Isotopic Coefficients Research Articles

Operando Isotopic Exchange in Solid Oxide Fuel Cells: Oxygen-Transport Dependency on Applied Potential.

The oxygen isotopic exchange technique is a powerful tool to investigate the oxygen transport kinetics in an oxide solid. In a solid oxide fuel cell, isotopic surface exchange and diffusion coefficients are classically determined by using the Isotopic Exchange Depth Profiling method followed by ex situ SIMS characterizations. Despite its relevance, the utilization of in situ or operando techniques to measure the isotopic exchange under an electrical bias remains marginal. We developed here a set-up which enables operando monitoring of oxygen exchange in SOFC type cells under polarization. The system has been used for studying the oxygen mobility dependency upon polarization on a symmetrical Pt/YSZ/Pt cell (YSZ: yttria-stabilized zirconia). Homomolecular and heterolytic exchange reactions were undertaken to investigate the oxygen activation step and discriminate the limiting step among the sequence of elementary steps which constitute the oxygen transport process in the SOFC system. Oxygen ions incorporation into the dense ionic conductor was identified to be the rate determining step, and its first order rate constant dependency on applied potential was established.

An analytical formulation of isotope fractionation due to self-shielding

Isotope fractionation due to photochemical self-shielding is believed to be responsible for the enrichment of inner solar system planetary materials in the rare isotopes of carbon, nitrogen, and oxygen relative to the Sun. Self-shielding may also contribute to sulfur isotope mass-independent fractionation in modern atmospheric sulfates, although its role in the early Earth atmosphere has not yet been convincingly established. Here, I present an analytical formulation of isotopic photodissociation rate coefficients that describe self-shielding isotope signatures for 3 and 4-isotope systems broadly representative of O and S isotopes. The analytic equations are derived for idealized molecular spectra, making an analytic formulation tractable. The idealized spectra characterize key features of actual isotopologue spectra, particularly for CO and SO2, but are applicable to many small molecules and their isotopologues. The analytic expressions are convenient for evaluating the magnitude of isotope effects without having to pursue involved numerical solutions. More importantly, the analytic expressions illustrate the origin of particular isotope signatures, such as the previously unexplained large mass-dependent fractionation associated with photodissociation of optically-thick SO2. The formulation presented here elucidates the origin of some of these important isotopic fractionation processes.

Modeling the carbon isotope signatures of methane and dissolved inorganic carbon to unravel mineralization pathways in boreal lake sediments

Vertical profiles of the concentration and isotopic composition (δ13C) of methane (CH4) and dissolved inorganic carbon (DIC), as well as of ancillary parameters, were obtained in the top 25 cm of a sediment column in a seasonally anoxic basin from an oligotrophic boreal lake. Modeling the profiles of CH4 and DIC concentrations and those of their δ13C signatures with reaction-transport equations allowed us to determine the organic matter (OM) degradation rates according to various reactions and to constrain the in situ isotopic fractionation factors and diffusivity coefficients of CH4 and DIC. This exercise reveals inter alia that (i) CH4 production occurs below a depth of 5 cm, with the highest production rate between 5 and 7.5 cm depth, (ii) all CH4 is produced through hydrogenotrophy, and (iii) methanogenesis yields a production rate of CH4 about three times greater than that of DIC. This latter observation indicates either that fermentation of OM is not the exclusive source of H2 sustaining hydrogenotrophy, or that the commonly assumed model molecule CH2O does not adequately represent the fermenting OM, since its fermentation yields identical rates of CH4 and DIC production. The porewater profiles of Fe and SO42- suggest that some H2 may be produced during the reoxidation of reduced sulfur by Fe(III), but the rate of H2 production via this process, if active, would be insignificant in comparison to that required to sustain the estimated rate of hydrogenotrophy. We deduce that the imbalance between CH4 and DIC production rates is rather due to the fermentation of organic substrates that are more reduced than CH2O, i.e., having a negative average carbon oxidation state (COS). From the constraints on reaction rates and on fermentation pathways imposed by the δ13C data, we infer that the organic substrate fermenting between depths of 5 and 7.5 cm should have a COS of −1.87. We thus submit that CH4 is produced in the sediments of the seasonally anoxic basin of our boreal lake through hydrogenotrophy coupled to the fermentation of reduced organic substrates that can be represented by a mixture of fatty acids (e.g. C16H32O2; COS of −1.75) and fatty alcohols (e.g., C16H34O; COS of −2.00). This study emphasizes the importance of characterizing the sedimentary OM undergoing mineralization in order to improve diagenetic model predictions of CH4 cycling in boreal lakes and of its significance in climate change.

