Automated Radioanalytical System Incorporating Microwave-Assisted

Publication Date (Web): March 22, 2012 ... Continuous, automated operation was demonstrated over the course of 54 h, which resulted in the analysis of...
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Automated Radioanalytical System Incorporating MicrowaveAssisted Sample Preparation, Chemical Separation, and Online Radiometric Detection for the Monitoring of Total 99Tc in Nuclear Waste Processing Streams Oleg B. Egorov,†,‡ Matthew J. O’Hara,† and Jay W. Grate*,† †

Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: An automated fluidic instrument is described that rapidly determines the total 99Tc content of aged nuclear waste samples, where the matrix is chemically and radiologically complex and the existing speciation of the 99Tc 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 99Tc. 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.

be coupled directly to detection instrumentation, such as flow through scintillation detectors or inductively coupled plasma mass spectrometers (ICPMS). However, sample preparation prior to automated separations is still generally carried out as a manual step in the radiochemistry laboratory. In a previous paper, we described the investigation of an automated sample preparation system for the determination of the total 99Tc in nuclear waste.5 The 99Tc can exist in multiple valence states, which must all be converted to pertechnetate, 99 Tc(VII)O4−, for a subsequent anion exchange separation. A system was developed using a digestion vessel in a microwave cavity to which samples, standards, and reagents could be fluidically added and then heated. These reagents acidified the matrix and oxidized reduced technetium species to pertechnetate. The sample chamber was designed to enable fluidic retrieval of the prepared sample and automated delivery of a measured volume of digestate to a separation column. In this paper, we are concerned with the development of automated radiochemical analysis instrumentation that is suitable for at-site use to monitor the radiochemical solutions

Radiochemical analysis−for the determination of radionuclides that cannot be nondestructively determined by gamma spectroscopy−entails sample preparation, radiochemical separations, and detection. Whether radiometric, optical spectroscopic, or mass spectrometric, detection is subject to potential interferences from other radionuclides or elements in the sample and from matrix effects on the detection process. Separations are therefore required to capture the analyte(s), separate them from interferences, and deliver the analytes in a matrix suitable for subsequent source preparation and/or detection. The separations themselves may require that the analyte be present in a particular valence state or speciation; therefore, sample preparation steps must often precede the separation. Though time-consuming and tedious, these radioanalytical chemistry steps are essential, or as another author has put it: “inconvenient, but indispensable”.1 The development of programmable digital fluidic instrumentation approaches has enabled the automation of radiochemical separations. Such efforts have been described in the past and were recently summarized in a comprehensive review.2−4 Fluidic automation approaches have been developed for the separation and analysis of fission products such as 90Sr and 99Tc, as well as various actinides.2−4 These automated separations can be used to generate separated fractions or can © 2012 American Chemical Society

Received: October 19, 2011 Accepted: March 9, 2012 Published: March 22, 2012 3090

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on detecting its β particle emission (βmax = 294 keV). The overall monitor approach has been briefly described previously.7−9 Here we provide a detailed description of the monitor design, its operation, and the accurate determination of total Tc in a variety of nuclear waste sample matrixes with quantification over a very large dynamic range. This instrument performs programmed spike additions periodically to automatically obtain matrix matched calibration data, while simultaneously obtaining a self-diagnostic parameter to assess the overall performance of the automated separation and quantification. Results are presented that demonstrate extended unattended operation of the instrument with excellent precision and accuracy.

associated with large-scale nuclear processing operations. Key challenges in this effort are the need to perform the complete analyses very rapidly, separating the analyte of interest from a radiologically and chemically very complex sample matrix, and fully integrating sample preparation, separation, and detection in one automated sequence. In conventional radioanalytical chemistry, these analytical processes would be carried out in a laboratory over the course of hours or days. Here we describe the design and demonstration of a prototype process-monitoring instrument for an at-line/online analysis that fully integrates radiochemical sample preparation, separation, and detection in an automated fluidic format. This monitor was developed to support a column-based processscale 99Tc removal operation that was planned as part of the Hanford Waste Treatment Plant in Washington State. 99Tc is a high abundance fission product with a ∼6% fission yield that is present in nuclear waste from the production of plutonium and from power generation. Technetium is a concern in nuclear waste processing because of the volatility of its heptavalent oxide, Tc2O7, which has melting and boiling points of just 119 and 311 °C, respectively. Under nuclear waste vitrification conditions, losses of >95% have been reported due to volatilization of technetium species.6 A monitor was required immediately downstream of a column-based Tc removal process, in order to detect 99Tc breakthrough. When breakthrough was detected, the column would be switched out and the 99Tc-retaining sorbent material would be regenerated or replaced. Several monitoring performance criteria were established by the operators of the Hanford site (see Table 1), including continuous operation with at least four analytical results provided per hour.

