Isotope Exchange Method Research Articles

Leu-enkephalin homogeneously labeled with tritium in studying the Selank inhibiting effect on the enkephalin-degrading enzymes of human plasma

A method of analysis of enkephalinase activity in blood plasma based on the application of Leu-enkephalin generally labeled with tritium at all its amino acid residues was developed. The method allows the simultaneous estimation of activity of several peptidases in microquantities of tissues. [G-3H]Leu-enkephalin was prepared by the method of solid phase catalytic isotope exchange (120 Ci/mmol) and subjected to proteolysis by the treatment with blood plasma. The resulting radioactive metabolites were separated by HPLC in the presence of the mixture of unlabeled fragments of Leu-enkephalin as internal standards. It was shown that aminopeptidases, dipeptidylaminopeptidases, and dipeptidylcarboxypeptidases respond for approximately 80%, 2%, and 10% of the total enzymatic activity, respectively. The new pathway of degradation of Leu-enkephalin by carboxypeptidase that provides for approximately 6% of the total enkephalin-degrading activity was discovered. Bestatin was shown to predominantly inhibit aminopeptidases and carboxypeptidases, whereas Selank is more specific for carboxypeptidases and dicarboxypeptidases. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2004, vol. 30, no. 3; see also http://www.maik.ru.

Soil isotopically exchangeable zinc: A comparison between E and L values

The objectives of the present paper were: (i) to determine isotopically exchangeable zinc using two isotopic exchange methods (E and L values) in a series of polluted and non-polluted Swiss agricultural soils, and (ii) to evaluate the ability of chemical extraction methods to estimate plant-available soil Zn using isotopic techniques. The surface horizon (0–20 cm) of seven polluted and non-polluted soils representing a wide range in physico-chemical properties and Zn contents were sampled. An isotopic exchange kinetics (IEK) approach was used to assess, in a batch experiment, the isotopically exchangeable Zn content (E value). In order to determine the L values, a pot experiment was carried out with Lolium multiflorum (cv. Axis) in a growth chamber using a 65Zn-isotope dilution technique. Total Zn uptake and the isotopic composition (65Zn/stableZn) were determined in Lolium multiflorum for five successive cuts. The amounts of zinc extracted by different chemicals were compared with L values and regression parameters were estimated. The isotopic composition in soil extracted by DTPA and EDTAAc at the end of the pot experiment was also determined. Results showed that the equation describing the increase of isotopically exchangeable Zn with time could be extrapolated to three months for polluted and non-polluted neutral and acidic soils, and that the results were not different from the amount of isotopically exchangeable Zn experimentally determined with Lolium multiflorum (L value). In alkaline soils however, results suggest that either 65Zn sorption occurred in the batch experiment or that the concentration of Zn in the soil solution had been overestimated, leading to an overestimation of the E value compared to the L values. Furthermore, the specific activities measured in DTPA and EDTA extractions at the end of the pot experiment were significantly different compared to the specific activity of the plant, showing that both these chelating agents extract neither all the available soil Zn nor only the available soil Zn for plants.

Adsorption Assisted Desorption of NH3on γ-Alumina Studied with Positron Emission Profiling

Evidence for adsorption assisted desorption of 13NH3 in the presence of gas phase ammonia on γ-alumina is presented. Due to lateral interactions with physisorbed ammonia, the adsorption enthalpy of ammonia on γ-alumina was measured to be −11 ± 1kJ/mol, using a 13NH3/14NH3 isotopic exchange method. This adsorption/desorption exchange was monitored with positron emission profiling. Infrared spectroscopy showed ammonia to exchange preferentially on the Lewis acid sites of alumina.

Isotopic and Ion-Exchange Kinetics in the Nafion-117 Membrane

The exchange kinetics of Cs+, Cu2+, and Eu3+ ions in Nafion-117 were measured at different H+ ion concentrations in equilibrating salt solutions. The exchange kinetics were found to be independent of the H+ ion concentration in equilibrating salt solutions over the pH range of 5−2. The exchange kinetics were therefore used to evaluate self-diffusion coefficients of the metal ions using an analytical solution of Fick's second law. The self-diffusion coefficient of Cs+ in Nafion-117 was found to be in agreement with the value obtained in our earlier work (1.94 × 10-7 cm2 s-1) based on the isotopic-exchange method in the absence of H+ ions. This further confirmed that the exchange process in the Nafion-117 membrane represents the pure isotopic exchange in the said pH range. The self-diffusion coefficients of Cu2+ and Eu3+ were found to be 2.9 × 10-7 and 4.5 × 10-8 cm2 s-1, respectively. The smaller self-diffusion coefficient of Eu3+ reflects its increased electrostatic interaction with the membrane matrix. T...

