Simple Tracer Studies

The determination of radioactivity distributions and migratory aptitudes Before it is possible to consider the intimate mechanism of a reaction, it is important to know the identities and the yields of the products. When these products are dilferent chemical compounds, the isotope-dilution method, just discussed, is very useful for this purpose. There are many chemical reactions, however, which proceed by tvx or more different paths to the same chemical entity. By proper choice of an isotopic label, coupled with unequivocal degradative methods, the relative contributions of these different paths can often be evaluated. 

Very recently (Kwart et al. 1961) the rearrangements of alloxan and several of its derivatives have been shown to take place with nitrogen-to-carbon rather than with carbon-to-carbon rearrangement. Thus, alloxan-5-i C [24a, = R = H] was subjected to rearrangement 

By oxidation of the labeled products [28 and 29 or 30ab] to labeled benzoic and substituted benzoic acids followed by assay of these purified acids for carbon-14, the relative migratory abilities of the Ar groups with respect to phenyl were determined. Thus in the dehydration, with phosphorus pentoxide in xylene, of a series of carbinols (27, X = OH), the following migratory aptitudes (Ar Ph) were determined p-xenyl, 1-3 m-tolyl, 1-6 p-i-propylphenyl, l-8 3,4-dimethyl-phenyl, 1-9 p-tolyl, 2-0 p-ethylphenyl, 2-2 p-t-butylphenyl, 3-2 and p-anisyl, 21-2. Burr (1953) demonstrated the p-tolyl phenyl migration ratio to be 2-5 during the solvolysis of 2-phenyl-2- p-tolylethyl-l-i C tosylate (27, Ar = p-tolyl, X = tosyl). In a series of deaminations of the primary amines (27, X = NH ) and 31 (Ar = p-tolyl and o-tolyl) the p-tolyl phenyl (1 2), p-biphenyl phenyl (1 0) p-anisyl phenyl 

Upon oxidation of 32 with chromic acid to benzoic acid (35), followed by a Schmidt degradation of the benzoic acid, it was demonstrated that the carboxyl group of 35 contained 1/7, whereas the phenyl group contained 6/7 of the total molar radioactivity, a result consistent with the intervention of the tropylium ion (C). 

Since tritium was transferred from benzylmagnesium chloride (37) to the triphenyhnethane (38) produced despite work-up with untritiated aqueous acid, the intermediate D was proposed to explain this unusual observation. The reaction of triphenylacetonitrile with benzylmagnesium chloride had previously been considered as a simple functional exchange. 

---

Kinds oi Inputs Since a tracer material balance is represented by a linear differential equation, the response to anv one kind of input is derivable from some other known input, either analytically or numerically. Although in practice some arbitrary variation of input concentration with time may be employed, five mathematically simple input signals supply most needs. Impulse and step are defined in the Glossaiy (Table 23-3). Square pulse is changed at time a, kept constant for an interval, then reduced to the original value. Ramp is changed at a constant rate for a period of interest. A sinusoid is a signal that varies sinusoidally with time. Sinusoidal concentrations are not easy to achieve, but such variations of flow rate and temperature are treated in the vast literature of automatic control and may have potential in tracer studies. 

The engineer is offered a large variety of flow-modeling methods, whose complexity ranges from simple order-of-magnitude analysis to direct numerical simulation. Up to now, the methods of choice have ordinarily been experimental and semi-theoretical, such as cold flow simulations and tracer studies. 

The time variations of the effluent tracer concentration in response to step and pulse inputs and the frequency-response diagram all contain essentially the same information. In principle, any one can be mathematically transformed into the other two. However, since it is easier experimentally to effect a change in input tracer concentration that approximates a step change or an impulse function, and since the measurements associated with sinusoidal variations are much more time consuming and require special equipment, the latter are used much less often in simple reactor studies. Even in the first two cases, one can obtain good experimental results only if the average residence time in the system is relatively long. 

Adsorption is another phenomenon particularly suited to tracer studies. It plays an important role in the glueing and finishing of wood. In a preliminary study of the adsorption of poly(vinyl acetateJ4C) on smooth geometrically simple surfaces, Weatherwax and Tarrow (66) were able to show that swollen cellophane adsorbed 40 times as much as the other surfaces tested. In the course of this work they were able to measure specific adsorptions of as little as 0.2 pg/cm2 with good accuracy. 

Industrial reactors are usually more complex than the simple simulator library models. Real reactors usually involve multiple phases and have strong mass transfer, heat transfer, and mixing effects. The residence time distributions of real reactors can be determined by tracer studies and seldom exactly match the simple CSTR or PFR models. 

