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Spectrochimica Acta Part B 58(2003) 1757–1784

0584-8547/03/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0584-8547Ž03.00156-3

Review

Mass spectrometry of long-lived radionuclides

Johanna Sabine Becker*

Central Division of Analytical Chemistry, Research Centre Julich, D-52425 Julich, Germany¨ ¨

Received 3 April 2003; accepted 10 July 2003

Abstract

The capability of determining element concentrations at the trace and ultratrace level and isotope ratios is a mainfeature of inorganic mass spectrometry. The precise and accurate determination of isotope ratios of long-lived naturaland artificial radionuclides is required, e.g. for their environmental monitoring and health control, for studyingradionuclide migration, for age dating, for determining isotope ratios of radiogenic elements in the nuclear industry,for quality assurance and determination of the burn-up of fuel material in a nuclear power plant, for reprocessingplants, nuclear material accounting and radioactive waste control. Inorganic mass spectrometry, especially inductivelycoupled plasma mass spectrometry(ICP-MS) as the most important inorganic mass spectrometric technique today,possesses excellent sensitivity, precision and good accuracy for isotope ratio measurements and practically norestriction with respect to the ionization potential of the element investigated—therefore, thermal ionization massspectrometry(TIMS), which has been used as the dominant analytical technique for precise isotope ratio measurementsof long-lived radionuclides for many decades, is being replaced increasingly by ICP-MS. In the last few yearsinstrumental progress in improving figures of merit for the determination of isotope ratio measurements of long-livedradionuclides in ICP-MS has been achieved by the application of a multiple ion collector device(MC-ICP-MS) andthe introduction of the collision cell interface in order to dissociate disturbing argon-based molecular ions, to reducethe kinetic energy of ions and neutralize the disturbing noble gas ions(e.g. of Xe for the determination of I).129 q 129

The review describes the state of the art and the progress of different inorganic mass spectrometric techniques suchas ICP-MS, laser ablation ICP-MS vs. TIMS, glow discharge mass spectrometry, secondary ion mass spectrometry,resonance ionization mass spectrometry and accelerator mass spectrometry for the determination of long-livedradionuclides in quite different materials.� 2003 Elsevier B.V. All rights reserved.

Keywords: Environmental monitoring; Glow discharge mass spectrometer; Inductively coupled plasma mass spectrometry; Isotoperatios; Laser ablation ICP-MS; Long-lived radionuclides; Radioactive waste; Trace analysis

*Fax: q49-2461-61-2560.E-mail address: [email protected](J.S. Becker).

1758 J.S. Becker / Spectrochimica Acta Part B 58 (2003) 1757–1784

1. Introduction

The determination of long-lived radionuclidesw1–11x especially in environmental materials suchas waters, geological and biological samplesw12–24x, medical samplesw25–28x nuclear materialsand radioactive wastew29–36x and high-puritymaterialsw37–41x ceramics and glassw10,40,42xis of increasing importance. The analysis of long-lived radionuclides is also of interest in areasranging from radiobioassay, environmental moni-toring, decontamination and environmental reme-diation, health safety, nuclear wastecharacterization(radioactive waste control) andmanagement of radioactive waste of high radiolog-ical toxicity for storage and disposal. Expertsestimated that worldwide more than 160 000 tonsof highly long-lived radioactive waste(LLRW)have been created and some ten thousands of tonsare added every yearw43x. Furthermore, the mobi-lization of radionuclides in the environment hasbeen studied in order to trace the routes from soil,via plants into the food chain which is availablefor consumption.Inorganic mass spectrometry is a universal and

extremely sensitive analytical method for thesimultaneous determination of element concentra-tions in the trace and ultratrace range and theirisotope ratio measurements and has been estab-lished in the last few years for the determinationof long-lived radionuclides. The application fieldsof inorganic mass spectrometry for the determina-tion of long-lived radionuclides are expectedincrease with improvements in sensitivity and pre-cision and decreasing detection limits. The deter-mination of the concentration and the preciseisotopic analysis of radioactive elements(e.g. U,238

U, Th and the decay nuclides) by inorganic235 232

mass spectrometry as terrestrial sources of radio-activity is applied in environmental research, geol-ogy or in solid-state research and materialcontrolling(e.g. of high-purity metals, alloys, sem-iconductors and insulators for microelectronics).The characterization of radioactive waste isrequired especially in respect to long-lived tran-suranics Np (t : 2.1=10 a) Pu (t :237 6 239

1y2 1y2

2.4=10 a), Pu (t : 6.6=10 a), Pu (t :4 240 3 2421y2 1y2

3.8=10 a), Am (t : 7.4=10 a) and fission5 243 31y2

fragments and activated products Se(t :791y2

7=10 a), Zr (t : 1.5=10 a), Tc (t :3 93 6 991y2 1y2

2.1=10 a), Pd (t : 6.5=10 a), Sn (t :7 107 6 1261y2 1y2

1.0=10 a), I (t : 1.57=10 a) and Cs4 129 7 1351y2

(t : 2=10 a). Also Am with relative short6 2411y2

half-life (t : 432 a) can be determined in waste1y2

and environmental samples by mass spectrometry.The determination of Pu(t : 88 a) is difficult238

1y2

due to isobaric interference with U; therefore,238

an isotope selective technique such as resonanceionization mass spectrometry(RIMS) or acceler-ator mass spectrometry(AMS) is advantageous.Besides the analysis of radioactive waste the deter-mination of contamination and enrichments ofselected radioactive nuclides(e.g. I, which is129

one of the most important environmental indicatorof nuclear accidents, Se, Tc, Np, Pu, Pu79 99 237 239 240

and Am) at ultralow concentration levels is241

useful for environmental monitoring due to falloutfrom nuclear weapons testing, nuclear power plantsor nuclear accidents. Long-lived radionuclide trac-ers have also been used for tracer experiments inbiological, medical and geological research andcan also be applied for determining the concentra-tion of monoisotopic elements(e.g. iodine using

I) by the isotopic dilution methodw11x. In Fig.129

1 different applications for determination of con-centration and isotope ratio of long-lived radionu-clides are summarized.Inorganic mass spectrometry—especially induc-

tively coupled plasma mass spectrometry(ICP-MS)—has developed in the past decade into acongruent method of the well-established classicalradioanalytical techniques due to easier samplepreparation steps, excellent detection limits andthe ability to carry out precise isotope ratio meas-urements. Conventional radiochemical methods forthe determination of long-lived radionuclides atlow concentration levels require a careful chemicalseparation of the analyte, e.g. by liquid–liquid,solid phase extraction or ion chromatography. Thechemical separation of the interferents from thelong-lived radionuclide at ultratrace level and itsenrichment in order to achieve low detection limitsis often very time-consuming. Especially inorganicmass spectrometry is advantageous in comparisonto radioanalytical techniques for the characteriza-tion of radionuclides with long half-lives()104

1759J.S. Becker / Spectrochimica Acta Part B 58 (2003) 1757–1784

Fig. 1. Overview of application fields for determination of long-lived radionuclides.

a) at ultratrace level and very low-radioactiveenvironmental or waste samples. Henry et al.w2xreported that improvements in quadrupole ICP-MShave resulted in attogram mass detection capability.Following the analysis of radionuclides with short-er half-life is also possible(Fig. 2).The development of analytical methods for the

determination of long-lived radionuclides at ultra-trace concentration levels in high-radioactive mate-rials from nuclear reactors—which is alsoimportant for waste classification—is focused onimproving microanalytical techniques in order toreduce the sample volume(minimize radioactivecontamination of instrument and dose to the oper-ator), to improve the detection limits, the precision(relative standard deviation, R.S.D.) and accuracyof mass spectrometric measurements. For example,the quality control of radioactive waste packagesor health control of exposed persons(blood, urine,faeces, hair and tissue analysis) requires powerfuland fast analytical methods which allow manysamples to be measured in a short time with ahigh degree of accuracy and precision. This review

discusses the different inorganic mass spectromet-ric techniques and their application for quantitativeanalysis and determination of isotope ratios oflong-lived radionuclides.

2. Mass spectrometric techniques for determi-nation of radionuclides

Solid-state mass spectrometric methods withmultielemental capability—such as laser ablationinductively coupled plasma mass spectrometry(LA-ICP-MS) w35,36,44–46x, glow dischargemass spectrometry(GDMS) w47–50x and second-ary ion mass spectrometry(SIMS) w12,51x—allowthe direct sensitive trace element determinationand isotope analysis of long-lived radionuclides insolid samples without any chemical sample prep-aration. For the trace and ultratrace analysis ofselected radionuclides and the precise determina-tion of isotopic ratios in solid samples also, thermalionization mass spectrometry(TIMS) w12,52,53x,AMS w54,55x and RIMSw22,56x have been used.


