Superparamagnetic bi-functional composite bead for the thermal ionization mass spectrometry of plutonium(IV) ions

Abstract

Single resin bead-based thermal ionization mass spectrometry (TIMS) offers numerous advantages for Pu(IV) determinations in complex aqueous samples. These include removal of the matrix and interfering ions by a one-step process, selective preconcentration of Pu(IV) ions with a high chemical recovery, transportation without heavy shielding, avoiding the possibility of cross-contamination, and acting as a point source. The single-bead TIMS method reported in literature is based on an anion-exchange resin that lacks sorption-selectivity toward Pu(IV) ions, and where the beads are not easily retrievable from a large volume sample. Therefore, silica-coated superparamagnetic Fe₃O₄ embedded functionalized porous poly(ethersulfone) (PES) beads were developed for Pu(IV) preconcentration and analysis by TIMS. The beads were functionalized with a phosphate bearing monomer along with or without a quaternary ammonium bearing monomer by UV-induced surface grafting. Since the beads were used in a highly acidic solution, the Fe₃O₄ nanoparticles were protected with a silica coating formed by the hydrolysis and condensation of tetraethoxysilane. The PES porous beads were prepared by a phase inversion method. The monomers used for UV-grafting were 2-hydroxyethyl methacrylate ester and (3-acrylamidopropyl) trimethylammonium chloride. The functionalized beads were characterized by scanning electron microscopy, energy dispersive analysis, and vibrating sample magnetometry. The sorption studies indicated that the bi-functionalized PES beads consisting of phosphate and quaternary ammonium groups not only have a higher distribution coefficient (K_d) for Pu(IV) but also high selectivity toward Pu(IV) ions in the presence of a large excess of U(VI) ions (K_d(Pu(VI))/K_d(U(IV)) = 11.5). The phosphate-functionalized PES beads showed comparable selectivity (K_d(Pu(VI))/K_d(U(IV)) = 9.1), but a lower K_d value for Pu(IV). The quaternary ammonium-functionalized PES beads were found to have lower selectivity and K_d values toward Pu(IV) ions. The analytical performance of single bi-functionalized bead-based TIMS for the determination of Pu(IV) using isotope dilution was compared with the solution-based TIMS, validated using the Pu isotopic standard reference material NIST SRM-947 and applied to real samples such as dissolver solutions and soil leach liquors.

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Introduction

Knowledge about the Pu isotopic composition and its amount is important for nuclear energy utilization, nuclear safeguards, nuclear forensics, and environmental contamination. For nuclear forensics, the relative contents of the different Pu isotopes are useful as a finger print to identify the source of origin, as the abundances of the different isotopes are highly dependent on the type of nuclear reactor and nuclear fuel burn-up. Various Pu measurement procedures have been developed using alpha spectrometry, mass spectrometry, and other radiometric methods.¹ The three mass spectrometric methods used for the ultratrace determination of Pu concentrations in the complex aqueous samples are thermal ionization mass spectrometry (TIMS), inductively coupled plasma mass spectrometry (ICP-MS), and accelerator mass spectrometry (AMS).^1,2 Multiple collector mass spectrometers are capable of the simultaneous determination of a number of isotopes. Sector-field multi-collector inductively coupled plasma mass spectrometers (MC-ICP-MS) are capable of analyzing less than 10 fg of Pu with a high precision (3% 2 s).³ However, a chemical purification step is required to eliminate isobaric and polyatomic interferences such as ²³⁸UH interference in ²³⁹Pu determination.⁴ Chemical treatments using rapid column extraction with TEVA™ and TRU™ resin cartridges not only remove inferences but also make possible the simultaneous analysis of the different actinides with ICP-MS.⁵

A combination of anion-exchange and extraction chromatography is preferable for achieving a higher Pu chemical recovery and U decontamination factor.⁶ An automated sequential injection separation system in conjunction with MC-ICP-MS has been developed for the simultaneous analysis of ²³⁷Np and Pu isotopes in environmental samples.⁷ The high chemical recovery (>90%) and low detection limits, 2.5, 2.1, and 0.42 fg mL⁻¹ obtained for ²³⁷Np, ²³⁹Pu, and ²⁴⁰Pu, respectively, reduces the soil and sediment samples requirement to as low as 1 g.⁷ The chemical purification along with isotope dilution technique makes MC-ICP-MS highly reliable for the analysis of ultratrace concentrations of Pu in environmental samples.⁸

However, TIMS in conjunction with isotope dilution is also commonly used for the quantification of Pu in the ng range in nuclear facilities.^1,2 TIMS analysis gives a reproducibility of 0.1% (2 s) for ≈1 ng Pu, 1.5% (2 s) for 200–500 fg Pu, and 10% for <50 fg Pu.^3b In general, Pu analysis by TIMS requires an appropriate sample preparation procedure involving dissolution/leaching to bring Pu into solution, matrix elimination and the selective preconcentration of Pu, and finally, manual deposition of a small volume (≈10 μL) of solution onto a filament surface.⁹ The general comparison of TIMS and ICP-MS based analytical methods is given in Table 1.

