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Talanta 80 (2010) 1744–1749

Contents lists available at ScienceDirect

Talanta

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etermination of rare-earth elements in uranium-bearing materials bynductively coupled plasma mass spectrometry

solt Vargaa,∗, Róbert Katonab, Zsolt Stefánkab, Maria Walleniusa, Klaus Mayera, Adrian Nicholl a

European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, GermanyInstitute of Isotopes, Hungarian Academy of Sciences, Konkoly-Thege M. ut 29-33, H-1121 Budapest, Hungary

r t i c l e i n f o

rticle history:eceived 10 August 2009eceived in revised form8 September 2009ccepted 8 October 2009vailable online 14 November 2009

a b s t r a c t

A novel and simple analytical procedure has been developed for the trace-level determination of lan-thanides (rare-earth elements) in uranium-bearing materials by inductively coupled plasma sector-fieldmass spectrometry (ICP-SFMS). The method involves a selective extraction chromatographic separationof lanthanides using TRUTM resin followed by ICP-SFMS analysis. The limits of detection of the methodproposed is in the low pg g−1 range, which are approximately two orders of magnitude better than thatof without chemical separation. The method was validated by the measurement of reference material

eywords:nductively coupled plasma masspectrometryanthanidesranium oresuclear forensics

and applied for the analysis of uranium ore concentrates (yellow cakes) for nuclear forensic purposes, asa potential application of the methodology.

© 2009 Elsevier B.V. All rights reserved.

xtraction chromatography

. Introduction

Lanthanide or rare-earth elements (REE) are present in nuclearaterials (uranium and plutonium) in widely variable concen-

rations. Certain lanthanides may be added intentionally to theuclear materials. For instance, burnable poisons, such as erbiumr gadolinium, are added to nuclear fuel in order to control theeactivity of the fuel in nuclear reactors. REE are also formed in theeactor as a result of nuclear fission. Lanthanides can also be presentn nuclear materials in trace-level amounts either as residues fromhe raw material or as a contamination of the process.

There are several important applications of lanthanide deter-ination in nuclear materials. Their measurements are used for

he quality control and development of the nuclear fuel mate-ial [1], burn-up determination (e.g. via the isotopic analysis ofeodymium) [2,3] or for the nuclear forensic investigation of foundr confiscated illicit nuclear materials [4–6]. Therefore, accurateetermination of these elements is of high importance.

Several analytical techniques can be applied for such pur-oses with or without prior chemical separation of the analytes7]. Direct-current plasma emission spectrometry [8], atomicbsorption spectrometry [1] or inductively coupled plasma atomic

∗ Corresponding author. Tel.: +49 7247 951 491; fax: +49 7247 951 99491.E-mail address: zsolt.varga@ec.europa.eu (Z. Varga).

039-9140/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.talanta.2009.10.018

emission spectrometry (ICP-AES) [3,9,10] can be used for the lan-thanide analysis if the analyte is present at a concentration closeto or above �g g−1. Because of the complex emission spectrum ofuranium and the lanthanides, removal of uranium is usually neces-sary before the measurement. Recently mainly mass spectrometrictechniques, such as inductively coupled plasma mass spectrome-try (ICP-MS) or thermal ionisation mass spectrometry (TIMS) areused for the isotopic and elementary measurements of the lan-thanides in nuclear matrices [2,11–13]. Very low detection limitsand high precision can be achieved by mass spectrometry, however,the presence of molecular isobaric interferences and matrix sup-pression of signal intensity due to considerable amounts of matrixelements often hinder the low-level lanthanide determinations.The spectral interferences in the m/z = 146–170 mass region weregenerally caused by oxide and hydroxide molecular ions of bar-ium [14,15]. As Ba is usually present in high amount compared tolanthanides in most investigated materials and the oxide produc-tion rate is relatively high in ICP-MS for most sample introductionsystems (typically 1–2% of the parent ion), the isobaric interfer-ences can cause erroneous results. Spectral interferences can beeliminated by the use of higher instrumental mass resolution or by

the measurement of the doubly charged ions [14,16–18], however,these approaches also imply the strong decrease in the sensitivity(high increase in the detection limit). The mass resolution requiredfor the separation of the interfering oxide ions from lanthanides aretypically between 7000 and 10,000 [18].

