determination of plutonium in environmental samples with quadrupole icp-ms

DOI: 10.1007/s10967-006-7004-z Journal of Radioanalytical and Nuclear Chemistry, Vol. 275, No.1 (2008) 55–70

0236–5731/USD 20.00 Akadémiai Kiadó, Budapest © 2007 Akadémiai Kiadó, Budapest Springer, Dordrecht

Determination of plutonium in environmental samples with quadrupole ICP-MS

C. Greis,1* S. Karlsson,1 A. Düker,1 H. Pettersson,2 B. Allard1

1 Man-Technology-Environment Research Centre, Örebro University, SE-701 82 Örebro, Sweden 2 Radiofysikavdelningen,O-centrum US, Universitetssjukhuset, SE-581 85 Linköping, Sweden

(Received December 21, 2006)

A method for rapid determination of plutonium isotopes in environmental samples with ultrasonic nebulisation and quadrupole ICP-MS detection was established. Techniques for sample dissolution, pre-concentration and chemical separation were evaluated and the optimal scheme outlined. Comparisons with -spectrometry and high resolution ICP-MS confirmed the suitability of the method when applied to different environmental matrices within the global fallout concentration range in the northern hemisphere as well as more contaminated sites. Operational detection limits were 0.5–1.5 fg/l for fresh waters and 0.03–0.1 ng/kg for lake sediments and saline marsh sediments.

Introduction

At present there is a growing concern about the potential release of radionuclides from acts of terror, in addition to the well known risks from the nuclear power production cycle and nuclear ordnance. These potential sources are also of interest since they are considered as the main sources of the release of long term hazardous anthropogenic radionuclides, such Np, Pu and Am, in the environment. Thus, two different perspectives call for analytical procedures of monitoring of low level concentrations and high amounts with a short response time.

In this paper we focus on the rapid and rational determination of plutonium and its isotopes in water, sediment and soil.

The total amount of plutonium in nuclear weapon devices and in nuclear power production are estimated to be 260 40 ton and 650 ton, respectively.1 The main component of the most plutonium nuclear weapons is 239Pu, with a typical isotopic composition of 93.5% of 239Pu, maximum 6.0% of 240Pu and 0.5% of 241Pu with minor amounts of 238Pu and 242Pu.1 Energetic plutonium has a more complex composition of the mentioned isotopes, which differs between reactors but the 240Pu concentration in reactor grade plutonium is preferably less than 18%.2 In Sweden, environmental plutonium originates from atmospheric deposition after nuclear weapon testings mainly from 1963–1964, and with some minor contribution from the Chernobyl accident in 1986. The regional deposition of 239+240Pu varies and may reach levels around 40 Bq/m2.3Approximately 16.5 ng/m2 has been assumed from weapons fallout and some 5 pg/m2 from the Chernobyl accident, with isotopes ratios of 0.18 and 0.408, respectively.4,5 Relatively little is known about the

* E-mail: [email protected]

isotope ratios (abundances) in Swedish environmental samples.

Traditionally, actinides have been determined by radiometric methods. Depending on the specific technique of detection, some of the nuclides have to be separated since their energies are inseparable by the instrument resolution. For example, the determination of plutonium with alpha-spectrometry requires that it is separated from americium and thorium due to the similar alpha-energies for 238Pu, 241Am and 228Th (5.499, 5.486 and 5.423 MeV respectively).6,7 For environmental applications 228Th is present at such high amounts that it might interfere even after radiochemical separation.4 Also, 242Pu and 231Pa (4.901 and 5.014 MeV, respectively)6,7 have similar alpha-energies which require chemical separation prior to analysis.

In environmental matrices the 240Pu/239Pu ratio can be used to identify the source of the element, which is essential in, e.g., risk assessments and to trace plutonium pollution. Therefore, it is unfortunately that these two plutonium isotopes have inseparable alpha-energies (5.156 and 5.168 MeV respectively),6 and that a chemical separation is impossible. In recent years the refinement of inductively coupled plasma-mass spectrometry (ICP-MS) has made this technique an alternative to radiometric detection. ICP-MS is increasingly used for rapid analysis of actinides and other elements since it offers rapid determination by mass at ultra trace levels. In theory, the simplest design based on a quadrupole mass filter (ICP-QMS) would differentiate between 239Pu and 240Pu, provided that they are present in the sample at the low ng/l level or higher. Further, ICP-QMS would be suitable for the determination of actinides in environmental samples for emergency preparedness, where speed, simplicity and instrument availability should be decisive factors in the choice of technique. Since the ICP-QMS replaces many

C. GREIS et al.: DETERMINATION OF PLUTONIUM IN ENVIRONMENTAL SAMPLES

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atomic absorption units it would increase the number of laboratories that can be invoked and thereby increase the geographical resolution in national monitoring grids. Unfortunately, some inherent features of the quadrupole mass filter limit the quantification of some isotopes. This is overcome in the magnetic sector design because of its high resolution but these expensive instruments are quite rare, at least in Sweden. These arguments indicate that ICP-QMS could be an efficient alternative, faster than alpha-spectrometry and more widespread than the high resolution ICP-MS.