Since 1958, more than 20 nuclides of actinides ranging from neptunium to einsteinium were identified and prepared for tracer studies. From neutron-irradiated uranium samples 2 9Np was adjusted to the pentavalent state and separated by TBP extraction from perchloric acid media. Plutonium-239 was separated by TBP extraction from nitric acid solution followed by anion exchange in a system of Dowex-1 resin and nitric acid. Neptunium-237 was separated from a spent fuel solution of JRR-1 (Japan Research Reactor -1) using anion exchange and TBP extraction. The TBP extraction in the hydrochloric acid medium is a simple and effective technique to purify neptunium from plutonium contamination. On the other hand, both anion exchange and solvent extraction with HDEHP could be used to separate tracer scale plutonium from irradiated neptunium targets. 

The genesis of the tropolone ring was more problematic. Tracer studies on simple mold tropolones such as puberulic acid (CXII) (66) and stipita-tic acid (CXIII) (67) have indicated that these compounds are at least partly derived from acetate. However, sodium acetate-l- C was found (6, 61) to be incorporated entirely into the A-acetyl function of colchicine and not at all into demecolcine (6). 

With the closing of the low-energy accelerator at Michigan State University and the subsequent construction of a new heavy-ion facility there, it was desirable to investigate the feasibility of producing with the less optimal heavy-ion beams. Although it was anticipated that the heavy-ion reactions would produce substantial quantities of and F, we hoped that simple chemical techniques, which we had already developed, would be adequate to purify the sources. Thus, if sujBBcient quantities of were produced by heavy-ion reactions, it would be possible to continue our tracer studies with these beams. 

However, a Fe tracer study shows that the reaction is not a simple sequential process and is far more complex. Iron(iii) complexes of salicylaldehyde and pyruvic acid thio- and seleno-semicarbazones and salicylaldehyde-4-phenylthiosemicarbazone have been prepared. It was found that small changes... 

In order to perform a simple competition study it is necessary to first determine the minimal concentration of competitor needed to saturate its own transport. To calculate intracellular concentrations of microinjected macromolecules we have taken the accessible volume of the nucleus to be 40 nl and the cytosol to be 500 nl (Gurdon and Wickens, 1983). In practice, saturation concentrations are operationally defined as the minimal unlabeled substrate concentration necessary to reduce significantly the rate of labeled tracer substrate transport. If too high a concentration of unlabeled competitor is used, then nonspecific inhibitory effects on tracer substrate transport can be expected. 

Recent tracer studies suggest the same pattern for other simple coumarins as well. Aesculetin was reported in the early 1970s not to be formed from the analogously oxygenated caffeic acid (10) in Cichorium intybus (chicory), in which it occurs primarily as the 7-glucosyloxy derivative. 

Siegenthaler, U. and Joos, F. (1992). Use of a simple model for studying oceanic tracer distributions and the global carbon cycle, Tellus 44B, 186-207. 

Since the discovery of the first noble gas compound, Xe PtF (Bartlett, 1962), a number of compounds of krypton, xenon, and radon have been prepared. Xenon has been shown to have a very rich chemistry, encompassing simple fluorides, XeF2> XeF, and XeF oxides, XeO and XeO oxyf luorides, XeOF2> XeOF, and Xe02 2 perxenates perchlorates fluorosulfates and many adducts with Lewis acids and bases (Bartlett and Sladky, 1973). Krypton compounds are less stable than xenon compounds, hence only about a dozen have been prepared KrF and derivatives of KrF2> such as KrF+SbF, KrF+VF, and KrF+Ta2F11. The chemistry of radon has been studied by radioactive tracer methods, since there are no stable isotopes of this element, and it has been deduced that radon also forms a difluoride and several complex salts. In this paper, some of the methods of preparation and properties of radon compounds are described. For further information concerning the chemistry, the reader is referred to a recent review (Stein, 1983). 

Whether to use as perturbing function a step, pulse, or cycled feed depends on the particular system under study. For expensive tracers, a pulse is often mandatory. However, simple textbook relations based on a Dirac function do not usually apply, for a relatively long pulse may be required to get a good signal. A long enough pulse becomes two step functions, and as already men-... 

Radiotracers are uniquely well suited to such studies. The sensitivity of detection means that only very small amounts of tracer need be added to follow the chemical pathway of the relevant species. Furthermore, it matters little what the physical or chemical state of the tracer is, for measurements may be made on liquids, solids or gases. Chromatography, solvent extraction and precipitation are amongst separation methods widely studied by means of radiotracers. In the individual separation steps the distribution of the species may be studied by simple radioactivity measurements, and subsequently the tracer will serve as a yield indicator for the overall procedure. 

---