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Polyatomic interferences in plutonium determination in the femtogram rangeby double-focusing sector-field ICP-MS

Fabien Pointurier,* Am�elie Hubert, Anne-Laure Faur�e, Philippe H�emet and Anne-Claire Pottin

Received 30th July 2010, Accepted 7th March 2011

DOI: 10.1039/c0ja00097c

The determination of very low concentrations of plutonium, in the femtogram per millilitre range, in

environmental samples using ICP-MS can be interfered by polyatomic species containing heavy

elements, from hafnium to bismuth. Mass-to-charge ratios of these polyatomic species are very close to

the ones of the plutonium isotopes and cannot be separated from them in the low resolution mode

currently used for trace analysis. In this paper, we present an evaluation of the extent and impact of

these interferences on the background at masses of plutonium isotopes 239Pu and 240Pu, based on

measurement of environmental samples for three years using a double-focusing sector-field ICP-MS.

We demonstrate that these molecular interferences, especially the ones involving lead, mercury, and

iridium through species PbO2+, ArHg+, IrO3

+, must be considered as the additional background

induced by these interferences (on average for all interferences respectively �1.2 and �1.9 counts s�1 at

239 and 240 atomic mass units) is often higher than the instrumental background measured with

deionised water acidified at 2% by ultra-pure grade HNO3 (�0.4 counts s�1), and are generally of the

same order as other usual sources of background for low-level plutonium analysis by ICP-MS, i.e. 242Pu

tracer isotopic impurities, 238U hydrides and peak tail. Thus, if not corrected, these polyatomic

interferences may lead to false detection of femtogram amounts of plutonium or overestimation of

results. The extents of formation of the main polyatomic species are highly variable from one analysis

to the other, over one or two orders of magnitude, depending on instrumental settings. Chemical

purification procedures are very efficient in eliminating the greater part of heavy elements present in the

initial samples. However, these procedures also bring heavy elements into the final sample solutions

through contamination by the atmosphere, glassware and reagents.

Introduction

Numerous examples can be found in the literature about ICP-MS

with best detection limits for plutonium isotopes in the femtogram

range (10�15 g) for quadrupole-based instruments1–4 and, more

often, for sector-field instruments.5–25 However, these instru-

mental detection limits are obtained with synthetic solutions and

the analytical detection limits obtained with real-life samples may

be considerably higher, generally because of a loss of sensitivity

due tomatrix effects or tomemory effects.10With the exception of238U hydrides atmass 239, the possible occurrence of interferences

due to polyatomic species with total masses equal to masses of

actinide isotopes is rarely considered. At the high end of the mass

range, the actinides have typically been considered as far removed

from the notorious polyatomic interferences (PIs) that tend to

plague the lighter elements. Nevertheless, some authors

mentioned and, in some cases studied, possible occurrence of

some PIs in the actinide mass range6,11,13,17,22,23,26–31 containing

CEA, DAM, DIF, F-91297 Arpajon Cedex, France. E-mail: fabien.pointurier@cea.fr; Fax: +33 1 69 26 70 65; Tel: +33 1 69 26 49 17

1474 | J. Anal. At. Spectrom., 2011, 26, 1474–1480

a heavy element like platinum,28 lead,7,22,23,26,28 lanthanides,23,30,31

mercury22 or even of organic nature in samples with significant

organic content.13,29 Nygren and co-workers31 carried out an

extensive study on lanthanide phosphate molecular species which

interfere with plutonium isotopes extracted from soil and sedi-

ment samples. However, influence of PIs is regarded as very low,

significant only in special cases and a high confidence is put in the

chemical preparation to remove the interfering species. We

observed that with a sector-field ICP-MS which exhibits a very

high sensitivity (typically a few million counts s�1 per ng ml�1 of

plutonium), despite a neat chemical purification with many

precautions to limit contamination, pure background values

measured in the actinide mass range with real-life samples

(1 counts s�1 or more) are systematically higher than the instru-

mental background (IB) recorded with deionised water and ultra-

pure grade nitric acid (�0.4 counts s�1). We established that this

additional background is due to polyatomic species that are

combinations of one heavy element (HE) like Pb, Hg, W, Ir, Pt,

etc., with some of the most abundant atoms in the plasma (O, N,

H, Cl, Ar).32 In a previous paper, we proposed a method to esti-

mate and to correct these interferences,32 which, if not identified

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and corrected, may lead to false detection of femtograms of

plutonium with high sensitivity ICP-MS. This method has been

now applied to real samples for about three years. So experience

has been gained on the degree of influence of polyatomic species

on the background in the actinide mass range.

