round-robins in the area of uranium and plutonium bulk analysis of environmental samples

Round-robins in the area of uranium and plutonium bulk analysisof environmental samples

Fabien Pointurier • Ross W. Williams •

Stephen P. LaMont • Robert E. Steiner •

Debbie A. Bostick • Khris B. Olsen • Ned A. Wogman

Received: 4 July 2012 / Published online: 8 August 2012

� Akademiai Kiado, Budapest, Hungary 2012

Abstract In the framework of a collaboration between

laboratories involved in bulk U and Pu analysis of envi-

ronmental samples (DIF centre of the French Commissariat

a l’Energie Atomique, US National Laboratories of New

Brunswick, Lawrence Livermore, Pacific Northwest, Oak

Ridge, and Los Alamos), two round-robins were organised,

each one consisting of the complete analysis (chemical

preparation and isotope measurement) of three Quality

Control samples. The samples were 10 9 10 cm cotton

tissues (‘‘swipe samples’’) containing low amounts of U

(from *20 to *150 ng) and Pu (from *0.15 to *10 pg).

Despite using different spikes, different methods of sample

preparation and different analytical instrumentation, the

results for U and Pu contents and isotopic compositions

reported by all laboratories are globally in good agreement.

All laboratories are able to measure sub-pg amounts of U

and Pu isotopes with acceptable accuracy and reproduc-

ibility, even if limited discrepancies are observed affecting

one or other measurement and/or laboratory. General and

laboratory specific recommendations were discussed and

adopted to continue to improve the accuracy and precision

of the measurements.

Keywords Uranium � Plutonium � Round-robin �Inter-comparison � Swipe samples � Bulk analysis

Introduction

Action Sheet 4 (AS 4) between the United States Department

of Energy/National Nuclear Security Administration (DOE/

NNSA) and the Commissariat a l’Energie Atomique (CEA) of

France for cooperation in the area of Bulk Environmental

Sampling was signed in July 2007. All participating labora-

tories are members of the IAEA’s NetWork of Analytical

Laboratories (NWAL) for bulk analysis. Environmental

sampling is considered to be one of the key tools in safeguards

for the detection of undeclared activities. Inspectors from the

IAEA (International Atomic Energy Agency) collect small

amounts of nuclear material (U, Pu, fission products, etc.) by

wiping various surfaces inside or around nuclear facilities,

using small pieces of cotton cloth, referred to as ‘‘swipe

samples’’. These samples are sent for analysis to a few labo-

ratories that are members of the NWAL of the IAEA in sup-

port of the safeguards. When ‘‘U–Pu bulk analysis’’ is

requested by the IAEA, the whole ‘‘swipe sample’’ is reduced

to ashes, digested, U and Pu are purified from the matrix and

from various impurities, and U and Pu content as well as

isotopic ratios are measured using mass spectrometry tech-

niques. The laboratories involved in the collaboration are the

following: the DIF centre of the CEA (CEA/DIF), Lawrence

Livermore National Laboratory (LLNL), Pacific Northwest

National Laboratory (PNNL), Oak Ridge National Laboratory

(ORNL) and Los Alamos National Laboratory (LANL).

F. Pointurier (&)

CEA, DAM, DIF, 91297 Arpajon, France

e-mail: [email protected]

R. W. Williams

DOE/LLNL, P.O. Box 808, Livermore, CA 94551, USA

S. P. LaMont � R. E. Steiner

DOE/LANL, MS-J514, P.O. Box 1663, Los Alamos, NM

87545, USA

D. A. Bostick

DOE/ORNL, MS-6050, P.O. Box 2008, Oak Ridge, TN

37831-6050, USA

K. B. Olsen � N. A. Wogman

DOE/PNNL, P.O. Box 999, Richland, WA 99354, USA

123

J Radioanal Nucl Chem (2013) 296:599–608

DOI 10.1007/s10967-012-1985-6

In the framework of AS 4, laboratories have exchanged

their analytical procedures related to both sample prepa-

ration (chemical purification) and isotopic measurement,

and organised two round-robins (RRs) (i.e. analysis of

Quality Control Samples—QCS). First RR took place from

January to September 2008 and the second one from July

2010 to March 2011. Additionally, several meetings were

organised, to discuss analytical procedures, results, rec-

ommendations, etc. These RRs have several specificities.