Here, we report the possibility to use ICP-QMS for the determination of actinides, notably 239,240,242Pu, in environmental matrices with focus on emergency preparedness. Special attention is paid to the performance in terms of interferences such as isobaric mass overlaps and how to resolve them by different commonly used chemical and instrumental features.

Analytical background

In Sweden, the quadrupole design dominates the market why it is interesting for emergency preparedness as well as routine environmental monitoring. Typically, the limit of detection for the quadrupole when operated with common nebulizer designs (cross-flow, V-groove, concentric) is in the low ng/l range for environmental matrices, with an operational resolution of 0.6–1.0 atomic mass units (amu). This performance would be sufficient for the determination of plutonium at heavily contaminated areas while at reference levels as well as low contaminated sites would require enrichment. Isobaric interferences are another drawback that limits the applicability. For plutonium the interferences from formation of hydrides, potentially also oxides and argides, constitutes the primary problem. Notably the formation of 238UH on 239Pu and 239PuH on 240Pu are in focus.

Isobaric interferences can be resolved by increasing the mass resolution of the system, such as in the magnetic sector design that can operate at some 0.0001–0.01 amu. This is sufficient to resolve 238UHfrom 239Pu and 239PuH from 240Pu. These interferences are unresolved in ICP-QMS why it is necessary to adapt the analytical procedure to these instruments. Argide interferences are unavoidable since the plasma is operated with argon gas but oxides and hydrides emanate mainly from water, which is the preferred solvent. The impact of water can be lowered substantially or even eliminated with a desolvator which reduces oxygen, nitrogen and hydrogen to trace levels. These interferences can be further suppressed by the use of high purity argon, but unfortunately to a very high cost, why they usually have to be accounted for in the optimization of analytical schemes and evaluation of the analytical signal.

In environmental samples other masses with or causing potential isobaric interferences are 238Pu, 238U,241Pu, 241Am and theoretically also 239U and 239Pu. The latter is negligible since 239U has a half-life of 23.5 minutes.8 The high concentrations of 238U in relation to 239Pu, constitutes the real challenge since there are possibilities of mass overlap in the detector as well as formation of 238UH. Other possible interfering molecular ions are 207Pb16O14NH2 and 208Pb16O14NHor 238UH2 and 208Pb16O14NH2,9 for 239Pu and 240Pu, respectively. Interferences by lead are important to resolve since the element lead is common in environmental matrices at variable concentrations. In addition, 241Pu (T1/2 = 14.4 y)8 decays by –-emission to 241Am. Thus, 241Am is an indicator of previously deposited plutonium and interfering with the determination of 241Pu as well as 242Pu by formation of 241AmH. For 241Am, -spectrometry can be a preferable technique since the technique is fast and do not waste the sample.10 In order to ascertain the quality of ICP-MS measurements it is necessary to include a chemical separation of Am, Pb, Pu and U.

Chemical separation can be made by different techniques. Both cation- and anion-exchange are feasible but the latter is usually preferred since the actinides form stable anionic complexes in mineral acid solutions.11 The most common functional groups are quaternary amines (Dowex and BioRad) and separation is obtained by changing the acid medium in combination with redox control.12

Liquid-liquid-extraction has been reported to be an efficient technique to separate actinides. Frequently used ligands are organic phosphorus compounds, amines and ketones,11,12 and common organic phases includes benzene, xylene or cyclohexane. The extraction is based in selective changes of the analytes redox state.

Although extraction chromatography initially was developed for industrial applications it can be used for actinide separation in analytical chemistry. Resins with high specificity for actinides are commercially available, e.g., TEVA (aliphatic quaternary amines), UTEVA (diamyl, amylphosphate) and TRU (octophenyle-N,N-di-isobutyl carbamoyl phosphine oxide) from Eichrom Technologies. Also for extraction chromatography, the separation relies on redox control.

To conclude, several in principal different chemical separation schemes are available. However, the majority of them have been developed for radiometric detection. The principles of separation can be used for ICP-QMS, but it is necessary to optimize the final matrix composition. From a practical point of view an aqueous solution of 0.3M HNO3 is optimal.

In environmental samples and for emergency preparedness at low contaminated sites the element concentration might require enrichment. Although the separation procedure results in a reduced volume an


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initial co-precipitation is often preferred. Rare earth fluorides of neodymium and cerium have been used. Other common precipitants with well documented properties include manganese dioxide and ferric hydroxide.13

Experimental

Alpha-spectrometry

Plutonium was electrodeposited on stainless steel disks mounted as cathodes in cells and with Pt wire as anodes. The samples were electroplated according to standard procedures, see TALVITE14 and HALLSTADIUS.15 For alpha-detection, an Ortec Octête Plus alpha-spectrometry system equipped with Si solid state detectors was used. Counting times were in the range of 300000–600000 s.

