TECHNICAL NOTE www.rsc.org/jaas | Journal of Analytical Atomic Spectrometry

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High sensitivity analysis of plutonium isotopes in environmental samples usingaccelerator mass spectrometry (AMS)

D. P. Child,* M. A. C. Hotchkis and M. L. Williams

Received 12th October 2007, Accepted 25th January 2008

First published as an Advance Article on the web 28th February 2008

DOI: 10.1039/b715788f

This article presents a methodology for the determination of the concentration and isotopic ratio of

plutonium occurring at femtogram levels in environmental matrices such as soils and sediments by

accelerator mass spectrometry (AMS). Results on analyses of a number of reference materials

(IAEA-375, IAEA-135, IAEA-300, IAEA-327, NIST 4350, NIST 4353b) are presented as validation of

the method in reproducibly measuring the plutonium isotopic ratio 240Pu : 239Pu in a variety of

environmental sample matrices.

Introduction

Mass spectrometry techniques such as sector field inductively

coupled plasma mass spectrometry (SF-ICP-MS) and thermal

ionisation mass spectrometry (TIMS) have enabled the sensi-

tivity of measurements of long lived actinides to be improved

at least three orders of magnitude compared to alpha count-

ing1–3 and in addition allow measurement of 239Pu : 240Pu : 241Pu

concentration ratios. The application of accelerator mass

spectrometry (AMS) to the analysis of actinides offers potential

further improvements to sensitivity through its ability to reduce

isobaric molecular interferences and spectral interferences

caused by abundant adjacent mass peaks,4–6 particularly in the

case of samples with complex matrices.

In the measurement of nuclear safeguards environmental

samples it has been necessary to conduct simultaneous extraction

and measurement of plutonium, uranium and iodine isotopes on

the one sample, necessitating a specialised sample preparation

methodology. Iodine is required because the fission product129I can be a useful signature of nuclear activities.7 In order to

validate this method and to ensure that disproportionation

between native and tracer plutonium was not occurring,

a number of standard reference materials with published

plutonium concentrations were prepared and analysed.

Analytical method

Materials used

The laboratory used for sample preparation is ventilated with

HEPA-filtered air at positive pressure. All chemical reagents

used are of high purity ARISTAR grade or equivalent unless

otherwise noted. All water used during sample preparation was

deinionised water (18.2 MU) from a Milli-Q water purification

system. All equipment is washed successively with an acid

wash (HNO3 or H3PO4), a neutralising agent plus deconta-

minating detergent (Decon 90�), and then rinsed with deionised

water.

Australian Nuclear Science and Technology Organisation (ANSTO),Private Mailbag 1, Menai, NSW 2234, Australia. E-mail: dpc@ansto.gov.au; Fax: +61 2 9717 9265; Tel: +61 2 9717 3851

This journal is ª The Royal Society of Chemistry 2008

For AMS determination of plutonium concentration and

isotopic ratios (240Pu : 239Pu) the technique of isotope dilution

is used. NIST SRM 4334 G 242Pu is added to all samples as the

reference isotope. Additions of 1 pg 242Pu per sample were used

in the present work. An in-house standardised 238Pu tracer was

used as a yield monitor in tests of the extraction procedures

described below. The 238Pu was analysed by alpha spectrometry.

Matrix destruction

Prior to processing, soil and sediment samples are checked for

homogeneity, and if necessary coned and quartered to ensure

homogenous representation of the sample. A combustion

method was chosen to enable collection of volatile iodine from

the bulk sample matrix prior to liberation of the actinides. The

method shown in Fig. 1 was then used for the separation and

extraction of plutonium and uranium from sample matrices.

The combination of high temperature combustion with only

partial sample dissolution (acid leaching) poses a risk of

incomplete dissolution of native plutonium oxide and therefore

incomplete mixing of tracer plutonium (242Pu) and native

plutonium (239Pu and 240Pu).8 Plutonium of safeguards interest

is expected to be present as very fine micron sized oxide particles

condensed from the exhaust in reprocessing facilities or adsorbed

to the surface of aerosol particles, and therefore should be

susceptible to aqua regia leaching. This partial dissolution

method was therefore chosen to utilise the reduction in potential

ingress of matrix elements into the sample (particularly silicon

and uranium), reduction in chemical manipulation and therefore

lower blanks, and a simplified, more rapid sample preparation

process for plutonium analysis. To minimise the potential for

disproportionation between fallout plutonium and tracer

plutonium the temperature of combustion was kept below

900 �C and the duration of the high temperature combustion

was kept to �30 min.9

Plutonium purification

Environmental samples are expected to contain uranium at

much higher concentrations than plutonium. As the presence

of uranium may interfere with the detection of plutonium

J. Anal. At. Spectrom., 2008, 23, 765–768 | 765

Fig. 1 Schematic of sample processing.

