Radiotracer Measurements

noble metal diffusion in DIP and Pc were carried out. As Ag diffusion in polymers is well studied and understood [12, 13, 15-17, 32, 33], depth profiles for Ag diffusion in Pc and DIP were obtained. Comparison with the depth profiles of polymers will help to further the understanding of the nature of noble metals in these organic semiconductors. 

The Ag atoms and clusters in these parts are also sputtered prior to the rest of the metal atoms. Instead of a sharp peak the metal concentration is smeared out over a depth range which correlates with the surface roughness of 40 nm due to the non-uniform sputtering of the interface. 

This similarity of the depth profiles for the DIP and the TMC-PC indicates that the interplay of diffusion and aggregation in organic crystalline films and polymers is basically the same. This was expected as the cohesive energy of DIP and TMC-Pc is of the same order. The region with decreased slope is as pronounced as in polymers, suggesting a similar interpretation. In contrast 

The film with the smaller grains has a higher grain boundary density. The radiotracer measurements can help to determine their influence on the diffusion. A higher grain boundary density might lead to more metal atoms diffus- 

The crystallites of the film deposited at -180 °C are half as large as the film thickness. The rocking width of the XRD spectra is a measure for the structural order of the film. 

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Marcus P, Teissier A and Oudar J 1984 The influence of sulfur on the dissolution and the passivation of a nickel-iron alloy. 1. Electrochemical and radiotracer measurements Corrosion Sc . 24 259... 

Applications Radiotracer measurements, which combine high sensitivity and specificity with poor spatial resolution, have been used for migration testing. For example, studies have been made on HDPE, PP and HIPS to determine effects of manufacturing conditions on migration of AOs from plastic products into a test fat [443]. Labelled antioxidant was determined radio-analytically after 10 days at 40 °C. Acosta and Sas-tre [444] have used radioactive tracer methods for the determination of styrene ethyl acrylate in a styrene ethyl acrylate copolymer. 

This theory of an equilibrium of one species between each side of the membrane was formulated by Donnan in 1925 and from then until 1955, it reigned as the theory of membrane potentials. Its demise came when radiotracer measurements showed that all relevant ions (e.g., K+, Na+, and Cl ) permeated more than a dozen actual biological membranes, although each ion had a characteristic permeability coefficient in each membrane (Hodgkin and Keynes, 1953). 

In the Goldman equation use the diffusion coefficients in question 1 in place of permeation coefficients. The rate of permeation is -D(dc/dx). The dx is essentially the membrane thickness and its neglect will cancel out of the equation (cf. the Goldman equation), (c) Is this flux consistent with actual radiotracer measurements of the movement concerned (In the Hodgkin-Huxley and Katz work, arbitrary values were used for the P s to ensure that the equation replicated the experiment. This, of course, makes it difficult to check its validity. Assume the starting concentration of ions on either side of the membrane is that shown in the text. The average internal diameter of a squid axon is about 1 mm.)... 

In the case of indirect radiotracer methods, instead of labeling the species to be studied another adequately chosen labeled species (the indicator species) is added to the system and the adsorption of this component is followed bythe usual radiotracer measuring technique. The adsorption of the indicator species should be related to that of the species studied. The nature of this link could vary in different systems. 

In experiments involving radiotracer measurements of diffusion in molten salt, the Stokes-Einstein equation has been found to be roughly applicable. For a series of ions, in molten salts it was found that the product D /T = 10 dyn mol . From this information, find whether the best form of the coefficient in this expression for this case is nearer to 6 or 4. 

In a molten salt solution of CdClj in KCl, radiotracer measurements of the diffusion coefficient of Cd at 470 °C showed the heat of activation to be 23.0 kJ moC. A rough calculation of the entropy of activation showed this to be small, about 41.8 J mol K ... 

Weber A., Riess W., Wenzhoefer F., and Jorgensen B. B. (2001) Sulfate reduction in Black Sea sediments in situ and laboratory radiotracer measurements from the shelf to 2,000 m depth. Deep-Sea Res. I 48, 2073 -2096. 