Table 1 Comparison of TIMS and ICP-MS based analytical methods

Parameter	TIMS	ICP-MS
Ionization	Thermal ionization (produces ion beam with very high ionization yield and selectivity)	Inductively coupled plasma source at normal pressure
Sample	Amenable to solid samples (≈10 μL of an aqueous sample solution is deposited on a high-purity Re filament surface or resin/bead mounted on Re filament)	More suited for liquid sample injection
Interference	Formation of plutonium oxide can be prevented by filament carburization	Isobaric (e.g. ^238U and ^238Pu, ^241Am, and ^241Pu) and polyatomic (e.g. ^238UH and ^239Pu) interferences
Sample purification methods	Ion-exchange and extraction chromatography	Ion-exchange and extraction chromatography
Analytical performance	Advantage: excellent sensitivity, precise isotopic ratio determination, small amount of analyte needed	Advantage: excellent sensitivity, low detection limit, small amount of analyte, excellent simultaneous multi-element analysis ability, on-line separation and analysis
	Disadvantage: lacks multi-element capability, complicated sample preparation, restricted to elements with ionization potential <6 eV	Disadvantage: solid samples cannot be used, matrix effect
Major applications	Isotopic composition determination of actinides	Quantification of isotopes in various environmental, biological, and geological samples

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There is a possibility to use solid samples in TIMS instead of liquid samples. A single anion-exchange resin-bead-based TIMS method has been developed for the isotopic analysis of U and Pu in safeguarded nuclear materials.¹⁰ In this method, Pu(IV) ions are loaded in to a bead either from the original sample or after purification using an anion-exchange or extraction chromatography, and the Pu-loaded-bead is then fixed on a Re filament and pyrolyzed during TIMS. Thus, the bead acts as the matrix for chemical separation, helps to secure physical transportation, hence reducing the possibility of instrument contamination or cross-contamination of the sample, and it is a point source for thermal ionization. Sequential mass spectrometric analysis of uranium and plutonium was carried out in our laboratory employing a single Dowex 1X8 resin bead loaded with about 10 ng of plutonium.^10a To enhance the Pu(IV) sorption kinetics in a single bead for TIMS, the acoustic streaming was used for small sample volumes.¹¹

In the present study, the Fe₃O₄ nanoparticles (NPs) embedded bi-functional poly(ethersulfone) (PES) beads were developed for the single-bead-based TIMS analysis of Pu in aqueous samples. The choice of quaternary ammonium and phosphate functional groups bearing monomers for UV-grafting on the PES beads was based on the high affinity of the phosphate group toward Pu(IV) ions,¹² and the quaternary ammonium group facilitates the sorption of hexanitrato anionic complex, Pu(NO₃)₆²⁻, existing in a solution with a high concentration of 7–8 mol L⁻¹ HNO₃. The PES polymer was used as the base matrix due to its amenability for UV grafting¹³ and easy formation of the porous beads by the phase inversion method. The superparamagnetic Fe₃O₄ NPs were immobilized in the bi-functional PES beads for their easy retrieval from large volume samples using an external magnet. This concept is similar to magnetically assisted chemical separation for remediation and analytical applications.¹⁴ For the single-bead TIMS analysis of Pu, the isotope dilution method was employed for the quantification to make the results independent of chemical recovery and unknown variations in the experimental parameters.¹⁵

Experimental

Reagents and apparatus

Analytical reagent grade solvents, suprapure grade nitric acid (Merck, Mumbai), and deionized water (18 MΩ cm⁻¹) purified by Quantum from Millipore (Mumbai, India) were used. Hydroxylamine hydrochloride was obtained from SISCO Research laboratories Pvt. Ltd. (Mumbai). Hydrogen peroxide (30%) was procured from Merck Specialties Pvt. Ltd. (Mumbai). The Fe₃O₄ nanoparticles were obtained from J.K. Impex, Mumbai, India. These particles were characterized by X-ray diffraction (XRD) and field emission gun scanning electron microscopy (FEG-SEM). The size distribution of Fe₃O₄ nanoparticles ranged from 20 to 30 nm.

