The Radiotracer Methods

The plot of the various reactants is most revealing. It is compared in Figure 6 with the radiotracer and potentiodynamic methods. It will be seen that there is a rough kind of agreement between the data here obtained and the radiotracer method data, though the potentiodynamic method is evidently less sensitive. On the other hand, the FTIR data distinguishes between the two, CO in the triple bond form and CO in the double bond form. 

Finally, it should be noted that the structure of the DHP varies greatly depending on the polymerization conditions employed. Additionally, the yield and chemical and physical properties of these DHP preparations differ substantially from protolignin. Further improvements in simulation of the lignification process are therefore needed, and the radiotracer method can be employed as one approach to solve such problems. 

It was demonstrated that the radiotracer method, using labeled anions, is an adequate tool to follow anion adsorption in the course of voltammetric measurements and to gain simultaneous information on hydrogen and anion adsorption [163]. Coupling voltammetric and radiometric measurements in the study of platinized platinum electrodes gave insight in the anion-hydrogen atom coadsorption process. 

The radiotracer method for estimating efficiency of initiation was applied by Bevington and Eaves (32) to polymerization in benzene and in carbon tetrachloride. Whereas they had calculated that about 47% of the radicals from AIBN initiate polymer chains in DMF solvent, efficiency in benzene was about 50% and in carbon tetrachloride about 30%. This low efficiency in carbon tetrachloride is attributed to attack of radicals from AIBN on the carbon tetrachloride solvent, especially at high concentrations of solvent. Chains initiated by secondary radicals derived in this way from the solvent would not be detected by the tracer method. 

The adsorption of Cd(II) and oxalate or citrate ions was calculated from an uptake of their concentration from solution. The ion concentration in solution was determined by the radiotracer method. For labelling the solution, 14C or 115Cd isotopes were used. Radioactivity of the solutions before and after adsorption was measured using a liquid scintillation counter (LS5000 TD Beckmann). Co-adsoprtion of Cd(II) and carboxylate ions was determined simultaneously. The adsorption of measured ions was conducted in a pH range from 3.0 to 10.0. 

As to the role of the labeled species in the radiotracer study of adsorption phenomena, two different versions of the method may be distinguished. In the first, the direct method, the species to be studied is labeled and the radiation measured gives direct information on the adsorption of this species. However, this method cannot be used in several cases owing to technical restrictions related to the very nature of the radiotracer method (the available concentration range is limited no distinction can be made between the adsorption of the labeled compound studied and that of a product formed from it the number of commercially available labeled compounds is restricted.)... 

One of the advantages of the radiotracer method is that it can be used to follow adsorption as a function of time, namely to study the kinetics of adsorption. 

Medical science provides a framework or paradigm in which 10 understand disea.se and to maintain health. Nuclear medicine is the branch of medical science that contributes to medicine by the use of the radiotracer method for diagnosis and use of in vivo unsealed radioactivity for therapy. 

Figure 5.3 shows calculated curves for the adsorptions of surfactant, F, (the full lines), and counterions, F (the dotted lines), vs. the SDS concentration, These lines represent the variation of Fj and Fj along the experimental curves, which correspond to the lowest and highest NaCl concentrations in Figure 5.2, that is, = 0 and 115 mM. We see that both F, and are markedly greater when NaCl is present in the solution. The highest values of F, for the curves in Figure 5.3 are 4.2 x 10 mol/m and 4.0 x 10 mol/m for the solutions with and without NaCl, respectively. The latter two values compare well with the saturation adsorptions measured by Tajima et al. - for the same system by means of the radiotracer method, that is, F, = 4.3 x 1(F mol/m and 3.2 X 10 mol/m for the solutions with and without NaCl. 

Among the ex situ methods that can be employed in surface analysis, low-energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS) can give the crystal structure and the nature of the surface ad-layers after the electrochemical and adsorption experiments as explained in this chapter [31,32]. Among the in situ non-electrochemical techniques, the radiotracer method [33] gives information about the adsorbed quantities however, infrared spectroscopy in FTIR mode [34] allows the identity of the bonding of the adsorbed molecules, and finally ellipsometry [35] makes possible the study of extremely thin films. Recently, some optical methods such as reflectance, x-ray diffraction, and second harmonic generation (SHG) [36] have been added to this list. 