Alkalization of larval mosquito midgut and the role of carbonic anhydrase in different species of mosquitoes

We have previously demonstrated the involvement of carbonic anhydrase (CA) in the alkalization mechanism of the Aedes aegypti larval midgut. In this study, we used Hansson's histochemistry to examine the distribution of the enzyme in the midgut of six different species of mosquito larvae ( Aedes aegypti, Aedes albopictus, Culex quinquefasciatus, Culex nigripalpus, Ochlerotatus taeniorhynchus, Anopheles quadrimaculatus). Additionally, we quantitated CA content in the gastric caeca, anterior and posterior midgut of fourth instar larvae from these species using the 18O isotope exchange method coupled to mass spectrometry. We also tested the effect of CA inhibitors such as methazolamide and acetazolamide in the alkalization of the midgut for these species. Our results indicate that CA is present in the larval midgut of the species studied and that it appears to be associated with the posterior midgut and gastric caeca in some species and with the anterior midgut in others. CA inhibitors appear to have a profound effect on the alkalization mechanism of the midgut with lethal consequences for most of the species tested.

Response of excluder, indicator, and hyperaccumulator plants to nickel availability in soils

Availability is a key property for the assessment of soil-to-plant transfer of heavy metals. This work was conducted to determine whether the available pool of Ni differs according to the ability of plants to take up and accumulate the metal. An excluder plant species (Triticum aestivum L.), an indicator (Trifolium pratense L.), and 3 populations of the Ni-hyperaccumulator Alyssum murale (Waldst. & Kit.) were grown for 90 days on 4 soils with a gradient of concentrations of total and available Ni. Isotopic exchange methods with 63Ni ions were used to measure the exchangeable soil Ni (E-value, intensity, and capacity factors), to monitor its uptake by plants and to determine the size of the available pool (L-value). Results showed that, for a given soil, the L-values were similar for all plant species, showing that they all access the same Ni exchangeable pool regardless of their Ni uptake capacity. Also, L-values for a given soil were equal to the E-value calculated for a 90-day period, demonstrating that plant Ni originated from the isotopically exchangeable soil Ni. This pool can be accurately and simply determined with the isotopic exchange kinetic methods run on soil–solution batch systems without plants. Moreover, the results indicate that the plant species take up Ni as a response to ‘intensity’, ‘capacity’, and ‘quantity’ soil factors and that E-value alone is not enough to predict plant uptake. This work suggests a uniform behaviour of the plants tested towards soil Ni and may have practical applications in phytoextraction and phytomining, as the plants removed Ni exclusively from the exchangeable pool.

Methods for estimating adsorbed uranium(VI) and distribution coefficients of contaminated sediments.

Assessing the quantity of U(VI) that participates in sorption/desorption processes in a contaminated aquifer is an important task when investigating U migration behavior. U-contaminated aquifer sediments were obtained from 16 different locations at a former U mill tailings site at Naturita, CO (U.S.A.) and were extracted with an artificial groundwater, a high pH sodium bicarbonate solution, hydroxylamine hydrochloride solution, and concentrated nitric acid. With an isotopic exchange method, both a KD value for the specific experimental conditions as well as the total exchangeable mass of U(VI) was determined. Except for one sample, KD values determined by isotopic exchange with U-contaminated sediments that were in equilibrium with atmospheric CO2 agreed within a factor of 2 with KD values predicted from a nonelectrostatic surface complexation model (NEM) developed from U(VI) adsorption experiments with uncontaminated sediments. The labile fraction of U(VI) and U extracted by the bicarbonate solution were highly correlated (r2 = 0.997), with a slope of 0.96 +/- 0.01. The proximity of the slope to one suggests that both methods likely access the same reservoir of U(VI) associated with the sediments. The results indicate that the bicarbonate extraction method is useful for estimating the mass of labile U(VI) in sediments that do not contain U(IV). In-situ KD values calculated from the measured labile U(VI) and the dissolved U(VI) in the Naturita alluvial aquifer agreed within a factor of 3 with in-situ KD values predicted with the NEM and groundwater chemistry at each well.