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EXPERIMENTAL SECTION Reagents, Standards, and Samples. Reagents were of analytical grade. The activity of standards was verified by ICPMS or liquid scintillation counting; 99Tc standards were prepared by dilution of NIST traceable stock standards. Nuclear waste samples were described previously;5 briefly, tank waste simulant (AN-105) and Hanford tank waste samples (such as AZ-102 and others, see Table 2) were used for method development and testing. The solutions were normalized to ∼5 M Na+, and bulk of the 137Cs activity in the tank waste solution was removed by an inorganic ion exchange process. 99Tc concentrations in the tank waste samples were determined by ICPMS analysis according to procedures described previously.5,10 Monitor Components. The monitor instrument in Figure 1 was assembled using the following components. The microwave unit was a STAR 2 open vessel instrument (CEM Corporation, Mathews, NC) using a 7-mL PTFE Teflon concave-bottom digestion vessel (CEM) that was modified with inlet and outlet tubes as described previously.5 The separation column body was put together from the OmegaChrom column system (Upchurch Scientific, Oak Harbor, WA) with internal dimensions of 4.6 × 50 mm (0.83 cm3 bed volume). The column packing was AG MP-1 M strongly basic macroporous anion exchange resin of particle size 38−75 μm (Bio-Rad Laboratories, Hercules, CA). Online detection was provided by a Beta-Ram 2B flow-through scintillation detector (IN/US Systems, Inc., Tampa, FL) equipped with a 1 mL pore volume flow cell containing Lidoped glass scintillator particles. This glass scintillator is sensitive to beta particles while being less prone to contamination than materials such as yttrium silicate. The fluidic components and instrument control were described previously.5 The major components, shown in Figure 1, included Kloehn digital syringe pumps (48,000 step resolution, zero-dead-volume syringes, and attached distribution valves) (The Kloehn Company, Las Vegas, NV) and Cheminert valves (Valco Instrument Company, Houston, TX) in several configurations. These included conventional twoposition 4- and 6-port valves and multiposition selection valves designated S1 and S2 (model C5F) where the rotors connect nonselected lines to individual waste ports. Reagent and sample transport lines were constructed of PEEK and Teflon FEP 1/ 16” tubing, connected using PEEK 10−32 and 1/4−28 fittings (Upchurch Scientific). The sample loop typically had a volume of ∼0.5 mL; in the course of the research, sample loop volumes of 0.736, 0.495 mL, and 0.286 mL were used. Safety Considerations. Highly radioactive nuclear waste samples used in this work present severe radiological and

Table 1. Monitor System Goals and Performance parameter

goal

analysis time detection limit

15 min per sample 37 Bq/mL of 99Tca

precision

10% RSD

accuracy

15%

extended operation

40 h continuous

demonstrated performance 12.5 min 23.5 Bq/mL of 99Tc in LAW with 5 M Na+ concentration using 0.495 mL sample volumeb better than 10% RSD at concentrations above the quantification limit as defined by the clientc better than 15% for the analysis of AN-102, AN107, AZ-101, AZ-102, and AP-104 LAW matrices at concentrations above the quantification limitb 54 h continuous

a

This detection limit was specified in Ci/L corresponding to the round number 1.0 × 10−6 Ci/L of 99Tc. bDetection limit calculated as 3σ of the background counts during 99Tc elution, corrected using eqs 3-6 (see “Detection and Quantification” section below). cAs defined by the funding client, quantification limit is 10 times the detection limit of 99 Tc, yielding a quantifiation limit of 235 Bq/mL by this criterion.