At first, no target values are defined, as masses of U and Pu

added to the samples were not certified. Therefore, results

are compared to the global average. Second, despite pre-

cautions, there is no guarantee that samples are perfectly

identical. However, considerable care was taken to prepare

homogeneous and uniform samples for distribution in these

RRs, so that differences between results can reliably be

attributed to differences in the metrology. The excellent

agreement observed between most of the quantification

results, both for U and Pu, suggests that samples may be

considered identical both for mass of nuclear material and

for isotopic composition within the precision of the mea-

surements. Third, these exercises, in contrast to other

international RRs [1–4], involve chemical purification and

extraction of U and Pu. Because all the laboratories have to

carry out a chemical preparation, discrepancies can be due

to both errors in isotopic measurements and to contami-

nation during the chemical purification procedure. Fourth,

the amounts of U and Pu contained in the samples for these

RRs were uniquely low. Finally, total Pu contents ranged

from *0.15 to *10 pg, and total U contents ranged from

*20 to *150 ng. Lastly, the type of sample, the ‘‘swipe

samples’’ which consist in 10 9 10 cm cotton tissue,

identical to the ones used by the IAEA to collect dust in

inspected facilities, is not common for RR samples. Other

RRs usually consist of nuclear materials [3, 4] or solutions

containing high concentrations of actinides [1, 2], or urine

samples [5, 6]. However, all participating laboratories, as

members of the NWAL, are accustomed to analysing such

samples.

This paper is focused on the two RRs carried out in the

framework of this collaboration. The RR samples were

prepared at LLNL by adding acidic solutions containing U

and Pu to the cotton swipes. The swipes were dried, placed

in double plastic bags, labelled, and sent to the partici-

pating laboratories as environmental samples. In this paper,

procedures and instruments used by the laboratories are

briefly presented. Then, results obtained by the laboratories

are described and commented upon, both for U and Pu

quantifications and isotope composition measurements.

The last part of the paper gives the conclusions drawn from

this collaboration, expresses recommendations to improve

laboratory performances for bulk analysis of environmental

samples. In general, despite using different spikes, different

methods of sample preparation and different analytical

instrumentation, the differences between the results

reported by all laboratories are quite small. So, this report

focuses on those small differences to aid each laboratory to

continue to improve their analytical methods, and serves as

a benchmark for further RRs or analytical developments.

Experimental

Methods used by all the participating laboratories are

similar. They basically consist in a two-step process: (i) a

chemical purification to eliminate most of the sample

matrix and to concentrate as much as possible the elements

of interest (U, Pu), and (ii) an isotope measurement using a

mass spectrometry technique. All procedures, for sample

preparation and for mass spectrometry measurement as

well, are fully documented in QA written procedures.

Comparison of the chemical purification treatment

For all of the participating laboratories, the chemical

purification step starts with combustion of a sample swipe

into ash in an oven, and is followed by dissolution and

chromatographic separation of U and Pu from the major

elemental components of the matrix thanks to an oxida-

tion—reduction cycle. However, type and quantity of resin,

as well as types, volumes and concentrations of reagents

vary from one laboratory to the other.

Main features, common points and differences between

labs of these chemical purification treatments are given

hereafter. All labware is single-use. All DOE labs use

quartz, Teflon and/or poly fluoro alkyl (PFA) labware,

whereas CEA uses borosilicated glass. The combustion to

ash step is carried out by all laboratories in dedicated

ovens, although a large range of temperature, from 400 to

750 �C, is defined. Regarding the chemical separation

technique, all laboratories use chromatography to separate

U and Pu from the sample matrix. However, some labo-

ratories use solid phase extraction chromatography (PNNL

[7], LLNL, ORNL), whereas the others use ion exchange

chromatography (CEA [8], LANL). It should be mentioned

that all laboratories clean the resins beforehand. Quantities

of resin used by the labs are rather small: from 0.5 to 2 mL

for DOE labs. However, for the initial U/Pu separation,

CEA uses a 20-mL column filled with resin. All laborato-

ries use the isotope dilution technique to quantify U and

Pu. 233U is used by all labs for U quantification, whereas242Pu (CEA, LANL) or 244Pu (LLNL, PNNL, ORNL) can

be used for Pu quantification. It is preferable to use 244Pu

rather than 242Pu because environmental samples never

contain 244Pu but usually contain small amounts of 242Pu.