Poly(ethersulfone) was obtained from Goodfellow Cambridge Ltd. (England). Tetraethoxysilane, phosphoric acid 2-hydroxyethyl methacrylate ester (HEMP) containing 25% diester, 3-(acrylamido propyl) trimethylammonium chloride (APTAC), and N-N′-dimethylformamide (DMF) were obtained from Sigma-Aldrich (Steinem, Switzerland). The chemical structures of the monomers are given in Scheme S1 (ESI†). The surface grafting of monomers on the PES beads was carried out in a photo-reactor (Heber Scientific, Chennai, India) consisting of eight 8 Watt 365 nm UV lamps (Sankyo Denki, Japan) fitted in a circular geometry.

The purified stock solutions of ^mixedPu, ²⁴¹Am, and ²³³U were obtained from Fuel Chemistry Division, B.A.R.C., Mumbai, India. Pu was generated in an Indian Pressurized Heavy Water Reactor (PHWR) with an average burn-up of ∼10 000 megaWatt day per ton of U. The isotopic compositions of these tracers were determined by TIMS. The isotopic composition of Pu(IV) was ²³⁸Pu (0.16%), ²³⁹Pu (68.79%), ²⁴⁰Pu (26.94%), ²⁴¹Pu (2.09%), and ²⁴²Pu (2.02%). The irradiated UO₂ fuel dissolver solution was obtained from the Fuel Reprocessing Division, BARC, Mumbai, India, and contained the fission products, major actinides (U and Pu), minor actinides, and activation products. The alpha activity of actinides in an aqueous solution was measured by a home-built liquid scintillation counter with an EMI 9514 photomultiplier tube using an Ultima Gold AB scintillation cocktail (Perkin Elmer). The gamma activity measurement was carried out using a well-type 3′′ × 3′′ NaI(Tl) detector coupled to a single channel analyzer (NUCLEONIX). The TIMS analyses were carried out using a model MAT-261 (Finnigan, Germany) equipped with 9 Faraday cups, each cup connected to a resistor of 10¹¹ ohm. The specifications of the TIMS instrument used in the present study are given in Table 2. The Pu⁺ ion currents were measured under a static mode of multi-collection, and a double Re filament assembly was used for the sample loading.

Table 2 Specifications of the Finnigan MAT-261 TIMS instrument used in the present study

Parameter	MAT-261 TIMS instrument
Acceleration voltage	10 kV
Ion source & filament	Thermal ionization with a high-purity rhenium double-filament assembly
Analyser	Nine variable Faraday cup detectors, designated as FAR2 to FAR10, each coupled to a 10^11 ohms resistor
Isotopic mass assigned to the Faraday cup	^239Pu to L1, ^240Pu to axial/centre cup, ^241Pu to H1 and ^242Pu to H2
Mass resolution (ΔM/M) at 10% valley	450
Abundance sensitivity	10 ppm (at m/z = 237/238)

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Synthesis of functionalized magnetic PES beads

The Fe₃O₄ nanoparticles (NPs) were coated with tetraethoxysilane (TEOS) dissolved in ethanol while constantly being shaken at room temperature for 12 h. The magnetic PES beads were synthesized by a phase inversion method.

Typically, PES granules were dissolved in an appropriate volume of DMF to obtain a saturated solution. To this, the TEOS-coated Fe₃O₄ NPs (Fe₃O₄@SiO₂) were homogeneously dispersed by sonication for 30 min. The Fe₃O₄@SiO₂ dispersed PES solution was added dropwise to water with continuous stirring to produce nearly spherical magnetic PES beads with diameters ranging from 0.5 to 1 mm, as shown in Scheme 1. For grafting functional groups, the Fe₃O₄@SiO₂-loaded PES beads were equilibrated with an equimolar mixture of the two monomers HEMP and APTAC in a solution consisting of 1 : 1 v/v water : ethanol. After equilibration, the polymerizing solution filled beads were irradiated for 15 min in a UV reactor (λ_max = 365 nm). During irradiation, the poly(ethersulfone) polymer chains underwent photolysis to generate free radicals,¹³ which initiated graft polymerization of HEMP and/or APTAC monomers on the PES beads. The poly(HEMP), poly(APTAC), and poly(HEMP-co-APTAC) grafted magnetic PES beads obtained after irradiation were washed with ethanol to remove unreacted monomers, then air-dried and conditioned with 0.1 mol L⁻¹ NaCl before use. A detailed schematic of the steps involved in formation of the bi-functional magnetic PES bead is given in Scheme 1.