ICP-QMS

Mass spectrometric measurements were performed with an ICP-QMS (Agilent 4500). The instrument had an ultrasonic nebulizer with a membrane desolvator (USN; CETAC 6000AT+). The USN evaporator was operated at 140–150 °C and the sweep gas flow in the desolvator was daily optimized in relation to the plasma argon flow and instrument tuning. Operating parameters for the ICP-QMS were typically: RF power 850 W, nebulizer gas flow 0.8–1.2 l/min and sample flow 1.0–2.0 ml/min. Ion optic voltages were daily optimized with aspiration of 238U. During the procedure interferences from 238UH and 238U16O were monitored.

The signals from the three channels in the centre of the mass signal in this instrument design were recorded and used in the evaluation. The detector was operated in automatic mode with a change from pulse to analogue counting at high signal intensity. Acidified aqueous solutions (0.3M HNO3) containing known concentrations, 1–70 g/l and 0.09–9.0 ng/l of 238U and 242Pu, respectively, were included in each sample batch. Evaluation was performed by external calibration with the known concentrations and signal intensity (2–240 Mcps and 0.25–24 kcps, respectively). The impact of sample matrices on the response was quantified by standard addition. To minimize sample carryover and memory effects, the system was flushed with 5% HNO3 followed by 5% NH3 and water between each sample.

To evaluate the suitability of a transient signal an ion chromatography (IC) pump and injector was tested. This would also lower the sample loading and allow for lower sample volumes. A 100 l sample was injected into a stream of acidified (0.3M HNO3) 18 M water and then lead to the USN. This would give a short pulse of sample into the carrier stream and possibly improve the

performance. This kind of sample introduction also allows for a continuous monitoring of the background both before and after the transient signal. Instrument and USN settings were identical to ordinary USN operations and the masses (232, 238, 239, 240, 242) were recorded continuously during a 1.2-minute run. Also the 129Xesignal was included in an attempt to monitor the impact of the sample matrix on plasma efficiency.

ICP-SFMS

The validation of ICP-QMS measurements was performed with an Element2 ICP-SFMS instrument (Thermo-Finnigan, Bremen, Germany) equipped with a semi-demountable Fassel quartz torch and a CD-2 guard electrode. Sample introduction was made by self aspiration ( 0.2 ml/min) in a conical nebulizer with a cyclonic spray chamber (GlassExpansion, Melbourne, Australia) (c.f. NYGREN et al.)16 The detector was operated at low resolution (m/ m = 300) mode. Tuning on 238U was performed daily for the torch position, ion optic voltages and nebuliser gas flow rates. Calibration was made as for ICP-QMS.

Enrichment and separation

Chemicals and lab-ware: Washing of labware and preparation of solutions were made with 18 M water. The tracer solutions of 242Pu were prepared from a Standard Reference Material 4334G Plutonium-242 Radioactivity Standard (National Institute of Standards and Technology). Reagent grade chemicals [HNO3,HCl, H2SO4, H2O2, ascorbic acid, HF (Scharlau Chemine), NaNO2 (Merck) and NH4I (Acros Organics)] were used throughout. Solutions containing NaNO2,H2O2 and NH4I were freshly prepared before use. Prior to ICP-MS analysis the samples were evaporated to dryness on a hot plate and dissolved in hot subboiled distilled (from analytical grade) 0.3M HNO3, allowed to cool to room temperature and transferred to 15 ml polyethylene test tubes. Sample preparation was performed in an ordinary laboratory while ICP-QMS measurements were made in a clean room (class 10–100).

Leaching: Sediment and soil samples were either leached in glass beakers on a hot plate or in closed perfluoroalkoxy vessels in a microwave reaction system (CEM MARS 5). On the hot plate at least 10 ml concentrated HNO3 was added to 1 g of sample. The samples were leached for 6 hours with addition of H2O2and intermittently stirred. At all time, the vessels were covered with a watch glass. In the microwave 0.5–1 g of sample was leached with 10 ml concentrated HNO3,HNO3/HCl (1:3, v/v) or HNO3/HF (1:3, v/v). The microwave was programmed to ramp from room temperature to 175 °C during 6 minutes, hold for


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1 minute, ramp to 190 °C during 2 minutes and finally hold at 190 °C for 14 minutes.

Co-precipitation: The ferrihydroxide co-precipitation was originally designed for enrichment of actinides in sediment and soil samples after acid leaching,17 but here it was also used for water samples. The Fe3+ solution was prepared from FeCl3 6H2O(Fluka, p.a.). The original method for neodymium fluoride co-precipitation was developed for seawater.18

Here, it was modified and used for enrichment of acid digested sediment samples (Fig. 1). The Nd was added as a nitrate solution, prepared from Nd2O3 (Acros organics).

During the precipitation for enrichment of soil and sediment samples pH was monitored with pH-paper while for large water samples a pH-meter was used. The geological samples were centrifuged in polyallomer (ethylene-propylene copolymer) centrifuge tubes (40 ml) and the water samples were centrifuged in polyallomer centrifuge bottles (250 ml) at 12,000g for 10 minutes.

Ion-exchange: An anion-exchange resin (AG 1-X4, 100–200 mesh; Bio-Rad) was packed in glass columns (diam. 10 mm) to a height of 50 mm and conditioned with 40 ml 8M HNO3. The flow rate was kept at 1 ml per minute. Conditions for the separation are given in Fig. 2 (original method by HOLM and BALLESTRA).17 In order to optimize the volumes required in the washings, 10 ml fractions were collected and their element content was determined.

