Tracer Penetration Research Articles

Modeling tracer diffusion in fractured and unfractured, unsaturated, porous media

A proposed tracer diffusion test for the Exploratory Shaft Facility at Yucca Mountain, NV, is modeled. For the proposed test, a solution containing conservative tracers will be introduced into a borehole in the geologic medium of interest. The tracers will diffuse and advect from the saturated source region into the unsaturated matrix in the surrounding tuff. After some time, the borehole is to be overcored, and tracer concentrations in the fluid will be measured in the core as a function of distance from emplacement. The data will be used to evaluate diffusive behavior and to derive effective diffusion coefficients for the tracers in the specific tuff. Numerical simulations are used to study the effects of effective diffusion coefficient, porosity, saturation, and fracturing on tracer transport. Results are reported for numerical simulations of tests in the Topopah Spring Member and the Tuff of Calico Hills, which have significantly different porosities and saturations. The simulations make the following predictions: The spread of tracer during the test will be sensitive to the effective diffusion coefficient of the tracer. Tracer will diffuse farther in the Topopah Spring Member than in the Tuff of Calico Hills because of the former's lower porosity and saturation. Tracer transport by advection into the Topopah Spring Member will be greater than that into the Tuff of Calico Hills because of capillary effects. While advection will be a significant mechanism for tracer penetration into the Topopah Spring tuff, it will be less significant for tracer penetration into the Calico Hills tuff. The proximity of a single vertical fracture to the source region determines its effects on tracer transport, especially if the fracture diverts fluid flowing from the source region into the matrix.

Topography of the rat blood-testis barrier after intratubular administration of intercellular tracers

Intratubular injection of electron-opaque tracers (lanthanum hydroxide, peroxidase) by micromanipulation showed that the intercellular spaces of the adluminal compartment are in continuity with the lumen of the rat seminiferous tubule at all stages of the spermatogenic cycle. This continuity involves the intercellular spaces which surround zygotene spermatocytes and late leptotene spermatocytes in stages X–XI. The present observations would seem to cast doubt on the real existence of a third compartment, or intermediate compartment, in the seminiferous epithelium. In the material which, in addition to intratubular administration, had been treated with intravascular tracer perfusion, we found the presence of a parabasal region of interSertoli junctions where the tracer stopped whatever its direction. Freeze-fracture replicas exhibit interSertoli tight junctions arranged, from basal lamina to lumen, in three well-defined patterns: in the most basal area, which is the zone of free penetration of tracers from the peritubular environment, junctions are very sparse or absent; a belt of closely apposed continuous and parallel junctions, also parallel to the basal lamina, is the exclusion zone which prevents tracer penetration from either direction; the most apical membrane areas display irregularly arranged, loosely spaced and frequently interrupted rows of particles, most tending to be perpendicular to the basal lamina. This area corresponds to the territory where the intraluminally injected tracer permeates the interSertoli space.

Tracer diffusion studies in NiO bicrystals and polycrystals

The tracer self-diffusion of nickel in two NiO bicrystals and a polycrystalline specimen at 700° C has been studied using sectioning and autoradiography. The two bicrystal boundaries were 〈110〉 symmetrical tilt boundaries with misorientations of 39.5° and 99.5°, respectively. After diffusion measurements the bicrystals were fractured and the boundaries studied by a variety of surface analysis techniques. The tracer penetration profiles were analysed in terms of lattice diffusion and diffusion along low angle boundaries composed of dislocation arrays introduced by mechanical polishing. The coefficients for lattice and low angle boundary diffusion are in acceptable agreement with previous determinations. Diffusion along high angle boundaries could not be detected by profiling, but could be detected by autoradiography in some boundaries after annealing at high temperature (1400° C) before the diffusion anneal. Autoradiography also revealed that the boundary diffusion was not homogeneous and was sensitive to both thermal history and boundary geometry. The bicrystal boundaries were found to be extensively contaminated by calcium and silicon compounds. It is concluded that high angle boundary diffusion in the present experiments is dominated by these impurities and their distribution. The implications for other measurements of grain boundary properties are discussed and the need for chemical characterization of the actual boundaries used in measurements of boundary properties is emphasized.