All tracers are as pure as possible, although they all contain

600 J Radioanal Nucl Chem (2013) 296:599–608

123

traces of other isotopes from the same element. These

contributions are corrected for and amounts of tracers

added to the samples are limited, both for economic rea-

sons and for limiting degradation of the accuracy and

precision of isotope measurements due to correction from

tracer impurities. It should be noted that some laboratories

(ORNL, PNNL) sample an aliquot of each sample after

dissolution to screen the U and Pu concentrations in the

samples. CEA/DIF, LANL (most of the time), LLNL, and

ORNL perform at first a U–Pu separation, and then purify

separately on U and Pu fractions. On the contrary, for

PNNL and LANL in particular cases (high Pu and/or U

contents), samples after dissolution are divided in two

parts, one for U the other for Pu. All labs use various acids

of ultra-pure grade, and all laboratories take many pre-

cautions to protect samples from airborne contamination

during critical steps. Most of them use laminar flow hoods

to protect sample from airborne contamination. LANL,

LLNL and ORNL even use clean room facilities for sample

preparation. All laboratories consider that their sample

preparation techniques based on chromatography with

limited volumes of resins are the most adaptable to the

trace measurement of actinides (low blank levels are

achievable and smaller amounts of reagents are needed).

Comparison of the mass spectrometry measurements

The participating laboratories use two types of mass

spectrometers: inductively coupled plasma-mass spec-

trometers (ICP-MS) and thermal-ionisation mass spec-

trometer (TIMS). All laboratories have at their disposal

several more or less recent high performance ICP-MS and/

or TIMS. These are quadrupole-based ICP-MS (CEA/DIF

for U [9], PNNL, LANL), double focusing single-collector

ICP-MS (CEA/DIF for Pu) or multi-collection ICP-MS

(LLNL, PNNL, ORNL) or TIMS (LANL). Common trends

in analytical methodology include the fact that most of the

instruments now used for sample analysis are ICP-MS.

Sample introduction into the ICP-MS is commonly

achieved with the use of micro-nebulizers, desolvating

membranes to enhance sensitivity of small volumes of

solutions (mL range) by concentrating as much as possible

the elements of interest. Moreover, laboratories tend now

to be equipped with multiple collector instruments, which

allow improving reproducibility of isotope ratios and

measurement of very low abundant isotopes like 236U in

some samples. All laboratories, except CEA/DIF, now use

multiple collector instruments for bulk analysis, and carry

out multiple ion–counting whenever possible. Other com-

mon practices are the use of home-made software for cal-

culations, including an uncertainty budget, instead of

software provided by the manufacturer with the instrument.

In the same way, all laboratories correct raw data for U

hydrides, both at the mass to charge ratio of 236U for 235UH

and at the mass to charge ratio of 239Pu for 238UH. Besides,

LLNL, ORNL and CEA/DIF consider—and correct for if

appropriate—the possible influence of various polyatomic

species at the mass-to-charge ratios of Pu isotopes which

may cause significant isobaric interferences when Pu is in

the fg (10-15 g) range [8, 10]. Lastly, all laboratories

correct raw signal counts from background induced by

peak tailing from high intensity neighbouring peaks (gen-

erally 238U). Thus, all participating laboratories are in

compliance with what can be called ‘‘good laboratory

practices for precise and accurate measurements of isotope

ratios’’.

Description of the RR samples

The RR samples were prepared at LLNL in a clean envi-

ronment by adding acidic solutions containing U and Pu to

the cotton swipes. The swipes were dried, placed in double

plastic bags, labelled, and sent to the participating labora-

tories as environmental samples. The activities of U and Pu

in these samples were much lower than the definitions of

radioactive material for international and domestic US

shipping, and the samples were sent by common carrier.

Each laboratory was sent a duplicate set of samples.