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Table 1Optimized ICP-MS operating parameters.

Forward power (W) 1350

Cooling gas flow rate (L min−1) 16.0Auxiliary gas flow rate (L min−1) 0.92

Nebulizer gas flow rate (L min−1) 1.240Solution uptake rate (�L min−1) ∼100

Data acquisitionResolution 300

Runs and passes 5 × 5Mass window (%) 120Sampling time (s) 0.01Samples per peak 20

Search window (%) 80Integration window (%) 60

Sampling time (s) 0.1Integration type Average

Scan type E-scanMeasured isotopes 103Rh (internal standard),

137Ba, 139La, 140Ce, 141Pr, 146Nd,147Sm, 153Eu, 157Gd, 159Tb,

Z. Varga et al. / Tala

Chemical separation of lanthanides from the matrix is anotherption to reduce spectral interferences and matrix-induced sig-al suppression. It also enables to pre-concentrate the analyte tohigher degree and to protect the instrument from a contamina-

ion of e.g. uranium or, if present, of different radioactive activationr fission products other than lanthanides. Most frequently pre-ipitation (i.e. co-precipitation with carriers), solvent extraction,on exchange or extraction chromatographic methods are used toeparate and pre-concentrate the lanthanides. Co-precipitation isarely used as the carrier has to be removed prior to the analy-is, thus requiring an additional step [10,19]. Ion exchange offersrapid possibility for the separation of the trivalent lanthanides,

lthough it is less selective and robust to matrix effects comparedo solvent extraction [3,8,19,20]. Various extractants, such as TBPtri-n-butylphosphate), TBP–TOPO (tri-n-octylphosphine-oxide) orOPO alone (in CCl4, xylene or dodecane) are most often usedor the solvent extraction of rare-earths [3,9,10,19]. Although highelectivity can be achieved and the methods do not require spe-ial instrumentation, the cumbersome separation, the high amountf generated organic and/or radioactive waste makes its routinese less attractive. Extraction chromatography, on the other hand,ffers a possibility to combine the selectivity of solvent extractionethod with the ease of use of column separation [3,21]. Extrac-

ion chromatography has already been used for the separation ofrace-level activation products, such as Pu and Am from uranium

aterials [22–24].The aim of this study is the development of a straightfor-

ard and simple extraction chromatographic method followedy inductively coupled plasma sector-field mass spectromet-ic analysis for the determination of lanthanide elements fromranium-bearing nuclear materials. Measurement of a certified ref-rence material was used to validate the procedure. The methodas applied for the measurement of lanthanide composition

n uranium ore concentrates (yellow cakes) for nuclear foren-ic investigation. The sample preparation, elimination of possiblenterferences and figures of merit of the developed method areiscussed.

. Experimental

.1. Instrumentation

The mass spectrometric analysis was carried out using a double-ocusing magnetic sector inductively coupled plasma sector-field

ass spectrometer (ICP-SFMS) equipped with a single electronultiplier (ELEMENT2, Thermo Electron Corp., Bremen, Germany).ll measurements were carried out in low resolution mode

R = 300) to keep on sensitivity using a low-flow microconcen-ric nebulizer operated in a self-aspirating mode (flow rate was0 �L min−1). Optimized operating parameters are summarized inable 1. Prior to the sample analysis the instrument was tunedsing a 1 ng g−1 multielement solution (Merck, Darmstadt, Ger-any). The optimization was carried out with respect to maximum

ranium sensitivity and low UO+/U+ ratio. The sensitivity waspproximately 1.4 × 106 cps for 1 ng g−1 238U and the UO+/U+ ratio3.5 × 10−2.

