c) is a composition profile showing Ga and As concentrations across the interface. The approximate position of the interface is shown by arrows in these figures. Data are from Liddle et a/.130 (Courtesy of G. D. W. Smith.)... 

Further development of this concept (Long and Speight, 1989, 1990, 1997) involved the construction of a different type of compositional map using the molecular weight distribution and the molecular type distribution as coordinates. The separation involved the use of an adsorbent such as clay, and the fractions were characterized by solubility parameter as a measure of the polarity of the molecular types. The molecular weight distribution can be determined by gel... 

The slope and position of the cut line also depend on the solubility parameter of the precipitating solvent as well as the temperature of operation. For paraffin solvents, the overall solubility parameter of the solvent is to the left of the composition map and the cut line is the right-hand wing of the solubility bell curve as it cuts through the composition map. 

---