Oxygen diffusion in pure and doped ZnO

Oxygen diffusion coefficients in pure and doped ZnO polycrystals were determined by means of the gas-solid isotope exchange method using the isotope 18O as oxygen tracer. The diffusion experiments were performed from 900 to 1000 °C, under an oxygen pressure of 10(5) Pa. After the diffusion annealings, the 18O diffusion profiles were determined by secondary ion mass spectrometry. The results of the experiments show that oxygen diffusion in Li-doped ZnO is similar to the oxygen diffusion in pure ZnO, while in Al-doped ZnO the oxygen diffusion is enhanced in relation to that observed in pure ZnO, in the same experimental conditions. Based on these results is proposed an interstitial mechanism for oxygen diffusion in ZnO. Moreover, it was found that oxygen grain-boundary diffusion is ca. 3 to 4 orders of magnitude greater than oxygen volume diffusion in pure and doped ZnO, which means that the grain-boundary is a fast path for oxygen diffusion in ZnO.

Kinetic characterization of wild-type and proton transfer-impaired variants of β-carbonic anhydrase from Arabidopsis thaliana

We have cloned and overexpressed a truncated, recombinant form of β-carbonic anhydrase from Arabidopsis thaliana. The wild-type enzyme and two site-directed variants, H216N and Y212F, have been kinetically characterized both at steady state by stopped-flow spectrophotometry and at chemical equilibrium by 18O isotope exchange methods. The wild-type enzyme has a maximal k cat for CO 2 hydration of 320 ms −1 and is rate limited by proton transfer involving two residues with apparent p K a values of 6.0 and 8.7. The mutant enzyme H216N has a maximal k cat at high pH that is 43% that of wild type, but is only 5% that of wild type at pH 7.0. 18O exchange studies reveal that the effect of the mutations H216N or Y212F is primarily on proton transfer steps in the catalytic mechanism and not in the rate of CO 2–HCO 3 − exchange. These results suggest that residues His-216 and Tyr-212 are both important for efficient proton transfer in A. thaliana carbonic anhydrase.

Experimental and theoretical study of pressure effects on hydrogen isotope fractionation in the system brucite-water at elevated temperatures

A detailed, systematic experimental and theoretical study was conducted to investigate the effect of pressure on equilibrium D/H fractionation between brucite (Mg(OH)2) and water at temperatures from 200 to 600°C and pressures up to 800 MPa. A fine-grained brucite was isotopically exchanged with excess amounts of water, and equilibrium D/H fractionation factors were calculated by means of the partial isotope exchange method. Our experiments unambiguously demonstrated that the D/H fractionation factor between brucite and water increased by 4.4 to 12.4‰ with increasing pressure to 300 or 800 MPa at all the temperatures investigated. The observed increases are linear with the density of water under experimental conditions. We calculated the pressure effects on the reduced partition function ratios (β-factor) of brucite (300–800 K and P ≤ 800 MPa) and water (400–600°C and P ≤ 100 MPa), employing a statistical-mechanical method similar to that developed by Kieffer (1982) and a simple thermodynamic method based on the molar volumes of normal and heavy waters, respectively. Our theoretical calculations showed that the reduced partition function ratio of brucite increases linearly with pressure at a given temperature (as much as 12.6‰ at 300 K and 800 MPa). The magnitude of the pressure effects rapidly decreases with increasing temperature. On the other hand, the β-factor of water decreases 4 to 5‰ with increasing pressure to 100 MPa at 400 to 600°C. Overall D/H isotope pressure effects combined from the separate calculations on brucite and water are in excellent agreement with the experimental results under the same temperature-pressure range. Our calculations also suggest that under the current experimental conditions, the magnitude of the isotope pressure effects is much larger on water than brucite. Thus, the observed pressure effects on D/H fractionation are common to other systems involving water. It is very likely that under some geologic conditions, pressure is an important variable in controlling D/H partitioning.

Three kinetically different inorganic phosphate entities in bovine casein micelles revealed by isotopic exchange method and compartmental analysis

Much controversy exists concerning the way calcium phosphate is linked to milk phosphoproteins including caseins. Homoionic exchange of inorganic phosphate between micellar calcium phosphates (MCP) of casein micelles and solute phosphates in cows’ milk was investigated using H 32PO 4 2− as radiotracer. Compartmental analysis and modelling revealed the presence of three MCP-related inorganic phosphate compartments each representing a separate phosphate entity. The relative phosphate quantities per compartment, i.e. the quantities of kinetically identical phosphate ions per MCP-ion cluster, and their mean residence times are 2:1:1 and 818, 0.24 and 23 h, respectively. Hence each MCP-ion cluster comprises four inorganic phosphate ions divided over three intra-MCP binding sites each characterised by a mean residence time for homomolecular phosphate exchange at solution/MCP interface.