.2. Reagents and materials

For the lanthanide measurements external calibration standard

olutions and spike solution used for the optimization of the separa-ion were prepared from a 10 �g g−1 lanthanide standard solutionSpex, Metuchen, NJ, USA) by gravimetric dilution. Uranium andhodium monoelemental standard solutions were purchased fromlfa Aesar (Karlsruhe, Germany). The TRUTM extraction chromato-

163Dy, 165Ho, 167Er, 169Tm,172Yb, 175Lu, 235U

graphic column (100–150 �m particle size, active component:octylphenyl-N,N-di-isobutyl carbamoylphosphine oxide dissolvedin tri-n-butyl phosphate) were supplied by Eichrom (EichromEurope Laboratories, Bruz, France). For the preparation of columns,1.6 mL of the resin was placed in plastic Bio-Rad holders (diameter:8 mm) and plugged with porous Teflon frit (Reichelt ChemietechnikHeidelberg, Germany) on the top of the resin to avoid mixing. Thefree-column volume (FCV) of the column is approximately 1.05 mL.For dilution high-purity water was used (UHQ System, USF Elga,Germany). Hydrochloric acid used for the sample preparation wasof Suprapur grade (Merck, Darmstadt, Germany). Suprapur gradenitric acid was further purified by subboiling distillation in quartzequipment. Analytical grade phosphoric acid and sulphuric acidused for the robustness tests were purchased from Merck (Darm-stadt, Germany). The U3O8 certified reference material, Morille,(Cetama, France) used for the validation of the method is certi-fied for four lanthanide content (Dy, Gd, Eu and Sm). Uranium oreconcentrates (yellow cakes) analysed in this study originate fromthree different milling facilities (Beverley in Australia; Cluff Lakeand Stanrock in Canada).

2.3. Sample preparation for ICP-SFMS analysis

Approximately 300–500 mg of sample was weighed into aTeflon vial and dissolved in 9 ml 10 M ultra-pure nitric acid whileheating to 90 ◦C on a hot-plate for 6 h. After cooling to room tem-perature, the weight of the solution was calculated as the differenceof the weight of the vial containing the solution and the vial weight(Mstock). Approximately 300 �L of this stock solution was weighedgravimetrically into a polyethylene vial (Mload) and diluted four-fold using ultra-pure water in order to adjust the proper HNO3concentration. This aliquot was used for the lanthanide separation,corresponding to about 13 mg of sample (approximately 10 mg ofuranium). The lanthanide content of the sample aliquots was sep-arated using extraction chromatography by the selective retentionof trivalent lanthanides and actinides on the TRUTM resin in 3 Mnitric acid medium. After conditioning of the resin with 10 mL 2 MHNO3 the sample aliquot was loaded on the column. After washingthe column and removal of the non-retaining matrix components

with 2 mL of 2 M HNO3, the lanthanides were stripped from thecolumn into a Teflon beaker using 1 mL concentrated HCl followedby 4 mL of 4 M HCl. After the addition of 200 �L ultra-pure HNO3 tothe final fractions, the samples were evaporated to almost complete

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the analytes. Uranium content in the HCl fractions was very low(typically <50 pg g−1), due to the high retention of U (VI) on thecolumn in both HNO3 and HCl media.

746 Z. Varga et al. / Tala

ryness on a hot-plate in order to destroy the organic resin residu-ls. The residue was dissolved in 1 mL of 2% (m/m) ultra-pure nitriccid while heating slightly. After the gravimetric weighing of thenal fraction and the gravimetric addition of Rh internal standardmfinfr), the samples were analysed by ICP-SFMS using external cal-bration, i.e. the lanthanide concentrations were determined usingcalibration curve established by analysis of separately measured

uranium free) standards.For comparison purpose and to validate the separation proce-

ure, the lanthanide content of the samples was also measuredithout chemical separation. In this case the dissolved sample

olution was diluted to a uranium concentration of approximately00 �g g−1. After adding Rh internal standard, the lanthanide con-entration was measured in the sample by ICP-SFMS using externalalibration with matrix-matched standards. The matrix-matchedtandards were prepared by dilution of the monoelemental Utandard to 100 �g g−1 U concentration and addition of dilutedanthanide standard mixture. All dilutions were performed gravi-

etrically.