Diffusion of 55Fe in Fe 2O 3 single crystals

The diffusion of 55Fe has been measured parallel to the c axis of Fe 2O 3 single crystals at temperatures in the range 708–1303°C and at an oxygen activity of unity. The tracer penetration profiles were determined using sectioning techniques. For temperatures above 900°C the tracer diffusion coefficient is given by D ∗(Fe) = 1.6 × 10 9 exp[−6.0 (eV)/ kT] cm 2 s −1 and below 900°C by 2.8 × 10 −9 exp[−1.8 (eV) kT]. The high-temperature behaviour is probably characteristic of pure Fe 2O 3, whereas diffusion at lower temperatures may be influenced by impurities. The most likely defects responsible for diffusion of Fe are iron interstitials and, for oxygen, oxygen vacancies, and the observed activation energies are discussed in terms of the properties of these defects. The diffusion data and defect models have been used to predict the rate of growth of Fe 2O 3 and indicate that outward Fe diffusion is the dominant transport process. Previously published data for Fe 2O 3 growth in a variety of experimental situations have been corrected to a single rate constant using a model for multilayer growth. The corrected data are all in good agreement but are approximately two orders of magnitude greater than predicted from diffusion data, which suggests that grain boundary diffusion controls the growth of Fe 2O 3 in practice.

Comparison of four thermocycling techniques

This investigation revealed no significant difference among the four thermocycling techniques. The use of a dye or an isotope was equally effective and penetrated the tooth/restoration interface to a similar degree. The extent of tracer penetration appeared to be independent of the dwell time in the thermal baths. All procedures involving thermal changes were more potent in demonstrating leakage than the noncycled method.

Soil-water residence time and solute uptake: 2. Dye tracing and preferential flow predictions

The occurrence of preferential flow, by-passing soil peds, is predicted from the occurrence of rainfall intensities in excess of pedal sorption; depth of preferential tracer penetration is estimated from the displacement of this excess through a small operational volume of preferential flow, θ f. θ f-values estimated from column drainage experiments to lie between 0.01 and 0.03 cm 3 cm −3 for rainfall intensities from 1 to 10 mm hr. −1, increasing to ∼ 0.1 cm 3 cm −3 at saturation. Observed tracer penetration is compared with predicted tracer penetration and mean profile θ f-values are calculated. The preferential flow calculations discussed provide a useful approximation for characterising field tracer behaviour.

A mathematical analysis of diffusion in dislocations. II. Influence at low densities on measured diffusion coefficients

For pt.I see ibid., vol.14, p.3863 (1981). Previous calculations are applied to estimate the contribution relatively small concentrations of dislocations of dislocations make to the diffusion coefficients defined by appropriate gradients of the initial part of conventional tracer penetration plots and usually presumed to represent volume diffusion. A first-order expression is derived for the effective, or dislocation perturbed, diffusion coefficient as a function of dislocation density using an empirical analytic representation of the computed numerical values of mean tracer concentration as a function of penetration distance.

Impermeability of hagfish cerebral capillaries to radio labelled polythylene glycols and to microperoxidase

The rate and extent of penetration of radiolabelled polyethylene glycols (900 and 4000 Daltons) from plasma into brain was examined in the Atlantic hagfish ( Myxine glutinosa), a primitive vertebrate, and, for comparison, in the rat. Tracer penetration into brain was limited to a similar extent in hagfish and in rats, indicating a basic similarity between the two species in the blood-brain barrier to polar non-electrolytes. The permeability of hagfish brain capillaries to the electron microscopic tracer microperoxidase (2000 Daltons) was also examined. Intravenously injected microperoxidase remained confined to the capillary lumen and was not found in the basal lamina or surrounding brain parenchyma. In contrast to an earlier study, these results indicate that the hagfish has a blood-brain barrier to hydrophilic molecules larger than 900 Daltons. It is concluded that a blood-brain barrier is a general characteristics of all vertebrates.

A vertebrate-like blood-brain barrier, with intraganglionic blood channels and occluding junctions, in the scorpion

