Elemental identity, 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). 

In multiple-layer thin films, it is possible that some of the elements may be present simultaneously in two or more layers. XRF analysis of this type of film can be complicated and cannot be made solely from their observed intensities. Additional information, such as the compositions or thickness of some of the layers is needed. The amount of addidonal non-XRF information required depends on the complexity of the film. For example, in the analysis of a FeMn/NiFe double-layer film, the additional information needed can be the composition or thickness of either the FeMn or NiFe layer. Using the composition or thickness of one of the film predetermined from a single-layer film deposited under identical condidons, XRF analysis of the FeMn/NiFe film was successfiil. ... 

On a relative basis, i.e. residues per 1000, there is virtually no one species like the other. In contrast, different shell samples from the same species and obtained from the same natural habitat yield identical amino acid patterns. It is of interest that (1) the structure of carbonates (aragonite-calcite-vaterite), (2) the content in trace elements, and (3) the stable isotope distribution are markedly effected by fluctuations in salinity, water temperature, Eh/pH conditions, and some anthropogenic factors. The same environmental parameters determine to a certain degree the chemical composition of the shell organic matrix. This feature suggests a cause-effect relationship between mineralogy and organic chemistry of a shell. In the final analysis, however, it is simply a reflection of the environmentally-controlled dynamics of the cell. 

Cyclotriveratrylenes are made by the condensation of veratrole with formaldehyde and were a curiosity when they were first prepared by Robinson in 1915 [47], At the time she believed the compound to be a dimer, 2,3,6,7-tetramethoxy-9,10-dihydroxyanthracene. The reason for this was that the composition of the original compound was determined by elemental analysis. This technique determines the percentage of hydrogen and carbon in the sample which would be identical for a dimer, trimer or any other cyclic product with the same proportion of carbon to... 

The comparison of a number of dialytic extracts with the parent coals is given in Table 1L These results indicate that the elemental composition of the dialytic extract closely mirrors that of the organic fraction of the coal. Similar conclusions were reached when coal liquids were separated via the dialytic method. The conclusion that dialysis does not concentrate any particular compound type deserves further investigation, since obtaining a representative sample is crucial to the utility of the method. In Table H, it is particularly interesting to note that in each case the "organic sulfur 1 from the classical coal analysis is almost identical to the sulfur content directly determined on the dialytic extract. 

Other complications that arise are (a) that the surface compositions of glassy metals to be used as electrocatalysts are rarely identical with the corresponding bulk compositions, as was shown in recent Auger surface analysis experiments by Vracar and Conway (134), and (b) that when such alloys are used as anodes for O2 evolution in water electrolysis an oxide film of appreciable thickness is formed, and the distribution of elements of the alloy in the film is not usually the same as in the parent metal owing to some preferential anodic leaching of any base-metal components that are present in the alloy. 

Table III shows the XRF analysis of the fine-sediment fraction of the sediment samples listed in Table II made into 100-mg wafers. These samples were biased to the fine fraction, as were the F samples in Table II, and were analyzed on filter paper, identical to what would be prepared by the CS system. This information was used primarily to relate the concentration of the elements in the standard pellets of Table II of known thickness and composition to the CS wafers shown in Table IV.

The A A analyses shown in Table I were of samples from the uppermost sections of all seven cores, from the lowest section within Core 4, and from each quadrant of Cores 1 and 7. Table II summarizes the elemental analysis performed with the high-energy XRF system. The samples were identical to those for A A analysis except for their preparation. Comparison of Tables II and III indicates a very good agreement between the two analytical systems. Variations in composition from one station to another were exhibited by both systems for some of the major elements. In addition to the close agreement between the more common elements Fe, Mn, and Ti, very good agreement was found between the trace elements. These results confirmed the applicability and accuracy of both analytical systems for this type of sample measurement. 

Table IV shows good agreement between the CS heavy metals values and the ground-truth data shown in Tables I and II. With the exception of Pb, all the elements were determined with good accuracy and indicated a ground-truth agreement between the surficial sediments and the CS wafers produced from the same material. The XRF analysis made use of six different standards that were compared to NBS standard reference material 1646. This standard is a marine sediment of nearly identical mineral composition to that of the Baltimore Harbor samples.

Scientists synthesize new compounds for many uses. Once they make a new product, they must check its identity. One way is to carry out a chemical analysis that provides a percentage composition. For example, in 1962, two chemists made a new compound from xenon and fluorine. Before 1962, scientists thought that xenon did not form compounds. The scientists analyzed their surprising find. They found that it had a percentage composition of 63.3% Xe and 36.7% F, which is the same as that for the formula XeF4. Percentage composition not only helps verify a substance s identity but also can be used to compare the ratio of masses contributed by the elements in two substances, as in Figure 8. 

We often want to know not only the identity of chemical elements but also their concentrations in a specimen. Thus, quantitative elemental analysis is often required. The concentrations of elements must relate to their peak intensities in the spectrum, similar to the relationship between weight fractions of crystalline phases and their peak intensities in the XRD spectrum. Chemical compositions should be calculated by comparing the ratios of integrated peak intensities among elements in the specimen. In general, the weight fraction (C) of an element in relation to the relative intensities of its peaks (Ir) is mainly affected by the instrument factor (K), and the matrix factor of specimen (M). 

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