The automated monitor to be described in this paper completely processes, separates, and determines total 99Tc as 99 Tc(VII)O4− in less than 15 min. The monitor design is shown schematically in Figure 1. The monitor uses the previously described microwave-assisted sample preparation module as a front end,5 followed by an anion-exchange column separation, and finally uses a flow scintillation detector for quantification of the purified 99Tc(VII)O4−. 99Tc is a pure β emitter with a specific activity of 633 Bq/μg; radiometric determination relies 3091

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Figure 1. Schematic diagram of the process monitor incorporating sample preparation, column-based separation, and online radiometric detection. The distribution valves on the ends of the syringe pumps are simplified for the diagram: some of them have more than three ports to provide access to more than one reagent solution. The multiposition valves “S1” and “S2” are simplified to show the main connections. The abbreviation “W” stands for waste.

Table 2. Composition and Analysis Results of Hanford Tank Waste Samples sample (envelope)

nonper technetate,a %

overall measurement efficiency, %b

AN-107 (C) AN-102 (C) AP-104 (A) AZ-101 (B) AZ-102 (B)

50 48 72 0 0

59.0 54.1 57.0 52.6 56.6

99

Tc monitor Bq/mL (ppm) 1850 (2.96) 3070 (4.88) 6870 (10.9) 14900 (23.8) 16700 (26.6)

RSD, %c 3.4 1.5 0.6 0.9 1.6

99

Tc by ICPMS Bq/mL (ppm) 1830 (2.92) 2750 (4.38) 6600 (10.5) 13400 (21.4) 15800 (25.1)

monitor/ ICPMS 1.01 1.11 1.04 1.11 1.06

a

Nonpertechnetate determined from total Tc before and after removing pertechnetate using a Superlig 639 column. bOverall measurement efficiency represents the calibration parameter Emon obtained using the automated standard addition technique. cRSD is the relative standard deviation of the triplicate measurements.

can lead to reduced Tc species (nonpertechnetate) but is not the only source of nonpertechnetate species. The majority of Hanford waste falls in Envelope A. Envelope B has higher 137Cs concentrations than Envelope A. Further details on Hanford waste envelopes and waste simulants can be found in a report.18 In this paper we will demonstrate successful analysis of samples from all the compositions. Specific samples, their “envelope”, and percent nonpertechnetate are listed in the first two columns of Table 2. Samples from both envelopes A and C in this set have substantial percentages of nonpertechnetate. These samples are known as “feed” samples; dilutions of these feed samples with a matrix-matched diluent to get lower 99Tc activity test samples are referred to as “blend” samples. To be quantified using online scintillation detection, the 99Tc must be separated from other interfering radionuclides. The separation materials used in the 99Tc analysis do not retain and purify the nonpertechnetate species.12,15,16,19−23 Our monitor system is designed to process the waste sample to convert all 99 Tc species to pertechnetate and then deliver this species in a low acid matrix to an anion exchange column. Following purification of the pertechnetate on the column, the analyte is

chemical toxicity hazards and can be handled only in specialized nuclear facilities. Proper safety protocols for sample handling and waste disposal must be observed.

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RESULTS AND DISCUSSION

Sample Analysis Considerations. The aged nuclear waste in the storage tanks at the Hanford Site is of varying chemical and radiological composition in an alkaline, high salt matrix. Due to multiple nuclear processes, and waste management practices that moved wastes from tank to tank and mixed wastes from various processes, the waste composition and matrix cannot be predicted on the basis of the expected waste stream from a single process.11 The 99Tc can be present in a variety of oxidation states.12,13 In some aged wastes with high organic content 60−70% of the total 99Tc may be present as reduced nonpertechnetate species.12,14−16 Waste from the Hanford waste storage tanks is classified as Envelope A, B, or C, based on source and composition, with Envelope A waste noted for being particularly alkaline and Envelope C waste noted for high concentrations of organics.17 High concentrations of organics 3092