Liquid-liquid extraction: For liquid-liquid extraction the studied ligands were tributyl phosphate (TBP) (Sigma-Aldrich, 99%) thenoyl trifluoro acetone (TTA) (Sigma-Aldrich, 99%) and trioctyl amine (TOA) (Fluka, purum) with xylene (Scharlau, analytical grade) as organic phase. A summary of the procedures are provided in Figs 3 to 5. The aqueous sample was extracted in a Teflon funnel (600 ml) at contact times (for extraction and back extraction) that were varied during the experiments. For contact times of 2 minutes or less the funnel was shaken vigorously by hand while a shaker was used for longer contact times. Sample treatment continued immediately upon separation of the two immiscible phases.

Extraction chromatography: The resins (TRU or TEVA; Eichrom Technologies) were packed to a height of 40 mm in polypropylene columns with diameters of 8 mm. The resin was held in place with frits on both sides. The procedures are summarized in Figs 6 and 7. The initial conditioning was made with 10 ml 2M HNO3at a flow rate of 1 ml per minute. The Fe3+ carrier consisted of FeCl3.6H2O dissolved in water. Ammonium bioxalate was prepared by dissolving ammonium oxalate and oxalic acid dihydrate

(Merck, p.a.) in water. The pH in the hydroxylammonium chloride solution (Merck, pro analysi) was adjusted with HCl. Other chemicals that were used included Al(NO3)3 (Merck), hydroquinone (Aldrich) and amidosulfonic acid (Kebo Lab, p.a.). The method for Pu-separation using the TRU resin is originally a draft method19 for Am/Cm and Pu separation in aqueous samples. The separation scheme for TEVA is mainly adopted from Eichrom.20,21

Fig. 1. Outline of sample preparation during co-precipitation with Nd2+


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Fig. 2. Outline of sample preparation during ion-exchange with AG 1-X4; (a) Volume optimization. Every 10 ml eluted effluent

analyzed.; (b) 80 ml 1.2M HCl + 2 ml 30% H2O2;(c) 40 ml M HCl + 0.6 g NH4I

Fig. 3. Outline of sample preparation during liquid-liquid extraction with TBP; (a) Exclusion of the step; (b) back extraction: 25 ml 1.2M

HCl + 0.66 ml 30% H2O2. Second back extraction: 15 ml 1.2M HCl + 0.33 ml 30 %H2O2; (c) back extraction: 25 ml 0.5M HCl + 250 mg ascorbic acid. Second back extraction: 15 ml 0.5M HCl + 150 mg

ascorbic acid; (d) back extraction: 25 ml 9M HCl + 0.3 g hydroquinone. Second back extraction: 15 ml 9M HCl + 0.14 g hydroquinone; (e) back extraction: 25 ml 0.3M HNO3 + 0.3 g

hydroquinone. Second back extraction: 15 ml 0.3M HNO3 + 0.14 g hydroquinone; (f) back extraction: 25 ml 8M HCl + 0.2 g NH4I.

Second back extraction: 15 ml 8M HCl + 0.1 g NH4I

Fig. 4. Outline of sample preparation during liquid-liquid extraction with TOA; (a) contact times varying from 0.5 minutes to 240 minutes;

(b) back extraction: 25 ml 1.2M HCl + 0.66 ml 30% H2O2.Second back extraction: 15 ml 1.2M HCl + 0.33 ml 30% H2O2;(c) back extraction: 25 ml 0.5M HCl + 250 mg ascorbic acid.

Second back extraction: 15 ml 0.5M HCl + 150 mg ascorbic acid

Fig. 5. Outline of sample preparation during liquid-liquid extraction with TTA


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Fig. 6. Outline of sample preparation during extraction chromato-graphy with TRU; (a) 20 ml 4M HCl.0.1M hydroquinone; (b) 20 ml

1.2M HCl + 0.5 ml 30% H2O2; (c) 20 ml 0.5M HCl + 200 mg ascorbic acid; (d) 20 ml 0.1M hydroxyl ammonium chloride, pH 3.6;

(e) 20 ml 0.1 M hydroxyl ammonium chloride, pH 2.25;(f) 20 ml 0.1 M hydroxyl ammonium chloride, pH 1.7

Fig. 7. Outline of sample preparation during extraction chromatography with TEVA20,21


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Fig. 8. Concentrations of 239+240Pu in a salt marsh sediment core; (a) -spectrometry ( ) and USN-ICP-QMS ( );

(b) measurements by USN-ICP-QMS plotted against measurements by -spectrometry

For 242Pu the background was 5–100 cps during a working day, with an RSD about 1% to 10% in the concentration range of 0.9–9.0 ng/l. It was noted that the background counts were related to the preceding sample indicating that precautions are necessary in terms of rinsing between samples. The detection limit for 242Pu in 0.3M HNO3 was 10–30 pg/l according to the previous

definition. In a water sample that is pre-concentrated 20 times the detection limit would be 0.5–1.5 fg/l in the undiluted sample or 0.03–0.1 ng/kg in dry sediment. The impact of 238UH on the 239Pu signal was corrected for by measuring the contribution in a solution free from 239Pu.