Discerning the interactions between environmental parameters reflected in δ13C and δ18O of recent fluvial tufas: Lessons from a Mediterranean climate region

δ13C and δ18O of recent, continuous tufa records, obtained during a monitoring period spanning 3 to 13years, are compared with the corresponding, known environmental conditions. Three rivers in NE Iberia (located along a 200-km N–S transect) are used for this comparison. The isotopic variations through space and time are discussed in terms of the environmental and geological parameters that operate on different scales, focusing on discerning the interactions between these parameters and providing examples of possible misinterpretation of climatic conditions, which is important to past climate studies based on isotopic data.The calculation of the actual isotopic fractionation coefficients, and the comparison with the literature-derived coefficients, demonstrates that the studied tufa formation was close to isotopic equilibrium to reflect the water temperature. The difference between mean measured water temperature (Tw) and mean calculated Tw (based on δ18Ocalcite and measured δ18Owater) is less than 2.7°C. Tendencies of these calculated Tw are similar to the regional air temperature (Tair) tendencies through time, in particular in the case of the 13-year record, although certain deviations exist over shorter time spans. The best agreement between measured and calculated Tw and between δ18Ocalcite-based Tw tendencies and Tair tendencies corresponds to the tufa stromatolite facies.Differences between the δ18Ocalcite records of the three rivers cannot be attributed to temperature changes, but to the varying influences of groundwater inputs and isotopic rainfall composition in each river. Without considering these parameters, δ18Ocalcite-based Tw calculations yield inaccurate results when comparing the study sites.δ13Ccalcite values do not exhibit distinct patterns over time, and δ13Ccalcite variations are likely caused by local processes that do not reflect general environmental changes.These findings underscore the significance of accounting for both groundwater behaviour and rainfall stable isotope composition when interpreting climate parameters in carbonate systems, particularly when differences between the isotopic signatures of deposits exist in the same region.

Microbial immobilisation of phosphorus in soils exposed to drying-rewetting and freeze-thawing cycles

Drying-rewetting and freezing-thawing can drastically alter P availability in soil. We studied how these weather events affect microbial immobilisation/mobilisation of P on the four soil types from a climatic gradient with increasing annual mean temperatures and a progressive decrease in precipitation: Podzol, Phaeozem, Chernozem and Kastanozem. Soils were exposed to (1) optimal moisture and temperature, (2) drying-rewetting and (3) freezing-thawing. Soils were treated with a 33P spike immediately after rewetting or thawing to simulate P pulse. Thereafter, P immobilisation by soil microorganisms was estimated by direct fumigation and anion exchange membrane techniques. To ensure correct estimation of microbial P (Pmic), the conversion factors kP were determined individually for each soil by 33P labelling with the correction for 33P sorption and 31P–33P isotopic exchange. The membrane extraction minimised both sorption and isotopic exchange of P released with both sorption and isotopic exchange coefficients close to 0.9 irrespectively of the soil. Specific kP varied from 0.19 to 0.38. Pmic values followed the pattern freezing-thawing < drying-rewetting < optimal conditions, varying from 2.0 to 36.6 mg P kg−1. Intensive microbial immobilisation of 33P after rewetting (up to 41 %) demonstrated a conversion of dissolved P to Pmic potentially available for plant nutrition. Remarkably, no detectable microbial immobilisation of 33P was found in the freezing-thawing treatment. In contrast to drying, freezing decreased total Pmic by up to 7.5 times. Values of membrane-extractable 33P increased in the order drying-rewetting < control < freezing-thawing, and up to 77 % of added 33P was recovered in dissolved P forms after thawing, indicating the potential risk of P leaching after freezing-thawing events.