The purpose of this paper is to present an evaluation of the

extent and impact of PIs on the background at masses of

plutonium isotopes 239Pu and 240Pu. This evaluation is based on

the measurement of plutonium in environmental samples for the

years 2007, 2008, and 2009, following the method developed in

the laboratory. For all measured samples, the sum of the back-

ground due to PIs at mass 239 and 240 is systematically

compared to other known sources of background (electronic

background, 238U hydrides, 238U peak tailing, tracer impurities,

etc.). Average values and variations of the extents of formation

of the most relevant polyatomic species are given. We also

present an evaluation of the efficiency of the chemical purifica-

tion procedure in the case of cotton wipe samples. This work

demonstrates that, for many of the analyses, total count rates

arising from polyatomic interfering species are higher than the

instrumental background and cannot be neglected for other

sources of background. The intention of this study is to show

that when appropriate conditions are encountered (very sensitive

instrument, samples with ultra-low plutonium contents), the

correction of polyatomic interference must be considered to

avoid false detections of plutonium at femtogram level.

Experimental

Instrumentation

For trace analyses of long-lived actinides, we routinely use

a double-focusing sector field ICP-MS (‘‘Axiom SC’’, VG

Elemental,Winsford, Cheshire, UK).Operation and optimisation

of this instrument for measurement of low amounts of actinides

had already been thoroughly described.9,12Briefly, this instrument

is equippedwith nickel cones, a platinum guard electrode andwith

the S-option, an additional primary pump that decreases vacuum

in the interface andprovides an increase in sensitivity by a factor of

three compared to a non-S option. A Teflon micro-concentric

nebuliser (‘‘PFA 100’’, Elemental Scientific, Inc., Omaha, NE,

USA) in the self-aspiration mode (�100 ml min�1) is used for

sample introduction. The nebuliser is routinely operated in

combination with two on-line spray chambers (‘‘cyclonic’’ and

Table 1 Instrument and data acquisition settings for the double focusingsector-field ICP-MS VG ‘‘Axiom’’

Instrument settingsForward power 1300 WCool gas 14 L min�1

Auxiliary gas 0.85–0.95 L min�1

Nebuliser gas 0.8–0.9 L min�1

Resolution 360Data acquisition settingsAcquisition mode E-scan, peak jumpingMonitored masses 233–247 (15 masses)Number of points per peak 1Dwell time per mass 100 msNumber of sweeps 100Number of repetitions 5Total integration time per mass 50 sTotal analysis time per sample 15 minutes

This journal is ª The Royal Society of Chemistry 2011

‘‘impact bead’’, Thermo-Fisher, LesUlis, France) cooled to 12 �C.Performance is daily optimised with respect to sensitivity, short

term stability (10 minutes), and background. The instrument is

operated in electric scanning by varying the accelerating voltage,

with the intensity of the magnetic field set to a fixed value. Typical

instrument and data acquisition settings for the acquisition of

plutonium isotopes are given in Table 1. Typical performance of

the instrument is given in Table 2. The instrumental detection limit

for plutonium isotopes calculated according to the 3s criteria

based on the background obtainedwith deionisedwater and ultra-

pure grade HNO3 at 2% is about 0.25 to 0.5 fg ml�1.

Sample preparation and purification procedure

All the acids used for sample preparation and purification are of

ultrapure grade (Merck, Darmstadt, Germany). 233U and 242Pu

are used as isotopic dilution (ID) tracers for uranium and

plutonium quantitative analysis. The standard solutions were

diluted (by weight) to obtain stock solutions between 0.2 and

2 ng ml�1. Heavy element (HE) solutions used for the determi-

nation of PI extents of formation and efficiency of the chemical

purification are mono-elemental certified solutions

(Spex, Longjumeau, France).