.4. Data evaluation

The concentration of the lanthanide elements in the startingample material was calculated as follows:

Ln = mfinfrcfinfrMstock

MsampleMloadR(1)

here cLn is the concentration of the lanthanide element in thetarting material, cfinfr is the concentration of the lanthanide ele-ent in the final fraction measured by ICP-SFMS, mfinfr is the weight

f the final fraction, Mstock is the weight of the stock solutionbtained after the dissolution of the material, Mload is the weightf the stock solution aliquot used for the separation, Msample is theeight of the dissolved sample, R is the chemical recovery esti-ated by the measurement of certified reference material. The

verall uncertainty of the result was calculated taking into accounthe uncertainties of the measured concentration values, weight

easurements and chemical recovery.

. Results and discussion

.1. Optimization of the separation scheme

Relatively few commercially available extraction chromato-raphic resins retain lanthanides in nitric acid medium. For ourork we chose TRUTM resin because of the following reasons:ighly retains uranium (its distribution coefficient is typicallyigher than 900 at nitric acid concentrations in excess of 2 M [21]),elatively high retention of lanthanide in nitric acid medium, noetention of alkali metals, alkali earth elements and most transitionetals. The lanthanides show relatively high retention in the range

f 1–5 M nitric acid concentration and they can be stripped from theolumn using HCl [21]. In order to optimize the separation scheme,laboratory-prepared standard solution containing 100 �g g−1 U

nd 1 ng g−1 of lanthanides was prepared and its elution behaviourn TRUTM column was investigated. The test solution was loadedn the columns in 2 M HNO3 and eluted with HNO3 followed byCl. After loading, 8 mL of 2 M HNO3 was added to the column

n 1 mL portions, thereafter the resin was converted to chloride-orm with 1 mL of concentrated HCl. Finally, the lanthanides weretripped with eight times of 1 mL of 4 M HCl. The aliquots were

ollected separately and their uranium and lanthanide contentsere analysed by ICP-SFMS. The elution profile of the elements is

hown in Fig. 1. We observed uranium visibly formed a yellow bandn the upper part of the column, since under such circumstancesranium highly retains on the column (only a very small portion

Fig. 1. Elution profile of lanthanides and uranium on TRUTM column. Column: 1.6 mLTRUTM, diameter: 8 mm. Elution after load: 8 × 1 mL 2 M HNO3, 1 mL ccHCl, 8 × 1 mL4 M HCl.

appears in the load fraction). As shown in Fig. 1, all lanthanides canbe quantitatively recovered using the HNO3 elution and the firsttwo HCl strip steps (total chemical recovery was between 95 and102%).

Lanthanides sequentially elute from the column: the less-retaining heavy lanthanides start to elute from the column afterabout 2 mL 2 M HNO3. As our aim was to measure all lanthanidessimultaneously, in our final method described in Fig. 2 we used only2 mL of 2 M HNO3 in order to avoid any chemical fractionation oflanthanides (partial elution of heavy lanthanides), followed by thestripping using HCl. Using this scheme all the lanthanides can becollected in a single fraction, while uranium and most metal ions,which can cause interference or matrix effects, are separated from

Fig. 2. Elution profile of the final method used for the separation of lanthanides.Column: 1.6 mL TRUTM, diameter: 8 mm. Elution after load: 2 × 1 mL 2 M HNO3, 1 mLccHCl, 4 × 1 mL 4 M HCl.

Z. Varga et al. / Talanta 80 (2010) 1744–1749 1747

Table 2Limits of detection (LOD) of the method developed in comparison with the direct measurement using external matrix-matched calibration (inng g−1 of the original material).