Scheme 1 Schematic of the synthesis of bi-functional magnetic PES beads by a phase inversion method and UV-grafting.

Sorption and desorption studies

The sorption and desorption studies of Pu(IV), Am(III), and U(VI) ions from the bi-functional superparamagnetic PES beads were carried out using the same procedure, as described in our earlier study.^12,15a Briefly, the sorption efficiencies of the actinide ions were measured by equilibrating 100 mg beads for 3–4 h with 5 mL of the well-stirred solutions containing 3 to 8 mol L⁻¹ HNO₃ and spiked with fixed concentrations of ^mixedPu/²⁴¹Am/²³³U. The sorption efficiency of actinides ions in the beads were determined from the difference in α-activity (^mixedPu and ²³³U) or γ-activity (²⁴¹Am) of the aliquots taken from the solution before and after equilibration. For desorption, the beads were equilibrated with 3 mol L⁻¹ HNO₃, 1 mol L⁻¹ NH₂OH·HCl and 0.5 mol L⁻¹ Na₂CO₃ for Am(III), Pu(IV) and U(VI), respectively. The Pu(IV) sorption kinetics was studied by equilibrating 200 mg of PES beads with 30 mL of 3 mol L⁻¹ HNO₃ solution spiked with an appropriate amount of Pu radiotracer, with continuous stirring. The uptake of Pu(IV) by the beads was monitored by taking 25 μL aliquots from this solution at regular intervals, and the α-activities in these aliquots were measured as a function of equilibration time. For determination of the distribution coefficients (K_d) of the actinide ions, the PES beads were equilibrated with a mixture of ²³³U and ^mixedPu ions in 100 : 1 mol proportion, and the K_d values for Pu(IV) and U(VI) ions were obtained using the following equation:where A₀ and A_e represent the radioactivities of ²³³U/^mixedPu before and after equilibration, respectively, W is the weight of the PES beads and V is volume of the equilibrating solution.

Thermal ionization mass spectrometry

The analytical performance of the single-bead-based thermal ionization mass spectrometry (SB-TIMS) for Pu analysis was studied using a NIST SRM-947 Pu isotopic standard. The various steps involved in Pu analysis by the SB-TIMS method are depicted in Scheme 2. The isotopic composition of Pu in the sample was determined by SB-TIMS by equilibrating a single magnetic bi-functional PES bead in 5 mL of the well-stirred 3 mol L⁻¹ HNO₃ solution, spiked with ∼1 μg of Pu for 1 h. After equilibration, the bead was brought close to the solution surface using an external permanent magnetic bar and collected from the solution using tweezers. The collected bead was washed roughly with water and 3 mol L⁻¹ HNO₃, then air-dried and directly loaded onto a high-purity rhenium filament for the TIMS analysis. Prior to the Pu analysis, the ionization filament (IF) was heated to obtain ∼200 mV equivalent of ¹⁸⁷Re⁺ ion current. Thereafter, the vaporization filament (VF) was heated from 0 to 2 A in 900 s and degassing was carried out at 2 A for 1800 s. The degassing temperature (VF temp. 2 amp) was sufficiently lower than the temperature required for the evaporation of Pu atoms loaded on the Re filament (VF temp. 2.25 amp). Therefore, practically no loss of Pu during the degassing period was observed. After degassing, VF heating was increased to obtain the optimum Pu⁺ ion current. During each determination of Pu, the blank beads were subjected to TIMS to observe the significant Pu⁺ ion current, if any. This ensured that there were no Pu contaminations in the beads.

Scheme 2 Depiction of different steps involved in the determination of Pu(IV) ions using single-bead-based isotope dilution thermal ionization mass spectrometry developed in the present study.