Validation of USN-ICP-QMS

The validation of USN-ICP-QMS with -spectrometry was performed on salt marsh sediments, matrix and sample preparation are further described in GREIS et al.23 The sediment samples were leached with concentrated HNO3 in a microwave system, a 242Pu-spike was added and the elements separated with ion-exchange. 75% of each sample after ion-exchange were electroplated and analyzed by alpha-spectrometry. 25% was evaporated to dryness, dissolved in 10 ml 0.3M HNO3 and analyzed by USN-ICP-QMS. The chemical yield was calculated from the 242Pu. The relative standard deviation (RSD) in the USN-ICP-QMS measurements were in the range of 2–6% and 2–10% for

-spectrometry. The difference in 239+240Pu concentration detected by -spectrometry and ICP-QMS varied between 0 and 13% (one outlier at 25%) (Fig. 8a). In two thirds of the analyses ICP-QMS gave a higher value. Considering the high signals from 238 (90–1300 kcps) possible reasons could be tailing from the 238U channel or mass overlap from 238UH. In Fig. 8b the results from alpha-measurements are plotted against the ICP-QMS measurements, which gives a fairly linear relation (k = 1.037, R2 = 0.964).

Fresh water sediments (Lake Stensjön, boreal Sweden) were used for the validation of ICP-QMS with ICP-SFMS. The sediment samples were leached in concentrated HNO3 in a microwave system, 242Pu-spike was added and the elements were separated with ion-exchange. Matrix and sample preparation in further described in GRAHN et al.24 The concentrations of 239Pu and 240Pu in two sediment cores were analyzed both with USN-ICP-QMS and ICP-SFMS. In Fig. 9 the concentration of 239Pu and 240Pu in one of the cores measured with ICP-SFMS is plotted against ICP-QMS. The relation between the techniques is fairly linear for both isotopes (k = 0.938, R2 = 0.95 and k = 0.830,R2 = 0.975).

Table 1. Concentrations of plutonium in lake water (Lake Svartsjön, Sweden) measured by USN-ICP-QMS

and ICP-SFMS

Method 239Pu, fg/l 240Pu, fg/l 240Pu/239Pu USN-ICP-QMS 34.7 1.4 7.5 1.1 0.215 0.005ICP-SFMS 29.3 1.3 6.6 0.5 0.229 0.002

Samples pre-concentrated 20000 times prior to analysis.


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Fig. 9. Concentrations of (a) 239Pu and (b) 240Pu in a fresh water sediment core (Lake Stensjön, core B). Measurements by USN-ICP-

QMS plotted against measurements by ICP-SFMS

Measurements of plutonium isotopes in lake water (Lake Svartsjön, south Sweden) with USN-ICP-QMS was compared with measurements with ICP-SFMS. Lake water samples were flocculated with Ca2+ and separated with ion-exchange. For further details of the sample preparation see GREIS et al.25 The determination with USN-ICP-QMS gave slightly higher concentrations of plutonium and a lower isotope ratio of 240Pu/239Pu (Table 1). A probable reason could be that interference from 238U (238.051 amu)8 or 238UH (239.059 amu)8

on 239Pu (239.052 amu)8 are possible to resolve in

ICP-SFMS ( mass 0.0065) operated at a nominal resolution of 0.01 amu.

In order to validate the developed ion-exchange scheme (Fig. 10) and the USN-ICP-QMS measurements, participation in the NUSIMEP 5 campaign (The Nuclear Signatures Inter-laboratory Measurement Evaluation Programme) was initiated. Four 1% saline samples, each of 20 ml, with unique isotope compositions of uranium (234, 235, 236, 238) were processed and analyzed with the objective to determine isotopic ratios. Two of the samples also contained nuclides of plutonium (238, 239, 240, 241, 242) and cesium (134, 137). Concentrations were approximately 5 ng uranium per g solution, as well as 0.05 Bq per ml solution of plutonium and cesium, respectively. The samples were evaporated to dryness and dissolved in 25 ml 8M HNO3. Uranium and plutonium were separated following the scheme in Fig. 10 and analyzed with USN-ICP-QMS. Values lying within 10% deviation from the reference values were considered to be acceptable results according to the final report.26 Results from the measurements of the isotopic ratios 235U/238U, 240Pu/239Pu and 242Pu/239Pu were reported. For the 235U/238U three out of four measurements were within the criterion for good results (reference values in brackets): 0.0024 (0.00215), 0.0075 (0.00726), 0.021 (0.02080), 0.0312 (0.03076). For the ratio 240Pu/239Pu one out of two measurements were considered as good results: 0.253 (0.2605), 0.255 (0.1840). For the ratio 242Pu/239Pu both measurements failed: 0.021 (0.00068), 0.747 (0.00047). The failure in some of the measurements could be due to either the separation or the analytical steps. There was only one opportunity for separation of each sample. Since the specific concentration levels of the plutonium isotopes were unknown, the processed samples were only dissolved in a few ml of 0.3M HNO3, resulting in only one chance for analysis with the USN-ICP-QMS. The probable reason for the consistently higher ratios of 242Pu/239Pu compared to the reference values is that the concentration of 242Pu was too close to the detection limit. In this study, cesium was not analyzed.