Element LOD using external matrix-matchedcalibration (ng g−1)

LOD of the developed method withchemical separation (ng g−1)

La 37.6 0.080Ce 12.7 0.082Pr 2.3 0.014

Nd 3.5 0.14Sm 3.5 0.20Eu 1.4 0.021Gd 1.3 0.048Tb 0.12 0.0087Dy 7.5 0.025Ho 1.55 0.0066

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.2. Figures of merit

The calculation of the limit of detection (LOD) was basedn three times the standard deviation of the method blank (3�riterium). The detection limits of the method developed in com-arison with the direct measurement of the sample after dilutionsing external matrix-matched calibration are shown in Table 2.he detection limit of the developed method is approximately tworders of magnitude better than that of without chemical sepa-ation. This is due to the higher pre-concentration (less dilution)nd the lack of signal suppression due to the removal of the ura-ium matrix. Considering that approximately 13 mg of the sampleas used for the analysis, the absolute detection limit corresponds

o approximately 0.02–2 pg. Another advantage of the developedethod is that the components of the possibly interfering molec-

lar ions, such as barium, which would cause barium-oxide andydroxide molecular ion interferences in the m/z = 146–170 region,re removed. This is often a serious problem with samples havingomplex matrixes, like uranium ores and spent nuclear fuels. Theypical intensity of 137Ba+ at m/z = 137 for these samples was lesshan 104 cps, thus the estimated level of oxides and other molecularons are negligible taking into account the applied oxide formationate. By the separation scheme the possible oxide interferencesf the lighter rare-earth oxides on the heavier rare-earth inten-ities cannot be avoided, as they are co-separated. For samplesith elevated light rare-earth content either mathematical correc-

ion of the oxide interference or proper modification of the samplereparation method by the separation of light and heavy rare-earthlements can be a possible option (Fig. 1).

.3. Validation of the method

For the validation of the method, Morille certified reference

aterial (CRM) was analysed by the method developed. In addi-

ion, the lanthanide concentrations were also determined in theissolved CRM after only diluting the sample to approximately00 �g g−1 U concentration without chemical separation. In thisase, external calibration with matrix-matched standards was used

able 3nalysis results of Morille reference material measured using the two methods and their

Element Developed method with chemical separation

Measured concentration (ng g−1 U) Chemical recovery (%)

Sm 482 ± 31 94 ± 3Eu 499 ± 26 97 ± 4Gd 503 ± 33 92 ± 4Dy 525 ± 35 95 ± 3

0.0390.00240.120.0039

for the quantification. The measured concentrations of the four cer-tified lanthanides together with the certified contents are shownin Table 3. The concentrations presented are given relative to theuranium content, which was determined in an independent mea-surement. The measured values agree well with each other andalso with the reference values. The chemical recovery of the chem-ical separation (calculated by the measurement of the referencematerial in the same batch) was 94 ± 6% for all the lanthanides. Theoverall uncertainty of the concentration measurement is slightlyhigher than that of the direct measurement (Table 3), which is dueto the additional uncertainty of the chemical recovery (Eq. (1)).The overall uncertainties of the lanthanide determinations withand without chemical separation for the Morille reference materialare ranged in 7.1–8.9% and 2.4–6.6% relative standard deviation,respectively. It is noteworthy that the precision of the chemicalrecovery can be further improved if any of the lanthanides can bemeasured just by the dilution of the sample. The replicate analysisof the Morille reference material yielded an analytical precision of3.2–8.6% relative standard deviation for all the four certified lan-thanide elements. The procedural blanks of the method proposedare 0.1–3 pg. The decontamination factor for uranium (defined asthe ratio of uranium amount before and after the chemical sepa-ration) is approximately 2 × 106, thus the uranium concentrationin the final fraction (typically 1–10 ng g−1) is low enough to avoidany matrix effect and to protect the instrument from contami-nation. The concentration of uranium in the final fraction is alsolow enough to avoid the decrease in the sensitivity due to matrixeffects.

3.4. Robustness test of the method

The robustness of the method was tested by the measure-ment of a dissolved uranium ore concentrate sample from Cluff

Lake mine spiked with increasing amount of sulphate and phos-phate, which are reported to be possible interferences during theextraction chromatographic separation [21]. The uranium con-tent in the load was approximately 10 mg. Aliquots of the loadsolution were spiked with known amounts of sulphate and phos-

comparison to the certified values.