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Table 3. Fully Automated Total 99Tc Analysis Procedure operation sample load

sample digestion and spike addition for calibration samples

pertechnetate separation and quantification

a

step description and comments 1. Move autosampler probe to the sample tube 2. Aspirate sample to load sample loop (sample loop volume is 0.286 mL for analysis of feed samples or 0.495 mL for analysis of blended samples with lower Tc activities) 3. Dispense sample to the digestion vessel 4. Wash sample load pump and autosampler needle with 3 mL of water 1. Deliver required spike volume 99Tc standard in 1.61 M HNO3 to the digestion vessel if standard addition is being performed 2. Deliver 2.8 mL of 1.61 M HNO3 to the digestion vessel via sample loop and selection valve S1, or 2.8 mL minus spike volume during standard addition run 3. Aspirate and deliver 2.5 mL of air to flush transfer lines 4. First digestion with gas agitation 5. Deliver 0.7 mL of 2.02 M Na2S2O8 6. Second digestion with gas agitation 7. Deliver 1.6 mL of the digested sample to the column. 8. Wash digestion vessel with 7 mL of 1.61 M HNO3; aspirate and dispense to waste 9. Wash digestion vessel with 1 mL of 1.61 M HNO3; aspirate and dispense to waste 1. Condition column using 2 mL of 0.2 M HNO3 2. Deliver the digested sample to the column 3. Wash column with 5 mL of 0.2 M HNO3 4. Wash column with 6 mL of 1 M NaOH 5. Wash column with 6 mL of 0.2 M HNO3-0.5 M H2C2O4 6. Wash column with 5 mL of 2 M HNO3 7. Begin detector data acquisition 8. Elute Tc(VII) with 5.6 mL of 8 M HNO3 at 1.5 mL/min Flow direction through the column is reversed during elution step 9. Terminate detector data acquisition 10. Process and save detector data 11.a Wash separation column with 2 mL of 0.2 M HNO3 12.a Wash detector cell with 4 mL of DI water

Used on the last run before idle.

automatically eluted and then quantified with an online scintillation detector. Although 99Tc has a very long half-life of 2.11 × 105 years, with a relatively low specific activity of 633 Bq/μg, it can therefore be determined using mass spectrometric methods such as ICPMS that unambiguously determine total 99Tc at the required detection levels, we considered scintillation detection to be a more practical approach. ICPMS involves high-cost, complex instrumentation requiring frequent maintenance that is difficult to implement in a continuous, no-down-time, process-monitoring application. Although ICPOES has been demonstrated as a feasible method for 99Tc assay in waste samples,24,25 it also entails a plasma source and is problematic to implement and maintain for continuous monitoring in a process environment. Radiometric detection with a flowthrough scintillation detector represents a much simpler detection approach for an online monitor, it is low-cost, and it provides sufficient sensitivity. Overall System Design. The apparatus in Figure 1 uses the sample load pump to pull a sample from an autosampler into a sample loop configured on a 6-port 2-position valve. The use of a fixed sample loop provides highly reproducible sample volumes. The autosampler was an efficient method to provide samples during the research and demonstration. In a process monitoring application, the sample would be pulled into the sampling loop from the process stream (e.g., from the Tc extraction column effluent pipeline). Once the sample loop is loaded, the sample prep pump pushes the sample through the S1 selection valve to the digestion vessel in the microwave cavity. This pump also selects

and delivers reagents to the digestion vessel. The syringe pumps in the system are configured with distribution valves that can select and aspirate reagent solutions into the syringe and dispense them with minimal carryover from one operation to the next. (For simplicity, the distribution valves on the pumps in Figure 1 do not show all the ports to all the reagents.) During the matrix spike addition calibration routine, the standard addition pump delivers an aliquot of 99Tc standard to the digestion vessel immediately following sample delivery to the microwave digestion chamber. During microwave-enhanced digestion operations, compressed gas (air or nitrogen) is delivered to the digestion vessel via selection valve S1 to agitate and mix the solutions. Agitation gas and gases/vapors generated during sample preparation are vented via the vent line. The sequence of operations for these and the remaining steps are given in Table 3, which provides the procedure as it was used for the extended unattended operation to be described below. After completion of the sample preparation steps, the sample prep pump retrieves the sample from the digestion vessel, via selection valve S1 and the 6-port 2-position valve associated with the sample loop, and delivers it to the separation column via the selection valve S2 and the column flow reversal valve. Since total sample and reagent volumes are potentially variable, and hence the final digested volume may be different each time, we aspirated an excess 5 mL volume from the digestion vessel into the syringe resulting in both liquid and air in the syringe. Next, we dispensed all but the 1.6 mL of the syringe contents to a waste line (which got rid of the air plus excess sample 3093

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through the column for the elution step. A comparison of forward- and reversed-flow elution is shown in Figure 2.