Sulfur isotopic fractionation and source appointment of PM2.5 in Nanjing region around the second session of the Youth Olympic Games

Sulfur isotopic compositions (δ34S) of PM2.5 in Nanjing region were determined in order to evaluate sulfur sources of PM2.5 around the second session of the Youth Olympic Games (YOG). Meanwhile, δ34S values from different potential sources of PM2.5, such as coal combustion, vehicle exhaust, and straw burning, were synchronously measured for the first time. The results showed that PM2.5 concentrations and δ34S values were much lower during the YOG (2 August to 11 September 2014) compared to those before (6 July to 1 August 2014) and after (15 September to 20 September 2014) the YOG. δ34S values of PM2.5 were generally located in a small range of 3.5‰ to 4.6‰, suggesting a stable sulfur sources in Nanjing region around the YOG. Combining with δ34S values of potential sources and SO2, ion concentrations of PM2.5 and MODIS fire spot photos, we inferred that sulfur in PM2.5 was mainly from direct emission of coal combustion and vehicle exhaust as well as the secondary sulfate from SO2 oxidation in the atmosphere. Besides, biologic sulfur release might make a contribution to sulfur content in PM2.5. High [NO3−]/[SO42−] ratio of PM2.5 indicated that vehicle exhaust emission was predominant over coal combustion during the YOG. In addition, we studied sulfur isotopic fractionation coefficients during SO2 oxidation to sulfate in PM2.5 and found the contribution ratio (51.3%) of SO2 homogeneous oxidation was slightly higher than that (48.7%) of SO2 heterogeneous oxidation around the YOG.

A sharper characterization of the geographical origin of Lebanese wines by a new interpretation of the hydrogen isotope ratios of ethanol

The hydrogen isotope ratios of the methyl [(D/H)I] and methylene [(D/H)II] groups in ethanol can be used as geographical origin tracers of wines. These ratios reflect not only the deuterium content of sugars but also that of water in the grape medium, and hence are influenced by the irrigation, the degree of ripeness at harvest, and other viticultural and enological practices. Thus, the isotopic redistribution coefficients between water in the grape must and the methyl and methylene sites of ethanol were determined and then used to compute an isotopic ratio [(D/H)Is] representing the contribution of the fermentable sugars to (D/H)I. To evaluate the discrimination power of (D/H)Is, the ethanol D/H ratios of 78 Lebanese wine samples from different vineyards were determined using the SNIF-NMR method. An improved classification at the subregional level was obtained using (D/H)Is which exhibits an inverse linear correlation with the annual precipitation amount. Furthermore, the variations of the ethanol D/H ratios with the degree of grape ripeness and with the juice fractions obtained from the inner and peripheral pulps of the grape berries were studied on a small sample set. These factors should be considered when using the SNIF-NMR results to characterize the geographical origin of wines.

Denitrification and inference of nitrogen sources in the karstic Floridan Aquifer

Abstract. Aquifer denitrification is among the most poorly constrained fluxes in global and regional nitrogen budgets. The few direct measurements of denitrification in groundwaters provide limited information about its spatial and temporal variability, particularly at the scale of whole aquifers. Uncertainty in estimates of denitrification may also lead to underestimates of its effect on isotopic signatures of inorganic N, and thereby confound the inference of N source from these data. In this study, our objectives are to quantify the magnitude and variability of denitrification in the Upper Floridan Aquifer (UFA) and evaluate its effect on N isotopic signatures at the regional scale. Using dual noble gas tracers (Ne, Ar) to generate physical predictions of N2 gas concentrations for 112 observations from 61 UFA springs, we show that excess (i.e. denitrification-derived) N2 is highly variable in space and inversely correlated with dissolved oxygen (O2). Negative relationships between O2 and δ15NNO3 across a larger dataset of 113 springs, well-constrained isotopic fractionation coefficients, and strong 15N:18O covariation further support inferences of denitrification in this uniquely organic-matter-poor system. Despite relatively low average rates, denitrification accounted for 32 % of estimated aquifer N inputs across all sampled UFA springs. Back-calculations of source δ15NNO3 based on denitrification progression suggest that isotopically-enriched nitrate (NO3–) in many springs of the UFA reflects groundwater denitrification rather than urban- or animal-derived inputs.

Theory of isotopic fractionation on facetted ice crystals

Abstract. The currently used "kinetic-fractionation" (KF) model of the differential incorporation of water-molecule isotopologues into vapor-grown ice omits surface processes on crystal facets that may be important in temperature reconstructions. This article introduces the "surface-kinetic" fractionation model, a model that includes such surface processes, and shows that differences in deposition coefficients for water isotopologues can produce isotopic fractionation coefficients that significantly differ from those of KF theory. For example, if the deposition coefficient of H218O differs by just 5% from that of ordinary water (H216O), the resulting fractionation coefficient at 20% supersaturation may deviate from the KF value by up to about ±17‰, and even more at greater supersaturation. As a result, the surface-kinetic theory may significantly change how fractionation depends on supersaturation. Moreover, the model introduces possible new temperature dependencies from the deposition coefficients. These parameters need to be constrained by new laboratory measurements.