Statistical tests

Statistical tests were carried out to detect outlying obser-

vations. In this study, the following statistical tests were

carried out: (i) Comparison with arithmetic mean with

variance estimated using the sample; (ii) comparison with

arithmetic mean: Grubbs test; (iii) Dixon’s test. The cor-

responding test statistic functions and their distributions for

various levels of significance (5 and 1 % in this study) are

given in [11].

Results

Masses of U and Pu are expressed in ng and in pg. Isotopic

ratios are expresses as atomic ratios. The uncertainties are

expanded standard uncertainties, given with a coverage

factor of 2 (*95 % confidence level assuming a Gaussian

distribution of the results). The following abbreviations are

used below: QCS1-1, QCS1-2 and QCS1-3 refer, respec-

tively to QC samples of the RR #1; QCS2-1, QCS2-2 and

QCS2-3 refer, respectively to QC samples of the RR #2;

PB1 and PB2 refer to ‘‘Process Blank’’ (or ‘‘Reagent

Blank’’) of the 2 RRs. BS1 and BS2 refer to ‘‘Blank

Swipes’’, i.e. cotton swipes distributed with the QCS, for

the RR #1 and #2; lastly, NU refers to natural U. For

confidentiality reasons, names of the five participating

J Radioanal Nucl Chem (2013) 296:599–608 601

123

laboratories are replaced by letters ‘‘A’’ to ‘‘E’’. Average U

and Pu masses, and average U and Pu atomic ratios mea-

sured by the participating laboratories for the 2 RRs are

given in Table 1. Standard uncertainties for each value are

calculated as the standard deviations over all the results

obtained by the participating laboratories. However, only

average values for which reasonably low relative combined

uncertainties were obtained are given: the relative expan-

ded uncertainties with a coverage factor of 2 should be

below 100 %.

Plots of normalised results for all QCSs from the 2 RRs

are given in Fig. 1 for U analyses and in Fig. 2 for Pu

analyses. All results are normalised to the average values

calculated from the results obtained by all laboratories for

the same QCS. Only the results for which relative expan-

ded uncertainties are below 25 % in the case of U analyses

and below 100 % in the case of Pu analyses are plotted.

However, a Pu contamination occurred during the prepa-

ration process of RR #2, more precisely when the cotton

samples were packaged. Consequently, all laboratories

detected various Pu amounts in the BS2 samples. As this

contamination is likely variable from one QCS to the other,

it is not possible to draw conclusions from these mea-

surements. Therefore, only results from the RR #1 are

plotted in the case of Pu analyses. Results from statistical

tests are given in Table 2 for U and Pu analyses. Main

comments about results are given below.

U quantification

U contents in BSs are rather homogeneous, between 4.2

and 4.6 ng for the RR #1 and *3 ng for the RR #2. This U

content is not negligible and this prevents proper mea-

surements if only low amounts of U (hundreds of pg to a

few ng) are sampled. This is not surprising as cotton used

for these RRs is known for containing a relatively high

amount of NU. Good agreements are observed between all

laboratories for all the samples, although results are less

homogeneous for the RR #2 than for the RR #1. However,

results from laboratory C for all QCSs of the RR #1 are

slightly lower than the average values although the dif-

ferences are not significant. For the RR #2, laboratory B

shows a tendency to be slightly overestimated with respect

to the average values. According to statistical tests, no

outliers are detected.

234U/238U atom ratios

It should be noted that laboratories B and C probably

overestimated 234U/238U ratios in the PBs. This can be

due to contamination or to inaccurate corrections from

background or isotopic dilution tracer. 234U/238U ratios Ta

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123

measured in the BS are in relatively good agreement

(considering the low 234U amounts in the samples) for all

laboratories. However, the average 234U/238U average ratio

is surprisingly high (*9 9 10-5) in comparison to the

common ratio for NU (*5.5 9 10-5) even if it is well

known that this isotopic ratio can vary largely in the

environment because of geochemical effects. For all QCSs

from both RRs, results are in good agreement. Results from

lab E are slightly above the average ratios for all QCSs of

the RR #1 and also significantly below the average value

for QCS2-3 of the RR #2. Results for QCS1-2, QCS1-3 and

QCS2-3 obtained by lab E are identified as outliers

according to statistical tests. Moreover, these results from

lab E have also higher uncertainties than results from other

laboratories. By contrast, results from labs B and D show

an excellent reproducibility and accuracy (with respect to

the average ratios).