The developed ion-exchange scheme (Fig. 10) and the USN-ICP-QMS measurements were also validated with a Fangataufa Lagoon Sediment Sample (IAEA-384). Approximately 1.3 g of sediment was dissolved in hot concentrated HNO3, 0.046 ng 242Pu was added and uranium and plutonium were separated following the scheme in Fig. 10. In Table 2 the results are presented together with recommended values. Both the concentrations of 239Pu and 240Pu agree fairly well, as well as the atom ratio 240Pu/239Pu.


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Table 2. Concentrations of plutonium in Fangataufa Lagoon Sediment Sample IAEA-384 (n = 4)

239Pu 240Pu 240Pu/239Pu ng/kg Bq/kg ng/kg Bq/kg isotope ratio

USN-ICP-QMS 38.6 0.9 88.8 2.2 2.0 0.1 16.6 0.8 0.0490 0.0007Ref., median28 100 18.0 0.0497 0.0018Ref., conf. int.a28 85–109 14.0–19.2

a = 0.05.

Fig. 10. Established sample preparation during ion-exchange with AG 1-X4

Co-precipitation

Co-precipitation with Fe was performed both for water and sediment samples for enrichment of the analyte. The technique was time consuming and the precipitate was sticky and difficult to recover quantitatively. Since the acid leached sediment samples have a proportionately small volume, the technique seems to be unnecessary for pre-concentration. There is also a risk of loss of nuclides or cross contamination during the process. For water, the pH was set to 6–7 in order to enhance the co-precipitation and was settled overnight before centrifugation and phase separation. Although, the treatment adds another day to the total sample preparation time it is necessary in order to reduce large sample volumes.

As Nd-co-precipitation was used for organic rich sediment samples in this study, instead of water samples, the original method18,27 was not exactly followed since HClO4 had to be avoided due to practical reasons. The precipitate was sticky and the final

precipitate almost impossible to dissolve if following the scheme used in this study.

Neither Fe-precipitation nor Nd-precipitation can be used alone for purification prior to analysis by ICP-QMS. Due to practical laboratory work and a simpler scheme, Fe-co-precipitation is to be preferred in front of Nd. There are also more references on Fe-co-precipitation.

Leaching

Processing sediment (salt mash) samples in a microwave system (HNO3) did not increase the leached amount of plutonium into the solution compared to leaching on a hot-plate (H2O2 in HNO3). If time is the criteria, microwave is faster for samples smaller than 14 g. Although, these amounts of sediments leached on a hot plate do not require as much laboratory work as microwave leaching. Leaching in HNO3/HCl gave slightly higher concentrations of detected plutonium than leaching in concentrated HNO3 in the microwave, although HNO3/HCl was less pleasant to work with. Leaching in HNO3/HF did not improve the leached concentrations. No matter what acid solution or leaching technique used, the samples were not totally dissolved. Since only deposited plutonium is of concern for emergency preparedness this is not considered as a drawback. For samples including hot particles (PuO2),leaching with HNO3 or HNO3/HCl is insufficient. The abundance of Pu in hot particles varies considerably in environmental matrices within the range of 8–31%.27

For emergency preparedness the time for sample treatment and analysis would be more important than to quantitatively include hot particles. Treatment of matrices including these particles (HNO3 and HF) is further discussed by ERIKSSON.27

Ion-exchange

In summary, plutonium adsorbs to the resin as Pu(IV) and is eluted as Pu(III) after reduction. Since this is a crucial step, this study focuses on the suitability of reduction/elution with 1.2M HCl containing H2O2 and 9M HCl containing NH4I. For defined samples of 242Pu in 0.3M HNO3 both treatments gave recoveries between 80–95% for 242Pu.


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When applying these methods to the salt marsh sediment samples an important difference was noted. For reduction/elution with HCl/H2O2 the recovery of the 242Pu spike was quantitative (95–105%).Reduction/elution with HCl/NH4I resulted in gas formation in the column and gave a large variation of the recoveries (95–130%). Another drawback is that iodine has to be removed from the solution because of its adverse effects on the ICP-system, which takes some extra time. During this treatment a precipitate that was difficult and time consuming to dissolve was formed. In addition, these samples had to be diluted four times more than the samples eluted with H2O2 in order to be suitable for analysis with USN-ICP-QMS.

After establishing the appropriate reduction/elution procedure the volumes in the washing step were optimized with respect to efficiency and time consumption using defined solutions. Each 10 ml fraction of effluent was analyzed for Th, U, and Pu. In Fig. 11 normalized eluted amounts of 232Th, 238U, 239Pu and 242Pu are shown for each 10 ml of washing solution. For elution of thorium 30 ml of 10M HCl was sufficient. Some thorium appeared in the uranium and plutonium fractions but this is minor problem since thorium does not interfere in the analysis of plutonium with ICP-MS. After addition of 50 ml of 8M HNO3 no more uranium was detected. However, some uranium co-eluted with plutonium. This was observed for reduction/elution with HCl/H2O2 as well as the HCl/NH4I. For quantitative recovery of plutonium 40 ml of either HCl/H2O2 or HCl/NH4I was optimal. However, the evolution of gas during elution with iodine is unsuitable for ICP-MS analysis of environmental samples. In Fig. 10 the established method is outlined.