India ink and ionic lanthanum injections have revealed that the central nervous system (CNS) of the scorpion possesses a highly vascularized cephalothoracic ganglionic mass. It, together with other abdominal ganglia which form a ventral nerve cord, are all ensheathed by an outer layer of modified glial, or perineurial, cells. These cells resemble those which line the blood channels permeating the CNS, in exhibiting both inverted gap and tight junctions. Although the latter show close or fused membrane appositions, lanthanum appears to penetrate past a number, but not all, of them. Freeze-fracturing reveals that these junctions are composed of E-face particles aligned into a network of rows, or ridges, which are frequently discontinuous, especially near the periphery of the perineurium. This produces a somewhat ‘leaky’ system but occlusion to tracers occurs ultimately, for in the CNS none can be found beyond the perineurium. The existence of this perineurial blood-brain barrier is also demonstrable electrophysiologically where cations such as Mg 2+ are unable to penetrate beyond the perineurial layer although they can, it seems, leak in via the blood vascular system. Relative differences in tightness between the perineurium and the cells lining the blood channels may be attributed to differences in the relative number of discontinuous ridges. This is borne out by the observation that the peripheral nervous system has a highly attenuated perineurium with many fewer junctions, and some of these nerves tend to be leaky with respect to tracer penetration. In fixed material the junctional ridges may fracture on to the E-face or partly on both the EF and PF, while in unfixed tissue they are usually found on the PF. In both cases they exhibit complementary grooves that are coincident with the ridges across membrane transitions; in such cases the cell membranes are fused with concomitant obliteration of the intercellular space. These tight junctions, often closely associated with EF gap junctional particle aggregates which may be very loosely clustered, appear to form the basis of the observed blood-brain barrier in the scorpion CNS.

The evaluation of radio-frequency sputtering as a microsectioning technique for the tracer diffusion studies in oxides

Tracer penetration profiles in NiO diffusion specimens were determined using the radio-frequency sputtering technique. The sources of error introduced by the technique were investigated, and a comparison was made between the profiles determined using this method and those determined in similar specimens using a polishing technique. With sectioning by sputtering it is possible to analyse diffusion profiles having Dt > 10 -13 cm 2, and diffusion coefficients greater than 10 -19 cm 2 s -1 can be measured with an accuracy to ±10%.

Tracer Concentration Gradients for Diffusion Coefficients Exponentially Dependent on Concentration

A diffusion equation having a tracer diffusion coefficient exponentially dependent on solute concentration, D*=D0* exp (βC), was solved numerically for thin‐film and thick‐film boundary conditions and varying values of annealing time, film thickness, and β. For values of β in the range 3 to 10 mol fraction−1, the calculated distribution of solute remained remarkably Gaussian. The apparent diffusion coefficient evaluated from such solute gradients was greater than the true tracer diffusion coefficient D0* by ≤20% for large β and shallow tracer penetration. The results were used to estimate possible errors in data which have been reported for several oxide systems, and may be used as a guide for the selection of experimental parameters so that corrections will not be necessary.

The effect of benzalkonium chloride on the electropotential of the rabbit cornea.

The effect of benzalkonium chloride on the electropotenial of the cornea has been examined. The anterior surface of the in vivo or in vitro cornea was exposed to various concentrations of the surfactant, from 0.005% to 0.02%, for either 1 or 2 min. The initial effect is a hyperpolarization lasting up to 30 sec, followed by a rapid fall in potential difference with a subsequent recovery. The degree of potential difference decrease and the recovery rate was dependent upon both the concentration of the detergent, and the exposure time. There is excellent correlation between the previous anatomical and physiological studies on tracer penetration across the in vivo and in vitro cornea and our present work. The data indicate that benzalkonium chloride acts by breaking down the physiological and anatomical diffusion barrier to solute and solvent which is located in the outer layer of the epithelium.

Geosecs North Atlantic radiocarbon and tritium results

Radiocarbon data for 11 stations and tritium data for 16 stations in the North Atlantic Ocean from 74°N to 3°N are presented. For radiocarbon, normal errors in Δ 14C are± 4‰ , and in tritium, ± 0.09TU or± 3%, whichever is larger. There is a remarkable, but not simply linear, correlation between oceanic bomb transients in 14C and 3H. The deep convective mixing in the Greenland Sea is reflected in substantial bomb tracer penetration to all depths, with residence time for the deep, cold core water that seems to be 20 to 30 years. The outflow in the bottom layer southward over the sills of the Denmark Strait and Faroe Passage carries significant tritium concentration, at least to 40°N. Complicated, but coherent, profile structures in the subtropical Atlantic suggest effects of large-scale lateral advection. In particular, a pronounced minimum in both 14C and 3H might be associated with the Antarctic intermediate water.