Fig. 2. Comparison of sensitivity for U in ICP-MS using different nebulizers(from Ref. w127x with permission).238

An advantage of solid state mass spectrometryis that sample preparation steps are reduced to aminimum and, therefore, possible contaminationduring sample preparation can be avoided. Oneproblem is the quantification of analytical resultsin solid state mass spectrometry, which is difficultif no suitable(matrix-matched) standard referencematerials are available. The quantification problemcan be solved, for example, by the preparation andapplication of synthetic matrix-matched laboratorystandardsw36,57x.

2.1. Thermal ionization mass spectrometry

For many decades TIMS was a frequently usedisotope analytical technique allowing isotopicratios of long-lived radionuclides to be measuredwith a precision of better than 0.01%w53x. Thermalionization mass spectrometers with a multiple ioncollector system yield the most precise isotoperatios down to 0.001%(R.S.D.). In TIMS a smallvolume (down to 1ml) of aqueous solution con-taining the analyte in the nanogram to microgramrange is deposited on a cleaned filament surface(mostly high-purity Re) and evaporated to dryness.The most frequently applied technique in TIMSworks with two heated filaments(one for evapo-

ration of the sample the other for ionization ofevaporated atoms) which are arranged opposite toeach another. Due to the low initial energies(0.1–0.2 eV) of the ions formed on the hot thermalsurface mostly single magnetic sector field massspectrometers have been used for ion separation.The limiting factors for the accuracy of meas-

ured isotopic ratios in TIMS are mass discrimina-tion in the TIMS instrument(e.g. ion opticalsystem or ion detector) and mass fractionationeffects(caused during the evaporation of sample,where the measured isotope ratio changes withtime). These inherent effects limit the capabilityof isotope ratio measurements by TIMS can beconsidered by different internal calibration tech-niquesw11x or by using isotopic standard referencematerials with well-known isotopic ratios for anelement (e.g. from NIST—National Institute ofStandards and Technology, Gaithersburg, USA—or IRMM—Institute of Reference Materials andMeasurements, Geel, Belgium).The capability of TIMS in precise isotopic ratio

measurements—as the major application field ofthis analytical technique—is used in the accuratetrace element determination of radiogenic elementsfor determining element concentration by the iso-tope dilution method using high-enriched isotopic


Table 1Application of inorganic mass spectrometry in determination of uranium and thorium in high-purity metals, alloys and glass

Samples Equipment Concentration range Detection limit References

Indium ICP-QMS U 0.03mg g (U)y1 Grazhulene et al.w62xPlasmaquad after traceymatrix separation

Platinum LA-ICP-QMS U 3 ng g (U)y1 Becker et al.w63xELAN 6000

Aluminium TIMS, THQ U and Th 0.018 ng g (U)y1 Beer and Heumannw37xisotope dilution after traceymatrix separation 0.06 ng g (Th)y1

Titanium TIMS, THQ U and Th 0.07 ng g (U)y1 Beer and Heumannw64xisotope dilution after traceymatrix separation 0.07 ng g (Th)y1

Cobalt TIMS, THQ U and Th 0.007 ng g (U)y1 Beer and Heumannw65xafter traceymatrix separation 0.017 ng g (Th)y1

Molybdenum TIMS, MAT 261 U and Th 0.006 ng g (U)y1 Herzner and Heumannw38xTungsten isotope dilution after traceymatrix separation 0.008 ng g (Th)y1

Tantalum ICP-QMS U and Th 0.02mg g (U)y1 Panday et al.w66xELAN 5000 after traceymatrix separation 0.01mg g (Th)y1

Zircaloy ICP-QMS U and Th 0.01mg g (U)y1 Panday et al.w67xELAN 5000 after traceymatrix separation 0.02mg g (U)y1

Glass fibers LA-ICP-SFMS U and Th 0.03 ng g(Th, U)y1 Becker et al.w68xELEMENT

spikes. For example, Heumann’s group determinedU and Th(and other elements) in, e.g. high-puritymetals and silicides of Mo, W and Ta and siliconoxide after separation of analytes by TIMS andIDA w38,39,59x. Impurities of the naturally occur-ring radioactive elements uranium and thorium andtheir decay products at the picogram per gramlevel in high-purity refractory metals and theirsilicides, which have been increasingly used forgate electrodes, interconnections and diffusion bar-riers in integrated circuits, affect the electronic andphysical properties of integrated circuits. The iso-tope dilution technique using TIMS together withdifferent applications, also for the determinationof long-lived radionuclides, is reviewed by Heu-mannw60x.

TIMS was used by Aggarwal and Crainw61x forthe determination of the half-life of several tran-sactinium isotopes(e.g. Pu, Pu, Am,241 242 243

Cm) and of the fission yield of stable and long-242

lived fission products in thermal neutron inducedfission of U, Pu and Pu.233 239 241

Selected applications of TIMS in comparison to

other inorganic mass spectrometric techniques inthe determination of U and Th in high-puritymetals are summarized in Table 1.TIMS has a number of disadvantages: it requires

an often time-consuming sample preparation(including digestion of solid samples and trace-matrix separation), it lacks multielement capabilityand is restricted to elements with ionization poten-tial -6 eV; as a result this technique is beingreplaced by the more sensitive ICP-MS in recentyears.

2.2. Glow discharge mass spectrometry

GDMS was established as a powerful and effi-cient analytical method for the direct trace elementand depth profile analysis of solidsw47,49,50x. InGDMS an argon gas glow discharge at a pressureof 0.1–10 Torr is used as an ion sourcew58x. Thecathode surface consisting of the sample materialis sputtered by Ar ions, which are formed inq

low-pressure argon plasma and accelerated towardsthe cathode. Sputtered neutral particles of the


sample are ionized in the glow discharge plasma(‘negative glow’) by Penning andyor electronimpact ionization and charge exchange processes.For the direct analysis of solid samples the com-mercial direct current glow discharge mass spec-trometer VG-9000 (VG-Elemental, ThermoInstruments, UK)—a double-focusing sector fieldmass spectrometer with inverse Nier-Johnsongeometry—has been available on the analyticalmarket for many years. This instrument was usedmainly for the determination of trace elements inelectrically conducting materials with detectionlimits in the nanogram per gram concentrationrange and lower(reproducibility of f"10%R.S.D.). The analysis of nonconducting materialsby d.c. GDMS is difficult due to charge-up effectson the sample surface. Different techniques suchas mixing nonconducting powdered samples witha high-purity metal powder(or high-purity graph-ite) or the use of a secondary cathode have,therefore, been applied for the analysis of electri-cally insulating samples by d.c. GDMS. Betti andco-workers w47–49,69x used the VG 9000 fordifferent applications to characterize radioactivewaste materials in electrically conducting and non-conducting materials, e.g. for different types ofnuclear fuels, alloys containing Pu and U, claddingmaterials, nuclear waste glasses. It was thus pos-sible to determine the neptunium concentration inIrish Sea sediment samples with d.c. GDMS witha detection limit of 80 pg g w48x. In this work,y1

a certified marine sediment doped with Np was237

used for the calibration strategy. The Np con-237

centrations in sediments determined in the lownanogram per gram range are in good agreementwith results ofg spectrometry. Detection limits inthe low picogram per gram range measured byd.c. GDMS on soil, sediment and grass werereported by Betti et al.w47x. Table 2 summarizesselected applications of mass spectrometry for thedetermination of long-lived radionuclides in bio-logical, geological and environmental samples.

2.3. Resonance ionization mass spectrometry

RIMS is a highly selective and ultrasensitivemethod for ultratrace and isotope analysis of espe-cially radiotoxic isotopes( Ca, Sr, Tc, Cs,41 90 99 135

Pb, U, Pu, Pu, Pu, Pu, Pu) in210 236 238 239 240 242 244

environment, cosmochemistry, radiodating, nutri-tion and biomedical researchw22,56,75–78x. InRIMS the solid or liquid samples are vaporizedand atomized by an atomic beam source(e.g. inan atomic beam oven by thermal vaporization ona hot Re filament or by evaporation of sampleusing an electron beam). One or in most casesmore lasers are tuned precisely to the wavelengthrequired for the excited states and ionization ofevaporated atoms in order to obtain a highlyselective resonance ionization of the element ofinterest. RIMS has been successfully applied inenvironmental samples at the ultratrace concentra-tion level with detection limits of;3=10 atoms6

Sr per sample(;2 mBq) and the isotopic90

selectivity of010 w56,75x. The determination of10

Pu after electrolytic separation from soil, air filtersor urine was described by Erdmann et al.w78xwith detection limits of 10 –10 atoms. Nunne-6 7

mann et al.w22x determined Pu isotope ratios inenvironmental samples in order to distinguishbetween Pu from nuclear power plants and fromglobal nuclear fallout due to nuclear weapons testsor from Chernobyl nuclear fallout. Recently, Traut-mann w79x described the progress of RIMS in thedetermination of extremely small isotope ratios fordifferent applications. Besides the determination of

Pu in soil, sea water and sediment, house238–244

dust and urine, RIMS was used at the Universityof Mainz for the determination of Tc in sea99

water, Sr in aerosols from Chernobyl samples,90,89

soil, plant, milk and urine and for the surveillanceof nuclear reactor coolant, Ca in nuclear reactor41

concrete, meteorites and biomedical samplesw76,77x.