volume) and delivered a constant 1.6 mL of sample to the separation column. A volumetric factor D, to be described below in the quantification section, was developed to account for the variable total reagent volumes; in practice, our D value was always the same for a given sample loop size. For example, using the 0.495 mL sample loop, our total volume was 0.495 mL of sample plus 2.8 mL acid (with or without the spike addition) plus 0.7 mL persulfate solution. (Once the sample has been retrieved, the digestion vessel is washed twice by the sample preparation pump’s delivery of clean nitric acid solution followed by aspiration of the rinse solution and disposal of rinse solution to waste.) The sample at this point is approximately one molar in hydrogen ion concentration;5 pertechnetate is retained on the anion exchange resin under low to moderate strength acid conditions. The column separation procedure then proceeds with the delivery of a sequence of wash solutions to remove unretained matrix components from the column and to selectively elute slightly retained radiochemical interferences prior to the elution of the pertechnetate. These solutions are delivered using the wash reagents pump via the selection valve S2 and the column flow reversal valve. Column effluents are delivered to waste via the diverter valve. To elute the retained and purified pertechnetate and deliver it to the detector, the column flow reversal and diverter valves are both switched, and the Tc eluent is supplied by the eluent pump. The scintillation detector peak areas are determined automatically in software. Accurate results in a variable sample matrix are assured by the periodic use of a matrix spike addition method. The standard is automatically added to the sample in the digestion vessel; the standard volume can be varied in software. In addition to providing for accurate analyses, the standard addition procedure provides a method to assess the continuing performance of the instrument. Microwave-Assisted Sample Preparation. The sample preparation steps were designed to transform the combination of pertechnetate and nonpertechnetate species in an alkaline sample matrix to an acidic matrix containing all the 99Tc in the pertechnetate form. We developed a rapid sample preparation procedure, investigated in detail previously,5 that first acidifies the sample by the addition of nitric acid. Then peroxydisulfate was added to oxidize all nonpertechnetate to pertechnetate, rapidly yielding precipitate-free solutions for subsequent separations.5 The solution chemistry steps for sample preparation are given in Table 3; the microwave parameters are given in Table S1 in the Supporting Information. The selected parameters provide a significant excess of reagent and time to ensure effective conversion of nonpertechnetate to pertechnetate.5 Separation Method and Selectivity. Analytical separations for 99Tc determination can be carried out by a variety of methods including ion exchange, solvent extraction, precipitation, and extraction chromatography, and often multistep combinations of such methods are employed.21,26−31 For our automated instrument, we selected a macroporous polymeric analytical grade anion exchange resin, AG MP-1M. In our application, the separation column will be used over and over again. Extraction chromatographic materials,32−36 such as TEVA-resin,22,37−44 are known to leach extractant and we viewed this property as disadvantageous for extended use. With AG MP-1M, we found that efficient separations were possible at flow rates as high as 8 mL/min. In addition, we found that we could obtain narrower elution peaks by reversing the flow

Figure 2. Comparison of pertechnetate elution with continuous forward flow and reversed flow, illustrating faster elution using flow reversal.