For each analysis, a preparative chemistry based on ion-

exchange chromatography is carried out on the samples. The

procedure in the case of cotton wipe samples, used to sample dust

inside nuclear facilities, is described hereafter. Samples are

transferred to Pyrex beakers, and 10 ml of concentrated HNO3

are added. The mixture is boiled on a hot plate at 120–150 �C) forat least 2 hours. During heating, the beaker is covered with

a watch glass to prevent significant evaporation. The mixture is

evaporated to dryness and the sample is then reduced to ash at

550 �C for 12 hours in an electric furnace to decompose organic

matter. Dissolution of the ashes is accomplished by repetition of

acid digestions: 20 ml of aqua regia, evaporation to dryness, 10

ml of concentrated HNO3 + 1 ml of H2O2, evaporation to

dryness, 10 ml of concentrated HCl and evaporation to dryness.

Isotopic dilution (ID) tracers 233U and 242Pu are added in

appropriate quantities to the sample solutions before separa-

tions. Spiked samples are evaporated to dryness and 20 ml of

concentrated nitric acid are added. Adjustment and redox state

stabilisation of plutonium are performed by the addition of

nitrite followed by evaporation. 20 ml of 8 N HNO3 are added

and the solution is passed through the conditioned ion-exchange

Table 2 Typical instrumental performance of the double-focusing sector-field ICP-MS VG ‘‘Axiom’’. Background count rates are evaluated fordeionised water acidified with HNO3 at 2%. Instrumental detection limitfor the plutonium isotopes are defined as 3 times the standard deviationover the instrumental blanks

Instrumental performance

Solution introduction rate �100 ml min�1

Sensitivity for 238U 2–4 � 106 counts s�1 (ng ml�1)�1

Background at masses 240–242 0.1–0.5 counts s�1

U hydride ratio 2–4 � 10�5

U oxide ratio 2–3%238U peak tail at mass 237 25–35 � 10�6

238U peak tail at mass 236 4–6 � 10�6

Detection limit for 239Pu and 240Pu 0.25–0.5 fg ml�1

J. Anal. At. Spectrom., 2011, 26, 1474–1480 | 1475

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resin columns. The first plutonium purification is performed with

a 20 ml column filled with Dowex AG1X8 anion-exchange resin

(10 ml of 50/100 mesh resin at the bottom and 10 ml of 100/200

mesh resin on the top). The plutonium fraction is eluted with

HCl/NH4I. Further U–Pu purification is achieved by using a 2 ml

column filled with Dowex AG1X4 anion-exchange resin for Pu

or AG1X8 100/200 mesh for U. These resins are washed with 8 N

HNO3 (U fraction), 10 N HCl (Th fraction) and, finally, NH4I

(1.5%)–12 N HCl solution is added to the eluted Pu fraction. The

final solution is evaporated to dryness and recovered with 5 ml of

deionised water acidified by HNO3 at 2%. In addition, one or

more process blanks are prepared in the same conditions as the

samples for each series of samples.

Fig. 1 Example of comparison between the instrumental background

(IB), estimated from instrumental blanks made with 2% ultra-pure grade

nitric acid in deionised water, and the background induced by mercury-

containing (ArHg+) and lead-containing (PbO2+) polyatomic species. PB1

and PB2 are process blanks and S1, S2, S3, S4 are environmental samples

(cotton wipes).

Method of analysis including correction of polyatomic

interferences

Briefly, themethodnowused for over three years in the laboratory

for correction of PIs32 requires first the determination of the count

rates of at least one isotope of each HE present in the sample

solutions, from hafnium to bismuth. This allows identification of

the heavy elements whose concentrations in the sample solutions

may result in a significant contribution to the background at

masses of the plutonium isotopes, knowing the order of magni-

tudes of the extents of formation of the inducedbackground.Then

the extents of formation of the background induced by poly-

atomic species made with the HEs identified in the first step must

be measured as accurately as possible with mono-elemental

standard solutions using the same settings of the instrument and

the same analytical conditions, ideally just after the measurement

of the sample solutions. The background induced by polyatomic

interferences including heavy element HEj at mass i is given by:

ni;pa ¼Xj

fi;HEj$nHEj

(1)

where fi,HEjis the extent of formation of heavy element HEj at

mass i, and nHEjis the count rate of the heavy element HEj in the

sample solution.

