Separation and recovery of exotic radiolanthanides from irradiated tantalum targets for half-life measurements

The current knowledge of the half-lives (T_1/2) of several radiolanthanides is either affected by a high uncertainty or is still awaiting confirmation. The scientific information deriving from this imprecise T_1/2 data has a significant impact on a variety of research fields, e.g., astrophysics, fundamental nuclear sciences, and nuclear energy and safety. The main reason for these shortcomings in the nuclear databases is the limited availability of suitable sample material together with the difficulties in performing accurate activity measurements with low uncertainties. In reaction to the urgent need to improve the current nuclear databases, the long-term project “ERAWAST” (Exotic Radionuclides from Accelerator Waste for Science and Technology) was launched at Paul Scherrer Institute (PSI). In this context, we present a wet radiochemical separation procedure for the extraction and purification of dysprosium (Dy), terbium (Tb), gadolinium (Gd), and samarium (Sm) fractions from highly radioactive tantalum specimens, in order to obtain ¹⁵⁴Dy, ^157-158Tb, ^148,150Gd, and ¹⁴⁶Sm samples, needed for T_1/2 determination studies. Ion-exchange chromatography was successfully applied for the separation of individual lanthanides. All separations were conducted in aqueous phase. The separation process was monitored via γ-spectrometry using suitable radioactive tracers. Both the purity and the quantification of the desired radiolanthanides were assessed by inductively coupled plasma mass spectrometry. Test experiments revealed that, prior to the Dy, Tb, Gd, and Sm separation, the removal of hafnium, lutetium, and barium from the irradiated tantalum material was necessary to minimize the overall dose rate exposure (in the mSv/h range), as well to obtain pure lanthanide fractions. With the herein proposed separation method, exotic ¹⁵⁴Dy, ^157-158Tb, ^148,150Gd, and ¹⁴⁶Sm radionuclides were obtained in sufficient amounts and purity for the preparation of samples for envisaged half-life measurements. During the separation process, fractions containing holmium, europium, and promethium radionuclides were collected and stored for further use.

Immobilization of Eu and Ho from synthetic acid mine drainage by precipitation with Fe and Al (hydr)oxides

Use of lime to mitigate acid mine drainage is, in general, accompanied by precipitation of iron (Fe) and aluminium (Al) (hydr)oxides which may increase the removal of trace elements from water. This work aimed to evaluate the precipitation of Fe/Al (hydr)oxides to remove rare earth elements (REE) from contaminated water and the stability of precipitates. Two sets of 60-day syntheses were carried out using different Fe/Al/REE molar ratios, for europium (Eu) and holmium (Ho). The pH was periodically adjusted to 9.0, and the stability of the resulting precipitates was evaluated by water-soluble and BCR extractable phases, namely (1) acid soluble, extracted by 0.11 mol L^-1 acetic acid; (2) reducible, extracted with 0.5 mol L^-1 hydroxylamine hydrochloride; and (3) oxidisable, extracted with 8.8 mol L^-1 hydrogen peroxide efficiencies of the water treatments for both Eu and Ho that were higher than 99.9% irrespective to the Fe/Al/REE molar ratios. Water-soluble phases of Eu and Ho were lower than 0.01% of the total contents in the precipitates. Recoveries from precipitates by Bureau Communautaire de Référence (BCR) sequential extractions increased with increasing concentrations of Eu and Ho. Acetic acid extracted higher amounts of REE, but Eu recovery was superior to Ho. Lepidocrocite was formed as Eu concentration increased which decreased its stability in the precipitates.

Carbon footprint assessment of recycling technologies for rare earth elements: A case study of recycling yttrium and europium from phosphor

