Atom concentrations, measurement

What are the limitations of quaniiiaiive analysis with XPS Why might the atomic concentrations measured not correspond lo the bulk composilion For polyurethane, the sloichiomclric atomic conceiitralions are C 76.0%. N S.0%. and O 16.0%. Calculate the percentage error in the values obtained in part (e) for each clement. 

Most investigations have used a modulated scanning magnetic field and phase sensitive detection of the signal. For relative determinations of atom concentrations, measurements of peak heights suffice under most conditions (see below) but absolute concentrations are derived from the ratio of integrated intensities for the atom and O2, respectively. 

The problem of line widths, as applied to relative atom concentration measurements, has been discussed in a recent paper." For all species (except S-state atoms) in dilute concentration, the true unsaturated line width is determined simply by the total pressure (i.e. by the frequency of collision with any particle), and the signal heights are thus directly proportional to the relative concentration of atoms. This theoretical result has been verified experimentally for 0( Pg i) and CK Pg/g) atoms." For S-state atoms (H( S) and N( S)), the true line width depends on the atom concentration via spin-exchange collisions, unless a paramagnetic diluent is present." " This complication probably does not lead to great difiiculty in the majority of commonly encountered chemical systems. 

For O and N atoms, a cross-check of absolute atom concentrations measured by e.p.r. with those measured by O -F NOg and N -F NO titrations has satisfactorily verified the quantitative validity of both techniques." The e.p.r. method has been fruitful over the last few years in yielding rate constants of a number of simple atom transfer reactions at 3(X) K these include the reactions... 

For a detailed discussion on the analytical teclmiques exploiting the amplitude contrast of melastic images in ESI and image-EELS, see chapter B1.6 of this encyclopedia. One more recent but also very important aspect is the quantitative measurement of atomic concentrations in the sample. The work of Somlyo and colleagues [56]. Leapman and coworkers and Door and Gangler [59] introduce techniques to convert measured... 

Mineral and Chemical Composition. X-ray diffraction is used to determine the mineral composition of an Mg(OH)2 sample. Induced coupled plasma (icp) spectrophotometry is used to measure the atomic concentrations present in a sample. X-ray fluorescence analysis is another comparative instmmental method of determining chemical composition. 

The mechanism of the synthesis reaction remains unclear. Both a molecular mechanism and an atomic mechanism have been proposed. Strong support has been gathered for the atomic mechanism through measurements of adsorbed nitrogen atom concentrations on the surface of model working catalysts where dissociative N2 chemisorption is the rate-determining step (17). The likely mechanism, where (ad) indicates surface-adsorbed species, is as follows ... 

Although the most sensitive line for cadmium in the arc or spark spectmm is at 228.8 nm, the line at 326.1 nm is more convenient to use for spectroscopic detection. The limit of detection at this wavelength amounts to 0.001% cadmium with ordinary techniques and 0.00001% using specialized methods. Determination in concentrations up to 10% is accompHshed by solubilization of the sample followed by atomic absorption measurement. The range can be extended to still higher cadmium levels provided that a relative error of 0.5% is acceptable. Another quantitative analysis method is by titration at pH 10 with a standard solution of ethylenediarninetetraacetic acid (EDTA) and Eriochrome Black T indicator. Zinc interferes and therefore must first be removed. 

Three common uses of RBS analysis exist quantitative depth profiling, areal concentration measurements (atoms/cm ), and crystal quality and impurity lattice site analysis. Its primary application is quantitative depth profiling of semiconductor thin films and multilayered structures. It is also used to measure contaminants and to study crystal structures, also primarily in semiconductor materials. Other applications include depth profilii of polymers, high-T superconductors, optical coatings, and catalyst particles. ... 

As in RBS analysis, ERS can provide information on the atomic concentration of hydrogen as a function of depth (measured in atoms/cm ). This is derived from the height Aobs°f ERS spectrum (counts per channel), at energies corresponding to particular depths within the sample (see Figure 3c). For a sample consisting of H and another material X, with composition the spectrum height... 

Thus, as for XP S, the average surface concentration Na can, in principle, be calculated by measurement of the Auger current, according to Eq. (2.13). Again, as in XPS, relative sensitivity factors are generally used. The Auger current for the same transition XYZ in a standard of pure A is measured under the same experimental conditions as in the analysis of A in M, whereupon the ratio of the atomic concentrations is... 

The Fe-B nanocomposite was synthesized by the so-called pillaring technique using layered bentonite clay as the starting material. The detailed procedures were described in our previous study [4]. X-ray diffraction (XRD) analysis revealed that the Fe-B nanocomposite mainly consists of Fc203 (hematite) and Si02 (quartz). The bulk Fe concentration of the Fe-B nanocomposite measured by a JOEL X-ray Reflective Fluorescence spectrometer (Model JSX 3201Z) is 31.8%. The Fe surface atomic concentration of Fe-B nanocomposite determined by an X-ray photoelectron spectrometer (Model PHI5600) is 12.25 (at%). The BET specific surface area is 280 m /g. The particle size determined by a transmission electron microscope (JOEL 2010) is from 20 to 200 nm. 

Figure 7.9 (a) Echo-detected X-band EPR spectrum of a powder sample of Gd001 Y099W30 at T = 6 K. (b) Decoherence times 7, (solid symbols) and T2 (open symbols) measured at fi0H = 0.347 T, as a function of the atomic concentration of Gd ions. 

Polymer Mole % Acetyl Ti Concentration Measured by XFS (atoms/cm2) ... 

Polymer Ti Concentration Measured by XFS (atoms/cm2 x 1(T15) Before Etching After Etching Rate 0 (A/min) ATi Upon Etching (%)... 

During the last several years, a number of new instrumental surface techniques have been developed that are quite effective in detecting changes in the surfaces of minerals that have undergone chemically induced or natural geologic alteration. These techniques are quite sensitive (approximately 0.1-0.5% atomic concentration for x-ray photoelectron and Auger spectroscopy, for example), and they make it possible to monitor very small amounts of elements that may be present in the near surface material. Any change in the surface with respect to chemical composition may readily be measured qualitatively... 

Prompt NO mechanisms In dealing with the presentation of prompt NO mechanisms, much can be learned by considering the historical development of the concept of prompt NO. With the development of the Zeldovich mechanism, many investigators followed the concept that in premixed flame systems, NO would form only in the post-flame or burned gas zone. Thus, it was thought possible to experimentally determine thermal NO formation rates and, from these rates, to find the rate constant of Eq. (8.49) by measurement of the NO concentration profiles in the post-flame zone. Such measurements can be performed readily on flat flame burners. Of course, in order to make these determinations, it is necessary to know the O atom concentrations. Since hydrocarbon-air flames were always considered, the nitrogen concentration was always in large excess. As discussed in the preceding subsection, the O atom concentration was taken as the equilibrium concentration at the flame temperature and all other reactions were assumed very fast compared to the Zeldovich mechanism. 

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