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isotopes in mass spectrometry analysis by way of peak overlap,

it is necessary to separate plutonium from uranium. This is

achieved in our facility by ion exchange chromatography as

detailed in Fig. 1. The method developed at ANSTO is based

upon those given in ref. 10 and 11.

All samples are processed by the method described in Fig. 1

with the following exceptions. If samples contain large quanitites

of aluminium, the coprecipitate formed after digestion is washed

with 6 M NaOH to dissolve bulk aluminium before addition of

the samples to ion exchange columns. If samples are suspected

of having high phosphate content, the solutions are made up

to 1 M Al(NO3)3 to reduce the possibility of phosphate

complexed actinides prematurely eluting from the columns.

For introduction into the AMS ion source, the element of

interest must be purified and concentrated into a small pellet

of material of no more than a few milligrams. For measurement

of small quantities of actinides (<1 mg), the most suitable form

for AMS has been found to be as a co-precipitate with

�1.5 mg of iron oxide.12,13 Therefore approximately 1 mg of

iron(III) nitrate (Choice Analytical, 1000 ppm ferric nitrate) is

added and the Fe/Pu co-precipitated with NH4OH. This precipi-

tate is collected, washed with deionised water, dried and calcined

for loading into AMS sample holders.

Yields on samples processed using the methodology outlined

in this paper varied between 60–80% (determined by alpha

spectroscopy measurements of reference samples spiked with238Pu) and were deemed acceptable for the sensitivity require-

ments of this technique. Higher and more reproducible yields

are desirable for the future however, so further investigation

will be conducted to determine where the remainder of the

plutonium is being lost.

766 | J. Anal. At. Spectrom., 2008, 23, 765–768

AMS instrumentation and analysis method

Samples are measured by AMS on the ANTARES FN tandem

accelerator at ANSTO, using the ‘actinides beamline’ previously

described for analysis of 236U/238U isotopic ratios.13 For the

measurement of Pu isotopes on this system, ions of PuO� are

injected, a terminal voltage of 4 MV is used, and ions of Pu5+

are extracted giving a beam energy of 23.75 MeV. However,

the method for obtaining isotopic ratios of Pu differs to that

previously published.

With AMS, the ion beam is mass-analysed twice using sector-

field magnets: first as negative ions before acceleration, and then

as positive atomic ions after acceleration. To obtain isotopic

ratios for uranium on ANTARES, the rare isotope 236U5+ is

counted in a gas ionisation detector and the 238U5+ is measured

in an off-axis position using either a Faraday cup or a secondary

electron multiplier in current-amplifying mode. Thus the

uranium isotopes are measured at separate points on the focal

plane of the analysing magnet without changing magnetic field

or terminal voltage. The ions injected into the accelerator are

rapidly switched between 236UO� and 238UO� by modulating

the beam energy in the negative ion analyser (‘fast-cycling’). In

contrast, to measure Pu isotopic ratios, it is necessary to count

each Pu isotope sequentially in the same gas ionisation detector,

as all Pu isotopes are at low levels. This requires changing the

magnetic field of the positive ion analyser between each measure-

ment for the ANTARES instrument. This process is relatively

slow (‘slow-cycling’) as the magnet was not designed for rapid

switching. The configuration of the ANTARES system necessi-

tates this mode of operation, in contrast to the configuration

described in ref. 12, where isotopes can be switched by

modulating the terminal voltage.

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Test results and discussion

Determination of ability to discriminate plutonium in the

presence of uranium

A group of samples containing from 0 to 200 ng total uranium as

natural uranium (NIST 4321c), were analysed. A fixed amount

of 242Pu tracer was added to each sample. This experiment

allowed the impact of uranium concentration on plutonium

sensitivity to be determined; in particular 239Pu, which is the

isotope most likely to be affected by interferences from uranium.

The samples were analysed to derive the apparent amount of239Pu present in each sample. The results are shown in Fig. 2.

Fig. 2 Interference of uranium with mass 239 sensitivity. The dotted line

represents a simulated result assuming a 239 : 238 abundance sensitivity

of 9 � 10 �7 and an ‘offset’ of 3.5 fg (see text).

Table 1 Plutonium content of a collection of reference materials comprising rin this work are based upon single measurements and errors represent 1s dreported in the cited publication

Referencematerial

Samplesize/g

239Pu activity/Bq kg�1

240Pu activityBq kg�1

IAEA-135 0.48–0.64a 126 � 19 86 � 17Irish Sea sediment

127 � 8 96 � 6

IAEA-375 2.06 0.113 � 0.010 0.124 � 0.02Russian soil 0.49 0.108 � 0.025 0.119 � 0.04

IAEA-300 1.08 2.41 � 0.09 1.51 � 0.11Baltic Sea sediment

IAEA-327 1.55 0.282 � 0.020 0.205 � 0.03Soil

NIST SRM-4353 0.93 4.74 � 0.24 1.14 � 0.14Rocky Flats soil

NIST SRM-4350b 1.47 0.364 � 0.024 0.117 � 0.02River sediment 1.41 0.342 � 0.022 0.152 � 0.02

a This range represents an average of results from 3 samples (11 measuremenaverages calculated from 2 reported results. Errors include only counting starepresents a mean of 3 measurements.