Phosphorus fertility recapitalization of nutrient-depleted tropical acid soils with reactive phosphate rock: An assessment using the isotopic exchange technique

A “soil P fertility recapitalization” initiative utilizinglarge rates of phosphate rocks (PRs) was proposed to improve the soil P statusand increase the sustainable food production in acid and P-deficient tropicalsoils. Two series of experiments were carried out using five tropical acidsoilstreated with heavy applications of Gafsa phosphate rock (GPR). In the firstseries, the soils were mixed with GPR at the following application rates: 0,500, 1000 and 2000 mg P·kg−1, andincubatedfor one month in moist conditions. In another series, 1000 mg Pkg−1 applied as GPR was added to three soils andincubated for 1.5 month; thereafter 50 mg P kg−1as triple superphosphate (TSP) were added. The 32P isotopic exchangemethod was utilized to assess the contribution of GPR to the available soil P.Changes in amounts, E, of P transferred with time as phosphate ions from thesoil particles to the soil solution as well as changes in pH, calcium andphosphate concentrations in soil suspensions were determined. It was foundthat:(i) the contribution of P from GPR to recapitalization of soil P fertility wasmainly assessed by E pool size, pH, calcium and phosphate concentrations; othervariables were not significant at the 0.1 level; (ii) heavy applications of GPRdid not saturate all the P sorption sites, P freshly applied as water-soluble Pwas still sorbed; (iii) recapitalization of soil P fertility using GPR waspartly obtained in some acid tropical soils; (iv) Upon dissolution, GPRprovidedcalcium ions to crops and to soils, thus reducing Al toxicity, but its limingeffect was limited. To explain these effects with heavy application rates ofGPR, it was postulated that a coating of Al and Fe compounds is formed aroundPRparticles with time, thus reducing further dissolution.

PHYSICO-CHEMICAL AND ENGINEERING PRINCIPLES OF BORON ISOTOPES SEPARATION BY USING BF3–ANISOLE·BF3 SYSTEM

This review is devoted to consideration of physico-chemical and engineering data required for realization of boron isotopes separation by the method of chemical isotope exchange between BF3 and the complex anisole·BF3. The properties of the initial substances and anisole·BF3 complex required for calculation of the separation process are considered. The data on phase equilibrium in the system, isotope equilibrium, kinetics of interphase isotope exchange, and implementation of phases conversion are presented. This article considers comprehensively the reasons for anisole decomposition and the measures for its prevention. The problem of hydrodynamics and choice of packing are investigated. The conditions of initial substances preparation are treated. Perspective trends of separation process improvement are discussed.

Time scales for sorption-desorption and surface precipitation of uranyl on goethite.

The sorption of uranium on mineral surfaces can significantly influence the fate and transport of uranium contamination in soils and groundwater. The rates of uranium adsorption and desorption on a synthetic goethite have been evaluated in batch experiments conducted at constant pH of 6 and ionic strength of 0.1 M. Adsorption and desorption reactions following the perturbation of initial states were complete within minutes to hours. Surface-solution exchange rates as measured by an isotope exchange method occur on an even shorter time scale. Although the uranium desorption rate was unaffected by the aging of uranium-goethite suspensions, the aging process appears to remove a portion of adsorbed uranium from a readily exchangeable pool. The distinction between sorption control and precipitation control of the dissolved uranium concentration was also investigated. In heterogeneous nucleation experiments, the dissolved uranium concentration was ultimately controlled by the solubility of a precipitated uranyl oxide hydrate. The X-ray diffraction pattern of the precipitate is characteristic of the mineral schoepite. Precipitation is kinetically hindered at low degrees of supersaturation. In one experiment, metastable sorption controlled dissolved uranium concentrations in excess of the solubility limit for more than 30 d.

Oxygen mobility in layered cuprates La2−xSrxCuO4−δ

Oxygen mobility in layered lanthanum cuprates La2−xSrxCuO4−δ is studied by dynamic-thermal isotope exchange method and coulometric titration. Depending on the temperature, different preferred surface exchange reactions are realized. Such characteristics of bulk diffusion process as activation energy and diffusion coefficient are stated for the cuprates. Theoretical modeling of diffusion process in La2CuO4 and LaSrCuO3.5 phases is carried out using molecular dynamics simulations. The data obtained are compared to the ones previously obtained experimentally.