Single-bead-based isotope dilution thermal ionization mass spectrometry (ID-SB-TIMS) technique was employed for the determination of Pu concentration in the dissolver solution of irradiated UO₂ fuel and in the soil leach liquor samples spiked with a known amount of Pu(NO₃)₄. The Pu concentration was determined by an isotope dilution (ID) technique, which involved the addition of a known weight W_sp of a pre-calibrated spike solution, with a Pu concentration C_sp, to a known weight W_sa of the sample solution. Each sample and its spiked mixture was equilibrated with a bi-functional magnetic PES bead and placed on a Re filament for isotopic composition analysis by TIMS. The Pu concentration C_sa in the sample can be correlated with the change in either the ²⁴⁰Pu/²³⁹Pu or ²⁴²Pu/²³⁹Pu atom ratio in the spiked mixture (R_m) with respect to that in the sample (R_sa) and spike (R_sp) using the following equation:

For determination of the Pu concentration, the single bi-functional magnetic PES bead was equilibrated with 3 mL of the dissolver solution (U : Pu mole ratio ∼1000 : 1) and its spiked mixture, separately, for 1 h and with continuous stirring.

After equilibration, the single bead was washed and loaded onto a Re filament for the isotopic composition analysis of Pu in the dissolver solution and its spiked mixture. For quantification of Pu(IV) in a large volume sample, a known Pu concentration and its spiked mixture were added to 50 mL of 3 mol L⁻¹ HNO₃ and then equilibrated with the single bi-functional magnetic PES bead for 24 h with continuous stirring. For determination of the Pu concentration in the soil samples, the solution containing a known Pu concentration was added to about 2 g of soil sample, collected from the B.A.R.C. premises after digging one foot, homogenized manually, and dried under an IR lamp for 2–3 h. The solution was sufficient to soak 2 g of soil. Then, the soil sample was treated with 50 mL of 8 mol L⁻¹ HNO₃ containing 2–3 mL of 30% H₂O₂, and heated (65–70 °C) under an IR lamp for 8 h. The obtained soil leach liquor was used for the Pu quantification.

The same procedure was repeated for a spiked mixture of Pu. The leach liquor (50 mL) was equilibrated with a single bifunctional magnetic PES bead for 24 h with continuous stirring. After preconcentration of Pu(IV) ions, the beads were collected using a magnet, washed, dried, and loaded onto a Re filament for Pu isotopic composition analysis by TIMS. The Pu concentrations in various samples were calculated from the change in isotope ratios in the spiked mixture with respect to that in the sample and the spike.

Results and discussion

Characterization of the functionalized magnetic beads

The functional monomers were anchored using the free radical formed during UV-light exposure of the PES polymer chains.¹³ The free radicals initiate graft polymerization of the monomers. The monomers APTAC and HEMP used for the preparation of bi-functional PES beads are acrylate based and, therefore, are expected to graft-polymerize with comparable efficiencies, as observed in our previous study.¹² The HEMP monomers have three or two double bonds, which may lead to the formation of a dense cross-linked grafted co-polymer on the surface layer of PES beads. The surface grafting of monomers on the PES was expected as 365 nm UV light would not penetrate deep into the interior of the matrix. The representative chemical changes involved in the grafting of monomers on the PES beads are shown in Scheme 3.

Scheme 3 The representative chemical modifications of PES beads during UV-grafting of phosphoric acid 2-hydroxyethyl methacrylate ester (HEMP) and 3-(acrylamido propyl) trimethylammonium chloride (APTAC).

The physical and elemental characterization of the bi-functionalized superparamagnetic beads was carried out with a scanning electron microscope (SEM) with an energy dispersive spectrometer attached to it. As can be seen from the SEM images given in Fig. 1, the PES beads prepared in the present study had a dense surface and highly porous interior. The physical structure of the beads did not change during grafting of the HEMP monomer with or without the co-monomer APTAC. The silica-coated Fe₃O₄ NPs were not visible as these may be embedded in the PES matrix. The phosphorus elemental mappings at different locations across a bead showed that the grafting occurred uniformly.

Fig. 1 SEM images of poly(HEMP-co-APTAC) grafted superparamagnetic bead under different magnifications.

The presence of elements such as S from the base PES matrix, Fe and Si from the silica-coated Fe₃O₄, phosphorus and nitrogen from the grafted poly(HEMP-co-APTAC) was observed in the EDS spectra. As can be seen from Fig. S1 (ESI†), the nitrogen (two N atoms in APTAC) and phosphorous (one P in HEMP) contents are comparable, indicating that both the monomers polymerized with the same efficiency. As beads have to be equilibrated with a solution containing a high concentration of HNO₃, the beads were immersed in 3 and 8 mol L⁻¹ HNO₃ for 24 h, and then subjected to elemental composition analyses. It is evident from Fig. 1S (ESI†) that the elemental composition of beads did not change during equilibration with a high concentration of acid. Thus, all the components, including silica-coated Fe₃O₄ in the functionalized beads, are highly stable against leaching in a solution with a high concentration of HNO₃. The changes in the superparamagnetic properties of Fe₃O₄ after coating silica and immobilizing in the bifunctional beads were studied by vibrating sample magnetometry (VSM).