This journal is ª The Royal Society of Chemistry 2008

The data can be interpreted in terms of two components: (i) an

abundance sensitivity for 239Pu : 238U of 9 � 10 � 7 and (ii) an

‘offset’ of 3.5 fg. The abundance sensitivity is limited by the

interference of 238U, which may be caused by a combination of

the finite resolving power of the magnetic analysers, the injection

of 238U as molecular ions of same mass as 239Pu16O (e.g. 238U16OH

or 238U17O), and the occurrence of scattering and charge

changing collisions in the beam tubes. The ‘offset’ can be

explained as being due to a fixed level of contamination in the

samples, of either 238U at a level of around 3.9 ng, or 239Pu at

a level of around 3.5 fg. The contamination could be from the

reagents and materials used in the preparation of samples for

this test, or result from ion source memory effects from previous

analysis of uranium-bearing samples.

Analysis of standard reference materials

A number of reference materials certified for plutonium content

were analysed. These were prepared and analysed using the

method described above. Results of the analyses as specific

activities of 239Pu and 240Pu as well as 239 + 240Pu activity and

240 : 239 atom ratios are given in Table 1 where they are

compared to published values. Some of the ANTARES AMS

results in this table were published previously in ref. 14 and 15.

In most cases the measured activities are consistent with

previously reported values for the reference materials. In addi-

tion there was good agreement between replicate analyses of

IAEA-135, demonstrating good internal reproducibility. A plot

of replicate analyses of this reference material is shown in Fig. 3.

adionuclide bearing environmental materials. All measurements reportedeviations unless otherwise noted. Where no error is cited, no error was

/ 239 + 240Pu activity/Bq kg�1

240Pu : 239Puatom ratio Reference

212 � 25 0.186 � 0.022 This workrange 205–225b Certificate

223 � 5 0.207 � 0.006 Lee et al.18

245 � 1.4c 0.212 � 0.004 c McAninch et al.5

0 0.237 � 0.022 0.296 � 0.054 This work9 0.226 � 0.055 0.299 � 0.140 This work

range 0.26–0.34b Certificate0.240 � 0.040c McAninch et al.5

3.92 � 0.14 0.170 � 0.014 This workrange 3.09–3.90 b Certificate

3.70 � 0.08d 0.1779d Sturup et al.19

0.19 � 0.02e Kim et al.2

0 0.487 � 0.036 0.198 � 0.032 This workrange 0.56–0.60b Certificate

5.88 � 0.28 0.065 � 0.009 This work8.03 � 0.60 0.055 Certificate

4 0.481 � 0.034 0.087 � 0.019 This work6 0.494 � 0.034 0.121 � 0.022 This work

0.508 � 0.029 0.105 Certificate

ts) combined. b This range represents a 95% CL. c These values representtistics. d These values represent means of 3 measurements. e This value

J. Anal. At. Spectrom., 2008, 23, 765–768 | 767

Fig. 3 Replicate analysis of IAEA SRM 135.

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The small size of samples that were used in this work, 0.5–2 g,

increases the risk that the results are affected by sample inhomo-

geneity. This is especially the case for very low activity samples

where the activity may reside in only a few hot particles per

gram of sample. In the case of SRM 4350b, mention is made

in the certificate of the existence of hot particles. This may

explain the deviation seen between our result and the published

value. The good agreement in other cases suggests the samples

are reasonably homogeneous. In the case of IAEA-375, our

successful analysis of the 0.49 g of the soil represents just 23 fg

and 7 fg of 239Pu and 240Pu respectively in the sample.

For several samples, isotopic information for Pu is reported

here for the first time. The soil IAEA 327 was collected in 1990

near Moscow. The Pu isotopic ratio measured is consistent

with that expected for global fallout. The Rocky Flats soil

(SRM-4353) has a low 240Pu : 239Pu ratio, showing evidence, as

expected, of local contamination with weapons-grade pluto-

nium. The high 240 : 239 ratio for IAEA-375 measured (0.296)

reflects contamination from the Chernobyl accident, being

distinctly higher than that measured for the average global

tropospheric fallout (0.18).16 However, the ratio is lower than

that of the Chernobyl source material (0.43)17 suggesting that

768 | J. Anal. At. Spectrom., 2008, 23, 765–768

the soil contains a mixture of Chernobyl fuel particles with

global tropospheric fall-out.

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