Elemental mapping

So far, it has been assumed that the result of microanalysis is the elemental composition of a small region of the specimen. This is obtained from the x-ray spectrum produced when the electron beam is stationary. It is often more useful to show the concentration of a specific element as a function of position on the specimen. This is elemental mapping. The map is formed by using the intensity of x-ray emission in a specific energy range to modulate the intensity on a display as the beam scans the specimen. The energy region, or window, is set to include 

Current systems allow simultaneous acquisition of maps for several different elements. Digital maps with colors assigned for each element permit a more rapid and detailed analysis. Superposition of the color maps is useful in determining associations between elements. This technique is more than simple elemental mapping and approaches more definitive compositional studies. Multiple maps may be obtained in the time previously used for mapping a single element. 

Energy dispersive x-ray spectroscopy can be conducted in the SEM STEM and AEM whereas wavelength dispersive spectroscopy is conducted only in the SEM or EPMA. For light element analysis, from boron to sodium, the WDS technique is preferred to ultrathin window EDS for polymers, so the AEM should not be used. 

If thin specimens are used in the AEM, high magnification images and diffraction information are accompanied by EDS of resolution about 10-100 nm. EDS of solid specimens in the SEM has micrometer resolution. Just as for imaging, this difference is due to the small interaction volume in thin films, where the beam does not spread out. Thin specimens also limit the need for absorption or fluorescence corrections, permitting the application of quantitative analysis techniques. 

There are three major problems with microanalysis of thin films in the AEM. 

Polymer specimens are particularly difficult to analyze in the AEM. Generally, there are small amounts of heavy elements in a polymer. These low levels are difficult to detect in a material that changes readily in the electron beam. These difficulties preclude routine quantitative analysis of polymers in either the SEM or AEM although microanalysis techniques can be applied. The major consideration for the polymer micro-scopist is that changes occur in the polymer during study. 

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Trebbia P. Quantitative elemental mapping of X-ray radiographs by ffactorial analysis of correspondence. Proc 5 th Eur.workshop on modern developments and applications in microbeam analysis, Torquay UK,149-173, 1997... 

High magnification imaging and composition (elemental) mapping... 

Figure 2 Micrographs of the same region of a specimen in various imaging modes on a high-resolution SEM (a) and (b) SE micrographs taken at 25 and 5 keV, respectively (c) backscattered image taken at 25 keV (d) EDS spectrum taken from the Pb-rich phase of the Pb-Sn solder (e) and (f) elemental maps of the two elements taken by accepting only signals from the appropriate spectral energy regions.

Care must be taken in interpreting the intensity distribution, because the electron intensity depends not only on the local concentration of the element but on the topography also, because surface roughness can affect the inelastic background underneath the line. Therefore elemental maps are customarily presented as variations of the ratio of peak intensity divided by the magnitude of the background on both or one side of the line this can easily be performed by computer. 

A variation on depth profiling that can be performed by modern scanning Auger instruments (see Sect. 2.2.6) is to program the incident electron beam to jump from one pre-selected position on a surface to each of many others in turn, with multiplexing at each position. This is called multiple point analysis. Sets of elemental maps acquired after each sputtering step or each period of continuous sputtering can be related to each other in a computer frame-store system to derive a three-dimensional analysis of a selected micro volume. 

Application of AES to zirconia ceramics has been reported by Moser et al. [2.146]. Elemental maps of Al and Si demonstrate the grain boundary segregation of small impurities of silica and alumina in these ceramics. 

If an incident electron beam of sufficient energy for AES is rastered over a surface in a manner similar to that in a scanning electron microscope (SEM), and if the analyzer is set to accept electrons of Auger energies characteristic of a particular element, then an elemental map or image is again obtained, similar to XPS for the Quantum 2000 (Sect. 2.1.2.5). 

Fig. 2.30. SAM map offractured SiC after sintering with B addition [2.167], (a)-(d) elemental maps in boron, potassium, sodium, and oxygen, respectively. (E), (F) point analyses at points A and B, respectively.