The comparison of magnetization curves is shown in Fig. 2. As can be seen from Fig. 2, the Fe₃O₄ NPs retained their superparamagnetic properties, although the saturation magnetization decreased due to magnetic shielding by the silica coating and PES matrix. For example, the saturation magnetization of Fe₃O₄ particles decreased from 50 emu g⁻¹ to 26 emu g⁻¹ after the silica coating, and further decreased to 9 emu g⁻¹ after embedding them (∼2 wt%) in the matrix of bi-functionalized PES beads. However, the saturation magnetization of beads was found to be good enough for withdrawing the single bead from an aqueous sample using an external permanent magnet kept outside.

Fig. 2 VSM magnetization curves showing the superparamagnetic properties, and change in saturation magnetization of the bi-functionalized PES bead with respect to Fe₃O₄ and Fe₃O₄@SiO₂ particles.

Sorption properties of functionalized magnetic beads

To understand the effect of bi-functionalization on the sorption of actinide ions, the uptake profiles of the representative actinides, namely, Pu(IV), Am(III), and U(VI), were studied as a function of HNO₃ concentration using the poly(HEMP)- and poly(HEMP-co-APTAC)-grafted magnetic PES beads. The Pu(IV), Am(III), and U(VI) uptake profiles given in Fig. 3 clearly show that Am(III), a representative of trivalent actinides, was not sorbed with any significant efficiencies in both mono-/bi-functionalized beads. Unlike Am(III), Pu(IV) and U(VI) ions were sorbed with a high efficiency. It is interesting to observe from Fig. 3 that the Pu(IV)-sorption efficiency did not change significantly in the HNO₃ concentration range from 2 to 8 mol L⁻¹ in both types of beads. However, the Pu(IV)-sorption efficiency of poly(HEMP-co-APTAC)-grafted PES beads was considerably higher (≈90%) than that in the poly(HEMP)-grafted PES beads (60%). This may be attributed to a fact that the anionic Pu(IV) complexes, such as [Pu(NO₃)₅]⁻ and [Pu(NO₃)₆]²⁻, exist in a higher HNO₃ concentration and, therefore, the presence of quaternary ammonium groups improved the Pu(IV)-sorption efficiency.

Fig. 3 The uptake profiles of actinides ions as a function of HNO₃ concentration in the poly(HEMP) functionalized (a) and poly(HEMP-co-APTAC) functionalized (b) magnetic PES beads.

In the case of U(VI) ions, the sorption efficiency of poly(HEMP)- and poly(HEMP-co-APTAC)-grafted beads remained high (80–90%) and decreased slightly with an increase in HNO₃ concentration.

Thus, both types of beads do not differentiate between U(VI) and Pu(IV) ions present in the aqueous sample. The selectivity of polymer sorbents toward Pu(IV) and U(VI) ions was studied in a competitive mode by measuring their K_d values in solutions containing a 100 : 1 mole ratio of U(VI) and Pu(IV) at 4 mol L⁻¹ HNO₃. It can be seen from the data given in Table 3 that the K_dPu(IV) values are higher in all sorbents, including commercially available anion-exchange resin Dowex 1X8 and home-made poly(APTAC) PES beads. The data given in Table 3 suggest that the K_d value for Pu(IV) and ratio of K_d(Pu(IV)) to K_d(U(VI)) is highest in the bi-functional PES magnetic beads.

Table 3 Distribution coefficient (K_d value) of Pu(IV) and U(VI) ions in the different sorbents from the solution containing 4 mol L⁻¹ HNO₃ and 1 : 100 mole proportion of Pu(IV) to U(VI) ions

Polymer sorbent	Kd (Pu(IV)) (mL g^−1)	Kd (U(VI)) (mL g^−1)	Kd(Pu(IV))/Kd(U(VI))
Dowex 1X8	197	32	6.15
Poly(APTAC)-PES	98	46	2.13
Poly(HEMP)-PES	227	25	9.09
Poly(HEMP-co-APTAC)-PES	462	40	11.53