Most laboratories measure 235U/238U in the PBs within

uncertainty of NU ratio (7.25 9 10-3). However, the

results from laboratories B, D and E are all slightly elevated

with respect to NU. Regarding the BSs, all laboratories

recovered the 235U/238U NU ratio, except lab D for which

result for BS1 shows an excellent reproducibility but is

clearly slightly elevated relative to the average. A possible

explanation is contamination by extraneous U during the

chemical preparation process. For all QCSs from RR #1,

most of the results are globally in good agreement. How-

ever, the result from lab D is slightly above average for

QCS1-1 and the result from lab A is slightly below average

for QCS1-3. This latter result is identified as an outlier

according to statistical tests. For QCSs from RR #2, most of

the results are also globally in good agreement. However,

biases for specific samples with respect to average values

are observed for lab C (QCS2-1, ?1.2 %), lab D (QCS2-1,

-2.4 %, identified as outlier according to statistical tests),

and lab E (QCS2-3, -2.8 %, identified as outlier according

to statistical tests). These non-systematic biases can result

from inaccurate correction from various effects, or by ran-

dom contamination during the sample spiking—packaging

process and during the sample treatment.


Two laboratories (B and D) detected 236U in the PBs and in

the BSs. These anomalous detections may result from

inaccurate corrections (235U hydrides, tailing from the 238U

peak, isotope dilution tracer impurities, background, etc.)

or contamination during the chemical purification process.

Laboratory E also systematically reported negative isotopic

ratios for both PBs and BSs. Regarding QCSs from RR #1,

only lab D was able to detect a very low (*2 9 10-7; i.e.

*5 fg of 236U) significant 236U/238U ratio in QCS1-1. As

the detected isotopic ratio is close to the ones detected in

PB1 and BS1, the hypothesis of a contamination by

extraneous U during the chemical preparation process

cannot be ruled out. Nevertheless, results of all laboratories

are in good agreement for QCS1-2 and QCS1-3. This

proves the ability of the participating laboratories to mea-

sure 236U/238U ratios in the 10-6–10-5 range. Although the

Fig. 1 Plots of all significant results for U content and U isotope ratio

measurements obtained by the laboratories for the 2 RRs. All results

are normalised to the average values calculated from the results

obtained by all laboratories for the same sample. Only the results for

which relative expanded uncertainties are below 25 % are plotted.

The results are given in the following order from left to right for each

laboratory: QCS1-1, QCS1-2, QCS1-3, QCS2-1, QCS2-2 and QCS2-

3. Uncertainties are expanded uncertainties (coverage factor of 2)


123

most precise of all, results from lab D are significantly

below the average ratios for QCS1-2 and QCS1-3, and may

be slightly underestimated with respect to the other labo-

ratories. However, these values do not appear as outliers

according to statistical tests. Regarding QCSs from RR #2,236U/238U ratios are very different from one sample to the

other: *3 9 10-4 in QCS2-1, *10-3 in QCS2-2, and an

extremely low value for QCS2-3. A good agreement

between laboratories is obtained for QCS2-1 and QCS2-2.

Laboratories B and E reported negative values for QCS2-3,

which is probably due to overcorrection from background.

The results from lab E is regarded as an outlier according to

statistical tests.

Pu quantification

One laboratory (D) reported detection of Pu in PB1 and

BS1 (resp. *15 and *30 fg), whereas laboratories B and E

reported negative values for PB1 and BS1. Results of the 3

QCSs from RR #1 are in good agreement, although results

Fig. 2 Plots of all significant

results for Pu content and240Pu/239Pu ratio measurements

obtained by the laboratories for

the RR #1. No results were

taken into account for the RR #2

as a Pu contamination occurred

during the sample packaging.

