Matthew J. O’Hara

Extraction chromatographic separations and analysis of actinides using sequential injection techniques with on-line inductively coupled plasma mass spectrometry (ICP MS) detection

A sequential injection (SI) separation system has been developed for separation and analysis of Am, Pu, and Np isotopes using on-line inductively coupled plasma mass spectrometry (ICP MS) detection. On-line actinide separations were carried out using an actinide-specific extraction chromatographic material (TRU-resin, Eichrom Industries, Inc. USA). Separations are demonstrated and characterized that are compatible with on-line ICP MS detection and address isobaric (241Am/241Pu, 244Pu/244Cm, 238U/238Pu), molecular (238UH/239Pu), and spectral (238U/237Np) interferences encountered in the analysis of Am, Pu and Np isotopic composition using ICP MS. The effects of reductive sample treatment on the extent of U matrix removal were investigated. Uranium separation factors were limited by the formation of U(IV) species, which exhibited separation behaviors similar to that of Pu(IV) and Np(IV). Nevertheless, separation factors as high as 3.0 × 105 can be achieved at the 0.1 mg ml−1 U concentration levels using Fe(II) sulfamate as a reductant. The sequential injection (SI) ICP MS technique has been applied towards analysis of Am, Pu, and Np isotopes in a dissolved vitrified nuclear waste sample. Results obtained using the automated separation technique with ICP MS detection were in satisfactory agreement with results obtained using standard analytical methodologies.

Automated radioanalytical system for the determination of 90Sr in environmental water samples by 90Y Cherenkov radiation counting.

Strontium-90 is an environmental contaminant at several U.S. Department of Energy sites, including the Hanford site, Washington. Due to its high biological toxicity and moderately long half-life of approximately 29 years, groundwater and surface water contamination plumes containing 90Sr must be closely monitored. The highly energetic beta radiation from the short-lived 90Y daughter of 90Sr generates Cherenkov photons in aqueous media that can be detected by photomultiplier tubes with good sensitivity, without the use of scintillation cocktails. A laboratory-based automated fluid handling system coupled to a Cherenkov radiation detector for measuring 90Sr via the high-energy beta decay of its daughter, 90Y, has been assembled and tested using standards prepared in Hanford groundwater. A SuperLig 620 column in the system enables preconcentration and separation of 90Sr from matrix and radiological interferences and, by removing the 90Y present in the sample, creates a pure 90Sr source from which subsequent 90Y ingrowth can be measured. This 90Y is fluidically transferred from the column to the Cherenkov detection flow cell for quantification and calculation of the original 90Sr concentration. Preconcentrating 0.35 L sample volumes by this approach, we have demonstrated a detection limit of 0.057 Bq/L using a 5 mL volume Cherenkov flow cell, which is below the drinking water limit of 0.30 Bq/L. This method does not require that the sample be at secular equilibrium prior to measurement. The system can also deliver water samples directly to the counting cell for analysis without preconcentration or separation, assuming that the sample is in secular equilibrium, with a detection limit of 7 Bq/L. The performance of the analysis method using a preconcentrating separation column is characterized in detail and compared with direct counting. This method is proposed as the basis for an automated fluidic monitor for 90Sr for unattended at-site operation.

Extraction Chromatographic Methods in the Sample Preparation Sequence for Thermal Ionization Mass Spectrometric Analysis of Plutonium Isotopes

A sample preparation sequence for actinide isotopic analysis by thermal ionization mass spectrometry (TIMS) is described that includes column-based extraction chromatography as the first separation step, followed by anion-exchange column separations. The sequence is designed to include a wet ashing step after the extraction chromatography to prevent any leached extractant or oxalic acid eluent reagents from interfering with subsequent separations, source preparation, or TIMS ionization. TEVA resin and DGA resin materials, containing extractants that consist only of C, N, O, and H atoms, were investigated for isolation of plutonium. Radiotracer level studies confirmed expected high yields from column-based separation procedures. Femtogram-level studies were carried out with TIMS detection, using multiple monoisotopic spikes applied sequentially throughout the separation sequence. Pu recoveries were 87% and 86% for TEVA and DGA resin separations, respectively. The Pu recoveries from 400 μL anion-exchange column separation sequences were 89% and 93% for trial sequences incorporating TEVA and DGA resin. Thus, a prior extraction chromatography step in the sequence did not interfere with the subsequent anion-exchange separation when a simple wet ash step was carried out in between these column separations. The average measurement efficiency for Pu, encompassing the chemical separation recoveries and the TIMS ionization efficiency, was 2.73% ± 0.77% (2σ) for the DGA resin trials and 2.67% ± 0.54% for the TEVA resin trials, compared to 3.41% and 2.37% (average 2.89%) for two control trials. These compare with an average measurement efficiency of 2.78% ± 1.70%, n = 33 from process benchmark analyses using Pu spikes processed through a sequence of oxalate precipitation, wet ash, iron hydroxide precipitation, and anion-exchange column separations. We conclude that extraction chromatography can be a viable separation procedure as part of a multistep sequence for TIMS sample preparation.