Rare earth elements are key raw materials in high-technology industries. Mining activities and manufacturing processes of such industries have caused considerable environmental impacts, such as soil erosion, vegetation destruction, and various forms of pollution. Sustaining the long-term supply of rare earth elements is difficult because of the global shortage of rare earth resources. The diminishing supply of rare earth elements has attracted considerable concern because many industrialized countries regarded such elements as important strategic resources for economic growth. This study aims to explore the carbon footprints of yttrium and europium recovery techniques from phosphor. Two extraction recovery methods, namely, acid extraction and solvent extraction, were selected for the analysis and comparison of carbon footprints. The two following functional units were used: (1) the same phosphor amounts for specific Y and Eu recovery concentrations, and (2) the same phosphor amounts for extraction. For acid extraction method, two acidic solutions (H₂SO₄ and HCl) were used at two different temperatures (60 and 90°C). For solvent extraction method, acid leaching was performed followed by ionic liquid extraction. Carbon footprints from acid and solvent extraction methods were estimated to be 10.1 and 10.6kgCO₂eq, respectively. Comparison of the carbon emissions of the two extraction methods shows that the solvent extraction method has significantly higher extraction efficiency, even though acid extraction method has a lower carbon footprint. These results may be used to develop strategies for life cycle management of rare earth resources to realize sustainable usage.

Fast and Selective Preconcentration of Europium from Wastewater and Coal Soil by Graphene Oxide/Silane@Fe3O4 Dendritic Nanostructure

In this study, nanocomposite of graphene oxide and silane modified magnetic nanoparticles (silane@Fe3O4) were synthesized in a form of dendritic structure. For this, silane@Fe3O4 nanoparticle gets sandwiched between two layers of graphene oxide by chemical synthesis route. The synthesized dendritic structure was used as a monomer for synthesis of europium ion imprinted polymer. The synthesis of imprinted polymer was contemplated onto the surface of the vinyl group modified silica fiber by activated generated free radical atom-transfer radical polymerization, that is, AGET-ATRP technique. The synthesized dendritic monomer was characterized by XRD, FT-IR, VSM, FE-SEM, and TEM analyses. The imprinted polymer modified silica fiber was first validated in the aqueous and blood samples for successful extraction and detection of europium ion with limit of detection = 0.050 pg mL(-1) (signal/noise = 3). The imprinted polymer modified silica fiber was also used for preconcentration and separation of europium metal ion from various soil samples of coal mine areas. However, the same silica fiber was also used for wastewater treatment and shows 100% performance for europium removal. The findings herein suggested that dendritic nanocomposite could be potentially used as a highly effective material for the enrichment and preconcentration of europium or other trivalent lanthanides/actinides in nuclear waste management.

Americium(III) capture using phosphonic acid-functionalized silicas with different mesoporous morphologies: adsorption behavior study and mechanism investigation by EXAFS/XPS

Efficient capture of highly toxic radionuclides with long half-lives such as Americium-241 is crucial to prevent radionuclides from diffusing into the biosphere. To reach this purpose, three different types of mesoporous silicas functionalized with phosphonic acid ligands (SBA-POH, MCM-POH, and BPMO-POH) were synthesized via a facile procedure. The structure, surface chemistry, and micromorphology of the materials were fully characterized by (31)P/(13)C/(29)Si MAS NMR, XPS, and XRD analysis. Efficient adsorption of Am(III) was realized with a fast rate to reach equilibrium (within 10 min). Influences including structural parameters and functionalization degree on the adsorption behavior were investigated. Slope analysis of the equilibrium data suggested that the coordination with Am(III) involved the exchange of three protons. Moreover, extended X-ray absorption fine structure (EXAFS) analysis, in combination with XPS survey, was employed for an in-depth probe into the binding mechanism by using Eu(III) as a simulant due to its similar coordination behavior and benign property. The results showed three phosphonic acid ligands were coordinated to Eu(III) in bidentate fashion, and Eu(P(O)O)3(H2O) species were formed with the Eu-O coordination number of 7. These phosphonic acid-functionalized mesoporous silicas should be promising for the treatment of Am-containing radioactive liquid waste.

Synergistic extraction of europium and americium into nitrobenzene by using hydrogen dicarbollylcobaltate and dodecaethylene glycol

Extraction of microamounts of europium and americium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-) in the presence of dodecaethylene glycol (DDEG, L) has been investigated. The equilibrium data have been explained assuming that the species HL+, H2L2+, ML3+ and MH-1L2+ (M3+ = Eu3+, Am3+; L = DDEG) are extracted into the organic phase. The values of extraction and stability constants of the complex species in nitrobenzene saturated with water have been determined. It was found that in this nitrobenzene medium, the stability constant of the EuL3+ complex is comparable with that of AmL3+.