In the scanning (or microprobe) mode the image is measured sequentially point-bypoint. Because the lateral resolution of the element mapping in scanning SIMS is dependent solely on the primary beam diameter, LMISs are usually used. Beam diameters down to 50 nm with high currents of 1 nA can be reached. 

Element mapping with non-resonant laser- SNM S can be used to investigate the structure of electronic devices and to locate defects and microcontaminants [3.114]. Typical SNMS maps for a GaAs test pattern are shown in Fig. 3.43. In the subscript of each map the maximum number of counts obtained in one pixel is given. The images were acquired by use of a 25-keV Ga" liquid metal ion source with a spot size of approximately 150-200 nm. For the given images only 1.5 % of a monolayer was consumed -"static SNMS". 

Analytical electron microscopy (AEM) can use several signals from the specimen to analyze volumes of catalyst material about a thousand times smaller than conventional techniques. X-ray emission spectroscopy (XES) is the most quantitative mode of chemical analyse in the AEM and is now also useful as a high resolution elemental mapping technique. Electron energy loss spectroscopy (EELS) vftiile not as well developed for quantitative analysis gives additional chemical information in the fine structure of the elemental absorption edges. EELS avoids the problem of spurious x-rays generated from areas of the spectrum remote from the analysis area. 

A position sensitive detector (PSD) is employed, of which there are several types used effectively around the world. One type is essentially a square array of multianodes, as shown in Figure 1.6. By measuring the time-of-flight and the coordinates of the ions upon the PSD, it is possible to map out a two-dimensional elemental distribution. The elemental maps are extended to the z-direction by ionizing atoms from the surface of the specimens. The z position is inferred from the position of the ion in the evaporation sequence, so that the atom distribution can be reconstructed in a three-dimensional real space. 

The sample environment was filled with He gas to prevent the argon X-ray emission from air. Beam scanning, data acquisition, evaluation and the generation of elemental maps were controlled by a computer. Micro-PIXE measurements were performed with a scanning 2.5MeVH+ microbeam accelerated by the 3 MV single-end accelerator. The beam diameter was 1-2 pm, so that individual particles could be analysed. The beam current was < 100 pA and the irradiation time was about 3(M0 min. 

The large amount of S in the particles suggested that S02 gas molecules or small sulfur-containing particles condense on to the surface of soil dusts during their transportation from China. Figure 4.22 illustrates an elemental map for Si distribution in coarse particles within a total scanning area of 25 pm x 25 pm. The scale bar shows the peak count of characteristic X-rays by pixel of the scan area. 

It is possible to measure nearly any type of sample for almost any element with little or no preparation. Only a few mg of sample is required, and the measurements are non-destructive in that the sample is generally undamaged. Measurements take only 1-20 min of beam time. Elemental mapping showing the variations in elemental concentrations can be measured over the surface of a sample using the ion microprobe for an area as large as 5 x 5 mm. 

Fig. 5.16 (A) Bright-field TEM image and (B) element mapping carbon (brighter contrast corresponds to higher concentration of carbon) of ZnO synthesized in aqueous solution at 37 °C in pH 8 buffer for 4 h in the presence of 1.2 mgmL-1 of gelatin. The inset shows the electron diffraction pattern taken parallel to the platelet normal. (Reprinted with permission from [77], Copyright (2006) American Chemical Society).

Fig. 12 EDX chlorine elemental maps showing (a) even dispersion of drug in tablet and (b) large concentrated areas of active drug.

Four samples were similarly selected for the EPMA experiments. The samples were dried and embedded in polished epoxy cylindrical plugs. Backscattered electron (BSE) images as well as elemental maps of As, Fe and Ni (EDS/WDS) were collected using a JEOL 8600 Superprobe electron microprobe analyzer (Dept, of Geological Sciences, University of Saskatchewan). 

Elemental maps and p-XRF transects confirmed the high affinity of the secondary minerals to adsorb or incorporate many of the elements released by sulfides or gangue minerals (Fig. 2). In particular, we routinely observed significant concentrations of Ni, Cu, Zn, and As in the goethite-bernalite layers... 

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