Studies on a synthetic sitinakite-type silicotitanate cation exchanger: Part 1: Measurement of cation exchange diffusion coefficients

A commercially available synthetic silicotitanate (IONSIV ® IE-911), with a sitinakite-like structure, is a candidate material for the remediation of aqueous nuclear waste streams. These streams are often complex in their cation composition. This work investigates the effect of the presence of Na, K, Mg, Ca and Ba on the rates of removal of Cs and Sr radioisotopes as cations of importance in aqueous nuclear waste streams. Results are compiled as isotopic diffusion coefficients, and by analysis of changes in pH with time, observed as the systems reach equilibrium. This kinetic treatment provides necessary data for use in the modelling of plant performance and for the accurate determination of distribution coefficients which will be the subject of the second part in this series.

Effect of N isotopic mass on the photoluminescence and cathodoluminescence spectra of gallium nitride

GaN is a wurtzite-type semiconductor at ambient conditions whose natural composition consists of almost pure 14N (99.63% 14N and 0.37% 15N) and a mixture of 60.1% 69Ga, and 39.9% 71Ga. We report a low-temperature photoluminescence and cathodoluminescence study of GaN thin films made from natural Ga and N, and from natural Ga and isotopically pure 15N. The contribution of the nitrogen vibrations to the bandgap renormalization by electron-phonon interaction has been estimated from the nitrogen isotopic mass coefficient of the bound exciton energy. The temperature dependence of the bandgap of GaN can be explained with the measured isotopic mass coefficients of Ga and N. We have estimated the aluminum and indium contribution to the bandgap renormalization in AlN and InN from the temperature dependence of the AlN and InN bandgap up to 300 K, assuming that the N contribution is similar to that found in GaN. The similar bandgap isotopic mass coefficients of C, N, and O, of Al, Si and P, of Zn, Ga and Ge, and of Cd and In suggests that elements of the same row of the periodic table have similar bandgap isotopic mass coefficients.

Effect of isotopic mass on the photoluminescence spectra of zinc oxide

ZnO is a wurtzite-type semiconductor at ambient conditions whose natural composition consists of almost pure 16O and a mixture of 48.6% 64Zn, 27.9% 66Zn, 4.1% 67Zn, 18.8% 68Zn, and 0.6% 70Zn. In this work, we report a photoluminescence study of different samples of isotopically pure and natural zinc oxide between 15 and 300 K. The isotopic coefficients of the bound exciton energy have been obtained and compared with previous values for the shift of the free A exciton energy. The isotopic coefficients of the bound exciton energy have allowed us to estimate the contribution of the zinc and the oxygen vibrations to the bandgap renormalization by electron–phonon interaction. A two-oscillator model based on the zinc and oxygen renormalization energies has been used to account for the temperature dependence of the bandgap in ZnO.

Site-Specific Hydrogen Isotope Fractionation in the Biosynthesis of Glycerol

The nuclear magnetic resonance study of site-specific natural isotope fractionation (SNIF-NMR) produced in the glycolytic conversion of glucose into ethanol and glycerol provides isotopic transfer coefficients, aij, which relate sites in the products to sites j in the reactants. The isotopic connection between the carbon-bound hydrogens of glycerol and those of glucose and water in fermentation reactions carried out with Saccharomyces cerevisiae has been investigated. The aij coefficients provide mechanistic information on the genealogy of the glycerol hydrogens, on the relative rates of triose phosphate isomerization and reduction of dihydroxyacetone phosphate into glycerol 3-phosphate, on the stereospecificity of the reduction, on the percentages of intra- and intermolecular hydrogen transfer occurring in the course of the reaction, and on the order of magnitude of the active overall kinetic and thermodynamic isotope effects. Thus a close connection is determined between sites 3 pro-R of glycerol (stereospecific numbering) and H6 pro-R and to a lesser extent H2 of glucose and between site 3 pro-S of glycerol and H6 pro-S and H1 of glucose. Moreover site 2 of glycerol, which exhibits a strong correlation with water is also partly connected with glucose. Possible changes in the values of the isotopic transfer coefficients, as a function of the composition of the medium or of the environmental conditions, enable mechanistic perturbations such as variations in the percentage of intermolecular transfers to be detected. Analyzed in terms of stereospecificity of the reduction step the isotopic results provide a direct chemical shift assignment of the enantiomeric pairs (1S, 3R) and (1R, 3S) of glycerol triacetate. The influence of added bisulfite, which strongly increases the yield in glycerol is estimated. The isotopic characterization of the bioconversion producing both ethanol and glycerol has been extended to the determination of the carbon isotopic parameters by isotope ratio mass spectrometry. Although it usually occurs as a by-product of the fermentation, glycerol can be considered as a useful complementary probe for characterizing the glycolytic pathway and for inferring various properties of the carbohydrate precursors.