Site-specific retention of colloids at rough rock surfaces

The spatial deposition of polystyrene latex colloids (d = 1 μm) at rough mineral and rock surfaces was investigated quantitatively as a function of Eu(III) concentration. Granodiorite samples from Grimsel test site (GTS), Switzerland, were used as collector surfaces for sorption experiments. At a scan area of 300 × 300 μm(2), the surface roughness (rms roughness, Rq) range was 100-2000 nm, including roughness contribution from asperities of several tens of nanometers in height to the sample topography. Although, an increase in both roughness and [Eu(III)] resulted in enhanced colloid deposition on granodiorite surfaces, surface roughness governs colloid deposition mainly at low Eu(III) concentrations (≤5 × 10(-7) M). Highest deposition efficiency on granodiorite has been found at walls of intergranular pores at surface sections with roughness Rq = 500-2000 nm. An about 2 orders of magnitude lower colloid deposition has been observed at granodiorite sections with low surface roughness (Rq < 500 nm), such as large and smooth feldspar or quartz crystal surface sections as well as intragranular pores. The site-specific deposition of colloids at intergranular pores is induced by small scale protrusions (mean height = 0.5 ± 0.3 μm). These protrusions diminish locally the overall DLVO interaction energy at the interface. The protrusions prevent further rolling over the surface by increasing the hydrodynamic drag required for detachment. Moreover, colloid sorption is favored at surface sections with high density of small protrusions (density (D) = 2.6 ± 0.55 μm(-1), asperity diameter (φ) = 0.6 ± 0.2 μm, height (h) = 0.4 ± 0.1 μm) in contrast to surface sections with larger asperities and lower asperity density (D = 1.2 ± 0.6 μm(-1), φ = 1.4 ± 0.4 μm, h = 0.6 ± 0.2 μm). The study elucidates the importance to include surface roughness parameters into predictive colloid-borne contaminant migration calculations.

Liquid-liquid extraction and flat sheet supported liquid membrane studies on Am(III) and Eu(III) separation using 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine as the extractant

Solvent extraction and supported liquid membrane transport studies for the preferential removal of Am(3+) from feeds containing a mixture of Am(3+) and Eu(3+) was carried out using 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (n-Pr-BTP) as the extractant. Diluent plays an important role in these studies. It was observed that the distribution coefficients deteriorate significantly for both Am(3+) and Eu(3+) though the separation factors were affected only marginally. The transport studies were carried out at pH 2.0 in the presence of NaNO(3) to result in the preferential Am(3+) transport with high separation factors. Effect of different experimental parameters, viz. feed composition, stripping agents, diluents of the organic liquid membrane and membrane pore size was studied on the transport and separation behaviour of Am(3+) and Eu(3+). The supported liquid membrane studies indicated about 85% Am(3+) and 6% Eu(3+) transport in 6h using 0.03 M n-Pr-BTP in n-dodecane/1-octanol (7:3) diluent mixture for a feed containing 1M NaNO(3) at pH 2 and a receiver phase containing pH 2 solution as the strippant. Consequently, a permeability coefficient of (1.75 ± 0.21) × 10(-4)cms(-1) was determined for the Am(3+) transport. Stability of the n-Pr-BTP and its SLM was also studied by carrying out the distribution and transport experiment after different time intervals.

Sorption of 137Cs, 133Ba and 154Eu by synthesized sodium aluminosilicate (Na-AS)

Sodium aluminosilicate has been synthesized by solution route for use as a sorbent for various radionuclides. It was characterized by XRD, zeta potential, BET surface area, FTIR spectroscopy and site density measurement. Sorption studies of (137)Cs, (133)Ba and (154)Eu on synthesized sodium aluminosilicate have been carried out at varying pH (3-10). Sorption of all the metal ions was found to increase with pH of suspension with the saturation value increasing with the oxidation state of metal ion. Effect of Aldrich humic acid (2mg/l) on sorption was also investigated. In case of (137)Cs, the sorption was not affected by the presence of humic acid, while in case of (133)Ba and (154)Eu, sorption was enhanced at lower pH and decreased at higher pH in presence of HA. A series of experiments were carried out for (154)Eu sorption on sodium aluminosilicate at various conditions. Sorption of europium was analyzed by different sorption isotherms, viz., Freundlich and D-R isotherm. Thermodynamic data reveal sorption phenomena as endothermic and spontaneous. Studies were further extended to find out effect of diverse ions (Ca(2+) and CO(3)(2-)) on sorption of europium.