2.4. Accelerator mass spectrometry

AMS started approximately 25 years ago atnuclear physics laboratoriesw55,80–84x as a highlyselective and ultrasensitive mass spectrometrictechnique. The sensitive determination of exoticradionuclides such as He, C, Be, Al, I,3 14 10 26 129

Si, Hf, Pb, U or Pu is carried out by32 182 210 236 244

ion sputtering (e.g. using a Cs primary ionq

source). The sputtered secondary negative ionswere extracted into a two-stage mass filter. The

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Table 2Application of inorganic mass spectrometry in trace and ultratrace analysis of long-lived radionuclides in biological, geological and environmental samples

Samples Equipment Radionuclides Detection limits References

Soil, sediments, GDMS Cs, Sr, Pu137 90 239 Low pg gy1 Betti et al.w47xgrass ‘VG 9000’ Pu, Pu, Th240 241 232

Zeolites LA-ICP-QMS U and Th 0.09mg g (U)y1 Pickhardt et al.w46x‘ELAN 6000’ 0.03mg g (Th)y1

Meteorites(chondrites) ICP-QMS U and Th 0.28 ng g (Th)y1 Shinotsuka and Ebiharaw70x

Irish sediments GDMS Np237 80 pg gy1 Aldave de las Herasw48x‘VG 9000’

Biological SRMs, ICP-QMS, ELAN 5000 U, Th238 232 0.9 pg ml ( U)y1 238 Truscott et al.w71xsea and river water ‘PlasmaQuad PQ2’ 0.3 pg ml( Th)y1 232

ETV, on-line solid phase extraction

Sediments, ICP-SFMS Ra, Th, U226 230 233, 4.0 pg l ( U)y1 235 Chiappini et al.

fish samples Element Np, Pu, Am237 239 241 1.2 pg l ( Pu, Pu, Am)y1 239 240 241 w72x

Thermal water ICP-QMSqUSN‘PlasmaQuad PQ2’ Ra226 2 fg gy1 Joannon and Pinw73xextraction, ion exchange

Food ICP-SFMSqUSN Pu 0.03 pg gy1 Evans et al.w74x‘ELEMENT’, ion extraction


mass-separated ions were accelerated in a tandemaccelerator to energies of some mega electron voltand were stripped in a gas target(or foil) stripperwhere they loose electrons and gain a high positivecharge states(e.g. U ) and, therefore, are236 5q

accelerated a second time by the same potential.The stripping process has the advantage that itdissociates molecular ions if enough electrons arestripped off which results in an elimination ofisobaric interferencesw55x. After the decelerationof ions and a final mass separation the ions weresensitively detected.Today with 63 accelerator mass spectrometers

installed worldwide (including recycled tandemaccelerator and new facility instruments) AMS hasbeen established as a powerful routine methodespecially for C dating( C: t s5.7=10 a),14 14 3

1y2

in which extremely small isotope ratios 10 –y12

10 can be measured very fast in routine modey16

in very small samplesw81x. Recently, Kutscheraand Muller w82x dated the Alpine Iceman Oetziänd associated materials using C dating by AMS.14

AMS revolutionized the use of long-lived radio-nuclides by detecting radioactive atoms directly atthe ultratrace level and it thus became possible tomeasure many long-lived radionuclides such asBe, Ca, U or Pu at natural abundances10 41 236 244

w55x. AMS can, therefore, be used to determineUy U isotope ratios in the range from 10236 238 y8

to 10 for evidence of U in environmentaly14 236

samplesw83x. However, since a standard materialfor U determination at this level is not estab-236

lished, the result of Uy U isotope ratio meas-236 238

urement by AMS of(6.1"0.4)10 for the ‘K.k.y11

Uranfabrik Joachimsthal’ uranium can be onlyconsidered preliminaryw55x. With AMS absolutedetection limits of 10 atoms are reached for6

isotopes such as Puw84x. Furthermore, with its239

capability of providing isotopic abundance ratiosas low as 10 for very small samples, AMS hasy15

been applied for the detection of extremely lowconcentrations of long-lived radionuclides forresearch in geochronology and archaeology.

2.5. Secondary ion mass spectrometry and second-ary neutral mass spectrometry

SIMS w85x is the most important mass spectro-metric surface analytical technique and can be

applied for analysis of long-lived radionuclides.Secondary neutral mass spectrometry(SNMS)w86x in less significant in this topic due to itslower sensitivity. In SIMS the solid sample surfaceis sputtered by bombardment with a focused pri-mary (keV) ion beam(Ar , Ga , Cs , O orq q q q

2

O ) and the sputtered ions are analyzed massy

spectrometrically. In contrast, in SNMS the sec-ondary neutrals, which were postionized, e.g. byelectron bombardment or using a laser beam, wereanalyzed mass spectrometrically. SIMS and SNMSare mainly applied for depth profiling, reachingdepth resolution in the low nanometer range. Bothtechniques are able to perform a microlocal anal-ysis in the sub-micrometer range(e.g. for analysisof local inclusion or impurities) and can be usedfor the determination of lateral element distributionand isotope analysis, e.g. for the characterizationof small particles, aerosols, and liquid or solidinclusions. Whereas in SNMS the detection limitsfor trace analysis are in the microgram per gramrange, SIMS can be applied for the characterizationof bulk material with detection limits down to thelow nanogram per gram range(but very elementdependent). Both surface analytical techniquesallow precise isotope ratio measurements withprecisions between 0.01 and 1%. By the applica-tion of multiple ion collector SIMS(NanoSIMS,CAMECA), e.g. in geochronology, a precision of0.002% can be reachedw11x. In contrast to SNMS,the quantification of analytical results in SIMS isvery difficult due to large matrix effects. Never-theless, if a matrix matched standard referencematerial is available for SIMS accurate analyticaldata were obtained. SIMS and SNMS are advan-tageous for determining surface contamination andinvestigation of interdiffusion of elements in solidmaterials or in layered systems. More details ofthis surface analytical mass spectrometric tech-nique are given in Refs.w85,86x.SIMS has been applied for the characterization

of single uranium and plutonium particles by Bettiet al. w87,88x. The identification of uranium micro-particles and the determination of their isotopiccomposition by SIMS is discussed by Tamboriniet al. w87x. The mapping of an entire samplesurface was performed by the use of the resistiveanode encoder, avoiding charging effects during


Table 3Figures of merit of ICP-MS for determination of long-lived radi-onuclidesw11x

Analytical method Detection limits

Solid state mass spectrometry (mg g )I1

SSMS 1–0.001GDMS 0.1–0.0001SIMS 10–0.002LA-ICP-MS 0.010–0.00001

ICP-MS (ng l )I1

Quadrupole ICP-MS 0.01–0.6ICP-SFMS(myDmy300) 0.00004–0.005ICP-QMS with collision cell 0.003–0.01ICP-TOFMS 0.1–1MC-ICP-MS (sector field) 0.0001–0.0002w75x

sputtering by a coating with 20 nm carbon. Work-ing in the microprobe mode the isotopic compo-sition of the particles was obtained with goodaccuracy and precision. A detection limit for ura-nium in the nanogram per gram to picogram pergram range was obtained by optimization of dif-ferent instrumental parameters. Erdmann et al.w89xdescribed a determination of isotopic compositionwith an accuracy better than 0.4% for U in235

single uranium oxide particles. Control particleswere produced consisting of isotopically certifiedmonodisperse uranium oxide microspheres of 1mm in diameter. These particles were obtained bydissolving uranium isotopic standard referencematerials, nebulizing the solution in droplets andcollecting the particles after their desolvation andcalcination.