Using AG MP-1M, pertechnetate can be purified by capture and washing on the anion exchange resin at low acid concentrations (e.g., 0.2 to 1 M HNO3) and eluted at high acid concentrations (e.g., 6 to 8 M HNO3). However, using only this simple low acid-high acid sequence, we observed a significant positive bias in the analysis of 99Tc in nuclear waste samples, indicating that the 99Tc-containing eluate at high acid concentration also contained other radionuclides as interferences. Ultimately, we developed a sequence of four wash steps, given in Table 3 and Figure 1, consisting of 5 mL of 0.2 M HNO3, 6 mL of 1 M NaOH, 6 mL of 0.2 M HNO3/0.5 M H2C2O4, and 5 mL of 2 M HNO3 . The base solution was added to dissolve and remove a radioactive solid that was sometimes captured on the column frit during sample load. This solid was likely a precipitate that formed following cooling of the microwave-enhanced digestate solution. The complexant solution containing oxalic acid (0.2 M HNO3/0.5 M H2C2O4) removed tetravalent actinides that can be retained on anion exchange material as nitrato complexes. Finally, the 2 M HNO3 wash solution was added to remove a significant interference due to 121m, 121Sn. The importance of the 2 M HNO3 wash solution to remove a radiological interference that caused inaccurate 99Tc determinations is illustrated by the following example. In experiments on Envelope C AN-102 samples, a significant positive bias of up to 45% was found in the determination of 99Tc when a 2 M HNO3 wash step was not included. Using a 2 M HNO3 wash step before the elution of 99Tc, an appreciable amount of activity could be removed from the column as shown in the online detector in Figure 3. The radioactivity present in the 2 M HNO3 fraction obtained in the analysis of the AN-102 samples is predominantly due to 121mSn. ICPMS analysis of the 2 M fraction indicated the presence of stable tin. Low energy photon spectroscopy (LEPS) data indicated the presence of energy peaks at 37.1 keV, 30.6 keV, and 26.4 keV that are consistent with the characteristic emission lines of 121mSn decay. The static liquid scintillation spectrum of the fraction contained a low energy transition electron peak and end point energy are consistent with the decay characteristics of 121mSn. Finally, gamma energy analysis indicated the presence of the 126Sn activity; 126Sn was present at levels approximately 20 times lower than 121mSn. 3094

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process. Standard addition was automatically performed after every sample during research and after every fourth sample during extended monitor operation. We defined a parameter, Emon, for the overall measurement efficiency of the monitor that serves as a calibration factor relating the 99Tc activity concentration in the sample in dpm/mL, Adpm/mL, to Ccpm/mL according to eq 6 Adpm / mL =

Emon =

Detection and Quantification. The eluent from the separation column was directed through a flow through solid scintillator detector; the area of the transient peak is the analytical signal for the determination of total 99 Tc. Immediately following the completion of the Tc elution/ detection cycle, the analysis software smoothed the data, determined peak start and end times, projected a baseline under the peak, subtracted the background signal, and calculated the net peak area counts, Ncts (see the Supporting Information for details). The elution peak area was converted to an instrument response in cpm/mL, Ccpm/mL, eq 3 (Ncts/tr ) Vs × D

Vd Vs + Vr

(3)

(4)

The counting residence time, tr is calculated as

V tr = c F

(6)

Csp , cpm − Cu , cpm Vsp , mL × Dsp × Asp , dpm / mL

(7)

Here, Csp,cpm and Cu,cpm are the instrument response values for the analysis of spiked and unspiked samples respectively, Vsp,mL is the spike volume in mL, Dsp is the spike volumetric factor, and Asp,dpm/mL is the known spike activity in dpm/mL. The volumetric factor is the same for sample runs (see also eq 3 and 4 above) and spiked runs if the volume of the nitric acid used in the analysis of the unspiked sample is equal to the volume of the nitric acid plus the volume of the spike in a spiked run, as indicated in Table 3 (line 2 of sample digestion section). We used a “smart” standard addition procedure where the volume of the spike was adjusted automatically based on the activity of the previous sample, so that the spiked sample signal exceeded unspiked sample signal by a factor of ∼3 (user adjustable). From the analysis of several blended tank waste samples, we obtained a detection limit of 23.5 Bq/mL, taking the detection limit as the activity concentration corresponding to a signal that was three times the standard deviation of the background counts. This detection limit is reported in Table 1. The background counts were determined as the average background counts underneath the Tc elution peak zone for the tested samples, i.e., 3σ is three times the square root of the average number of counts during a ∼2.4 min 99Tc elution sequence, where σ = (∼500 cpm background count rate × count time)1/2; 3σ was then utilized as Ncts in eq 3, and the detection limit was converted to activity concentration via implementation of eq 6 (when a 0.495 mL sample loop was utilized and Emon = 55.3 cpm/dpm). Automated Analyses on Tank Waste Matrixes. The precision and accuracy for the determination of total 99Tc in a variety of nuclear waste sample matrixes was evaluated on five Hanford nuclear waste sample matrixes spanning Envelopes A, B, and C in waste composition, using the procedure in Table 3. These vary in their percentage of nonpertechnetate 99Tc, and the presence of potential interferences such as 121mSn. The results for these samples are given in Table 2. The precision for triplicate measurements on each sample were all below 4% RSD. Furthermore, the results were consistent with the total 99 Tc determined independently by ICPMS; average monitor