All results are normalised to the

average values calculated from

the results obtained by all

laboratories for the same

sample. Only the results for

which relative expanded

uncertainties are reasonably low

(below 100 %) are plotted. No

results were given for the241Pu/239Pu and 242Pu/239Pu

ratios as no average values can

be calculated. The results are

given in the following order

from left to right for each

laboratory: QCS1-1, QCS1-2,

and QCS1-3. Uncertainties are

expanded uncertainties

(coverage factor of 2)

Table 2 Outlying values according to the statistical tests for U and Pu analyses, for both RRs

U (ng) 234U/238U 235U/238U 236U/238U Pu (pg) 240Pu/239Pu 241Pu/239Pu 242Pu/239Pu

BS1 – – – – – C – –

QCS1-1 – – – – – – B –

QCS1-2 – Ea – – – – – C

QCS1-3 – E A – – C – –

BS2 – – – – No test No test No test No test

QCS2-1 – – D – No test No test No test No test

QCS2-2 – – – – No test No test No test No test

QCS2-3 – E E E No test No test No test No test

The tests were not applied to Pu results of the RR #2, because of a Pu contamination occurred during the packaging process. If not outlying

values are detected, the corresponding cell is filled with a hyphena Only according to Dixon’s test


123

from lab E are almost systematically significantly lower

than the average values. This may be a spike calibration

issue or result from inaccurate measurement of the mass of

isotopic dilution tracer added to the sample.

240Pu/239Pu atom ratios

Results from all laboratories are in very good agreement

for QCS1-1 and QCS1-2. However, some discrepancies

between results are observed for QCS1-3. This is not sur-

prising as the quantity of 240Pu is lower for this QCS

(\10 fg) than for the two others QCSs (*400 fg for both

of them). Lab C measured 240Pu/239Pu ratio of (*2.0 ±

0.4) 9 10-2 in QCS1-3 is significantly higher than the

average value of *8 9 10-3, whereas lab B measured

ratio of (*1.7 ± 4.2) 9 10-3, despite its very large

uncertainty, is significantly lower than the average. The

results from lab C for BS1 (-4.0 ± 13.4) and for QCS1-3

are identified as outliers according to statistical tests. By

contrast, low uncertainty and excellent accuracy with

respect to the average ratios of lab D should be underlined.


Laboratory D detected 241Pu in PB1 and BS1 (resp. *6

and *9 fg), whereas lab C reported a negative 241Pu/239Pu

isotopic ratio in PB1. At this time, lab A was not able to

measure 241Pu and did not report any result. Only lab D

detected a significant amount of 241Pu in QCS1-3 (*2 fg).

Although most of the relative expanded (k = 2) uncer-

tainties are higher than 100 %, results from laboratories are

not fully consistent. Laboratories C and E tend to report

negative values, which is probably due to overcorrection

from background or other bias effect. The result from lab B

for QCS1-1 ((1.0 ± 0.6) 9 10-3) is identified as an outlier

according to statistical tests.


Laboratories A and E did not report any 242Pu/239Pu ratios

as these two laboratories use 242Pu as the isotope dilution

tracer. Laboratories C and D detected 242Pu in PB1

(*7 fg). Laboratory D is the only lab which reported

detection of significant 242Pu/239Pu ratios (respectively

(2.0 ± 0.3) 9 10-4, (1.1 ± 0.2) 9 10-3, and (4.5 ± 0.7)

9 10-3 for QCS1-1, QCS1-2 and QCS1-3, which corre-

spond to respective masses of 242Pu of *2 fg, *5 fg, and

*5 fg). Results from laboratories B and C show very large

relative expanded (k = 2) uncertainties and are often

negative. The result from lab C for QCS1-2 ((-4.9 ± 2.6)

9 10-3) is identified as an outlier according to statistical

tests.

Comparison of the relative standard uncertainties

Average relative standard uncertainties (RSU) obtained by

the laboratories for all U isotope ratio measurements

(234U/238U, 235U/238U and 236U/238U) and Pu isotope

measurements (240Pu/239Pu) for the 2 RRs are given,

respectively, in Figs. 3 and 4. Uncertainty budgets are not

compared for 241Pu/239Pu and 242Pu/239Pu measurements

as most of the results were not significant. Average RSUs

obtained by the laboratories for all U content and Pu

content measurements for the 2 RRs are given in Fig. 5.