2.6. Inductively coupled plasma mass spectrometry

ICP-MS w11,57,90x is at present the most fre-quently used mass spectrometric technique for fastsingle- and multielement determination and isotoperatio measurements in the trace and ultratraceconcentration range. This powerful analytical tech-nique is also increasingly used for analysis ofradionuclides at very low activity and low nuclideabundancesw1–11,70–74,91x. ICP-MS is appliedas well in aqueous radioactive solutions or on solidsamples after digestion. In contrast to conventionalinorganic solid mass spectrometric techniques,ICP-MS allows a simple sample introduction in anormal pressure ion source and an easy quantifi-cation procedure using aqueous standard solutions.The principle of ICP-MS is described in detail inRef. w90x. The chemical compounds contained inthe sample solution are decomposed into theiratomic constituents in an inductively coupledargon plasma at a plasma temperature of approxi-mately 6000–8000 K and ionized at a high degreeof ionization()90% for most chemical elements)with a low fraction of multiply charged ions(f1%). The positively charged ions are extractedfrom the inductively coupled plasma(at atmos-pheric pressure) into the high vacuum of the massspectrometer via an interface. The major problemin determining long-lived radionuclides in radio-active waste or environmental samples is the

appearance of isobaric interferences of long-livedradioactive nuclides and stable isotopes of otherchemical elements at the same mass. ICP-MSoffers some interesting advantages to solve theseinherent interference problems. Isobaric interfer-ences of radionuclides especially with molecularions can be resolved using double-focusing sectorfield ICP-MS at the required mass resolution.Furthermore, by the application of ICP-MS withcollision cell, disturbing interfering isobaric ionscan be suppressed or special sample introductionand coupling techniques such as high-performanceliquid chromatography(HPLC) and capillary elec-trophoresis(CE) can be helpful to avoid interfer-ence problems by separating the analytes.At present several quadrupole-based ICP mass

spectrometers without and with collision cell(e.g.Perkin Elmer Sciex, Agilent, ThermoElemental,Varian GmbH analytical instruments, Micromass,etc.), a time-of-flight ICP-MS from Leco andseveral commercial double-focusing sector fieldICP-MS with single and multiple ion collectors,e.g. ‘ELEMENT’ and ‘NEPTUNE’ (Thermo-Finnigan, Bremen, Germany) and a single mag-netic sector field ICP-MS with collision cell‘Isoprobe’ (Micromass Ltd, UK), ‘Nu Plasma’(Nu Instruments) are available on the internationalmarket. In Table 3 the detection limits for thedetermination of long-lived radionuclides meas-ured by ICP-MS are compared with those of solidmass spectrometry. In the low-resolution mode, theelement sensitivity of commercial double-focusingsector field ICP-MS is significantly higher than


conventional quadrupole ICP-MS. The extremeelement sensitivity of double-focusing sector fieldICP-MS permits ultratrace analysis down to thesub-femtogram per milliliter concentration rangein aqueous solutionw7x. Whereas the precision forisotope ratio measurements in quadrupole ICP-MSvaries between 0.1 and 0.5%, double focusingsector field ICP-MS with single ion detectionallows isotope ratio measurements with a precisionof 0.02% w11x. A better precision of isotope ratiomeasurements(one order of magnitude) wasachieved by the introduction of the multi-ioncollector device in sector field ICP-MS. The capa-bility of the double-focusing sector field ‘NuPlasma’ from Nu Instruments(with Nier-Johnsongeometry) with 12 Faraday cups is demonstratedby measurements from Haliczw92,93x on a leadand strontium NIST isotope standard referencematerial. With these measurements a precision at20 ppm(0.002% R.S.D.) and an excellent agree-ment with certified values was achieved. Alsosmall isotope ratios as demonstrated for Uy U234 238

w Uy U s63.0(0.5)=10 vs. Uy234 238 y6 234measured

U s62.58(0.12)=10 x in ocean238 y6recommended

water can be measured with good precision andaccuracyw94x.Similar to solid state mass spectrometric tech-

niques, ICP-MS possesses the multi-element capa-bility for the quasi-simultaneous determination oflong-lived radionuclides, but single element deter-mination was performed especially for selectedapplications(e.g. determination of I or Se) or129 79

in isotope ratio measurements.The oxide formation of long-lived radionuclidesRa, Th, Np, U, Pu and Am in226 230 237 238 239 241

double-focusing sector field ICP-MS and theirapplications is described by Becker and Dietzew95x. In ICP-MS and LA-ICP-MS the authorsfound a good correlation of oxide intensities oflong-lived radionuclides with bond dissociationenergies. From this correlation it was possible toestimate bond energies for AmO and NpO.

2.6.1. Application of collision cell in ICP-MS fordetermination of long-lived radionuclidesThe introduction of the collision cell in ICP

mass spectrometers is one of the most significantimprovements in ICP-MS instrumentation for more

sensitive analysis of radionuclides and more pre-cise determination of isotope ratios in comparisonto commercial quadrupole ICP-MS without colli-sion cell w96–102x. The principles and possibleapplication of collision-induced reaction in a gastarget for increasing the sensitivity in an inorganicmass spectrometer for ultrasensitive trace analysiswas already studied 20 years agow103x. Thecollision-induced reaction of ions—formed in theinductively coupled plasma—with molecules oratoms of the collision gas or gas mixture(e.g. Heandyor H , O , Xe, CH , NH) introduced by one2 2 4 3

or two mass flow controllers in the collision orreaction cell results in a reduction of the energyspread(‘cooling’) of the ions from several eV to-0.1 eV, in dissociation of the molecular ions andneutralization of disturbing atomic ions of noblegas of the plasma gas(Ar and Xe contaminants)used. Whereas the collision cell was introduced inICP-MS in order to dissociate disturbing argon-based molecular ions(ArX , XsO, N, C, H orq

Ar) and to neutralize the plasma gas ions(Ar ),q

more and more newly formed interference freemolecular ions were used as analyte ionsw11,98–101x. Tanner et al.w100x reviewed fundamentalsin the reaction chemistry and collision processesin the gas cell for resolving isobaric interferencesin ICP-MS.ICP-MS with collision cell is the method of

choice for the sensitive determination of the long-lived rare I(t f1.6=10 a), which is of great129 7

1y2

interest for environmental monitoring of nuclearfallout (e.g. as a result of the Chernobyl accident)and for monitoring of radioactive emissions fromnuclear facilities and for radioactive waste control.The main problem for I determination by ICP-129

MS is an impurity of Xe in the high-purity argongas used which results in an isobaric Xe129 q

interference, leading to a high background at mass129 u and an increased detection limit. Further-more, the determination of the Iy I ratio in129 127

environmental samples requires an abundance sen-sitivity down to 10 –10 . Analytical methodsy10 y11

which are mostly applied for the ultrasensitivedetermination of I—such as AMS and neutron129

activation analysis—are very expensive, requirespecial equipment and laboratory facilities. For thedetermination of I, ICP-MS with collision cell129


is a powerful tool for an ultrasensitive determina-tion of the extremely rare I radionuclide. Using129

He and H as the collision gases in ICP-MS with2

a hexapole collision cell(Platform, Micromass) anefficient reduction of the disturbing backgroundintensity is possible which results in a lowering ofthe detection limit in comparison to ICP-SFMS bynearly two orders of magnitude from 100 to 3ng l w11,104x. Recently, a further improvementy1

of the detection limit for I determination by129

ICP-MS with hexapole collision cell using oxygenas the collision gas was demonstrated in ourworking groupw105x. For the sample introductionof volatile I from solid geological material(soil)129

an electrothermal vaporization technique wasdeveloped. The detection limit for the analysis of

I in aqueous solution and in soils could be129

reduced to the sub-ppt and low ppt range,respectively.The determination of the long-lived radionuclideSe (t f65 000 a) can be performed by ICP-79

1y2

MS using a hydride generator for solution intro-duction in order to reduce the interference problem(possible isobaric interferences of Se with79

Br , K Ar , Cu O , Gd and79 q 39 40 q 63 16 q 158 2q

Dy ). The detection limit of Se determina-158 2q 79

tion in sector field ICP-MS was 100 pg mly1

w106x. An improvement of sample introduction byhydride generation coupled to ICP-QMS with hex-apole collision cell for selenium determination inbiological samples was described in Ref.w107x.By applying hydride generation in quadrupole ICP-MS with a hexapole collision cell the detectionlimit could be reduced to 5 pg ml because they1

disturbing argon hydride molecular ionsAr Ar H were additionally suppressedw11x.38 40 1 q

The application of a collision cell in ICP-MSresults in an improved sensitivity for heavy ele-ments due to increasing ion transmission as dis-cussed in Ref.w16x. An U ion intensity of up238 q

to 27 000 Mcpsyppm in ICP-QMS with hexapolecollision (using an ultrasonic nebulizer) cell wasfound.