Therefore, the net count rate at mass 239 is given by:

n0239 ¼ n239 � n239,IB � n239,pa � sUH + AS n238 � simp.239 n242(2)

where n239, n238, n242 are respectively the count rates at masses 239,

238 and 242 measured in the sample solutions, n239,IB the back-

ground at mass 239 in the instrumental blank (instrumental back-

ground), sUH + AS the extent of formation of 238U hydrides and the

peak tail of 238U atmass 239 (abundance sensitivity) and simp.239 the

impurity ratio of 239Pu of the 242Pu isotopic dilution tracer.

The uncertainty on the net count rate is:

sn0239

¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi�sn239

�2þ�sn239;IB

�2þ�sUHþAS$sn238

�2þðn238$ssUHþASÞ2þ�

simp:239$sn242�2þ�

n242$ssimp:239

�2þXj

�f239;HEj

$snHEj

�2

þXj

�nHEj

$sf239;HEj

�2s

and the detection limit according to the 3s criteria is:

DL239 ¼ 3$

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi�sn239;IB

�2þ�sUHþAS$sn238

�2þðn238$ssUHþASÞ2þ�

simp:239$sn2

s

1476 | J. Anal. At. Spectrom., 2011, 26, 1474–1480

A few practical precautions must be taken: (i) careful peak

centering during performance optimisation, for actinides but also

for heavy elements, to avoid bias of count rates; (ii) as stated

before, the same instrument settings or as close as possible when

the extents of formation are measured and during analysis of the

samples, to obtain relevant extents of formation; (iii) as HE

concentrations may be very high in order to estimate precisely the

extents of formation, it is necessary to consider a bias introduced

by saturation of the detector or acquisition carried out with

another detector (Faraday cup instead of ion counter) or another

counting mode (so-called ‘‘analog mode’’). It should also be

noted that the additional background induced by PIs can be in

some cases higher for the process blanks (PB) than for the

samples themselves. This unfortunately precludes correction by

a simple subtraction of the background count rates of the PB,

because it might lead to missing detection by overcorrecting the

background. An example is shown in Fig. 1: the additional

background at mass 239 due mainly in this case to lead- and

mercury-containing species is higher for one PB (‘‘PB2’’) than for

the samples. Therefore, in order to determine trace amounts of

plutonium in samples and in the PB, the most accurate method is

to correct from PIs, for the samples as well as for the PB.

Results and discussion

The extents of formation of interfering species obviously vary

from one analytical procedure to another, although no imme-

diate correlation was found with a specific parameter. First, we

examine the extents of formation of these interferences and their

(3)

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi42

�2þ�n242$ssimp:239

�2þXj

�f239;HEj

$snHEj

�2

þXj

�nHEj

$sf239;HEj

�2

(4)

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range of variability. Starting from all the results obtained with

the above-mentioned analytical procedure for years 2007 to

2009, i.e. 137 low-level plutonium measurements, we compare

the interference induced background with the IB and other

sources of background (ID tracer impurities, 238U hydrides, 238U

peak tail), and we discuss the importance and benefits of the

correction of PIs. Lastly, we give some results relative to the

efficiency of the chemical purification procedure for the elimi-

nation of HEs.

Fig. 2 Range of the extents of formation for the background induced by

polyatomic species made with heavy elements fromHf to Bi at masses 239

(a) and 240 (b) recorded for the years 2007, 2008, and 2009.

The extents of formation of the background induced by

polyatomic interferences

The extents of formation are evaluated using mono-elemental

standard solutions containing only a given HE in appropriate

concentration. For instance, extent of formation of the back-

ground at mass 239, induced by the polyatomic species including

the heavy element E is defined by:

f239;E ¼ n239;std � n239;IB

nmE;std � nmE;IB

(5)

where n239,std and n239,IB are the count rates at mass 239 in the

standard mono-elemental solution and the instrumental blanks

(deionised water acidified at 2% by nitric acid), nmE,stdand nmE,IB

are the count rates recorded for the selected isotope of mass m of

the heavy element E.