Matrix-matched method Certified concentration (ng g−1 U)

Measured concentration (ng g−1 U)

492 ± 18 500 ± 120507 ± 14 520 ± 30487 ± 21 560 ± 60514 ± 16 500 ± 120

1748 Z. Varga et al. / Talanta 80 (2010) 1744–1749

Fig. 3. Effect of sulphate (a) and phosphate (b) as possible interfering anions on the lanthanides separation method.

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ig. 4. Lanthanide pattern of three uranium ore concentrates of different origin. Theilutions with matrix-matched calibration (b). The lanthanide concentrations are n

hate prepared from diluted H2SO4 and H3PO4, respectively. Theample was then subjected to the chemical separation describedbove. The effect of sulphate and phosphate on the measuredoncentration of selected lanthanides is shown in Fig. 3a and b,espectively. As seen, the polyatomic anions have no adverse effectn the lanthanides retention even in case of Ho and Lu, whichave the lowest retention coefficients to the resin and which areost prone to matrix constituents during the chemical separa-

ion. The lanthanide content of the sample was also determinedithout chemical separation performing only dilution of the sam-

le and matrix-matched measurement by ICP-SFMS. The La, Nd,o and Lu concentrations were 963 ± 48 ng g−1, 3178 ± 201 ng g−1,04 ± 8 ng g−1 and 102 ± 4 ng g−1, respectively. The values obtainedre in good agreement with the results measured for the robustnessest samples.

anide concentrations were measured with the method developed (a) and after onlyized to chondritic values [25].

3.5. Application of the method

The method developed was applied for the analysis of threeuranium ore concentrates (yellow cakes) originating from dif-ferent mills in order to compare their lanthanide profiles (alsocalled rare-earth pattern) for nuclear forensic studies. The mea-sured lanthanide concentrations presented were normalized tochondrite values as a general practice in geochemistry in orderto eliminate effects related to nuclear stability [25]. This nor-malization produces a smooth pattern. The lanthanide patterns

of the samples after chemical separation are shown in Fig. 4a.For these samples the lanthanide content was high enough todetermine it also after only diluting the sample (Fig. 4b). More-over, as the samples are relatively pure intermediate productsof uranium production, the level of possibly occurring interfer-

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nts (especially barium) is relatively low. The Ba/La ratios in theamples investigated are between 0.2 and 16, thus the effect ofsobaric interferences on the measured isotopes is negligible inhis case even without chemical separation of the analytes. The

easured values agree well using the two different methods. Theanthanide profile obtained for the yellow cake samples can besed to verify the origin of the material, as the rare-earth elementattern reflects the formation conditions of the uranium deposits26].

. Conclusions

A novel analytical method for the determination of lanthanidesn uranium-bearing materials has been developed. The method,nvolving a rapid chemical separation of lanthanides by TRUTM

xtraction chromatography resin followed by ICP-MS analysis, hasreat advantage over direct measurement of the analyte especiallyhen analysing lanthanides in difficult matrixes at trace-level (con-

aining rare-earth at pg g−1 concentration level). For instance, ifossible interferents are present at high level or in radioactiveaterials (e.g. U ores, spent fuels). The method also allows the

eduction of the sample size due to the pre-concentration. Theethod described in the present paper separates lanthanides with-

ut any chemical fractionation and with good chemical recoveryabove 94%) from uranium matrices and interfering elements. Themount of chemicals consumed for the separation and thus theaste generated is much less (only a few mL per sample) com-ared to other separation methods, e.g. ion exchange or solvent

xtraction, which is of vital importance for radioactive and nuclearaterials. This also allows decreasing the background originating

rom the chemicals. As the TRUTM resin used in this study stronglyetains Th and Pu, the method can be extended for the lanthanideetermination from other types of nuclear materials and fuels. The

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(2010) 1744–1749 1749

method has potential application also in the field of nuclear foren-sics, where rare-earth element pattern can be used for the originassessment of unknown samples.

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