One of the older techniques for measuring directly the adsorbed amoimt of surfactant molecules or polymers at liquid interfaces is the radiotracer technique. Its idea is the measurement of the radiation emitted by radio-labelled molecules, adsorbed at an interface (Sally et al. 1950, Flengas Rideal 1959). Because of the background radiation the method yields relative data only. Using equilibrium adsorption isotherms, the dynamics of adsorption can also be followed by the radiotracer method. Experiments were performed with various surfactant systems (Matuura et al. 1958, 1959, 1961, 1962, Tajima 1970, Konya et al. 1973, Muramatsu et al. 1973, Okumura et al. 1974) and adsorbed polymers (Frommer Miller 1966, Adams et al. 1971, Graham Phillips 1979b). Due to the development of more efficient methods the use of this technique has been reduced. 

In the following, we will use data of Jorgensen et al. (2004) from the Black Sea as an example to illustrate the differences between the two approaches. Fig. 8.18 shows results from the deep sulfidic part of the Black Sea where macrofauna are unable to live and where bioirrigation is therefore absent (although current-induced advective pore water transport may take place near the sediment surface). Sulfate reduction rates were measured experimentally by the radiotracer method down to 20 cm depth in the sediment (Fig. 8.18 A). 

Radiochemical methods form an important class of in situ electrosorption studies. The radiotracer methods furnish direct information about the surface concentration of adsorbed species ensuring a relatively easy way for the determination of concentration and potential dependence of the adsorption. The exchange rate of sorbed labeled species with nonlabeled species added to the solution phase gives an important information on the mobility of the adsorbed molecules. 

Various techniques were elaborated for the elimination of these problems. The most sensitive technique indicating and monitoring the anion adsorption on adatoms or the coadsorption of metal ions and anions is the radiotracer method labeling the anions. ... 

The radiotracer method has also been used in hiunan subjects if the test compoimd is not toxic (Feldmann and Maibach, 1969, 1970, 1974). This approach was employed to assess percutaneous absorption of pesticides in swine (Qiao etal., 1994), in human testing of ultraviolet filters (Benech-Kieffer et al., 2003), and for determination of topical absorption of cosmetics (Malhotra et al., 2001) for clinical safety assessment. 

Agents used for the drying of some important solvent types are listed in Table 9.1. However, despite the existence of an arsenal of desiccants, the presence of water in some systems continues to be a problem. Recommended agents for the removal of water and polar impurities from other solvents may not be suitable, e.g. for amines. The radiotracer method for water assay developed by Burfield et al. [Bu 77] can be applied to obtain quantitative data on the drying of some representative solvents. In the case of amine type solvents, e.g. only molecular sieves and CaH2 were shown to have efficient siccative effect [Bu 81]. 

Using the radiotracer method, the strength of the ion-polymer interactions can also be studied, which is certainly a special advantage of this technique. When unlabeled species are added to the solution phase in great excess, the sorbed species are exchanged provided that there are no strong interactions (chemical bonds) between the ions or molecules and the polymer. 

Hieland, Gileadi, and Bockris have recently studied the adsorption of benzene on platinum, using the radiotracer method. Typical results are shown in Figure 9. The coverages indicated here are higher than those obtained with ethylene for a given bulk concentration of the organic. 

The merits of the radiotracer method for studying phy sieal interactions (including sorption) with hair were recognized quite early (3). In fact, up to 1980 it was the chief method that was used to study such interactions that involved hair-conditioning polycations. Some advantages of the method are ... 

The results obtained by the radiotracer method by Karman et al. (1991) showed that HEDP forms a loosely bound adsorption layer on the iron oxide surface at 3-6 x 10 " M concentrations. The potential dependence of the adsorption in this concentration range was found to be very weak, and an increased amount of HEDP on the metal surface could be observed with increasing HEDP concentration in the solution. 

Horanyi G (1980) Recent development in the application of the radiotracer method to the investigation of adsorption and electrocatalytic phenomena. Electrochim Acta 25 43-57... 

The competitive technique is poorly suited for situations that involve widely different reaction rates. Because of this factor along with the other experimental difficulties that have been enumerated below, the achievement of accurate (kg/ki) results for CHFs/CsFe competitors using the radiotracer method has proven to be unusually difficult. 

Some early results for adsorption of ethylene on Pt, obtained by the radiotracer method, are shown in Figure 13.3. 

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