2.6.2. Application of ICP-MS for determination oflong-lived radionuclides after traceymatrixseparationLong-lived radionuclides occur at extremely low

concentrations especially in environmental sam-

ples; therefore, matrix separation and enrichmentof the analytes is proposed for their analysis byseveral authors w19,71–74,108–117x. Traceymatrix separation, which is performed off-line oron-line, is also advantageous in order to avoidpossible isobaric interferences, matrix effects andto reduce the detection limits for the determinationof long-lived radionuclides.For example, Yamamoto et al.w91x suggested

for the determination of Tc in environmental and99

radioactive waste samples by ICP-MS—which isdisturbed by isobaric interference with Ru(and99 q

MoH ) ions—a separation of Tc using differ-98 q 99

ent solvent extraction and purification techniqueswith anion exchange. In this work, the determina-tion of Tc in sediments from the Irish Sea is99

described with an absolute detection limit of 0.25pg, (0.16 mBq), using a double-focusing sectorfield ICP-MS (PlasmaTrace, VG Elemental Ltd).

Tc in a highly radioactive evaporator concen-99

trate from a nuclear power plantw118x was meas-ured in our laboratory after chemical separationprocedures using ICP-MS. The detection limit forTc determination in separated solutions was99

determined by double-focusing ICP-SFMS to be 5pg l , corresponding to an activity of 3y1

mBq ml , in comparison to quadrupole-basedy1

ICP-MS (ELAN 6000, Perkin Elmer Sciex) witha detection limit of 0.1 ng l w7x.y1

Eroglu et al.w19x studied separation and enrich-ment of Tc from sea water by anion exchange99

with a detection limit of 0.03 ng l using ay1

quadrupole-based ICP-MS(HP 4500, Hewlett-Packard). An IC-ICP-MS spectrum for the sepa-ration of Ru and Tc in an environmental sample99 99

is demonstrated by Betti in Ref.w14x.Barrero Moreno et al.w108x determined neptu-

nium and plutonium in the presence of highconcentrations of uranium by ion chromatographycoupled to ICP-MS. The determination of naturaluranium and thorium in environmental samples byETV-ICP-MS after matrix removal by on-line solidphase extraction was described by Truscott et al.w71x.Recently, Evans et al.w74x developed a rapid

and accurate method for the determination ofplutonium in food using double focusing ICP-SFMS with an ultrasonic nebulizer with desolva-


Table 4Concentration of different spallation nuclides of the lanthanides(mg g ) in an irradiated tantalum targetw114xy1

Nuclide Half-life Sample 1 Sample 2 Sample 3

Pr141 Stable 9.6"1.7 2.1"0.6 0.7"0.2Pm145 17.7 y 21.2"3.6 4.4"1.1 0.9"0.3Gd150 1.8=10 y6 28.7"4 6.3"1.2 1.1"0.3Ho163 33 y 80.3"10.4 21"4 3"0.6Yb173 Stable 111.1"13.3 29.6"5.3 4.6"0.8Lu173 1.4 y 16.5"2.8 5"1 0.8"0.2

tion unit (Cetac USN 6000 ATq) and ionchromatography. The samples were prepared byHNO closed vessel microwave digestion, evapo-3

rated to dryness and diluted into a mobile phase(1.5 M HNO and 0.1 mM 2,6-pyridinedicarbox-3

ylic acid). By on-line separation using a polysty-rene–divinylbenzene ion chromatography column

Pu and U were separated in order to reduce239 238

the U H interference. A further reduction of238 1 q

U H interference was achieved by application238 1 q

of an ultrasonic nebulizer(USN). The detectionlevel for Pu of 0.020 pg g (4.6=10 Bq kg )y1 y2 y1

is significantly below 1y10 of the European Unionlegislation for baby food (1 Bq kg –0.436y1

pg g ) w74x.y1

An analytical method for Ra determination in226

environmental samples(highly saline thermalwaters) by ICP-QMS with ultrasonic nebulizationwere developed by Joannon and Pinw73x. Radiumwas preconcentrated and isolated from the matrixelements by selective extraction using a radium-specific solid phase extraction membrane diskdesigned for radioactive counting method. A verylow detection limit was achieved in quadrupoleICP-MS when the pressure in the interface wasreduced from approximately 2 to 0.85 mbar. Lari-viere et al.w112x developed a selective extraction`procedure for preconcentration of Ra from ura-226

nium ores and biological samples. The measure-ments were performed by ICP-QMS with hexapolecollision cell in order to reduce possible interfer-ences. An absolute detection limit of 0.02 fg(0.75mBq) was obtained using less than 4 mg of solidsample or 25 ml liquid sample.In past years especially on-line ion chromatog-

raphy ICP-MS has been increasingly used for the

characterization of radioactive materials andenvironmental samples using ICP-MSw111,117,119,120x. For example, we developedanalytical procedures for the determination of spal-lation nuclides in the tantalum target of a spallationneutron source irradiated with 800 MeV protonsw32,119,120x. In order to separate isobars of rareearth elements, such as radioactive Lu, from173

stable Yb, on-line HPLC(for a chromatographic173

separation of a lanthanide mixture into the individ-ual elements) was coupled to the mass spectrom-eter. The concentrations of different spallationnuclides (Table 4) in the first tantalum plate ofthe spallation neutron source(sample 1 was col-lected from the centre of the plate, sample 3 isfrom the edge, sample 2 was collected between)were measured by HPLC-ICP-MS using reverseisotope dilution techniquew114x. With increasingdistance from the centre of the plate, that meanswith decreasing irradiation density of 800 MeVproton beam on the tantalum target, the concentra-tion of the spallation nuclides decreases. We alsocoupled CE to a double-focusing sector field ICP-MS for the same analytical taskw120x. Thisapproach reduces the sample volume from the 100ml range using HPLC-ICP-MS to the nanoliterrange, which is extremely important for the anal-ysis of high-radioactive solutions. The spallationnuclides in the irradiated tantalum were measuredusing both mass spectrometric coupling techniquesafter dissolution of high-radioactive tantalum in aHNO yHF mixture and after(off-line) matrix3

separation by liquid–liquid extraction of the tan-talum matrix (in order to reduce the high Ta182

activity). The theoretical results of spallation yieldsof tantalum are verified by mass spectrometricmeasurements of the concentration of spallationnuclides of the irradiated tantalum target by HPLC-ICP-MS and CE-ICP-MS as wellw32,119,120x.One example is demonstrated in Table 5 where thenuclide abundances of gadolinium produced viaspallation reactions in an irradiated tantalum targetand measured by HPLC-ICP-MS and CE-ICP-MSare compared.The simultaneous separation and determination

of lanthanides and actinides by ion chromatogra-phy ICP-MS combined with the isotope dilutiontechnique was studied by Perna et al.w109x. A


Table 5Nuclide abundances of gadolinium(%) produced via spallationreactions in an irradiated tantalum target via spallationreactions

Nuclide Nature Theory CE-ICP-MS HPLC-ICP-MS

Gd148 – 15.8 19.4 20.7Gd150 – 18.3 18.8 18.4Gd152 0.2 27.2 22.1 22.7Gd154 2.15 2.6 -3.4 -1.1Gd155 14.7 34.4 33.4 33.9Gd156 20.5 0.57 – -1.1Gd157 15.7 0.40 – -1.9Gd158 24.9 – –Gd160 21.9 – –

Table 6Application of inorganic mass spectrometry in trace and ultratrace analysis of long-lived radionuclides in nuclear fuel, radioactivewaste solid samples and radioactive solutions

Samples Method Radionuclides Detection limits References

UO fuel2 ICP-QMS, ELAN 5000 U, Np, Pu238 237 239 0.06mg l (Np)y1 Barrero Moreno et al.w108xion chromatographyisotope dilution

Radioactive waste ICP-SFMS Ra, Th, U226 230 233 0.05 pg l ( Am)y1 241 Becker and Dietzew7xsolution Element,(USN) Np, Pu, Am237 239 241 0.04 pg l ( Pu)y1 239

Radioactive waste ICP-SFMS Ra, Th, Th, U226 230 230 233 0.1 pg l ( Am)y1 241 McLean et al.w124xsolution Element,(DIHEN) Np, U, Am237 238 241 0.1 pg l ( Np)y1 237

Spent uranium ICP-SFMS U,236 0.2 pg l (solution)y1 Boulyga and Beckerw125xElement,(Aridus) Pu239 0.04 pg g (soil)y1

ion exchange

Radioactive concrete LA-ICP-SFMS Tc, Th, U99 232 233, 0.02 ng g ( U)y1 236 Becker et al.w35x