Here, Vs is the original sample volume in mL, and D is a volumetric factor accounting for dilution of the sample with reagents and the transfer of a fraction of this total volume to the separation column, eq 4, where Vd is the volume of the digested sample delivered to the column, Vs is the sample volume, and Vr is the total volume of reagents. For the 0.495 mL sample loop, D = 0.4005; for the 0.286 mL sample loop, D = 0.4226 D=

Emon

If rearranged, this equation simply gives the response (Ccpm/mL) as a product of the sensitivity times the activity concentration (Adpm/mL), where the sensitivity (Emon) incorporates all factors in the overall analytical sequence that affect the final response. Here we use the nonSI units of dpm for simplicity and clarity in relation to the conventional instrument response in cpm. In SI units, Emon would be expressed in cps/Bq, where cps is counts per second. The Emon calibration factor determined for a spiked sample in a standard addition run was calculated according to eq 7

Figure 3. Forward-flow two step elution of species retained from nuclear waste on the anion exchange column, revealing a separate radionuclide in the 2 M HNO3 fraction, later determined to be 121mSn. Separation flow rate is 1 mL/min.

Ccpm / mL =

Ccpm / mL

(5)

where Vc is the detector flow-cell pore volume, and F is the eluent flow rate. At a 1.5 mL/min flow rate and a 1.0 mL flowcell pore volume, the residence time was 0.667 min. The observed instrument response, Ccpm/mL, is dependent on the 99Tc activity concentration in the sample, the dilution and transfer of a fraction to the sample to the separation column, efficiency of recovery of the 99Tc from the separation column, and the detection efficiency of the flow-through scintillation spectrometer. We used the method of standard addition to provide a matrix-matched measurement to calibrate this 3095

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Figure 4. Comparison of 99Tc analyses from the automated monitor with baseline ICPMS values for the five tank waste matrixes, including the waste samples and blended test samples. Each point is the average of triplicate measurements. Error bars are plotted but are typically smaller than the markers and hence not visible. The inset is a 10× expansion to show the data at lower activity concentrations in blended samples, except the single highest point which is a feed sample. The dashed lines represent perfect agreement.

Table 4. 99Tc Activities in Blended AZ-102 Tank Waste Samples As Determined in Extended Continuous Monitoring sample ID AZ-102 AZ-102 AZ-102 AZ-102 a

#1 #2 #3 #4

blending factor 40.4 35.2 29.0 24.3

Tc by ICPMS Bq/mLa

99

390 448 543 648

number of samples 52 47 48 68

99

Tc monitor Bq/mL 395 470 547 688

precision, %RSD

relative analytical difference, %

6.2 5.3 8.0 6.7

+1.3 +4.8 +0.6 +6.2

Converted from measured mass concentrations from a specific activity of of 633 Bq/μg.

accuracy also confirms the success of the anion exchange separation method, using the elution sequence described above, to recover the pertechnetate from the sample matrix while removing radiological interferences. The performance of the instrument relative to the original goals is summarized in Table 1. Extended Automated Performance. We also demonstrated extended monitor operation. The automated monitor system was operated for a continuous 54 h using a set of tank waste samples (AZ-102) that had been blended to known ratios with AN-105 tank waste simulant. Table 4 provides the activity of 99Tc in the four blended solutions, and the monitor analysis results. The extended operation resulted in 215 sample analyses plus 54 hly matrix spike addition runs to determine new Emon values during operation. As compared to the ICPMSdetermined 99Tc values, the process monitor provided overall results within our performance goals. The precision values were well within the ±10% precision limits (Table 1). The accuracy, as indicated here by relative analytical difference values in percentage, was well within the ±15% goal, and relative analytical difference values were less than the precision requirement of ±10%. The overall measurement efficiency, Emon, (55.3 ± 2.6%, n = 54) was stable over the duration of the test, as shown in Figure 5, indicating consistent instrument analytical performance in the repetitive processing and analysis of nuclear waste samples. In addition, it should be noted that these results were obtained for a single separation column used repeatedly throughout the test period, indicating than an anion