Site-specific Eu(III) binding affinities to a Datura innoxia biosorbent

The binding of Eu(III) to a biosorbent derived from cultured cells of the plant Datura innoxia, have been investigated through elucidation of apparent affinity constants associated with different chemical environments present on the cell wall. Adsorption isotherms for separate types of binding sites were generated using metal ion luminescence measurements. Application of regularized regression analysis to these isotherm data for four chemically distinguishable sites revealed the presence of sites exhibiting both low (mean log K(app)=-0.3 to 0.6) and higher (mean log K(app)=3.2-3.5) apparent affinities for pH conditions of 2.0, 4.0, and 5.0. Low affinity sites were observed for all pH conditions and attributed to non-specific binding of the metal ions to the negatively charged biomaterial. The pH-dependent higher affinity sites are ascribed to specific sites involving either an ion-exchange mechanism or formation of weak surface-metal ion complexes. These results differed significantly from a similar analysis of total metal binding isotherms that indicated mean log K(app) values of -0.5 to 0.25 (low affinity) and 5.6-6.0 (high affinity).

Adsorption of Eu(III) onto TiO2: effect of pH, concentration, ionic strength and soil fulvic acid

The effects of pH, initial Eu(III) concentration, ionic strength and fulvic acid (FA) on the adsorption of Eu(III) on TiO(2) are investigated by using batch techniques. The results indicate that the presence of FA strongly enhances the adsorption of Eu(III) on TiO(2) at low pH values. Besides, the adsorption of Eu(III) on TiO(2) is significantly dependent on pH values and independent of ionic strength. The adsorption of Eu(III) on TiO(2) is attributed to inner-sphere surface complexation. The diffuse layer model (DLM) is applied to simulate the adsorption data, and fits the experimental data well with the aid of FITEQL 3.2. X-ray photoelectron spectroscopy (XPS) is performed to study the species of Eu(III) adsorbed on the surfaces of TiO(2)/FA-TiO(2) hybrids at a molecular level, which suggest that FA act as "bridge" between Eu(III) and TiO(2) particles to enhance the ability to adsorb Eu(III) in solution.

Studies on the interaction of olive cake and its hydrophylic extracts with polyvalent metal ions (Cu(II), Eu(III)) in aqueous solutions

The paper presents and discusses the chemical affinity of two different forms of natural organic matter (e.g. in solid (SOM) and soluble form (DOM)) for Cu(2+) and Eu(3+) ions. In the this study SOM is represented by olive cake and DOM by its hydrophilic extracts. The investigations were performed in aqueous 0.1M NaClO(4) solutions at pH 6, 23 degrees C and under atmospheric conditions by means of potentiometry using a copper ion selective electrode and competitions reactions between the two metal ions. The experimental data were evaluated according to an "operational" approach, which is based on the charge neutralization model and the Scatchard approach. Evaluation of the data resulted in the determination of conditional formation constants (beta(*)), which are a measure for the chemical affinity of natural organic matter for a metal ion. For the olive cake metal ion binding the values of log beta(*) for Cu(II) and Eu(III) were evaluated to be 5.1+/-0.3 and 5.4+/-0.5, respectively, whereas for the hydrophilic extracts the corresponding values for the Cu(II) and Eu(III) complexes were evaluated to be 5.3+/-0.3 and 6.3+/-0.5, respectively. The beta(*) values show that natural organic matter in the two different forms presents almost similar affinity for the Cu(2+) ion, whereas the affinity of DOM for the Eu(3+) ion is significantly higher than the corresponding affinity of SOM. This is ascribed to the higher flexibility of DOM, which allows better coordination of its active sites around the Eu(3+) ion.

Use of activated carbon in removal of some radioisotopes from their waste solutions

Removal of some radioisotopes namely (152+154)Eu and (65)Zn from radioactive solutions by activated carbon using both batch and column techniques has been performed. Experimental studies were conducted to evaluate and optimize the various process variables, i.e., equilibrium time, carbon dose, solution pH. Sorption data have been interpreted in terms of both Freündlich and Langmuir isotherms. The fixed-bed results indicate the high capacity of the activated carbon for the removal of europium and zinc ions. The data suggest the possible use of activated carbon for the removal of these cations from radioactive waste solutions.

