Tracer molecules

A related mechanism of degradation involves the direct interaction of the radioactive emission with other tracer molecules in the preparation. This phenomenon is likely to occur in high specific activity compounds stored at high radiochemical concentrations in the absence of free-radical scavengers. 

The distribution of tracer molecule residence times in the reactor is the result of molecular diffusion and turbulent mixing if tlie Reynolds number exceeds a critical value. Additionally, a non-uniform velocity profile causes different portions of the tracer to move at different rates, and this results in a spreading of the measured response at the reactor outlet. The dispersion coefficient D (m /sec) represents this result in the tracer cloud. Therefore, a large D indicates a rapid spreading of the tracer curve, a small D indicates slow spreading, and D = 0 means no spreading (hence, plug flow). 

Perfect tracer Tracer molecules behave identically with the process fluid molecules within the process unit. 

Tracer molecules originally in the system at time t = 0 gradually wash out. The exponential form of Equation (15.1) is specific to a CSTR, but the concept of washout applies to any flow system. Consider some time t > 0 when the fraction of molecules remaining in the system is W(t) = C ut(0ICo- These molecules must necessarily have entered the reactor before time t = 0 since no tracer was fed... 

Washout experiments can be used to measure the residence time distribution in continuous-flow systems. A good step change must be made at the reactor inlet. The concentration of tracer molecules leaving the system must be accurately measured at the outlet. If the tracer has a background concentration, it is subtracted from the experimental measurements. The flow properties of the tracer molecules must be similar to those of the reactant molecules. It is usually possible to meet these requirements in practice. The major theoretical requirement is that the inlet and outlet streams have unidirectional flows so that molecules that once enter the system stay in until they exit, never to return. Systems with unidirectional inlet and outlet streams are closed in the sense of the axial dispersion model i.e., Di = D ut = 0- See Sections 9.3.1 and 15.2.2. Most systems of chemical engineering importance are closed to a reasonable approximation. 

Impulse Response and the Differential Distribution. Suppose a small amount of tracer is instantaneously injected at time 1 = 0 into the inlet of a reactor. All the tracer molecules enter together but leave at varying times. The tracer concentration at the outlet is measured and integrated with respect to time. The integral will be finite and proportional to the total quantity of tracer that was injected. The concentration measurement at the reactor outlet is normalized by this integral to obtain the impulse response function. ... 

Experimental determination of the density function requires rapid injection of tracer molecules at the inlet to the system. Ideally, a finite number of molecules will be injected in an infinitesimal period of time. Think of quick injection using a syringe. 

For the special case of self-diffusion (tracer molecules dynamically... 

A mass-isotope dilution method for determining the gamma isomer of benzene hexachloride, in which gamma-hexadeuterobenzene hexachloride is used as a tracer molecule and the dilution is determined by use of infrared spectrophotometry, has been developed by Trenner et al. (52). Impurities have no effect on the accuracy of this method. 

Unlike the situation in a plug flow reactor, the various fluid elements mix with one another in a CSTR. In the limit of perfect mixing, a tracer molecule that enters at the reactor inlet has equal probability of being anywhere in the vessel after an infinitesimally small time increment. Thus all fluid elements in the reactor have equal probability of leaving in the next time increment. Consequently there will be a broad distribution of residence times for various tracer molecules. The character of the distribution is discussed in Section 11.1. Because some of the... 

Radical trapping studies, 14 277 Radicidation, 8 655 Radioactive decay, 21 287—288 particles associated with, 21 291 Radioactive decay properties of uranium isotopes, 25 393 Radioactive emission, interaction with tracer molecules, 21 276 Radioactive iodine, protection from,... 

We point out that the variances of Bx,By, andB are also the variances of the concentration distribution in the three coordinate directions. This result is physically plausible since J , represents the random distance that a fluid particle travels in the x direction between times t and t, and this distance is precisely that which a tracer molecule travels. Consequently, the variance of B is equivalent to the x-direction variance of the concentration distribution. 