values for each sample type were accurate to 11% or better (assuming ICPMS results to be the true values), well within the 15% accuracy goal. For each of the five tank waste matrixes, two to five additional test samples were prepared by blending these samples with a pertechnetate−free matrix of the same waste sample source; the pertechnetate had been stripped from the blending matrix by perfusion through a column of SuperLig 639. This modification preserves the overall matrix but enabled us to vary the total 99Tc and the ratios of pertechnetate to nonpertechnetate in each matrix. The %RSD values in the determination of 99Tc in triplicate measurements of all these blended samples with detectable 99Tc were below 10%. For 16 out of 21 samples with detectable 99Tc, the %RSD was less than 5%. For samples with total Tc over the quantification limit of 235 Bq/mL (as defined by the client to be ten times the observed detection limit), the ratio of the monitor determination (average of triplicate measurements) and the independently determined 99Tc values by ICPMS were as follows: Matrix (number of blended down concentrations):average ratio, AZ102(3):1.15, AZ-101(3):1.08, AP-104(5):1.05, AN102(2):0.953, and AN-107(5):1.11. These results are again consistent with the accuracy goal of 15%. In Figure 4, we show the analyzed values for all the waste matrixes and blended test samples plotted against the baseline values obtained by ICPMS (mass units converted to activity units for the comparison). The results clearly show the accuracy of the monitor analyses over a range of 99Tc activities in multiple matrixes. The 3096

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safeguards.8,9,47 The overall system described is designed to be suitable for at-site measurements to determine a radionuclide activity concentration in a process stream from an industrial scale nuclear process. It thus represents a new approach for online monitoring using fluidic automation to link wet chemistry procedures for sample preparation and separation to a detector.

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ASSOCIATED CONTENT

* Supporting Information S

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 5. The overall measurement efficiency, Emon, determined from the spike addition samples in extended automated operation. Overall Emon average was 55.3 ± 2.6%.

AUTHOR INFORMATION

Corresponding Author

*Phone: 509-371-6500. E-mail: [email protected]. Present Address

exchange separation column can be used in automated monitoring for extended time periods and over 200 sample analyses. At the completion of the 54 h continuous operation test, we calculated the overall detection limit based on the 215 sample analyses and 54 matrix spike additions that were performed. Using the 3σ methodology described in the previous section, we obtained a value of 27.5 (±1.8) Bq/mL. This value is ∼17% higher than the previously reported detection limit because of increased radiation levels in the laboratory workspace (∼630 cpm background count rate versus ∼500 cpm observed previously due to an increased sample load in the space), leading to a greater detector background signal. Based on same data set, we also calculated a more rigorous detection limit based on the Currie’s work,45 using a ∼4.65σ treatment of the detector background counts in lieu of the 3σ treatment described above. This approach yielded a detection limit of 43.3 (±2.8) Bq/mL for the final extended operation tests. Currie’s work defines the quantif ication limit as ∼14.1σ, which results in a value of 129 (±10) Bq/mL. Discussion. The vast majority of reports on the automation of radiochemical analysis focus on automating column separations, with or without a detection method that is fluidically linked.4 This paper describes a methodology that automates sample preparation prior to, and in conjunction with, an automated separation and detection process. In the determination of total 99Tc in nuclear waste samples, this approach successfully adjusted the speciation of the analyte, separated it from a chemically and radiologically complex sample matrix, and quantified the 99Tc concentration activity. Accurate results were obtained in less than fifteen minutes. In addition, the use of an on-board spike addition calibration maintains the accuracy, provides confidence in the results, and provides an ongoing assessment of the separation and detection performance (monitor efficiency). The overall approach is modular and could be adapted to other sample matrixes or analytes by changing the chemistry used within the sample preparation-separation sequence; for example, different separation materials and separation eluents could be used to purify other radionuclides. In recent work with DeVol et al., we demonstrated a sample preparation and detection method for measuring Sr-90 in simulated Hanford tank waste AN-105 using a separation based on SuperLig 620.46 We have also discussed how monitoring technologies such as this could be used for destructive assay and analysis of nuclear process samples to support national or international

‡

EHScientific, Russia.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge funding from US Department of Energy Environmental Management Science Program, the Environmental Remediation Science Program, and from Bechtel National, Inc. operating on the Hanford Site. J.W.G. acknowledges the William R. Wiley Environmental Molecular Sciences Laboratory, a US DOE scientific user facility operated for the DOE by PNNL. The Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute.

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REFERENCES

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