Depth distribution, elemental

Nuclear reaction analysis (NRA) is used to determine the concentration and depth distribution of light elements in the near sur ce (the first few lm) of solids. Because this method relies on nuclear reactions, it is insensitive to solid state matrix effects. Hence, it is easily made quantitative without reference to standard samples. NRA is isotope specific, making it ideal for isotopic tracer experiments. This characteristic also makes NRA less vulnerable than some other methods to interference effects that may overwhelm signals from low abundance elements. In addition, measurements are rapid and nondestructive. 

SNMS is suitable for quantitative element depth profiling of metallic and electrically insulating samples. Laser-SNMS enables the additional acquisition of 2D element distributions with HF-plasma SNMS bulk analysis is also feasible. 

Figure 8.5 shows the LEIS spectra of ZnAl204 and ZnO as a characteristic example of a multicomponent system analyzed by this technique [Brongersma and Jacobs, 1994]. Since only the surface peaks of A1 and O were detected for ZnAl204, the Zn atoms must be located in the subsurface layers. The onset of the tail agrees between the spectra, indicating that Zn is present in the second and deeper layers. This example illustrates the strength of the LEIS technique, in that characteristic peaks from different elements can be used to selectively analyze the atomic composition of the topmost surface. In addition, the shape of the tails could provide information on the in-depth distribution of the elements. 

Quantitative elemental analysis often limited in practical situations by the combination of an unknown depth distribution over the probing depth, which itself may not be known to better than a factor of 2. 

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). 

Using XRF and EPMA in conjunction with SEM and EDX the antiwear films were found to consist of P, S, O, and Zn (Brown et al., 1992 Rounds, 1993). The application of XPS and AES surface techniques promoted deeper understanding of the antiwear mechanism elemental composition of the chemical species, valence of the elements, and depth distribution. Chemical speciation, e.g., phosphate and S (sulfide or sulfate) can be obtained from binding energies. 

We conducted some ERD measurements to study the hydrogen depth distribution of Ni(ya-Al203 samples after annealing in steam (fig. 7). A 2.2 MeV 4He+ beam was used and a 8.9 Jim Mylar foil. The recoil angle was 36q, and the angle between the beam direction and the sample normal was 72D. It is evident that ERD is a very powerful technique, since it is one of the few methods capable of quantitative depth profiling light elements in a heavy matrix. 

It should be noted that this quantifleation approach assumes that the sample is homogeneous in depth. If this were the case however, the use of a surface analysis technique would not be Justifled. The approximation involved is applied in the absence of other information that can be used to describe the depth distribution of the elements. In particular, if a surface contamination layer (for example, atmospheric hydrocarbons) is present on the sample this will influence the intensity of the peaks to an extent which depends on the energy of the pho-toclcctron (through the dependence of X on the kinetic energy) and thus the clement. 

The biological pump influences, to varying degrees, the distribution of many elements in seawater besides carbon, nitrogen, phosphorus, and silicon. Barium, cadmium, germanium, zinc, nickel, iron, selenium, yttrium, and many of the REEs show depth distributions that very closely resemble profiles of the major nutrients. Additionally, beryllium, scandium, titanium, copper, zirconium, and radium have profiles where concentrations increase with depth, although the correspondence of these profiles with nutrient profiles is not as tight (Nozaki, 1997). 

Metastable Iron Sulfides Organic Sulfur Elemental Sulfur MECHANISM OF PYRITE FORMATION. 4.1 Evidence from Experimental Studies. 4.2 Isotope Effects during Experimental Pyrite Formation. 4.3 Origin of Morphological Variations in Pyrite SULFUR DIAGENETIC PROCESSES IN MARINE SEDIMENTS. 5.1 Depth Distribution of Diagenetic Sulfur Products. 5.2 Rates of Sulfate Reduction... 

Figure 6. This figure shows the depth distribution of Pb, Mo, and As in whole rock and insoluble-residue samples of rocks from borehole S-35 in the Ozark region. See Fig. 3 for the location of core S-35. The Pb (insols). Mo (insols), and As (insols) data are from emission spectroscopy analyses of insoluble-residues from the core. The Pb and Mo data are shown because these elements often correlate with As in midcontinent region rocks. The As (whole rock) column is data from whole rock analyses of the same depth intervals by Atomic Absorption Spectroscopy. The overall core interval spans the upper Cambrian and lower Ordo vician eras. The purpose of this plot is to show that insoluble residue data reflect, and even provide an enhanced distribution of As in rock column. The enhancement is because As is localized in the sulfide fraction that is concentrated in the insoluble residues.

Chemical characteristics and the oxidation state of elements in the nearsurface layer ( 5 nm) of a sample are recorded by photoelectrons that are produced by an X-ray beam. When this technique is combined with intermittent ion sputtering, data on depth distribution can be obtained. X-ray photoelectron spectroscopy goes beyond elemental analysis to provide chemical information such as distinguishing Si-Si from Si-O bonds. Elements from Li to U may be analysed with detection levels at 0.5% under high vacuum conditions. Raster scanning techniques produce images with a spatial resolution of 26 pm and depth profiles of 1 pm thick are possible (Mossotti et al., 1987 Wilson and Bums, 1987). 

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