Natural Abundance Isotopic Fractionation in the Fermentation Reaction: Influence of the Fermentation Medium

The isotopic parameters of fermentation products provide reliable criteria for characterizing their carbohydrate precursors on the condition that the isotopic coefficients which relate individual sites in the products and in the reactants have strictly reproducible or predictible values. Since fermentation may be performed either in natural media (such as fruit juices) which exhibit variable compositions or in “synthetic” media prepared from appropriate nutrients, it is necessary to estimate the influence of the medium on the carbon and hydrogen isotope ratios. By combining isotope ratio mass spectrometry and nuclear magnetic resonance investigations it has been shown that variations in the reaction rates induced by the medium are not connected with isotopic fractionation effects. At natural concentrations the nutrients present in juices, for instance, ensure a significant acceleration of the bioconversion with respect to pure water without introducing undesirable isotopic effects. The role of different nutrients added in relatively large amounts to synthetic media has been analyzed. It has been shown in particular that a significant decrease in the isotope ratio of the methylenic site of ethanol is due to the combined influence of both phosphates and assimilable nitrogen which are also the key components governing the growth and activity of yeasts. For a given nutrient composition the effect of the medium on the isotopic coefficients which connect the individual sites of ethanol to the starting water can be estimated. The observed behavior can be explained by modifications in the percentages of intra- and intermolecular hydrogen transfers (methyl site) and by variations in the kinetic isotope effects and exchange rates involving the hydrogens originating from water. Carbohydrates from different origins (beet and cane) have been fermented in different “natural” media prepared by eliminating sugars from orange, apple, and grape juices. In spite of very different fermentation rates the isotopic ratios of the products are then nearly independent of the composition of the medium. These results support the use of the ethanol probe for characterizing endogenous or exogenous sugars contained in different fruit juices.

The characteristics and site of inhibition of gluconeogenesis in rat liver cells by bacterial endotoxin. Stimulation of phosphofructokinase-1.

The characteristics and site of inhibition of gluconeogenesis by endotoxin were investigated in liver cells isolated from control and endotoxin-treated rats. Endotoxin treatment was associated with inhibition (40-50%) of gluconeogenesis from lactate plus pyruvate over a range of concentrations of substrate and of oleate and with or without glucose or glucagon. Similar inhibition was observed with asparagine, proline, glutamine, alanine and a substrate mixture, but not with glycerol, glyceraldehyde, dihydroxyacetone or endogenous substrates. There was no change in cellular ATP content or in the rates of ketogenesis or ureogenesis from asparagine, proline or glutamine. Other effects on isotopic fluxes, metabolite contents, enzyme activities and control coefficients were consistent with the suggestion that the effects of endotoxin on gluconeogenesis are exerted at the level of phosphofructokinase-1, and not at phosphoenolpyruvate carboxykinase, pyruvate kinase, pyruvate carboxylase or glucokinase.

Transport in aqueous solutions of Group IIB metal salts. Part 7.—Measurement and prediction of isotopic diffusion coefficients for iodide in solutions of cadmium iodide

Isotopic diffusion coefficients for iodide have been measured in aqueous solutions of cadmium iodide over the concentration range 0.1–1.0 mol dm–3, using the diaphragm cell technique. The iodide diffusion coefficients are shown to decrease smoothly but steeply from the infinite dilution value of 2.046 × 10–5 cm2 s–1 to less than half that value in the concentration range above 0.1 mol dm–3. This remarkable decline has been assigned to the presence of complexes of the type CdI+2–xx(x= 1, 2, 3, 4). The irreversible thermodynamic analysis developed previously for cadmium diffusion in this same system (part 4) has been extended to include isotopic diffusion of labelled ligands on multiply substituted complexes. Using this analysis the observed iodide diffusion coefficients have been predicted within close limits over the whole concentration range up to ≈ 0.5 mol dm–3.