We plot in Fig. 2 the range of the extents of formation at

masses 239 and 240 recorded for polyatomic species composed of

each HE, from hafnium to bismuth, from all analysis carried out

during the years 2007–2009. We observe that formation rates

vary widely—for some elements over several orders of magni-

tude—from one analysis to another. Obviously, instrumental

settings and plasma conditions are different and this may affect

recombination of atoms/ions in the plasma.

However, most of the extents of formation (for hafnium,

tantalum, tungsten, rhenium, platinum, gold, thallium, bismuth)

are very low, around 10�8 or even below. This means that the

background induced by these elements is negligible except if the

count rates of the HEs reach at least 107 counts s�1 meaning that

their concentrations are hundreds of mg l�1 or higher in the

sample solutions. This is highly unlikely after a chemical

purification.

However, the extents of formation of the background induced

at mass 239 by iridium (measured from isotope 193Ir) and

mercury (measured from 202Hg) can respectively be higher than

10�6 and 10�5. The involved polyatomic species are 191Ir16O3+ and

199Hg40Ar+. Similarly, the background induced at mass 240 by

mercury (measured from 202Hg) is between 10�5 and 10�4. In this

case, the corresponding polyatomic species is 200Hg40Ar+.

Consequently, concentrations in the mg l�1 range for these two

elements are enough to lead to a significant increase of the

background, at least in the order of one count s�1. Now, these

concentrations are occasionally really encountered in sample

solutions, even after chemical purification. Lead is a special case.

The extents of formation of the background induced by lead

(measured from the 208Pb isotope) at masses 239 and 240

respectively through the species 207Pb16O2+ and 208Pb16O2

+ are

quite low, ranging roughly from 10�9 to 2 � 10�7. However,

contrary to other HEs, lead is a ubiquitous element, relatively

This journal is ª The Royal Society of Chemistry 2011

abundant in some samples, in the atmosphere, and in glassware

and reagents used for chemical purifications as well. Its

concentrations in the sample solutions can thus reach tens of

mg l�1 or even hundreds of mg l�1, possibly giving rise to signifi-

cant background at masses 239 and 240. We report in Fig. 3 all

the extents of formation of background at mass 239 induced by

species made with mercury, iridium, and lead, measured for the

years 2007–2009. We observe that the extents of formation of the

background induced by these elements vary respectively from

6 � 10�6 to 6 � 10�5 for mercury, from 2 � 10�7 to 4 � 10�6 for

iridium, and from 7� 10�10 to 2� 10�7 for lead. These variations

of one or more than two orders of magnitude from one analysis

to another show the need to measure up-to-date extents of

formation valid for the current analysis. It should be noted that

the extents of formation certainly depend on the introduction

system: use of a desolvating device (membrane or heating/

condensing systems) could reduce the extent of formation of

some molecular ions, especially oxides. Such a system (‘‘Apex’’,

ESI, Omaha, NE, USA) will be implemented soon in the labo-

ratory and the extents of formation will be compared with the

ones obtained with the current conventional introduction

system.

Levels of background induced by polyatomic species and

comparison to other sources of background

The average, maximum, and minimum contributions of PIs

made with iridium, mercury and lead on the background at

masses 239 and 240 for plutonium measurements carried out for

the years 2007, 2008, and 2009 (i.e. 137 measurements) are

J. Anal. At. Spectrom., 2011, 26, 1474–1480 | 1477

Fig. 3 Various extents of formation of the background induced at mass

239 by polyatomic species made with mercury, iridium, and lead. The

extents of formation are ranked by increasing values and measured for

the years 2007, 2008, and 2009.

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reported in Table 3. The contributions of the other HEs (Hf, Ta,

W, Re, Pt, Au, Tl, Bi) are negligible (<0.01 counts s�1) and are

not reported. We also report in the table the average, maximum,

and minimum values of the IB, obtained from the instrumental

blanks. For both masses 239 and 240, sums of polyatomic species

induced-background are on average higher than average IB (1.23

counts s�1 versus 0.41 counts s�1 at mass 239, and 1.95 counts s�1

versus 0.43 counts s�1 at mass 240). The additional polyatomic

species induced-background is equivalent to plutonium concen-

trations of roughly 0.5 fg$ml�1.

