Tracer experiments, interpretation

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... 

The interpretation of the results of tracer experiments of this sort is sometimes complicated by, sO-exchange reactions. l80 from the solvent may be incorporated into the unreacted ester as hydrolysis proceeds (see below, p. 105), or into either or both of the products. The exchange reaction is significant with alcohols, such as triphenylmethyl alcohol, which give rise to relatively stable carbonium ions under acidic conditions (see, for example, refs. 67, 85), viz-... 

R. Shinnar, S. Katz, and P. Naor, Interpretation of Tracer Experiments in Recirculating Systems, and Application of Renewal Theory, unpublished report, Department of Chemical Engineering, City College, City University, New York (1970). 

The residence time distribution of the recycle reactor was determined by tracer experiments. This permitted the interpretation of the flow patterns in the reactor, so that the degree of mixing could be quantified. 

This concept was used for the study of the deactivation of n-hexane catalytic cracking on a US Y zeolite catalyst. The interpretation of the flow patterns in the recycle reactor, necessary for the quantification of the degree of mixing, was based upon tracer experiments. 

Intracellular fluxes can be estimated more precisely through 13C tracer experiments. Following 13C feeding to a cell it is possible to analyze metabolic products, such as amino acids, and measure 13C enriched patterns, so to be able to reconstruct the flux distribution from the measured data [91]. To obtain flux data from the labeling patterns, two techniques can be applied NMR [92, 93] and MS [94, 95]. Due to the low intracellular concentration of metabolites, these are often difficult to measure therefore the analysis of the labeling pattern of amino acids in proteins is used as input for flux quantification. Here proteins are hydrolyzed to release labeled amino acids and further analyzed by NMR of GC-MS. Once NMR or MS spectra are recorded, the next step is the quantitative interpretation of the isotopomer data by using mathematical models that describe the relationship between fluxes and the observed isotopomer abundance [96, 97], Some of the mathematical approaches used include cumulative isotopomer (cumomers) [98], bondomers [99], and fractional labeling [100], For a more comprehensive review on the methods we refer to Sauer [91]. 

To fully convert the temporal information from tracer experiments to the spatial distribution of continuous feed injection requires a knowledge of the entire flow field in the vessel. Of course, we do not now have this information in general. It is necessary then to either measure or model the flow field before these results can be fully interpreted. In the interim, tracer experiments are useful in developing mathematical models, model verification, and also in estimating model adjustable parameters. 

INTERPRETATION OF SOME IN-SITU TRACER EXPERIMENTS IN FRACTURED CRYSTALLINE ROCK AT ASPO HARD ROCK LABORATORY... 

The application of isotopes in science, in general, and in analytical chemistry, in particulate is based on two rather contradictory suppositions. Tracer experiments demand the same or very similar behavior of the isotopically modified and unmodified compounds. The study of the effects of isotopes requires the recognition, measurement, and interpretation of minute differences in the chemical and physical properties of compounds that differ only in their isotopic composition (isotopomers). These differences produce a different behavior of the isotopomers in all types of chromatographic processes, and these isotope effects have been demonstrated experimentally. 

Pittells in the Interpretation of Results From Tracer Experiments... 

Although developed in conjunction with radioisotope tracer experiments, these descriptions have broad application in studies of the trophic dynamics of animal communities. Functional interpretation of trophic level exchanges can similarly be made for the cycling of inqxntant nutrient elements, dry matter, and energy flow. Particularly valuable is the ability to predict the transient concentrations of such substances in successive trophic levels and the characteristic response times of interdependent populations to fluctuations elsewhere in the trophic structure. 

It may be noted that certain tracer experiments actually yield an interdiffusion coefficient between two tracers. The diffusion coefficient for oxygen in practice cannot and need not distinguish between the different isotopes in use (usually 0 and 0, more rarely O). However, for protons, the difference to deuteron (or triton) transport coefficients is considerable (e.g. a factor of 2) and should be taken into account when interpreting the results. 

Krambeck, F. J., R. Shinnar and S. Katz. Interpretation of Tracer Experiments in Systems with Fluctuating Throughput. Ind. Eng. Chem. Fundamentals 8 (1969) 431. 

Investigations may be carried out on the tracer level, where solutions are handled in ordinary-sized laboratory equipment, but where the substance studied is present in extremely low concentrations. Concentrations of the radioactive species of the order of 10 m or much less are not unusual in tracer work with radioactive nuclides. A much larger amount of a suitably chosen non-radioactive host or carrier is subjected to chemical manipulation, and the behavior of the radioactive species (as monitored by its radioactivity) is determined relative to the carrier. Thus the solubility of an actinide compound can be judged by whether the radioactive ion is carried by a precipitate formed by the non-radioactive carrier. Interpretation of such studies is made difficult by the formation of radiocolloids, and by adsorption on glass surfaces or precipitates. Tracer studies provide information on the oxidation states of ions and complex-ion formation, and are used in the development of liquid-liquid solvent extraction and chromatographic separation procedures. Tracer techniques are not applicable to solid-state and spectroscopic studies. Despite the difficulties inherent in tracer experiments, these methods continue to be used with the heaviest actinide and transactinide elements, where only a few to a few score atoms may be available [11]. 

Ogston, A.G. (1948). Interpretation of experiments on metabolic processes using isotopic tracer elements. Nature, London, 162, 963. 

Mechanistic Ideas. The ordinary-extraordinary transition has also been observed in solutions of dinucleosomal DNA fragments (350 bp) by Schmitz and Lu (12.). Fast and slow relaxation times have been observed as functions of polymer concentration in solutions of single-stranded poly(adenylic acid) (13 14), but these experiments were conducted at relatively high salt and are interpreted as a transition between dilute and semidilute regimes. The ordinary-extraordinary transition has also been observed in low-salt solutions of poly(L-lysine) (15). and poly(styrene sulfonate) (16,17). In poly(L-lysine), which is the best-studied case, the transition is detected only by QLS, which measures the mutual diffusion coefficient. The tracer diffusion coefficient (12), electrical conductivity (12.) / electrophoretic mobility (18.20.21) and intrinsic viscosity (22) do not show the same profound change. It appears that the transition is a manifestation of collective particle dynamics mediated by long-range forces but the mechanistic details of the phenomenon are quite obscure. 

The simplest and most direct way of finding the E curve uses a physical or nonreactive tracer. For special purposes, however, we may want to use a reactive tracer. This chapter deals in detail with the nonreactive tracer, and for this all sorts of experiments can be used. Figure 11.7 shows some of these. Because the pulse and the step experiments are easier to interpret, the periodic and random harder, here we only consider the pulse and the step experiment. 

Value of Isotopic Labeling. The experiments described demonstrate the usefulness of stable-isotope additions in understanding trophic interactions and biogeochemical fluxes in whole ecosystems. Using stable isotopes as chemical tracers in natural, undisturbed systems is especially helpful in interpreting the results of perturbation experiments. 

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