Transport in aqueous solutions of group IIB metal salts (298.15 K). Part 4.—Interpretation and prediction of isotopic diffusion coefficients for cadmium in dilute solutions of cadmium iodide

A method for predicting isotopic diffusion coefficients for component ions in a self-complexed electrolyte has been developed. In an irreversible thermodynamic analysis the diffusion coefficient has been shown to be a function of the mobility coefficients of the free ion and its complexes together with the interionic coupling coefficients between labelled and unlabelled species. It is shown that these coefficients may be evaluated by an extension of the methods developed earlier to predict isothermal transport in such systems. Cadmium ion diffusion coefficients in aqueous cadmium iodide, Daa, have been predicted (0.003–0.10 mol dm–3). These describe the maximum anticipated experimentally. Observed and calculated Daa agree to within 3 % at 0.1 mol dm–3.

Transport in aqueous solutions of group IIB metal salts (298.15 K). Part 3.—Isotopic diffusion coefficients for cadmium-115 ions in aqueous cadmium iodide

Cadmium isotopic diffusion coefficients (Daa) were determined in aqueous cadmium iodide in the concentration range 0.1–0.6 mol dm–3. Daa passes through two maxima, one below 0.1 mol dm–3, the other at 0.4 mol dm–3. Irreversible thermodynamic analysis shows that these maxima are due to positive fluctuations in the isotope-to-isotope coupling contributions to Daa. For these studies a modified diaphragm cell technique was developed.

Application of irreversible thermodynamics to isotopic diffusion. Part 1.—Isotope–isotope coupling coefficients for ions and water in concentrated aqueous solutions of alkali metal chlorides at 298.16 K

From literature sources irreversible thermodynamic parameters may be obtained for all alkali metal chloride salt solutions at 298.16 K in the concentration range 0–3.0 mol dm–3. Comparable data for isotopic diffusion coefficients of component ions and water are incomplete. These have been obtained in this study, using a diaphragm cell, for all but rubidium ion in rubidium chloride. It is shown that these two sets of data may be combined to yield Onsager frictional coefficients (rii*) or mobility coefficients (lii*) which measure the kinetic interaction between isotopes as a function of concentration for each salt solution. Interpretation of lii* for ions is made difficult because of large frame-of-reference contributions with water solvent, but these may be compared with values calculated from the Onsager limiting law. Frictional interpretations are more explicit. The main factor contributing to the experimental diffusion coefficient for ions, Dii, remains ion-water friction which amounts to some 90 % for many slats even at 3.0 mol dm–3. The primary cause is that interionic friction between ions of opposite charge is largely cancelled by isotope–isotope friction in the expression for Dii.

Sodium chloride, urea, and water transport in the thin ascending limb of Henle. Generation of osmotic gradients by passive diffusion of solutes.

Studies were designed to examine whether the thin ascending limb of Henle (tALH) decreases its luminal solute concentration by an active or a passive transport process. In all experiments isolated segments of rabbit tALH were perfused in vitro. When tubules were perfused with solutions identical to the bath, active transport of NaCl was excluded by the following: (a) osmolality of the collected fluid remained unchanged and the same as the bath. (b) net water reabsorption could not be demonstrated, and (c) transtubular potential difference was zero. Isotopic permeability coefficients (x 10(-5) cm s-1) were calculated from the disappearance rate of the respective isotope added to the perfusate. These values indicate that tALH is moderately permeable to [14C]urea (6.97 +/- 1.95) while having a higher permeability to 22Na (25.5 +/- 1.8) and [not readable: see text]Cl (117 +/- 9.1) than any other segment similarly studied. The influx (bath-to-lumen) isotopic permeabilities were not statistically different from the above efflux permeabilities. Osmotic water permeability was immeasurably small. When tALH were perfused with a 600 mosmol/liter solution predominantly of NaCl against a 600 mosmol/liter bath in which 50% of osmolality was NaCl and 50% urea (to simulate in vivo papillary interstitium), the collected fluid osmolality was decreased significantly below that of the bath (300 mosmol/liter/mm of tubule). The decrease in osmolality was due to greater efflux of NaCl as compared to influx of urea. We conclude that active transport of salt by the tALH was not detected by the experimental protocol of the current studies, and that the unique membrane characteristics of tALH allows for generation of osmotic gradients (lumen less concentrated than adjacent surroundings) on purely passive mechanisms when perfused with isosmolal salt solutions in a bath with appropriate salt and urea concentrations. These findings are consistent with the passive counter-current model previously proposed from this laboratory.