Liquid-liquid extraction

As in ion-exchange, plutonium has the highest extractability as Pu(IV) and back extraction is preferably achieved by reduction to Pu(III). Thus, the focus on these tests was put on the efficiency of the back extraction. The combinations of back extraction procedures that were chosen for TBP are partly based on their documented properties and their efficiencies in the previous ion exchange study. In the initial stage of the study, the extraction of 242Pu (2 ng/l) from defined solutions was evaluated and back extractions were performed with HNO3/H2O2 and HCl/ascorbic acid. The recoveries for the former combination varied anywhere between 45% and 115% while the latter gave 21–31%.

For acid digested salt marsh sediment samples the back extraction gave low recoveries (after an additional ion-exchange). The additional ion-exchange and reduction/elution with HCl/H2O2 was done as a precaution since precipitates were formed when the samples were evaporated to dryness. Recoveries of added 242Pu were 28%, 24%, 8%, 1% and 0% for HCl/H2O2, HCl/ascorbic acid, HNO3/hydroquinone, HCl/hydroquinone and HCl/HN4I, respectively. Since thorium is not a problem in the analysis of plutonium with USN-ICP-QMS (see above) the washing with 10M HCl could be excluded since it had no impact on the recovery of plutonium.

Extraction of 242Pu (defined solutions) with TOA in xylene and back extraction with HCl/H2O2 and HCl/ascorbic acid was evaluated. Recoveries for the former back-extractant were 38–71% while the latter gave 9–14%. Due to the large variations in recoveries for HCl/H2O2, the impact of contact time during back extraction was evaluated. The contact times were varied between 0.5 minutes and 240 minute. It does not seem to have a major impact on the recovery of 242Pu since the same recovery of 83% was obtained both after 15 minutes and after 240 minutes (Fig. 12).

After extraction with TTA precipitates formed when the samples were evaporated to dryness. Some insoluble matter remained after repeated treatments with hot concentrated HNO3 and H2O2. Analysis of the solution phase after this treatment had a severe effect on the USN and the recoveries for 242Pu fluctuated between 1% and 370%. In an attempt to elucidate the impact of the matrices the samples were gradually diluted with 18 Mwater and analyzed. Diluting the sample 20 times seemed to give less variable recoveries, 50–106% (sample RSD 33%, n = 8), but some outliers still remained. For most environmental samples, where the concentration of plutonium is low, this dilution would require a higher pre-concentration. The results indicate that similar, but less prominent, effects might influence the analysis after extraction with TBP and TOA.

The reason for the strong matrix effects on the sensitivity is partially unknown. It was noted that the signal intensity decreased during a sample run (Fig. 13). Several reasons are possible, such as reduced nebulizer and ionization efficiency, adhesion to tubings, deposition of residues on the membranes in the desolvator and on the cones, etc. Filtered samples gave less signal variation, Figure 13, and the signal variability seemed to be independent of the time, the sample had been standing in the autosampler prior to analysis.


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Fig. 11. Eluted (normalized values); (a) 232Th, (b) 238U, (c) 239Pu and (d) 242Pu from ion-exchange resin (AG 1-X4) per 10 ml washing solution. Error bars show the sample standard deviation, n = 3


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Fig. 12. Recovery of 242Pu from TOA as a function of contact times. Error bars show analysis standard deviation, n = 3

Fig. 13. Recovery of 242Pu after extraction with TPB and analysis with USN-ICP-QMS; (a) unfiltered samples; (b) filtered samples. The samples are shown in the analysis sequence run. Error bars show analysis standard deviation, n = 3

Thus, it would be preferable to remove particles from the solution. However, filtration is a complex process that can lead to loss of analytes that adsorb to particles or the filter surface. Furthermore, the rather insoluble precipitates that formed during evaporation suggest that there is a considerable risk that analytes are retained. Therefore, it was decided to exclude this sample treatment from the analytical scheme.

Extraction chromatography

In this study, only defined solutions were used. Reduction/elution of plutonium with 0.1M hydroquinone in 4M HCl19 gave a recovery of 131%. In addition, plutonium that eluted in the following and final ammonium bioxalate solution accounted for an additional recovery of 110%. As for some of the liquid-liquid extraction systems these erroneous results are attributed to variations in the nebulizing efficiency


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Fig. 14. Recovery of (a) 232Th, (b) 238U and (c) 242Pu after separation with TRU for four different sequences of washing solutions. Error bars show the sample standard deviation, n = 1 to 12


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Fig. 15. Overview of the evaluated sample preparation techniques. The preferred sequence is marked with the dense line

because of matrix effects. Precipitation occurred during evaporation of the samples during the pretreatment phase. Repeated treatment of the precipitate with hot concentrated HNO3 and H2O2 did not eliminate these effects.