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The kinetics of Pu(IV) sorption by the bi-functional magnetic PES beads was studied to ensure optimum recovery of Pu(IV) from the sample during equilibration. The rate of Pu(IV) sorption by the beads from 3 M HNO₃ as a function of equilibration time is shown in Fig. S2 (ESI†). As shown in Fig. S2 (ESI†), the Pu(IV)-sorption equilibrium was attained within 80 min. The Pu(IV)-sorption rate followed a pseudo-second order equation (see ESI†).¹² The initial Pu(IV)-sorption rate was fast but reduced thereafter due to the slow transfer of Pu(IV) ions from the surface to the interior of the matrix. The sorption capacity of the bi-functional magnetic PES beads determined from the slope of the plot of t/[Pu(IV)] vs. t/[Pu(IV)]_eq. was found to be 93 μg of Pu g⁻¹. This value is in a good agreement with the experimentally measured Pu loading capacity 92 μg of Pu g⁻¹.

The effect of volume of the equilibrating solution on the Pu(IV)-sorption efficiency of the bi-functional magnetic PES beads was also studied. The data presented in Fig. S3 (ESI†) indicate that the Pu(IV)-sorption efficiency was nearly constant up to 30 mL, but reduced thereafter because of the slow diffusion of Pu(IV) ions in the large volume of the equilibrating solution. Therefore, the preconcentration of Pu(IV) ions from a large volume sample, higher than 30 mL, would require a longer equilibration time (24 h for 50–100 mL) or a better equilibration method such as a flow cell.

Functionalized magnetic-bead-based thermal ionization mass spectrometry

For single-bead thermal ionization mass spectrometry (SB-TIMS), the bead was fixed on a thin Re filament and subjected to high temperature for pyrolysis of the bead, thermal desorption of the analyte, and subsequent ionization. The extent of ionization of the analyte is governed by the Saha–Langmuir equation,¹⁶ and the analyte should be in its chemically purest state in order to achieve optimum ionization. The rate of evaporation of Pu atoms, which is proportional to the Pu⁺ ion current, is important for the optimum analytical signal in TIMS. Therefore, the variations in ²³⁹Pu⁺ ion current as a function of vaporization filament temperature were studied in a solution and single-bead-based TIMS. The plots of ²³⁹Pu⁺ ion current vs. vaporization filament temperature (VF) are shown in Fig. 4.

Fig. 4 Variations of observed ²³⁹Pu⁺ ion current from the solution and bi-functionalized magnetic-based Pu loadings as a function of vaporization filament temperature.

With increasing temperature of the VF, the Pu⁺ ion current was found to increase according to the Saha–Langmuir equation. In the present study, no PuO⁺ ion current was observed during analysis due to the reducing atmosphere provided by the PES polymer present on the filament surface. The optimum VF heating current for the isotopic analysis of Pu was found to be about 4 amp, at which about 1 V signal corresponding to ²³⁹Pu⁺ ion current was obtained. The ionization efficiency of the SB-TIMS method was lower than that of the conventional solution-based loading method, but the ion collection efficiency was comparable for both the techniques. The magnetic PES bead on the Re filament did not decompose completely. Therefore, it was important to carry out degassing of the bead loaded onto the filament prior to TIMS analysis to remove volatile impurities and to obtain a stable Pu⁺ ion current. The effect of degassing on the precision of Pu isotopic atom ratio obtained from the single magnetic PES bead fixed on the Re filament was studied and compared with the precision obtained from the conventional TIMS method employing solution loading and 600 s degassing time.

As shown in Fig. S4 (ESI†), the best precision was obtained when degassing of the bead was carried out for a longer period of time. Therefore, the degassing was carried out for 1800 s prior to the analysis of each sample for obtaining a stable current. The comparison of Pu isotopic compositions in the standard NIST SRM-947 sample obtained by SB-TIMS and conventional TIMS is given in Table 4. The atom% of the two major isotopes, namely, ²³⁹Pu and ²⁴⁰Pu, determined by the two methods were found to agree within 0.2%, whereas the atom% of ²⁴¹Pu and ²⁴²Pu, which were less abundant isotopes, was found to agree within 0.5%.

