Tracers Experiments

Experimental methods for determining RTDs can be classified into three major categories, depending on how the tracer is introduced into the system. The use of all of these three methods yields specific variations in the concentration at the outlet. This in turn affects the mathematical treatment of the experimental measurement results. Some of the most common methods are briefly summarized below  

Use of deuterium in the mechanistic studies of the HER began shortly after its discovery in 1931. It was used in two directions (a) as a tracer to measure the unidirectional component rate of the reaction, and (b) for examination of the kinetic isotope effect, namely, the electrolytic separation factor. 

Results of the experiments have been summarized elsewhere, and only brief comments will be given here (a) results which support the Tafel-Volmer reaction route were obtained on Rh (in acid) and Rh and Ni (in alkaline solution), and partly on Pt, Pd, Ir, Au, and Ag (in acid solutions) (b) elementary step rates are frequently of comparable magnitudes, particularly on Rh and Ni (c) correspondingly, the stoichiometric numbers often deviate from integer values and (d) kinetics of the cathodic component rate of the Tafel step was second order in au, while that of the Volmer step indicated its symmetry factor (1 - /3) to be close to unity. Some results are summarized in Table 1/  

Results of Tracer Experiments. Values of mo = Iov/i ot and v. Hydrogen pressure  

As will be discussed later (Section 5.4), it is quite possible in such cases, with more than one irreversible step, to realize quasilinear Tafel lines which are usually accepted as characteristic of reactions with a unique rds. 

The principal objection of Edlbacher and co-workers to the hypothesis that histidine was catabolized by the way of urocanic acid was that their purified histidase preparations split the imidazole ring of histidine in the absence of urocanase and did not act on urocanic acid. Histidase prepared according to the best purification procedure from Edlbacher s laboratory by Mehler and Tabor yielded a product which had an eightfold increase in the ability to produce urocanic acid from histidine but did not split the imidazole ring. This preparation did not attack 

Kdlbacher, S., and Viollier, G., Hoppe-Seyler s Z. physiol. Chem. 276, 108 (1942). Morel, C. J., Helv. Chim. Acta 19, 905 (1946). 

Edlbacher and Viollier found that the two enzymes could be separated owing to the fact that histidase is selectively absorbed on Pbs(P04)2 and urocanase on Cas(P04). 

In Pseudomonas fluorescens extract, histidase free from urocanase can be prepared by heating the extract at 85°C. for 15 minutes. By this procedure all ability to destroy urocanic acid is lost, but the full capacity to deaminate histidine is retained when the product is supplemented with 10 M glutathione. 

Some of the reported purifications of histidase can not be judged for their freedom from urocanase, because, apparently, this was not determined. The test method employed was merely the rate of liberation of NHa from histidine. 

---

N. E. V. Rodrigues, B. A. Robinson, and E. Counce, "Tracer Experiment Results During the Long-Term Flow Test of the Fenton Hill Reservoir", Proceedings of the Eighteenth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, Calif., 1994, in press. 

The changeover from ROO radicals to HOO radicals and the switch from organic peroxides to HOOH has been shown as temperature is increased in propane VPO (87,141). Tracer experiments have been used to explore product sequences in propane VPO (142—145). Propylene oxide comes exclusively from propylene. Ethylene, acetaldehyde, formaldehyde, methanol, carbon monoxide, and carbon dioxide come from both propane and propylene. Ethanol comes exclusively from propane. 

Grinding-Rate Functions These were determined by tracer experiments in laboratoiy mills by Kelsall et al. (op. cit.) as shown in Fig. 20-19 and in similar work by Szantho and Fuhrmann [Aujhereit. Tech., 9(5), 222 (1968)]. These cuives can be fitted by the following equation ... 

Although extremelv useful, tracer experiments require considerable capital expenditures and personnel. In addition to the difficulties and uncertainty in making estimates of various parameters, especially cr, one of the fficulties in interpreting tracer studies is relating the atmospheric conditions under which the study was conducted to the entire spectrum of atmospheric conditions. For example, trying to interpret a series of tracer... 

Gryning, S. E., and Lyck, E., "Comparison between Dispersion Calculation Methods Based on In-Situ Meteorological Measurements and Results from Elevated- Source Tracer Experiments in an Urban Area." National Agency of Environmental Protection, Air Pollution Laboratory, MST Luft - A40. Riso National Laboratory, Denmark, 1980. 

A tracer experiment includes sampling on an arc at 1000 m from the source. If the horizontal spread is expected to result in a a- between 120 and 150 m at this distance, and if the wind direction is within 15° azimuth of that forecast, how many samplers should be deployed and what should be that spacing It is desirable to have above seven measurements within 2fTj, of the plume centerline and at least one sample on each side of the plume. 

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. 

Since NRA focuses on inducing specific nuclear reactions, it permits selective observation of certain isotopes. This makes it ideal for tracer experiments using stable isotopes. Generally, there are no overlap or interference effects because reactions have very different Qvalues, and thus different resultant particle energies. This permits the observation of species present at relatively low concentrations. A good example is oxygen and O can be resolved unambiguously, as they are... 