In addition to IB and to PI induced-background, other sources

of background are, on the one hand isotopic impurities of the

isotopic dilution (ID) tracer 242Pu, which contains small amounts

of 239Pu and 240Pu, and on the other, peak tail of 238U at masses

239 and 240 plus hydrides of 238U at mass 239. For each of the

measured samples, we determined the percentage of the back-

ground count rate due to each source of background. Then from

all these values we calculated the average percentages, plotted in

Fig. 4. At mass 239, the sum of PIs is one of the largest sources of

background (�22%) with impurities of the ID tracer 242Pu

(�24%) and sum of 238U hydrides and peak tail (�37%). IB at

mass 239 only accounted for �17% of the total background. At

mass 240, the sum of PIs represents the most important contri-

bution to the total background (�50%) well above IB, ID tracer

Table 3 Average, maximum and minimum contributions of polyatomicinterferences made with iridium, mercury and lead to the background atmasses 239 and 240 for plutoniummeasurements carried out for the years2007–2009 (137 measurements); comparison with the instrumentalbackground, noted ‘‘IB’’. The sum of all polyatomic interferences is noted‘‘Total’’. All values are given in counts s�1

Heavy element

Mass 239 Mass 240

AverageRangeof variation Average

Rangeof variation

Ir 0.16 0.00–3.78 0.03 0.00–0.80Hg 0.40 0.01–3.66 0.59 0.00–8.18Pb 0.67 0.00–14.05 1.33 0.00–29.68Total 1.23 0.01–14.34 1.95 0.01–30.10IB 0.41 0.00–1.67 0.43 0.00–1.75

1478 | J. Anal. At. Spectrom., 2011, 26, 1474–1480

impurities and 238U peak tail. By calculating the theoretical limit

of detection for each measurement only based on electronic

background, ID tracer impurities, 238U peak tail and 238U

hydrides, without taking into account the PIs, we show that false

detections of 239Pu and 240Pu may have occurred for respectively

15% and 22% of the samples.

Validation of this method is not easy, as we do not have

a certified reference material with very low amounts of pluto-

nium—in the femtogram range—at our disposal. However, a test

was done on a cotton wipe sample spiked with 10 fg of pluto-

nium. This test sample was prepared by another institution

(Khlopin Research Institute, St Petersburg, Russia). Calcula-

tions were made with and without correction of PIs (see Fig. 5).

The result obtained with correction of PIs is in better agreement

with the target value, whereas the result obtained without

correction of PIs is slightly overestimated.

Origin of heavy elements in the sample solution

Heavy elements responsible for PIs are largely eliminated during

sample preparation; this was also observed by other authors.22 In

fact, according to theoretical retention coefficients, with the

chemical media used in our purification procedures, a large part

of all the investigated HEs must be eliminated. To check this, we

applied the chemical purification procedure previously described

Fig. 4 Breakdown of the composition of the total background budget at

masses 239 (a) and 240 (b), average percentages over all plutonium

measurements carried out for the years 2007–2009 (137 measurements).

This journal is ª The Royal Society of Chemistry 2011

Fig. 5 Comparison of the results of the analysis of 3 process blanks

(PB1, PB2 and PB3) and of one quality control sample (‘‘S’’) spiked with

10 fg of 239Pu + 240Pu, using two calculation methods: without correction

of PIs (empty circles) and with correction of PIs (filled circles). Uncer-

tainties for the experimental results are given with a coverage factor of 2.

No uncertainty was provided for the target value.

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in section 2.1 for the analysis of cotton wipe samples on three

solutions spiked with relatively large amounts of certified mono-

elemental standard solutions of Hf, Ta, W, Re, Os, Ir, Pt, Au,

Hg, Pb, Tl, and Bi. The mass of each element added to the

solutions ranged from 120 ng to 10 mg. The efficiency of each

separation step was evaluated through measurement of the

remaining masses of each HE eluted. We give here the overall

efficiency of the purification procedure for plutonium analysis,

defined as the ratio of the masses of each HE in the final sample

solutions to the masses introduced in the initial solutions

(see Table 4). We give an upper value of the overall efficiency

when the element was not detected in the sample solutions. We

thus demonstrate that all HEs are more or less thoroughly

eliminated. However, as shown by the PB, the purification

procedure can also bring significant and variable amounts of

some HEs, through impurities in glassware, reagents and atmo-

spheric contamination. In the case of lead, efficiency of the

purification is probably better than the one given here, as a large

part of the lead measured in the sample solution is surely

Table 4 Overall efficiency of the elimination of HEs by the purificationprocedure for plutonium analysis starting from cotton wipe sample (seesection 2.1). Detection limits for the overall efficiency are defined as 3times the standard deviation over the process blanks. Uncertainties aregiven with a coverage factor of 2