Automated Radioanalytical System Incorporating Microwave-Assisted Sample Preparation, Chemical Separation, and Online Radiometric Detection for the Monitoring of Total 99Tc in Nuclear Waste Processing Streams

An automated fluidic instrument is described that rapidly determines the total (99)Tc content of aged nuclear waste samples, where the matrix is chemically and radiologically complex and the existing speciation of the (99)Tc is variable. The monitor links microwave-assisted sample preparation with an automated anion exchange column separation and detection using a flow-through solid scintillator detector. The sample preparation steps acidify the sample, decompose organics, and convert all Tc species to the pertechnetate anion. The column-based anion exchange procedure separates the pertechnetate from the complex sample matrix, so that radiometric detection can provide accurate measurement of (99)Tc. We developed a preprogrammed spike addition procedure to automatically determine matrix-matched calibration. The overall measurement efficiency that is determined simultaneously provides a self-diagnostic parameter for the radiochemical separation and overall instrument function. Continuous, automated operation was demonstrated over the course of 54 h, which resulted in the analysis of 215 samples plus 54 hly spike-addition samples, with consistent overall measurement efficiency for the operation of the monitor. A sample can be processed and measured automatically in just 12.5 min with a detection limit of 23.5 Bq/mL of (99)Tc in low activity waste (0.495 mL sample volume), with better than 10% RSD precision at concentrations above the quantification limit. This rapid automated analysis method was developed to support nuclear waste processing operations planned for the Hanford nuclear site.

Hydroxamate column-based purification of zirconium-89 (89Zr) using an automated fluidic platform

Zirconium-89 (89Zr) is a long-lived (t1/2 = 78.4h) positron-emitting isotope that is useful for positron emission tomography (PET) based diagnostic imaging using radiolabeled antibodies. Hydroxamate resin columns are predominantly used for the purification of 89Zr from cyclotron bombarded natY targets dissolved in strong HCl. 89Zr is conventionally eluted from the resin in 1M oxalic acid (H2C2O4), a complexant that is conducive to follow-on binding of 89Zr through a transchelation process to the deferoxamine siderophore. In the present study, we determined that a lower concentration of H2C2O4 eluent (0.8M) is adequate to efficiently remove 89Zr from a column containing 100mg hydroxamate resin. As a result, less buffering agents are needed to be added to the 89Zr product fraction prior to labeling. A simple automated fluidic system prototype has been developed to perform the steps required for 89Zr purification using a hydroxamate resin column (column conditioning in HCl, Y target dissolution, dissolved target solution load onto column, column washes using HCl and water, and 89Zr elution). The system performance was evaluated using several cyclotron bombarded Y targets; 89Zr product fractions demonstrated excellent chemical recoveries from these targets, with 1.0mL product volumes yielding 89±2% of the column elution peak activity and 84±2% of 89Zr recovered from the target (at EOB). These results compare favorably with previously published 89Zr product volumes and yields, despite the lower concentration of H2C2O4 eluent employed. Transchelation of resulting 89Zr product fractions was performed to assess product quality. The effective specific activity (ESA) ranged between 44(7) and 109(22) TBq·mmole-1, while the bindable metals concentration, a metric introduced for assessing and comparing product purity, ranged between 43(7) and 115(27) nmole·g-1.

Optimized anion exchange column isolation of zirconium-89 (89Zr) from yttrium cyclotron target: Method development and implementation on an automated fluidic platform

Zirconium-89 (89Zr), produced by the (p, n) reaction from naturally monoisotopic yttrium (natY), is a promising positron emitting isotope for immunoPET imaging. Its long half-life of 78.4 h is sufficient for evaluating slow physiological processes. A prototype automated fluidic system, coupled to on-line and in-line detectors, has been constructed to facilitate development of new 89Zr purification methodologies. The highly reproducible reagent delivery platform and near-real time monitoring of column effluents allows for efficient method optimization. The separation of Zr from dissolved Y metal targets was evaluated using several anion exchange resins. Each resin was evaluated against its ability to quantitatively capture Zr from a load solution high in dissolved Y. The most appropriate anion exchange resin for this application was identified, and the separation method was optimized. The method is capable of a high Y decontamination factor (>105) and has been shown to remove Fe, an abundant contaminant in Y foils, from the 89Zr elution fraction. Finally, the method was evaluated using cyclotron bombarded Y foil targets; the method was shown to achieve >95% recovery of the 89Zr present in the foils. The anion exchange column method described here is intended to be the first 89Zr isolation stage in a dual-column purification process.