Effects of fertility management strategies on phosphorus bioavailability in four West African soils

Low phosphorus (P) in acid sandy soils of the West African Sudano-Sahelian zone is a major limitation to crop growth. To compare treatment effects on total dry matter (TDM) of crops and plant available P (P-Bray and isotopically exchangeable P), field experiments were carried out for 2 years at four sites where annual rainfall ranged from 560 to 850 mm and topsoil pH varied between 4.2 and 5.6. Main treatments were: (i) crop residue (CR) mulch at 500 and 2000 kg ha −1 , (ii) eight different rates and sources of P and (iii) cereal/legume rotations including millet (Pennisetum glaucum L.), sorghum [Sorghum bicolor (L.) Moench], cowpea (Vigna unguiculata Walp.) and groundnut (Arachis hypogaea L.). For the two Sahelian sites with large CR-induced differences in TDM, mulching did not modify significantly the soils’ buffering capacity for phosphate ions but led to large increases in the intensity factor (CP) and quantity of directly available soil P (E1min). In the wetter Sudanian zone lacking effects of CR mulching on TDM mirrored a decline of E1min with CR. Broadcast application of soluble single superphosphate (SSP) at 13 kg P ha −1 led to large increases in CP and quantity of E1min at all sites which translated in respective TDM increases. The high agronomic efficiency of SSP placement (4 kg P ha −1 ) across sites could be explained by consistent increases in the quantity factor which confirms the power of the isotopic exchange method in explaining management effects on crop growth across the region.

Evaluation of bioavailable phosphorus in some acid soils of Ghana using <sup>32</sup>p isotopic exchange method

An adequate knowledge about bioavailable soil P of highly weathered tropical acid soils is a pre-requisite to their sustainable management for food production. The objective of this study was to characterize the bioavailable soil P status of six highly weathered low activity clay (LAC) soils from SW Ghana by using an isotopic exchange method. The bioavailable P was characterized by three factors, i.e intensity, quantity, and capacity inferred from experimental parameters that describe the dynamics of soil phosphorus through isotope exchange of phosphate ions and represented by a functional model with five compartments in which the pools have been defined according to their mean exchange time with the phosphate in soil solution. The results indicated that all the LAC acid soils had very low P fertility status as reflected by the low P concentration in solution, Cp ( -1 ), low exchangeable P, E1min ( -1 ), and high P fixation capacity, r1/R (0.24) with the exception of one soil. The compartmental analysis also showed that an average of 45 % of the total P pool could not be exchanged within 1 year while 0.53 % of it was exchangeable within 1 min. These results are consistent with the numerous exchange sites located on the oxides of the LAX acid soils. In addition, the study showed that the r1/R parameter correlated significantly with P max derived from P sorption isotherms and the SPR, i.e., the amount of P sorbed at a concentration of 0.2 mg P 1 -1 . It was concluded that the 32 P isotopic exchange method was useful for the determination of bioavailable P and also for estimating soil-fixing capacity of these low-activity clay acid soils of Ghana. Evaluation de phosphore biodisponible en quelques sols acides du Ghana employant la methode d'echange isotopique de 32 P Resume Une connaissance adequâte du P de sol biodisponible des sols acides de tropique fortement erodes est une condition prealable a leur exploitation soutenue pour la production alimentaire. L'Objectif de cette etude etait de caracteriser l'etat du P de sol biodisponible de six sols d'argile de moins d' activite (AMA) fortement erodes du Sud-Ouest du Ghana employant une methode d' echange isotopique. Le P biodisponible etait caracterise par trois facteurs, c.-a-d. intensite, qualite et capacite inferees des parametres d' experience qui decrivent le dynamisme de phosphore de sol a travers l' echange d'isotope d'ions de phosphate et represente par un modele fonctionnel avec cinq subdivisions dans lesquels les pools ont ete defini selon leurs temps d'echanges moyens avec le phosphate en solution de sol. Les resultats indiquaient que tous les sols acides d' AMA avaient de tres bas niveaux de fertilite de P comme traduit par la basse concentration de P en solution, Cp ( Ghana Jnl agric. Sci. Vol.33(2) 2004:139-146