Element U, U, U, Np235 236 238 237 1.4 ng g ( Tc)y1 99

Spent nuclear fuel ICP-QMS, ELAN 5000 U, Np, Pu,238 237 244 0.45 ng mly1 Perna et al.w109xon-line ion chromatography Am, Cm243 248

further application of ion chromatography for thedetermination of fission products and actinides innuclear applications is discussed by Betti et al.w14,121x. An on-line trace enrichment by flowinjection using a microcolumn of activated aluminaand mass spectrometric determination of uraniumin mineral, river and sea water was described byDadfarnia and McLeodw122x.Flow injection with on-line preconcentration

using solid-phase adsorption on a mini-column ofTc, Th and U at ultratrace level in soils is99 230 234

described by Hollenbach et al.w123x. Detectionlimits in the soil for Tc, Th and U were 1199 230 234

mBq g (0.02 ng g ), 3.7 mBq g (0.005y1 y1 y1

ng g ) and 0.74 mBq g (0.003 ng g ),y1 y1 y1

respectively.Different applications of mass spectrometry in

trace, ultratrace and isotope analysis of long-livedradionuclides in nuclear fuel, solid radioactivewaste samples and radioactive solutions are sum-marized in Table 6.

2.6.3. Ultratrace analysis of long-lived radionu-clides in very small sample volumesThe development and application of micro-ana-

lytical techniques for the precise isotope analysisand concentration determination of long-livedradionuclides at the ultratrace concentration levelis a challenging task for analytical chemistry.Micro-analytical techniques are of special impor-tance in order to reduce the radioactivity of thesample analyzed, the waste, contamination ofinstruments and tools, and the dose to the operator.Furthermore, for a multitude of applications espe-cially for biomonitoring(for monitoring contami-nation from radioactive waste in the environmentand for evidence of nuclear fallout) the concentra-tions of long-lived radionuclides are extremely lowand the sample amount is often restricted. In orderto analyze small sample volumes, micronebulizers(MCN Aridus, Cetac Technologies, USA and


Fig. 3. Comparison of UHyU formation rate on solution uptake rate in ICP-SFMS using DIHEN and Aridus microconcentricq q

nebulizers.

MicroMist, Glass Expansion, Australia)w7,124,126x have been increasingly used for deter-mining long-lived radionuclides by solution intro-duction into the ICP-MS instead of ultrasonicnebulizers, which consume high volumes of solu-tion. Using the direct injection high-efficiencynebulizer (DIHEN, J E Meinhard Associates,USA) w33,90x sample solution is introduced intothe inductively coupled plasma with an analytetransport efficiency into the plasma of 100%. Byreducing the solution uptake rate and the samplesize to the 1ml min and fm range, respectively,y1

very sensitive measurements of long-lived radio-nuclides in aqueous solutions are possiblew33,124x. In Fig. 2, the sensitivity(in Mcpsyppm)for U determination by ICP-MS using Aridus,238

Micromist, USN (Cetac Technologies, USA) andDIHEN is compared. For quadrupole-based ICP-MS vs. sector field ICP-MS with a single ioncollector (Element, Finnigan MAT) and multipleion collection (Nu Plasma, Nu Instruments) thelowest ion intensities for U were observed.238 q

The highest sensitivity was observed using USNin sector field ICP-MS but the solution uptake rateof 2 ml min for analyzing high-radioactive solu-y1

tions is relatively high. Considering the differentsolution uptake rate of several nebulizers, thehighest sensitivity for the determination of U238

(;3000 cps fg ) was measured using the DIHENy1

in sector field ICP-MS with a single ion detectordue to a low solution uptake rate of 0.01ml min . The problem with the DIHEN is they1

relatively high molecular ion formation rate,because the DIHEN works without spray chamberand desolvator. So far the Aridus microconcentricnebulizer with desolvator(sensitivity: ;2000cps fg ), which works at a solution uptake ratey1

of 0.1 ml min , is the micronebulizer of choicey1

especially when disturbing polyatomic ion forma-tion has to be suppressed. Nebulizers with lowuranium hydride formation rate are required inparticular for Pu determination, because due to239

a large excess of uranium in the presence of anextremely low concentration of plutonium a com-plete separation of Pu from U is very difficult. Acomparison of the uranium hydride formation rateof several nebulizers using different ICP-MS isdiscussed in Ref.w15x. As demonstrated by thesemeasurements aerosol desolvation(also producingdry aerosol, e.g. by laser ablation, which is notdemonstrated here) is advantageous for Pu and239

also U analysis. Fig. 3 compares the UHyU236 q q

formation rate on solution uptake rate in ICP-SFMS using DIHEN and Aridus microconcentricnebulizers. The highest hydride formation rate wasobserved for the DIHEN, whereas using the DIH-


Fig. 3. Comparison of UHyU formation rate on solution uptake rate in ICP-SFMS using DIHEN and Aridus microconcentricq q

nebulizers.

Table 7Characteristics of different ICP-MS instruments using microcentric nebulizer with desolvator(Aridus, Cetac Technologies)

Absolute sensitivity UH yUq q Abundance sensitivity(countsyatom)

ICP-QMSa 4=10y5 3=10y6 6=10y7

ICP-SFMS 7=10y4 3=10y5 5=10y6

MC-ICP-MSb 5=10y4 5=10y5 3=10y7

ELAN 6000.a

Halicz w94x.b

EN with decreasing solution uptake rate a decreas-ing hydride formation rate was observed. With theapplication of Aridus the hydride formation rateof approximately 3.6=10 is nearly constanty5

whereas the oxide formation rate increases withincreasing solution uptake rate. Characteristics ofdifferent ICP-MS instruments are summarized inTable 7 using probably the most useful microcon-centric nebulizer with desolvator currently availa-ble (Aridus, Cetac Technologies) for the analysisof small volumes of radioactive solutions.The low-flow microconcentric nebulizer with

desolvator(Aridus) coupled to ICP mass spec-trometers was successfully applied in our labora-tory for environmental monitoring of spent reactoruranium, evaluating origins of contamination withnuclear fuel in Chernobyl samples, studying radio-nuclide behavior in the environment or for the

analysis of depleted uranium in samples fromKosovo w15x.

The application of flow injection, as mentionedbefore, is extremely helpful for solution introduc-tion of small volumes into the ICP in order tominimize radioactive contamination in the instru-ment w5,126x. Microliter volumes of an aqueoussolution can be handled by a commercial HPLCinjection valve which was coupled, e.g. to amicroconcentric nebulizer (Micromist, GlassExpansion) for small droplet formation and aminicyclonic spray chamber. It is thus possible toanalyze small sample volumes(sample loop:)1ml) of radioactive waste solution introduced bythis HPLC injection valve into a continuous flowof 2% nitric acid. In Fig. 4 an example is givenof the application of flow injection ICP-MS(usinga quadrupole ICP-MS) for the determination of


Fig. 4. Application of flow injection ICP-MS for thorium determination in radioactive waste solution(from Ref. w126x withpermission).

Th in small volumes of aqueous solution. In the232

left part of Fig. 4, transient signals for 1, 2 and 4ng l (at a sample loop of 20ml) solutions arey1

demonstrated. The flow injection isotope dilutiontechnique(right part of Fig. 4) was developed forthe accurate determination of radionuclide concen-tration. In this experiment a Th solution(con-232

tinuous flow) was spiked with 20ml of 5 mg ly1

high-enriched Th(99.85%) for quantitative Th230

determination in radioactive solutionsw126x. Low-er analyte concentration in radioactive waste solu-tion can be measured by more sensitiveICP-SFMS. For example, transient signals of a

Np standard solution(sample loop: 20ml; Np237

concentration: 10 and 100 pg ml) were meas-y1

ured with a precision of 2.0 and 1.6%(R.S.D.,Ns5), respectively w5x. Another possibility ofanalyzing small quantities of sample is the appli-cation of electrothermal vaporization-ICP-MS forthe determination of long-lived radioisotopes,which is described in Ref.w128x.