Table 4 Determination of the isotopic compositions of Pu in the SRM-947 Pu standard by TIMS using a single bi-functionalized magnetic PES bead and solution-based loadings

Isotope	Certified value (atom%)	Bead loading (a) (atom%)	Solution loading (b) (atom%)
^239Pu	79.03 ± 0.02	78.96 ± 0.04	78.96 ± 0.03
^240Pu	19.02 ± 0.02	19.04 ± 0.012	18.99 ± 0.03
^241Pu	0.808 ± 0.006	0.735 ± 0.006	0.723 ± 0.006
^242Pu	1.238 ± 0.004	1.243 ± 0.008	1.225 ± 0.006

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Analytical performance of SB-ID-TIMS

The quantitative determination of the concentrations at the level of several μg L⁻¹ of Pu(IV) in the aqueous solution, irradiated fuel dissolver solution, and soil leach liquor samples was carried out by the SB-ID-TIMS method. The isotopic compositions of Pu in the samples and spiked solutions used for the isotope dilution, determined by conventional TIMS, are listed in Table 5. Each aliquot of dissolver solution, generally in 3 to 4 mol L⁻¹ HNO₃ medium, was equilibrated with a single magnetic PES bead. The bead was directly analyzed by TIMS after equilibration. For the dissolver solution, a PHWR grade Pu solution was chosen as a spike, which contained about 27 atom% of ²⁴⁰Pu and the change in ²⁴⁰Pu/²³⁹Pu atom ratio in the mixture with respect to that in the sample and the spike was monitored. The aqueous and soil leach liquor samples, 50–100 mL of each sample, were spiked with a known quantity of Pu and equilibrated with a single magnetic bi-functional PES bead for 24 h.

Table 5 Isotopic compositions of Pu in the samples and spike solutions used in the SB-ID-TIMS method

Isotope	Dissolver solution (atom%)	^240Pu spike used for dissolver solution (atom%)	(U, Pu)C (atom%)	^242Pu spike used for (U, Pu)C (atom%)
^238Pu	0.022 ± 0.001	0.192 ± 0.005	0.150 ± 0.005	0.02 ± 0.001
^239Pu	89.92 ± 0.04	67.73 ± 0.03	69.20 ± 0.03	1.54 ± 0.006
^240Pu	9.119 ± 0.009	27.35 ± 0.03	26.23 ± 0.03	2.97 ± 0.005
^241Pu	0.861 ± 0.003	2.574 ± 0.005	2.568 ± 0.005	0.66 ± 0.003
^242Pu	0.070 ± 0.001	2.15 ± 0.006	1.846 ± 0.006	94.81 ± 0.04

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The Pu spiked in aqueous samples and soil leach liquors contained ∼26 atom% of ²⁴⁰Pu, therefore a ²⁴²Pu-enriched spike was used and the concentration was determined from the change in ²⁴²Pu/²³⁹Pu atom ratio in the mixture with respect to that in the sample. The Pu concentrations in these samples determined by both SB-ID-TIMS and conventional ID-TIMS methods are given in Table 6. It is observed from this table that the results agree within 1%. This indicates that SB-ID-TIMS has a reasonably good analytical performance.

Table 6 Determination of Pu concentrations by the SB-ID-TIMS method and by conventional ID-TIMS

Sample	Pu concentration (μg g^−1)
	SB-ID-TIMS	ID-TIMS
Dissolver solution	11.8 ± 0.3	11.9 ± 0.2
4 mol L^−1 HNO3	3.37 ± 0.02	3.39 ± 0.02
Soil leach liquor	2.31 ± 0.02	2.32 ± 0.02

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Conclusions

A method was developed for the simultaneous preconcentration of Pu using a superparamagnetic bi-functional PES bead grafted with poly(HEMP-co-APTAC), followed by determination of the isotopic composition and concentration of Pu by the SB-ID-TIMS method. This method provides a single step purification and preconcentration of Pu from 5 to 100 mL in a solution. Thus, the sample manipulation steps, sample preparation time, exposure to radiation, and secondary waste generation could be minimized. The PES-bead-based loading technique was found to provide comparable accuracy and precision with respect to the conventional solution loading technique. The presence of superparamagnetic Fe₃O₄ NPs in the bi-functional PES beads allows easy retrieval of the bead from large volumes of aqueous solutions. In addition to this, the higher loading efficiency (K_d(Pu(IV)) value) and competitive-selectivity (K_d(Pu(IV))/K_d(U(VI))) of the bi-functional magnetic PES beads toward the Pu(IV) bead provide a possibility of using SB-ID-TIMS analyses for Pu(IV) determination in aqueous samples featuring a large excess of competing U(VI) ions.

Acknowledgements

The authors are thankful to Mr Santu Kaity, RMD, BARC for SEM imaging, and Dr S. M. Yusuf, SSPD, BARC for VSM analyses of the magnetic particles.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Kinetics experiments. See DOI: 10.1039/c5ra18419c

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