Self-Diffusion along Dislocations in Single-Crystal Au Films

Self-diffusion measurements over the temperature range 247-352\ifmmode^\circ\else\textdegree\fi{}C have been made on single-crystal Au films of $2\ensuremath{-}\ensuremath{\mu}$ thickness grown epitaxially onto (001) MgO substrates, using ${\mathrm{Au}}^{195}$ radioactive tracer and rf sputter-etching techniques for serial sectioning. The penetration profiles revealed shallow lattice diffusion and a much deeper tracer penetration down dislocations; the former is significantly enhanced by diffusion in dislocations. The activation energy ${Q}_{d}$ and the combined preexponential factor ${A}_{d}{D}_{d}^{0}$ for self-diffusion along dislocations are found to be 1.16 \ifmmode\pm\else\textpm\fi{} 0.02 eV and 5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{4}$/sec, respectively. From the enhanced lattice diffusion observed in the first stage of the penetration profiles, a dislocation density of \ensuremath{\sim}${10}^{11}$ lines/${\mathrm{cm}}^{2}$ could also be evaluated for the single-crystal films. The data compare well with other fcc metals and appear to favor a vacancy diffusion mechanism.

Atom Transport Under Varying Pressure * *

Abstract Study of the effect of hydrostatic pressure on the rate of diffusion-limited processes gives in­ formation regarding the intrinsic volume changes associated with formation and motion of defects. While there are basic problems associated with interpretation of the motional volume, such effects may be measured directly in some cases and subtracted to give unambiguous results for the defect formation volume. Experimentally, precise measurement of specimen temperature within a high-pressure environment poses the most severe practical constraint. Results are given for measurements to pressures of 7 kilobars of ionic conductivity in a series of pure and doped alkali halides: NaCl, KCl, NaBr, and KBr, all of which are predominantly cationic vacancy conductors. The activation volumes are found to be pressure dependent in the intrinsic regions. In all cases the Schottky pair vacancy formation volume is found to be larger than the molar volume, the ratio being largest for NaCl and smallest for KBr. Results are also given for the total activation volume for diffusion of Ti44 in beta-Ti, a case of "anomalous" bulk diffusion. The tracer penetration plots do not follow a perfect gaussian profile. However, if the penetration plots are fit to a single gaussian, the activation volume derived is consistent with that expected for a vacancy mechansm in bcc metals

Some Observations on the Effect of Surface Roughness upon Diffusion

The effect of surface roughness has been investigated experimentally and found to lead to spuriously high values of the diffusion coefficient. The magnitude of the error has been estimated as a function of Dt and the degree of roughness. The results indicate that surface roughness may contribute the major source of error in the determination of very small diffusion coefficients. The presence of an uneven source surface may also account for the anomalous tracer penetration profiles which are observed in the vicinity of the surface and, corresponding to which, anomalous values of D have been frequently reported.

Studies on Feline Distemper

Muscle and bone are known to act as a functional unit and communicate biochemically during tissue development and maintenance. Muscle-derived factors (myokines) have been found to affect bone functions in vitro. However, the transport times of myokines to penetrate into bone, a critical step required for local muscle–bone crosstalk, have not been quantified in situ or in vivo. In this study, we investigated the permeability of the periosteum, a major barrier to muscle–bone crosstalk by tracking and modeling fluorescent tracers that mimic myokines under confocal microscopy. Periosteal surface boundaries and tracer penetration within the boundaries were imaged in intact murine tibiae using reflected light and time-series xz confocal imaging, respectively. Four fluorescent tracers including sodium fluorescein (376 Da) and dextrans (3 kDa, 10 kDa and 40 kDa) were chosen because they represented a wide range of molecular weights (MW) of myokines. We found that i) murine periosteum was permeable to the three smaller tracers while the 40 kDa could not penetrate beyond 40% of the outer periosteum within 8 h, suggesting that periosteum is semi-permeable with a cut-off MW of approximately 40 kDa, and ii) the characteristic penetration time through the periosteum (~ 60 μm thick) increased with tracer MW and fit well with a relationship tcs=−4.43×104−0.57×MWDa−4×104−8.65×108MWDa−4×104, from which, the characteristic penetration times of various myokines were extrapolated. To achieve effective muscle–bone crosstalk, likely signaling candidates should have shorter penetration time than their bioactive time, which we assumed to be 5 times of the molecule's half-lifetime in the body. Myokines such as PGE2, IGF-1, IL-15 and FGF-2 were predicted to satisfy this requirement. In summary, a novel imaging approach was developed and used to investigate the transport of myokine mimicking-tracers through the periosteum, enabling further quantitative studies of muscle–bone communication in physiologically normal and pathological conditions.