The introduction of F in a tracer molecule can be used to block metabolism as,for example, with [ F]FDG,or to detect metabofic processes. Introduction of a fluorine also changes the fipophilidty of the compound. Correlations between hpophihcity (log P) and the abifity of a labelled compound to cross the blood brain barrier has been measured by PET (as an example see [59]). 

E = 137keV). The accompanying emission of 7-radiation can be used for scintigraphic imaging but also makes patient isolation necessary. The different half-lifes and /3 -energies allow individual therapeutic demands such as the pharmacokinetics of the tracer molecule, the linear energy transfer of the nuclides or the biodistribution and clearance of the radiolabeled drug to be met. The principles of the application of radioactive materials for therapy are summarized in an excellent review. ... 

J. Bergman, O. Solln, Fluorlne-18-labeled fluorine gas for synthesis of tracer molecules, Nucl. Med. Biol. 24 (1997) 677-683. 

Let us look at one face of our rectangular control volume (our box) and imagine that we put a tracer on the outside of the box, as shown in Figure 2.2. Initially, the tracer molecules will be distributed uniformly on the outside of the box, with a concentration distribution as shown in Figure 2.2a. However, all of these molecules are vibrating, with inertial movements back and forth. If we look at the tracer... 

On a larger scale, a similar process can occur. Fingering of the tracer is created by layered beds with a low conductivity and lenses with a high conductivity. The tracer that ends up in a lense travels at a relatively high speed. Those tracer molecules will reach the measuring point much sooner than the tracer molecules stuck in the low conductivity beds, thus creating longitudinal dispersion. As the lenses are not all parallel to each other, they will also create a lateral dispersion of the tracer. 

Let x0 and xe denote the respective initial and final (equilibrium) concentrations of the monomer and a0 and af their initial and final specific activities, expressed as mole fraction of the radioactive tracer. Consider an isotope effect which makes the rate of addition of the tracer molecule /-times greater than that of the non-radioactive monomer. Hence, the increase, da, in the specific activity of the residual monomer, arising from the polymerization of —dx moles of monomer (dx < 0), is given by... 

Theory for the self- and tracer-diffusion of a diblock copolymer in a weakly ordered lamellar phase was developed by Fredrickson and Milner (1990). They modelled the interactions between the matrix chains and a labelled tracer molecule as a static, sinusoidal, chemical potential field and considered the Brownian dynamics of the tracer for small-amplitude fields. For a macroscopically-oriented lamellar phase, they were able to account for the anisotropy of the tracer diffusion observed experimentally. The diffusion parallel and perpendicular to the lamellae was found to be sensitive to the mechanism assumed for the Brownian dynamics of the tracer. If the tracer has sufficiently low molecular weight to be unentangled with the matrix, then its motion can be described by a Rouse model, with an added term representing the periodic potential (Fredrickson and Bates 1996) (see Fig. 2.50). In this case, motion parallel to the lamellae does not change the potential on the chains, and Dy is unaffected by... 

To summarize this section, a prerequisite for chromatographic NMR based on solid phases is a fast exchange kinetics among slow and fast diffusion environments of the tracer molecule. This is another analogy with liquid chromatography, where strongly bound molecules are a nuisance to the separation performance, as they tend to remain trapped behind. 

If the concentration of the tracer molecules in the reactor effluent are known, then the measured response will depend on the length of the reactor, the rate of diffusion, and mean fluid velocity. The response... 

Null Claudin- 1 Death at birth, failure to form barrier against a 600-kDa tracer molecule... 

Initially, the deterministic theory was applied to describe the movement of a population of tracer molecules. Briefly, a drug administered as a bolus input into an... 

Sheikov, N., McDannold, N., Jolesz, F., Zhang, Y.Z., Tam, K., and Hynynen, K. (2006) Brain arterioles show more active vesicular transport of blood-borne tracer molecules than capillaries and venules after focused ultrasound-evoked opening of the blood-brain barrier. Ultrasound Med. Biol. 32,1399-1409. 

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