An automated flow system incorporating in-line acid dissolution of bismuth metal from a cyclotron irradiated target assembly for use in the isolation of astatine-211

Astatine-211 (211At) is a promising cyclotron-produced radionuclide being investigated for use in targeted alpha therapy. The wet chemical isolation of trace quantities of 211At, produced within several grams of Bi metal deposited onto an aluminum cyclotron target assembly, involves a multi-step procedure. Because the 211At isolation method is labor-intensive and complex, automation of the method is being developed to facilitate routine processing at the University of Washington and to make it easier to transfer the process to other institutions. As part of that automation effort, a module useful in the initial step of the isolation procedure, dissolution of the Bi target, was designed and tested. The computer-controlled module performs in-line dissolution of Bi metal from the target assembly using an enclosed target dissolution block, routing the resulting solubilized 211At/Bi mixture to the subsequent process step. The primary parameters involved in Bi metal solubilization (influent HNO3 concentration and flow rate) were optimized prior to evaluation of the system using replicate 211At-bearing cyclotron irradiated targets. The results indicate that the system performs in a predictable and reproducible manner, with cumulative Bi and 211At recoveries following a sigmoidal function.

Tandem column isolation of zirconium-89 from cyclotron bombarded yttrium targets using an automated fluidic platform: Anion exchange to hydroxamate resin columns

The development of a tandem column purification method for the preparation of high-purity 89Zr(IV) oxalate is presented. The primary column was a macroporous strongly basic anion exchange resin on styrene divinylbenzene co-polymer. The secondary column, with an internal volume of 33 μL, was packed with hydroxamate resin. A condition of inverted selectivity was developed, whereby the 89Zr eluent solution for the primary column is equivalent to the 89Zr load solution for the secondary column. The ability to transfer 89Zr from one column to the next allows two sequential column clean-up methods to be performed prior to the final elution of the 89Zr(IV) oxalate. This approach assures delivery of high purity 89Zr product and assures a 89Zr product that is eluted in a substantially smaller volume than is possible when using the traditionally-employed single hydroxamate resin column method. The tandem column purification process has been implemented into a prototype automated fluidic system. The system is configured with on-line gamma detection so column effluents can be monitored in near-real time. The automated method was tested using seven cyclotron bombarded Y foil targets. It was found that 95.1 ± 1.3% of the 89Zr present in the foils was recovered in the secondary column elution fraction. Furthermore, elution peak analysis of several 89Zr elution profile radiochromatograms made possible the determination of 89Zr recovery as a function of volume; a 89Zr product volume that contains 90% of the mean secondary column elution peak can be obtained in 0.29 ± 0.06 mL (representing 86 ± 5% of the 89Zr activity in the target). This product volume represents a significant improvement in radionuclide product concentration over the predominant method used in the field. In addition to the reduced 89Zr product elution volume, titrations of the 89Zr product with deferoxamine mesylate salt across two preparatory methods resulted in mean effective specific activity (ESA) values of 279 and 340 T Bq·mmole-1 and mean bindable metals concentrations ([MB]) of 13.5 and 16.7 nmole·g-1. These ESA and [MB] values infer that the 89Zr(IV) oxalate product resulting from this tandem column isolation method has the highest purity reported to date.

Uniform deposition of uranium hexafluoride (UF6): Standardized mass deposits and controlled isotopic ratios using a thermal fluorination method

We report a convenient method for the generation of volatile uranium hexafluoride (UF6) from solid uranium oxides and other U compounds, followed by uniform deposition of low levels of UF6 onto sampling coupons. Under laminar flow conditions, UF6 is shown to interact with surfaces within a fixed reactor geometry to a highly predictable degree. We demonstrate the preparation of U deposits that range between approximately 0.01 and 500ngcm(-2). The data suggest the method can be extended to creating depositions at the sub-picogramcm(-2) level. The isotopic composition of the deposits can be customized by selection of the U source materials and we demonstrate a layering technique whereby two U solids, each with a different isotopic composition, are employed to form successive layers of UF6 on a surface. The result is an ultra-thin deposit that bears an isotopic signature that is a composite of the two U sources. The reported deposition method has direct application to the development of unique analytical standards for nuclear safeguards and forensics. Further, the method allows access to very low atomic or molecular coverages of surfaces.