Using the IC for sample introduction improved the recoveries, two thirds of the added 242Pu was found in the desorption step and one third in the following ammonium bioxalate solution. Further improvements were achieved when the samples were diluted 5 times before analysis, which indicates that some solution component caused the loss of signal. Under these conditions the recovery stabilized in the range of 40–60% (Fig. 14). Although the analytical conditions improved, 0.1M hydroquinone in 4M HCl did not elute plutonium quantitatively. Still some 30% appeared in the ammonium bioxalate solution, why other solution compositions were evaluated.

Elution of plutonium with 0.5 ml 30% H2O2 in 1.2M HCl did not yield any detectable amounts of plutonium. However, it was an almost quantitative recovery (84%–102%) in the ammonium bioxalate solution that followed. Unfortunately, there was also a quantitative recovery of uranium in this fraction. Therefore, it is questionable if this method is suitable for samples with an inherent high level of uranium.

Reduction/elution of plutonium with 200 ml ascorbic acid in 0.5M HCl gave recoveries between 62% and 118% for samples diluted five times. As in the previous system, a white precipitate formed during evaporation that was insoluble in hot HNO3 with H2O2. Like the previous systems, dilution (5 times) resolved the problem. This solution composition seemed to be efficient in controlling the redox of the system since no plutonium was detected in the final ammonium bioxalate solution.

When plutonium was reduced/eluted with 0.1M hydroxyl ammonium chloride (pH 1.7, 2.2, 3.6) the recoveries were between 50% and 80%, independent of pH. These samples did not form any visible precipitate when evaporated to dryness. The recoveries did not increase upon dilution of the samples (5 times) or using the IC for sample introduction why interferences would be minor. The amount of plutonium recovered in the final ammonium bioxalate solution varied between 13% and 59%, and the total recoveries were almost quantitative. In summary, hydroxyl ammonium chloride performs best among the tested solutions. However, since roughly one third of the plutonium co-elutes with uranium there is still a problem to resolve the 239Pu from 238UH.

Plutonium is retained on the TEVA resin as Pu(IV) and unlike the procedure for TRU, NaNO2 was added to


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the sample to ensure redox control, before it was put on the column. U(VI) is not retained why it should be removed during washing. In this study, however, Th(IV) was eluted in a separate step, which is not necessary for analysis of plutonium with ICP-MS, but essential for detection with alpha-spectrometry.

Plutonium was reduced/eluted from the column with 0.1M NH4I in a mixture of 0.1M HNO3 and 0.05M HF. Although the sample was treated with H2O2 and hot HNO3, after evaporation to dryness, a precipitate formed in the USN. The precipitate could be seen with the naked eye as whitish particles in the tubing connecting the USN to the plasma torch. Solids were also deposited on the cones and ion lenses in the ICP-MS, and the signal became low and unstable. Anyhow, plutonium mainly (1–25%) appeared in the effluents of 20 ml 9M HCl and 3 ml 6M HCl (the two fractions were pooled) with a minor amount (less than 4%) in the fraction were plutonium was supposed to be reduced/eluted with 0.1M NH4I in a mixture of 0.1M HNO3 and 0.05M HF. Thorium was mainly eluted with HCl but one third was eluted in the last step. The main fraction of the eluted uranium was washed out by NH4I solution.

Another problem with the TEVA was the very slow and variable flow through the columns. Although the TEVA has been demonstrated to work well with -spectrometry and ICP-SFMS, it is not suited for USN-ICP-QMS. The precipitate that formed would possible not interfere with alpha-spectrometry. The impact on ICP-SFMS would be lower than for ICP-QMS under the present conditions since the sample loading rate is considerably lower on the former instrument. However, drying of the sample stream in the USN is most likely the critical step. This is required in ICP-QMS to minimize the interferences from 238UH on the 239Pu signal. Thus the TEVA procedure cannot be used without further development.

Conclusions

A ICP-QMS system equipped with an USN is sufficient for the determination of plutonium isotopes in environmental samples in emergency preparedness, provided an enrichment and separation from uranium. Accepting a limit of quantification of 100 cps under stable conditions in the USN a concentration of 10–30 pg/l can be analyzed.

Pre-concentration of water samples was preferably done by co-precipitation with ferrihydroxide. Sediment and soil samples can be leached in concentrated HNO3either on a hot plate or in a microwave system. Other acids might be considered depending on the source of pollution.

The USN showed to be very delicate and sensitive for the sample matrices. Due to insufficient removal of solid matrix components, liquid-liquid extraction and

extraction chromatography proved to be less suitable separation techniques. Ion-exchange did not cause any signal depression in the USN-ICP-QMS and the technique showed high efficiency in separation of plutonium from the original matrix as well as from uranium. The different evaluated sample preparation techniques are summarized in Fig. 15, with the preferred method marked.

*

Access to the ICP-SFMS at the Swedish Defence Research Agency (FOI), Umeå, Sweden and the assistance by Dr. H. RAMEBÄCK and Dr. U. NYGREN are gratefully acknowledged. The project is financed by the Swedish Radiation Protection Authority (SSI).

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determination of plutonium in environmental samples with quadrupole icp-ms

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