Yeager et al. [00] reported diffusion coefficients of water in Nafion 120 membranes containing various alkali-metal cations, determined by radiotracer measurements. In these studies, the diffusion coefficient of water was measured for fully hydrated (i.e., immersed) membranes. The water diffusion coefficient was found to be only slightly dependent on the cation present in the immersed membrane, with a... 

The same study reports Oq (pH) curves calculated from combined electrokinetic and radiotracer measurements (Na and Br adsorption) for two NaBr concentrations. 

The electrocatalytic oxidation of small organic molecules at noble metal electrodes has been intensively investigated over a period of many years, both by conventional electrochemical methods [59], including radiotracer measurements, and by an increasing battery of non-conventional techniques. The metal anodes in such reactions are rapidly poisoned by the formation of strongly adsorbed species that block the catalytic surface, and the identification of these species has become a major objective. 

There are three possible explanations for the gradual increase in the compressive stress as the potential is scanned towards the bulk silver deposition region. The first is that the silver monolayer enhances further adsorption of sulphate anions (this has been demonstrated on polycrystalline gold by radiotracer measurements [42 - 43]). Consequently there is a gradual build-up in compressive stress which continues until further silver deposition occurs near the bulk silver deposition region. 

Table 8.1 Mineralization of deposited organic carbon in marine sediments and the role of sulfate reduction. Data range from the shelf to the deep sea and are listed according to decreasing significance of sulfate reduction. The Black Sea is included at the end as an example of an anoxic basin totally dominated by sulfate reduction. The data are based on radiotracer measurements of sulfate reduction rates ( represents a modeled rate). ...

The data obtained from these radiotracer measurements are very useful in the discussion of the result of IR-spectroscopic studies. On the other hand, radiochemical measurements could be supported by ex situ electron spectroscopies of ultrahigh vacuum surface science. 

Some examples of PET images of the hydrodynamic flow in a soil column (length 1 m, diameter 10 cm) are given which demonstrate the possibilities of tomographic radiotracer measurements for studies of the heterogeneous mass flow. The local flow velocity distribution in different parts of the soil column varies strongly and has to be considered in the geochemical transport models. 

There is evidence now from direct radiotracer measurements and other investigations that metals of low reactivity, in particular Cu, Ag. and Au, diffuse into polymers at elevated temperatures and sometimes form clusters inside the polymer. Diffusion is most pronounced at low deposition rates, where a large number of isolated atoms impinge onto an initially almost metal-free surface. The mobility in the polymer matrix appears to be controlled by the availability of free volume. For reactive metals such as Cr and Ti, which form relatively sharp interfaces, no significant diffusion seems to occur due to strong binding to the polymer. 

The isotherms determined experimentally by radiotracer measurements for the adsorption of ethylene on platinum are in agreement with Langmuir-type adsorption. " ... 

One problem in calculating the solubility of small particles is that the surface energy is not accurately known. There are very few substances of which the solubility of particles of uniform size and less than 100 A diameter have been measured. According to Walton (174), a combination of these requirements has not previously been achieved, but recently using radiotracer measurements and the electron microscope, such measurements have been made on strontium sulfate particles, from which the solid-water interfacial energy was calculated to be 84 8 ergs cm". ... 

In a short but interesting paper, Wiegert and Odum (1969) discussed the use of radiotracer measurements in studies of food web diversity and utilized the data from the study discussed above (Wiegert et aL, 1967). They stated We feel that radionuclide tracers can simultaneously serve to identify the actual, as opposed to the merely possible trophic interactions, and tell us something about the quantitative importance of a given pathway. ... 

These advances in stable istopic tracer measurements are important because of the limitations in lead radiotracer measurements. Tracer studies of lead metabolism using radioisotopes ( Pb, Pb, and Pb) have been hindered by the relatively poor precision of the measurements, their relatively short ( Pb tin = 51.88 hr and "Pb tm = 10.64 hr) or long ( Pb tin 22.3 yr) half-lives, and the relatively weak gamma energy of Pb. Additionally, concerns about the radiation hazard of these isotopes have severely restricted their utilization in many laboratory and clinical studies. 

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