2.7. Laser ablation inductively coupled plasmamass spectrometry

To an increasing extent LA-ICP-MS is the meth-od of choice for the direct analysis of solid samples

with respect to the analysis of long-lived radionu-clides. This powerful analytical technique uses theevaporation of sample material by a focused laserbeam(mostly using a Nd–YAG laser withly4s266 nm) in an inert gas atmosphere(e.g. Ar)under normal pressure and postionization of evap-orated and ablated material in an inductively cou-pled plasma of the ion source of an ICP-MS.Commercial laser ablation systems(e.g. LSX-200or LSX-500, CETAC, USA and LUV 266, Mer-chantek, USA) are coupled to quadrupole analyz-ers. Most applications of LA-ICP-MS aredescribed with respect to the analysis of the natu-rally occurring radioactive elements U and Th ingeological and environmental samples. In our lab-oratory a noncommercial and a commercial laserablation system were coupled to a quadrupole-based ICP-MS (‘ELAN 6000’, Perkin ElmerSCIEX, Canada), a collision cell ICP-QMS(Plat-form, Micromass) and a double-focusing sectorfield mass spectrometer(‘ELEMENT’, FinniganMAT, Germany) for the characterization of radio-nuclides in geological samples and solid radioac-tive waste materials w11,35,36,41,45x. Forexample, for the determination of long-lived radio-onuclides in nonconducting materials a synthetic


Table 8Estimated limits of quantification of LA-ICP-MS for actinide radionuclides deposited on stainless steel plates after chemical sepa-ration w129x

Isotope Half-life (years) LOQ (g g )y1 LOQ (Bq)

LA-ICP-MS LA-ICP-MS Alpha spectrometry

Th230 7.54=104 3.8=10y15 2.9=10y6 2.0=10y5

Th232 1.41=1010 6.8=10y15 2.8=10y11 5.0=10y5

U233 1.59=105 3.8=10y15 1.4=10y6 –U234 2.44=105 3.8=10y15 8.8=10y7 2.0=10y5

U235 7.04=108 4.9=10y15 3.9=10y10 2.0=10y5

U236 2.34=107 4.0=10y15 9.5=10y8 3.0=10y5

U238 4.47=109 7.1=10y15 8.8=10y11 5.0=10y5

Np237 2.14=106 3.8=10y15 1.0=10y7 2.0=10y5

Pu239 2.41=104 3.9=10y15 9.0=10y6

Pu240 6.56=103 3.7=10y15 3.1=10y5 5.0=10y5

Pu241 1.49=101 3.8=10y15 1.4=10y2 –Pu242 3.87=105 3.6=10y15 5.1=10y7 2.0=10y5

Pu244 8.26=107 3.6=10y15 2.4=10y9 –Am241 4.32=102 3.7=10y15 4.7=10y4 8.0=10y5

Am243 7.37=103 3.6=10y15 2.7=10y5 4.0=10y5

Table 9Determination of Uy U and Uy U in isotope standard solutions(CCLU-500 and NIST U-020, U concentrations10 ppb)234 238 236 238

deposited on stainless steel targets by LA-ICP-SFMSw129x

Isotope Measured IR R.S.D.(%) Certified IR Accuracy(%)standard (ns6)

Uy U234 238

CCLU-500 1.1099=10y2 1.2 1.1122=10y2 y0.21NIST U-020 1.293=10y4 2.7 1.2756=10y4 1.4

Uy U236 238

CCLU-500 2.77=10y3 1.3 2.789=10y3 y0.67NIST U-020 1.639=10y4 2.7 1.6856=10y4 y2.8

laboratory standard with a concrete matrix wasdoped with low levels of long-lived radionuclides(e.g. Tc, I, Th, U, Np, U). The99 129 232 233 237 238

detection limits determined for Tc, U and Np in ablank concrete sample were in the low picogramper gram concentration range. The detection limitsare lower by more than one order of magnitudeusing double-focusing sector field ICP-MS(Ele-ment) in comparison to quadrupole LA-ICP-MS(ELAN 6000) w36x. The capability of LA-ICP-MSfor determining long-lived radionuclides for trace,ultratrace and isotope analysis in solid materials isdiscussed in Ref.w45x.Pu isotope ratios and americium were deter-

mined in moss samples which were collected from

the eastern Italian Alps(1500 m a.s.l.). The frozensamples were cut into 1–2 cm sections and ana-lyzed separately to obtain the distribution curvesof vertical concentrations. For plutonium andamericium isotope analysis 1–2 g of the sampleswere ashed, leached, separated with respect toanalytes and analyzed by alpha spectrometry andLA-ICP-MS after the plutonium or americium hadbeen electroplated on a stainless steel diskw129x.Estimated limits of quantification of LA-ICP-MSfor actinide radionuclides deposited on stainlesssteel plates after chemical separation are summa-rized in Table 8. For the longest-lived radionu-clides in moss samples, lower limits ofdetermination at 10 g g concentration levely15 y1


were found compared to those of alpha spectrom-etry. The accuracy of LA-ICP-MS measurementswas investigated on isotope standard referencematerials deposited on steel targets(Table 9). Theanthropogenic radionuclide concentrations( Pu,239

Pu and Am) in the samples are extremely240 241

low and these mosses appeared to be particularlysuitable for investigating atmospheric contamina-tion with actinides and providing a record of thehistory of atmospheric fallout. The Puy Pu240 239

isotope ratio was almost constant within experi-mental errors for all samples analyzed with aweighted average value of 0.212"0.003. Theprobable Pu contamination source was global fall-out after nuclear weapons tests in the sixties. Theanalysis of vertical distribution of different pluto-nium nuclides in moss profiles collected in Bellunoprovince yielded a correlation of maximum specif-ic activity of Pu and Pu with the maximum239 240

nuclear fallout from nuclear weapon tests in the1960sw129x.

Further applications of LA-ICP-MS for thetrace, ultratrace and isotope analysis of long-livedradionuclides were discussed in Ref.w45x.

3. Precise measurements of isotope ratios byICP-MS

Isotope ratio measurements of radiogenic ele-ments are of great importance in the nuclearindustry, where TIMS has occupied a favoredposition in the last few decades for routine meas-urements(e.g. quality assurance of fuel materialwith respect to isotopic composition of U and Pu;also for characterization of nuclear materials fromreprocessing plants and radioactive waste control).Callis and Abernatheyw52x reported on the devel-opment of a rapid high-precision analytical tech-nique for the determination of isotope ratios ofuranium and plutonium by total sample volatili-zation using multiple-filament TIMS with a com-mercial multicollector instrument(VG-354, VGIsotopes). Run-to-run reproducibilities of-0.02%R.S.D. have been obtained for isotope ratios of Uand Pu. At present TIMS is being increasinglyreplaced for precise isotope ratio measurements byICP-MS due to its excellent sensitivity and goodR.S.D. w9,53,61,130,131x. In particular, ICP-MS

has been applied more and more frequently inrecent years for environmental monitoring of acti-nides and evaluation of the contamination sources(nuclear weapons tests, nuclear power or reproc-essing plants accidents).The state of the art in precise and accurate

isotope ratio measurements by ICP-MS and LA-ICP-MS was reviewed recently in Ref.w11x. ICP-MS allows the determination of uranium andplutonium isotope ratios including Uy U and236 238

Puy Pu at the ultratrace level in small amounts240 239

of soil samples or hot particlesw13,15,16x. Deplet-ed uranium( Uy Us0.00202) was determined235 238

in penetrator samples and contaminated soil sam-ples from Kosovo by ICP-SFMS anda-spectrom-etry (after analyte separation) in good agreement.

U (3.1=10 g g ) and Am (1.7=10236 y5 y1 241 y12

g g ) have also been detected in penetrator sam-y1

ples, which indicates the previous existence ofneutron-related processes and points to a possiblepresence of spent reactor uranium in munitionsw15,132x. Isotope analysis of depleted uranium inuranium ammunitions and contaminated soil sam-ples collected during the Kosovo conflict wasdescribed also in Refs.w132,133x. The result of Puisotope analysisw Puy Pus0.35"0.1 at Pu240 239

concentration of(5.5"1.1)=10 g g x on ay13 y1

soil sample from Kosovo indicates a mixed falloutincluding spent reactor fuel due to the Chernobylnuclear power plant accident in 1986 and plutoni-um due to nuclear weapons test in the 1960sw15x.Isotope analysis of uranium and plutonium canalso be used for estimating the burn-up of spenturanium in contaminated environmental samplesas described in Ref.w125x. Table 10 summarizesthe results of calculating the percentage of spenturanium(%) in spentynatural uranium mixture insoil samples for different depths on the basis ofICP-MS measurements using instruments with sin-gle ion detection and multiple ion collectors. Themaximum level of contamination of soil sampleswith spent uranium on the soil surface is demon-strated. With increasing depth the contaminationof soil samples with spent uranium decreases.