Ameliorating acid infertile rice soil with organic residue from nitrogen fixing trees

Residues from two nitrogen fixing trees, Albizia zygia and Gliricidia sepium and one non nitrogen fixing tree Samania dinklagei and their combination with 50% of rice husk, were evaluated for their effect on growth and yield of rice (Oryza sativa) varieties (Rok 10 and Rok 23) grown in an Oxisol collected from a rice paddy in Inland valley swamps of Sierra Leone, West Africa. Ground and dried residues were mixed with soil and labelled with N15 ammonium sulfate (9.855% atom excess) at a rate of 10 mg N kg-1 soil. The 32P isotopic exchange method was used to determine the effect of organic residue incorporation on the bioavailable soil P status at five levels of P application. Albizia zygia and G. sepium had higher quality leaf litter (with relatively lower C/N, lignin/N and cellulose/N ratios) whereas Semania dinklagei and rice husk had high carbon, lignin and polyphenolic contents. Application of organic residue had positive effects on the soil available P status at varying degrees. Higher adsorption capacity, kinetic value and mean solution time of phosphates in solution were obtained with the A. zygia, and G. sepium residues. These values were lower with rice husk and S. dinklagei. Gliricidia sepium and A. zygia contributed more nitrogen derived from organic residue than rice husk and S. dinklagei but the mixture of A. zygia and G. sepium with rice husk enhanced N utilization.

Glycolide deuteriation by hydrogen isotope exchange using the HSCIE method

Although poly(α-hydroxy acids)s derived from lactic and glycolic acids (PLAGA) are currently used for many surgical and pharmacological applications, their behaviour and their fate in vivo are still a source of concern according to some authors. As part of our attempts to label these polymers and monitor their fate by using radioactivity, investigation was made of the possibility to exchange the protons present in glycolide molecules by hydrogen isotopes using the High-temperature Solid-state Catalytic Isotope Exchange (HSCIE) method which was previously successful in the case of lactide. Accordingly H→H exchanges were carried out on glycolide in the presence of Pd/CaCO3 at various temperatures and 550 mbar pressure of deuterium. Substitution yields as high as 24% were obtained, depending primarily on temperature. PLAGA polymers with lactyl and glycolyl repeating units labeled with deuterium were synthesized and characterized. Copyright © 1999 John Wiley & Sons, Ltd.

Characterization of Pesticide Desorption from Soil by the Isotopic Exchange Technique

The reversibility of pesticide sorption–desorption in soil is of fundamental importance in the understanding of the fate of these agrochemicals in the environment. We used an isotopic exchange method to characterize the irreversibility of the sorption–desorption process of the insecticide imidacloprid (1‐[(6‐chloro‐3‐pyridinyl)‐methyl]‐Nnitro‐2‐imidazolidinimine) and its degradation product imidacloprid‐urea (1‐[(6‐chloro‐3‐pyridinyl)‐methyl]‐2‐imidazolidinone) on a silty clay loam (SiCL) soil, and that of the metabolite imidacloprid‐guanidine (1‐[6‐chloro‐3‐pyridinyl)methyl]‐4,5‐dihydro‐1H‐imidazol‐2‐amine) on a loamy sand (LS) soil. The exchange between 12C‐pesticide molecules and 14C‐labeled pesticide molecules in soil suspensions preequilibrated for 24 h was monitored and indicated that a fraction of the sorbed chemicals was resistant to desorption. A two‐compartment model was applied to describe the experimental sorption data points of the sorption isotherms as the sum of a reversible component and a nondesorbable, irreversible component. The quantitative estimation of the irreversible and reversible components of sorption, experimentally derived from isotopic exchange experiments, indicated degree of irreversibility (percentage irreversibly bound) in the order: imidacloprid–SiCL soil (6–32%) < imidacloprid urea–SiCL soil (15–23%) < imidacloprid guanidine–LS soil (32–51%), with greater irreversibility at lower pesticide concentration. Increasing the preequilibration time and decreasing pH in the imidacloprid–SiCL soil system resulted in increased sorption irreversibility. The irreversible component of sorption determined by the isotopic exchange technique also allowed accurate prediction of the sorption–desorption hysteretic behavior during successive desorption cycles for all three soil–pesticide systems studied. The isotopic exchange technique appears to be a suitable method to quantitatively characterize pesticide desorption from soil, allowing prediction of hysteresis during sorption–desorption experiments.