Preparation and evaluation of cerium(IV) tungstate powder as inorganic exchanger in sorption of cobalt and europium ions from aqueous solutions

Cerium(IV) tungstate powder was chemically synthesized and exploited as adsorbent material for the decontamination study of cobalt and europium ions from radioactive waste solutions under simulated conditions using batch technique. The influences of pH, particle size and temperature have been reported. The uptake of europium was found to be slightly greater than that of cobalt and the apparent sorption capacity increases with increase in temperature. Thermodynamic parameters such as changes in Gibbs free energy (DeltaG degrees), enthalpy (DeltaH degrees), and entropy (DeltaS degrees) were calculated. The numerical value of DeltaG degrees decreases with an increase in temperature, indicating that the sorption reaction of each ion was spontaneous and more favorable at higher temperature. The positive values of DeltaH degrees correspond to the endothermic nature of sorption processes and suggested that chemisorption was the predominant mechanism. A comparison of kinetic models applied to the sorption rate data of each ion was evaluated for the pseudo first-order, the pseudo second-order, intraparticle diffusion and homogeneous particle diffusion kinetic models. The results showed that both the pseudo second-order and the homogeneous particle diffusion models were found to best correlate the experimental rate data. The numerical values of the rate constants and particle diffusion coefficients were determined from the graphical representation of the proposed models. Activation energy (E(a)) and entropy (DeltaS++) of activation were also computed from the linearized form of Arrhenius equation.

Separation of Am3+ and Eu3+ using an extraction chromatographic resin containing bis(2,4,4-trimethylpentyl)dithiophosphinic acid as the stationary phase

Sorption of Am3+ and Eu3+ onto an extraction chromatographic resin material, prepared by impregnating purified bis(2,4,4-trimethylpentyl)dithiophosphinic acid (commercially available as Cyanex 301) into Chromosorb W, was investigated. Separation factor (S.F. = K(d,Am)/K(d,Eu)) values tend to increase in the presence of complexing agents such as thiocyanate and nitrate. In presence of 1 M NaNO3 in the aqueous phase, a S.F. value of approximately 1000 was obtained. The nature of extracted species was ascertained by carrying out the sorption studies at different aqueous phase pH. A column made from 300 mg of the resin material was used for the separation of Am3+ and Eu3+. A synthetic solution containing 1x10(5) cpm each of Am3+ and Eu3+ when loaded on to the column, >99% Eu was eluted out in 60 mL of 1 M NaNO3 at a pH of 3.2 while Am was eluted out using 5 mL of 1 M HNO3. The performance of the resin material was found to be reasonably good even after three cycles.

Development of Soft-Based Double-Stranded Peptide Chelators which Selectively Separate Europium and Lanthanum Ions Based on the Hardness Concept

New double-stranded peptide chelators (1) conjugated Cat (2,3-dihydroxybenzoic acid) were synthesized and formed a molecular complex 1-Eu(3+) (or 1-Lu(3+)) with Eu(3+) and Lu(3+) but not La(3+). The double-stranded peptide chelator may prove to be useful tools for studying the selective separation of lanthanide ions.

Biosorption of La, Eu and Yb using Sargassum biomass

Biosorption of the lanthanides: Lanthanum (La(3+)), Europium (Eu(3+)) and Ytterbium (Yb(3+)) from single-component and multi-component batch systems using Sargassum polycystum Ca-loaded biomass was studied. The ion exchange sorption mechanism was confirmed by the release of calcium ions from the biomass that matched the total number of metal and protons removed from the solution. The metal binding increased with pH due to the decrease of proton concentration in the system, as they also compete for the binding sites. The maximum metal uptake capacity for pH 3, 4 and 5 ranged approximately between (0.8-0.9) mmol g(-1) for La (0.8-0.9) mmol g(-1) for Eu, and (0.7-0.9) mmol g(-1) for Yb. Biosorption from multi-component mixtures was examined at pH 4 using equimolar initial concentrations of the metals. The metal affinity sequence established was Eu>La>Yb, and the maximum metal uptake obtained was 0.29, 0.41, 0.28 mmol g(-1) for La, Eu and Yb, respectively.