Puy Pu isotope ratios in marine samples240 239

from areas affected by nuclear weapon tests ornuclear reprocessing plants and in surface soilsfrom Marshall Island measured by ICP-MS has


Table 10Results of calculation of spent uranium portion(%) in spentynatural uranium mixture in soil samples

Depth Calculated from results of

(cm) Single ion detector ICP-SFMS MC-ICP-MS

Via Uy U235 238

(R.S.D., %)Via Uy U236 238

(R.S.D., %)Via Uy U235 238

(R.S.D., %)Via Uy U236 238

(R.S.D., %)

0–5 61.2 (5.0) 67.5 (1.8) 61.7 (0.4) 67.5 (0.3)5–10 4.0 (64) 5.3 (5.7) 4.4 (7.9) 5.4 (0.6)10–15 2.0 (120) 1.64 (6.2) – 1.63 (0.6)

been reported in Refs.w134,135x, respectively. Awide range of Puy Pu isotopic ratios of 0.065240 239

(weapon grade Pu collected on Runit Island)–0.306 (contamination of BRAVO thermonucleartest, 1954, Bikini atoll) were found. The variationcan be related to specific events during the nuclearweapon testing programw135x.

Ketterer et al.w136x investigated naturally occur-ring radioactive material of U, U and Th238 235 232

which is released into the environment from manyanthropogenic sources by quadrupole ICP-MS afteruranium and thorium separation and enrichmentby selective extraction. Uy U was determined234 238

in natural waters after two stage extraction withChelex 20 and UTEVA(sample amount: 15–25mgU) with a precision of 0.3–0.5%. A correlationof Puy Pu and Puy Pu isotopic ratio was241 239 240 239

found by the study of nuclear fallout in environ-mental samples. Low isotope ratios are evidenceof global fallout, higher ratios were detected inChernobyl-derived Pu( Puy Puf0.3–0.35) in240 239

forest soils from Polandw137x.Characterization of airborne uranium and thori-

um contamination in Northern England wasdescribed by Bellis et al.w24,138x. The authorsmeasured significant enrichments of the natural

Uy U ratio in tree barks near the nuclear235 238

installations after bark digestion using quadrupoleICP-MS. Howe et al.w113x found a contaminationof sediments(Rivacre Brook) from the vicinity ofa uranium enrichment plant with U enriched235

uranium, via isotope ratio measurements in sequen-tial extracts using ICP-MS.Mass spectrometric techniques are also increas-

ingly being applied for the precise and accuratedetermination of Uy U isotope ratios at the235 238

trace and ultratrace level in medical samples, suchas urine, in order to demonstrate possible contam-ination with uraniumw25,28,139–143x. Inkret etal. w130x discussed applications of TIMS to thesensitive detection of Pu and Pu intakes. The239 240

determination of Pu concentration in urine samplesyielded an average measurement uncertainty of 3.8mBq 24 h , a 40-fold improvement over they1

measurement uncertainties associated with radio-chemistry anda-spectroscopy analytical methods.In order to reduce possible contamination problemsduring sample preparation an analytical methodusing LA-ICP-MS was developed in our laboratoryfor sensitive isotope ratio measurements on urinesamples after deposition on a quartz substratew144x.A determination of Pu isotope ratios at the

femtogram to nanogram level by multicollectorICP-MS with hexapole collision cell(Isoprobe,Micromass) using the Uq U double spike236 233

technique in order to correct for instrumental massbias and instrumental drift was described by Tayloret al.w9x. The results of isotope ratio measurementsare compared with those of TIMS in Fig. 5 anddemonstrate the capability of powerful ICP-MStechniques for extreme analysis.Thorium and uranium isotope ratios in low

concentration geological materials(-0.1mg g )y1

by MC-ICP-MS (‘Nu Plasma’ from Nu Instru-ments using an Aridus microconcentric nebulizerwith desolvator) were determined by Turner et al.w145x. Isotope ratio measurements of Thy230

Th—6=10 (at Th concentration: 5mg l )230 y6 y1

were performed with a precision of 1.1%.Stirling et al.w146x discussed the application of

laser ablation multicollector ICP-MS to certified


Fig. 5. Plutonium isotope ratio analysis at femtogram to nanogram levels by MC-ICP-CC-MS vs. TIMS(from Ref. w9x withpermission).

glass standards and naturally occurring opal for insitu uranium and thorium isotopic analysis. Theprecision of isotope ratio measurements of long-lived radionuclides obtained by ICP-QMS anddouble-focusing sector field ICP-MS with singleion collector and MC-ICP-MS together with dif-ferent applications has been demonstrated in sev-eral papers w11,87,147–156x. Recently, themonthly plutonium deposition fallout collected inTsukuba (Japan) was analyzed using the sectorfield ICP-MS ‘PlasmaTrace’ by Hirose et al.w157x.

Selected applications of mass spectrometry forthe determination of isotope ratios are summarizedin Table 11.

4. Conclusions

Inorganic mass spectrometric techniques of solidsamples allow the direct determination of long-

lived radionuclides in the picogram per gram rangewith a minimum of sample preparation. ICP-MSis an excellent tool for the analysis of aqueoussolutions, especially with on-line coupling tech-niques (ETV, HPLC, ion chromatography, flowinjection) achieving detection limits in the sub-femtogram per milliliter range. A wide variety ofapplications demonstrates the excellent capabilityof inorganic mass spectrometry techniques fordetermining very low levels of radioactivenuclides, due to their low detection limits, forevidence of contamination from radioactive wastein the environment(in biological and medicalsamples, waters or geological materials).The significance of ICP-MS in precise isotope

ratio measurements at ultratrace levels is increas-ing, especially when multicollector, andyor double-focusing sector field instruments are used.

1777J.S.

Becker

/Spectrochim

icaA

ctaP

artB

58(2003)

1757–1784

Table 11Application of mass spectrometry in isotopic measurements

Samples Method and Equipment Isotopic ratios Precision of isotopic Referencesanalysis(at concentration)

Standard solutions ICP-SFMS Uy U235 238 0.03 %(1 mg l )y1 Taylor et al.w147x‘Plasma 54’ prototype

Standard solutions ICP-SFMS Puy Pu240 239 2.0 % (Pu: 0.1 ng l ,y1 Becker and Dietzew7xWaste samples ‘ELEMENT’ Thy Ths10230 232 y3 50 fg) 0.34%(5 ng l )y1

Nuclear samples ICP-QMS, ‘ELAN Csy Cs135 138 0.92%(50 mg l )y1 Barrero Moreno et al.w150x5000’ on-line ion Csy Cs136 138 0.94%(50 mg l )y1

chromatography

Standard solutions ICP-SFMS, ‘Plasma 54’ Uy U235 238 0.014%(U: 1 mg l )y1 Walder and Freedmannw151x

Geological samples ICP-SFMS Thy Th230 232 0.04% Luo et al.w152x‘Plasma 54’ Uy U (0.000054)234 238 0.12%

Environmental samples ICP-MS, ‘HP 4500’ Puy Pu240 239 4.1% (at pg ml level)y1 Momoshima et al.w153x(sediments, soil, ‘PMS-2000’needles)

Environmental samples ICP-SFMS Puy Pu240 239 2% Sturup et al.w154x¨‘PlasmaTrace 2’

Soil samples ICP-SFMS ‘ELEMENT’ Uy U, Puy Pu,236 238 240 239 3.5% (Pu:f10 g g )y13 y1 Boulyga and Beckerw125x(Chernobyl fallout) after extraction Uy U, Uy U234 238 235 238

Tree barks ICP-QMS ‘HP 4500’ Uy U235 238 -1% Bellis et al.w24,138x(nuclear fuel fabrication plant)

Moss(environmental monitor LA-ICP-SFMS Puy Pu240 239 1.4% (Pu:f10 g g )y13 y1 Boulyga et al.w129xfor nuclear fallout) after separation and

electrolytic deposition


List of abbreviation

AMS accelerator mass spectrometryCE capillary electrophoresisDIHEN direct injection high-efficiency nebulizerETV electrothermal vaporizationGDMS glow discharge mass spectrometryd.c. GDMS direct current glow discharge mass spectrometryr.f. GDMS radio frequency glow discharge mass spectrometryHPLC high-performance liquid chromatographyICP-MS inductively coupled plasma mass spectrometryICP-QMS quadrupole based inductively coupled plasma mass

spectrometryICP-SFMS sector field inductively coupled plasma mass

spectrometryMC-ICP-MS multiple ion collectors inductively coupled plasma

mass spectrometryLA-ICP-MS laser ablation inductively coupled plasma mass

spectrometryLLRW long-lived radioactive wasteMCN microconcentric nebulizerRIMS resonance ionization mass spectrometrySIMS secondary ion mass spectrometrySNMS secondary neutral mass spectrometryTIMS thermal ionization mass spectrometryTOF time of flightUSN ultrasonic nebulizer

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