NRA is an effective technique for measuring depth profiles of light elements in solids. Its sensitivity and isotope-selective character make it ideal for isotopic tracer experiments. NRA is also capable of profiling hydrogen, which can be characterized by only a few other analytical techniques. Future prospects include further application of the technique in a wider range of fields, three-dimensional mapping with microbeams, and development of an easily accessible and comprehensive compilation of reaction cross sections. 

As we have said, the key to the analysis of asystemlike this one is tohave a function that approximates to the actual residence time distribution. The tracer experiment is used to find that distribution function,butwewillworkfroman assumed function to the tracer concentration-timecurvetoseewhattheexperimentaloutcomemightlooklike. 

While awaiting the results of tracer experiments, the present authors prefer to desist from assuming that the route from 24 to 25 via the addition product shown is the only pathway followed in the amination of 2-halogenopyridines. This is the more so since it seems probable that in the experiments described below derivatives of 2,3-pyridyne occur as intermediates. 

Tracer experiments can be employed to distinguish between these two possibilities. 

A linear mechanism similar to the accepted mechanisms for Ziegler-Natta polymerization, which has been proposed, e.g. by Marshall and Ridgewell (21), is also excluded by the tracer experiments. 

Schiesser and Lapidus (S3), in later studies, measured the liquid residencetime distribution for a column of 4-in. diameter and 4-ft height packed with spherical particles of varying porosity and nominal diameters of in. and in. The liquid medium was water, and as tracers sodium chloride or methyl orange were employed. The specific purposes of this study were to determine radial variations in liquid flow rate and to demonstrate how pore diffusivity and pore structure may be estimated and characterized on the basis of tracer experiments. Significant radial variations in flow rate were observed methods are discussed for separating the hydrodynamic and diffusional contributions to the residence-time curves. 

Various methods may be used for the determination of gas holdup—for example, displacement measurements and tracer experiments. Farley and Ray (F2) have described the use of gamma-radiation absorption measurement for the determination of gas holdup in a slurry reactor for the Fischer-Tropsch synthesis. 

Additional information concerning the mechanisms of solid—solid interactions has been obtained by many diverse experimental approaches, as the following examples testify adsorptive and catalytic properties of the reactant mixture [1,111], reflectance spectroscopy [420], NMR [421], EPR [347], electromotive force determinations [421], tracer experiments [422], and doping effects [423], This list cannot be comprehensive. Electron probe microanalysis has also been used as an analytical (rather than a kinetic) tool [422,424] for the determination of distributions of elements within the reactant mixture. Infrared analyses have been used [425] for the investigation of the solid state reactions between NH3 and S02 at low temperatures in the presence and in the absence of water. 

On the basis of these facts, it was speculated that plutonium in its highest oxidation state is similar to uranium (VI) and in a lower state is similar to thorium (IV) and uranium (IV). It was reasoned that if plutonium existed normally as a stable plutonium (IV) ion, it would probably form insoluble compounds or stable complex ions analogous to those of similar ions, and that it would be desirable (as soon as sufficient plutonium became available) to determine the solubilities of such compounds as the fluoride, oxalate, phosphate, iodate, and peroxide. Such data were needed to confirm deductions based on the tracer experiments. 

Lanthanum fission product, radioactive, tracer experiments, early work. 7, 10... 

The thiazolecarboxylic acid structure (40) was also guessed in a similar way, from tracer experiments. The unknown compound was converted into the thiamine thiazole by heating at 100°C and pH 2. On paper electrophoresis, it migrated as an anion at pH 4. Tracer experiments indicated that it incorporated C-l and C-2 of L-tyrosine, and the sulfur of sulfate. The synthetic acid was prepared by carboxylation of the lithium derivative of the thiamine thiazole, and the derivatives shown in Scheme 19 were obtained by conventional methods. Again, the radioactivity of the unknown, labeled with 35S could not be separated from structure 40, added as carrier, and the molar radioactivity remained constant through several recrystallizations and the derivatizations of Scheme 17. 

The significance of these metabolites in the biosynthesis of the thiamine thiazole in considered next. Although, from their constitution, and from the tracer experiments, the metabolites are undoubtedly the products of transformation of 1-deoxy-D-t/ireo-pentulose, their significance in the biosynthesis of the thiazole of thiamine is not clear. The thiazole glycol is not a product arising from a transformation of the thiazole (5) of thiamine. Reduction to this thiazole (5) occurs in dialyzed extracts of disrupted cells, in the presence of ATP, NADH, and NADPH, but only at 0.2% the rate of synthesis of the thiamine thiazole (5) by intact cells. The behavior of the thiazole glycol on plates is merely a consequence of the extreme sensitivity of the tetrazolium reagent. 

Heimann, M. and Keeling, C. D. (1989). A three-dimensional model of atmospheric CO2 transport based on observed winds 2. Model description and simulated tracer experiments. In "Aspects of Climate Variability in the Pacific and Western Americas," Geophys. Monogr. Ser. vol. 55 (D. H. Peterson, ed.), pp. 237-275, AGU, Washington, DC. 

The use of inert tracer experiments to measure residence time distributions can be extended to systems with multiple inlets and outlets, multiple phases within the reactor, and species-dependent residence times. This discussion ignores these complications, but see Suggestions for Further Reading. ... 

Thus, t can be found from inert tracer experiments. It can also be found from measurements of the system inventory and throughput since... 

---