Detailed study on simultaneous separation of rare earth elements by capillary electrophoresis

Separation of all rare earth elements (REEs) by capillary zone electrophoresis was investigated in a system of alpha-hydroxyisobutyric acid (HIBA) as a main complex reagent and acetic acid (HAc) as an assistant complex reagent. In the combined system, ligand Ac- plays an important role in improving separation of Eu and Gd, and Y and Dy. The calculated ratio of Ac- to HIB- concentrations was compared and demonstrated that Eu and Gd, and Y and Dy tend to be separated at lower, and higher ratio of the two free ligands, respectively. An operational buffer system was developed for a complete separation of all REE ions.

Production and separation of carrier-free 145,146Eu from a CsNO3 target using a 16O beam

Carrier free europium isotopes 145,146Eu have been produced by irradiating a CsNO3 target with an 80 MeV 16O beam. Radiochemical separation of the produced europium isotopes from the bulk target matrix of CsNO3 have effectively been done using HDEHP, the liquid cation exchanger.

Thiol-reactive, luminescent Europium chelates: luminescence probes for resonance energy transfer distance measurements in biomolecules

Lanthanide chelates have recently been shown to be extremely promising luminescence probes for distance measurements in biomolecules using luminescence resonance energy transfer measurements [P. R. Selvin, T. M. Rana, and J. E. Hearst (1994) J. Am. Chem. Soc. 116, 6029-6030; P. R. Selvin, and J. E. Hearst (1994) Proc. Natl. Acad. Sci. USA 91, 10024-10028]. In this work we describe simple procedures for preparing highly fluorescent thiol-reactive europium chelates. These new compounds contain a uv-absorbing coumarin group which sensitizes europium emission, diethylenetriaminepentaacetic acid or triethylenetetraaminehexaacetic acid groups which provide europium chelating function, and a pyridyl disulfide group which allows specific modification of thiol groups. These reagents can be used to label proteins at Cys residues or synthetic oligonucleotides which contain thiol groups. Modification can be reversed easily by treatment with a reducing agent (dithiothreitol). Luminescence energy transfer between these new chelates and CY5 fluorochrome attached to the opposite ends of 15-bp double-stranded DNA was measured to test their usefulness for distance measurements in macromolecules. The distance measured between the chelate (donor) and CY5 (acceptor) was in the range expected for the length of 15-bp DNA. The stability of europium chelates and their conjugates with a protein, the precision of distance measurements using these chelates, possible errors due to intramolecular energy transfer, and the modulation of the R0 value with deuterium oxide were tested. The results obtained fully confirmed the great potential of these new probes for sensitive, simple, and precise distance measurements in biomolecules using luminescence resonance energy transfer.

Enzymically accelerated biomineralization of heavy metals: application to the removal of americium and plutonium from aqueous flows

A biological process for the removal of heavy metals from the aqueous flows is described. Metals are precipitated on the surface of immobilized cells of a Citrobacter sp. as cell-bound metal phosphates. This uses phosphate liberated by the activity of a cell-bound phosphatase. Some radionuclides (e.g. 241americium) form metal phosphates readily; efficient removal of 241Am on a continuous basis is demonstrated. At low phosphatase activities, the efficiency of uranium removal correlates with enzyme activity. High phosphatase activities are not realised as an increase in metal removal, suggesting that chemical events become rate-limiting. Studies have suggested that maximal metal uptake occurs only after nucleation and the formation of precipitation foci. A model is presented to illustrate how nucleation and crystallization processes could enhance the removal of plutonium and neptunium from dilute solutions.

Preparation of europium-streptavidin in a time-resolved fluoroimmunoassay for interleukin-3

The preparation and immunoassay performance of europium-labeled streptavidin is described. The Eu-streptavidin conjugate can serve as a general detection reagent in time-resolved fluoroimmunoassays (TRFIA). The usefulness of such a strategy has previously been demonstrated with the Eu3+ chelate 4,7-bis(chlorosulfophenyl)-1,10-phenanthroline-2,9-dicarboxylic acid (Diamandis et al. 1988). In this report the conjugation of streptavidin was accomplished with the Eu3+ chelate of N1-(p-isothiocyanatobenzyl)diethylenetriamine-N1,N3,N3(+)-tetraace tic acid, using a 50 M excess of the label. The conjugation ratio of Eu3+/streptavidin was 16. The use of the Eu-streptavidin reagent in a two-site immunometric assay to measure human recombinant interleukin-3 in human plasma, showed that the useful range of the assay was 20-25,000 pg/ml.