Heavy elementOverallelimination efficiency

Hf <3 � 10�8

Ta <4 � 10�5

W <10�7

Re (8 � 2) � 10�9

Os <1.5 � 10�5

Ir (3 � 1) � 10�5

Pt <3 � 10�7

Au (6 � 3) � 10�9

Hg <10�4

Pb <10�3

Tl <5 � 10�6

Bi (8 � 4) � 10�4

This journal is ª The Royal Society of Chemistry 2011

introduced by the purification procedure itself. In particular,

impurities in reagents used for the last steps of the purification

procedure, like NH4I for the final plutonium elution, will

certainly be found in the final sample solutions. Investigations

are in progress to identify the main source of Pb, Ir and Hg in the

sample solutions. There is strong evidence that NH4I may be

responsible for a significant part of these contaminations. Use of

another reducing agent, like HBr or HI, is considered.

Conclusions

For three years we applied an original method for low-level

plutonium measurement using a very sensitive double focusing

sector-field ICP-MS (typically 2–4 million counts s�1 per ng ml�1

of plutonium). This method includes correction of polyatomic

interferences containing heavy elements from hafnium to

bismuth, which interfere at the masses of long-lived plutonium

isotopes 239Pu and 240Pu. We pointed out that the extents of

formation of these molecular species are highly variable, strongly

dependent on instrumental settings, and can vary over one or two

orders of magnitude from one analysis to another. Such extents

of formation are very low for most of the heavy elements and

should not lead to significant additional background in the

actinide mass range. Exceptions are mercury and iridium, which

show higher extents of formation for ArHg+ and IrO3+, respec-

tively of the order of 10�5 and 10�6, and may produce significant

additional background for limited concentrations—in the mg l�1

range—of the elements in sample solutions. The extents of

formation of PbO2+ molecular species are low, in the order of

10�9, but lead concentrations in sample solutions can be rela-

tively high, up to the mg l�1 range. Finally, we observed that the

corresponding additional background induced by these molec-

ular species is generally of the same order as, and in many cases

higher than, other usual sources of background for low-level

plutonium analysis by ICP-MS, i.e. electronic background, 242Pu

tracer isotopic impurities, 238U hydrides and peak tail. Thus, if

not corrected, these polyatomic interferences may lead to false

detection of femtogram amounts of plutonium or overestimation

of results.

Correction of these polyatomic interferences using the specific

methodology described in this paper serves to improve accuracy

and avoid false detections. However, corrections are not

a panacea as these corrections are tainted with analytical error

and result in an increase in the detection limits. A better way is of

course to minimise the concentrations of heavy elements in

sample solutions and/or the extents of formation of molecular

species. For this, the instrument should be optimised not only for

maximum analyte signal, but also for minimum oxide ratio to at

least minimise oxide interferences (PbO2+, IrO3

+, etc.) during

plutonium analysis. More generally, a thorough study of the

instrumental settings that minimise specific polyatomic interfer-

ences should be undertaken. In this respect, a desolvating

introduction system which in addition to enhancing the sensi-

tivity may decrease the oxide ratio, will be tested soon. Another

way is to use the high resolution capability of our instrument to

separate plutonium isotopes from PbO2+, ArHg+, IrO3

+ molec-

ular species (a moderate resolution of about 3500 is enough for

this) combined with a high-efficiency desolvating introduction

system to compensate at least in part for the loss of transmission

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in the medium-resolution mode. Lastly, our purification proce-

dures based on anion-exchange chromatography provide a very

efficient elimination of most of the heavy elements. However,

some of them are apparently introduced by the atmosphere, the

reagents and the glassware during the chemical purification.

Efforts must therefore be made to improve the chemical proce-

dures by using lower quantities of reagents, selecting purer

reagents and protecting the samples from atmospheric contam-

ination through use of clean-room-like conditions.

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