Regarding the RSUs for U atom ratio measurements,

laboratories B and D, and to a lesser extent laboratory C,

obtained the lowest standard uncertainty budgets for isotope

ratio measurements, probably thanks to the excellent

reproducibility achievable with the multiple collector

instruments used by these laboratories. Lowest relative

CSUs were close to 0.1 %. Uncertainty budgets obtained by

labs A, C and E, for isotope ratio measurements are in the

same range. The poorest precisions are obtained by lab E,

Fig. 3 Average relative

standard uncertainty (RSU)


all U isotope ratio

measurements for the 2 RRs.

Average RSUs for each

laboratory are given in the

following order: 234U/238U for

the RR #1, 234U/238U for the RR

#2, 235U/238U for the RR #1,235U/238U for the RR #2,236U/238U for the RR #1, and236U/238U for the RR #2. Each

value is the average of the RSU

calculated for the three

corresponding QCS


123

and, to a lesser extent, by lab A. This is probably due to the

use of single collector instruments by these two laborato-

ries. However, relative CSUs are globally very low taking

into account the low amounts of U in the samples: they

range from *0.4 to *9 % (average value of *3 %) for the234U/238U ratios, from *0.1 to *1.4 % (average value of

*0.45 %) for the 235U/238U ratios and from *0.4 to *8 %

(average value of *1.7 %) for the 236U/238U ratios.

RSUs for 240Pu/239Pu ratio measurements obtained by

the participating laboratories are rather homogeneous. For

the RR #1, they vary from *0.6 % (lab C) to *2 % (labs

A and D), with an average value of *1.3 %. RSUs of the

RR #2 are larger than the ones of the RR #1, because

amounts of Pu isotopes are significantly lower. They range

from *1.8 % (lab B) to *11 % (lab A), with an average

value of *5.6 %. Laboratories equipped with multi-col-

lector instruments obtain the lowest RSUs, provided the Pu

content is sufficient. Relative RSUs for total U content

determination are remarkably low for both RRs: they range

between *0.2 and *2 % with an average value of *1 %.

Higher and more variable RSUs are observed for total Pu

content measurements, especially for RR #2, for which

amounts of Pu isotopes are extremely low: average RSUs

for all labs are *1.5 % for RR #1 and *5 % for RR #2.

Discussion

Measurement of isotopic ratios of both U and Pu at low

concentrations is challenging for all laboratories, consid-

ering the extremely low amounts of both actinides in the

RR samples. In such conditions, concentrations of the

minor isotopes are in the fg to the pg range. However, in

general, the laboratories’ reported concentrations of U

and Pu in the RR samples were remarkably consistent.

Although this does not imply accuracy, taken as a whole, it

attests to very good agreement between the laboratories’

tracer (spike) calibrations.

It should be noted that some participating laboratories

measure elevated, non-natural 234U/238U, 235U/238U and236U/238U in PBs. In other aspects, all laboratories measure

elevated 234U/238U in blank cotton swipes relative to the

value for secular equilibrium. This observation can be

explained by the well-established fact of 234U/238U

enrichment in natural waters (both groundwater and river

water) that might be used for irrigation of cotton plants. All235U/238U measurements of the RR samples are very con-

cordant. The 236U/238U measurements of samples QCS1-2,

QCS1-3, QCS2-1 and QCS2-2 at approximately 5.1 9 10-5,

3.9 9 10-5, 3.4 9 10-4, and 10-5 are also concordant and

Fig. 4 Average relative standard uncertainty (RSU) obtained by the

laboratories for all 240Pu/239Pu isotope ratio measurements for the 2

RRs. Average RSUs for each laboratory are given in the following

order: 240Pu/239Pu for the RR #1 and 240Pu/239Pu for the RR #2. Each

value is the average of the RSU calculated for the three corresponding

QCSs

Fig. 5 Average relative

standard uncertainty (RSU)


all U and Pu content

measurements for the 2 RRs.

Average RSUs for each

laboratory are given in the

following order: U content for

the RR #1, U content for the RR

#2, Pu content for the RR #1,

and Pu content for the RR #2.

Each value is the average of the

RSU calculated for the three

corresponding QCSs


123

all laboratories could resolve this small difference. For these

samples, this translates to a demonstrated ability to measure

approximately 1 pg of 236U in the presence of 40 ng of 238U.

Where the 240Pu was larger than the laboratories’ individual

detection limits, the 240Pu/239Pu ratios are also very con-

cordant, i.e., for samples QCS1-1 and QCS1-2. All labora-

tories are able to measure Pu isotopic composition at the

sub-pg level. So, globally, results of the participating lab-

oratories are of high quality and demonstrate the laborato-

ries’ ability to measure precisely and accurately U and Pu

isotope compositions in the ‘‘swipe samples’’.

General and particular recommendations were discussed

and adopted to improve the accuracy and precision of the

measurements. General recommendations mainly consists

in investigating some subtle effects that may, if not properly

corrected, lead to small bias in isotope ratio measurements.

For instance, laboratories exchanged and discussed their

procedures for dead time correction. It is also recommended

all labs should break down their uncertainty budgets and

explain their calculation methods to estimate their sources

of errors, as in [12]. More generally, laboratories should

consider whether their reported uncertainties may be either

underestimated (have all the uncertainties on the compo-

nents of the results been considered?) or overestimated (are

components counted twice or estimated to be too large?).

A comparison of uncertainty budget calculation between two

laboratories (A and B) each applying its own calculation

procedure and starting from exactly the same set of raw data,

shows that uncertainties can differ in some cases by a factor

of two between the two laboratories.

Several specific recommendations for each laboratory

were proposed. Some of them are related to mass spec-

trometry measurements, for instance checking that count

rates stay in the linearity range of the detector (lab A),

improving the measurement protocol for optimising iso-

tope ratio measurements (lab C) and for better correcting

bias effects (labs C and E). Other recommendations con-

cern the chemical preparation: reducing the U level in

blanks by reducing amounts of chromatographic resins (lab

A), and avoiding cross-contaminations or memory effects

in the laboratories (labs B, D). Lastly, laboratories C and E

obviously overestimated uncertainties and should review

and re-evaluate sources of uncertainties for U isotopic ratio

measurements.

Conclusions

Results obtained in the frame of the two RRs by the partic-

ipating laboratories are globally in very good agreement,

except for Pu analysis in the case of RR #2, which is

explained by Pu contamination during sample packaging.

All laboratories prove their ability to analyse sub-pg amounts

of Pu and ng amounts of U with acceptable precision and

accuracy. Improvements in reproducibility and capability to

detect small isotopic differences between samples with

multi-collector instruments are clearly demonstrated. How-

ever, to translate improvement in reproducibility to more

accurate measurements requires optimised procedures, cor-

rection of all biases and high precision isotopic standards.

Although the participating laboratories have state-of-the-art

practices and large experience in this field, a few discrep-

ancies are observed for specific results, affecting one labo-

ratory or the other. Some of the participating labs, that have

to manage routine safeguard samples which contain larger

amounts of nuclear materials, may encounter limited con-

taminations by non-natural U and/or Pu from other samples.

Possible improvement and specific recommendations

were proposed for each laboratory. Common needs were

also identified. The main concern of the participating lab-

oratories is calculation of the uncertainty budgets. Close

examination of the results gives the feeling that some

uncertainties may be overestimated (results perfectly match

the average values but have surprisingly large uncertain-

ties) or underestimated (results with very low uncertainty

are biased with regards to the average values). However,

there is no ‘‘on-the-shelf’’ procedure for uncertainty cal-

culation by mass spectrometry, but only general recom-

mendations and guidelines provided by documents like

Guide to the expression of Uncertainty in Measurement

(GUM). As participating laboratories considered that soft-

ware provided with the instruments by the manufacturers is

not satisfactory for uncertainty calculations, each labora-

tory developed individually its own home-made software.

All laboratories deem it necessary to check, and, if nec-

essary, to improve their uncertainty calculations. There is

also a strong need for very high purity tracers. For instance,

some of the participating laboratories do not have at their

disposal high purity 244Pu, which would allow all labora-

tories to measure 242Pu. Similarly, there is a need for

environmental reference materials for which both actinide

content and isotopic composition are certified.

Acknowledgments Many thanks to all the people who make this

collaboration possible and to the people who actively participate in

the collaboration, through meetings, administrative matters, and, of

course through technical work in the facilities. Special thanks to Ross

Williams and his colleagues from LLNL, who assumed the difficult

task to prepare and dispatch the QC samples.

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