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

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Measuring 0.01& to 0.1& isotopic variationsby MC-ICPMS—testing limits for the first time with Pb d-iCRMs†

Christophe R. Qu�etel,*a Emmanuel Ponzevera,ab Ilia Rodushkin,c Axel Gerdes,d Ross Williamse

and Jon Woodheadf

Received 28th November 2008, Accepted 6th February 2009

First published as an Advance Article on the web 3rd March 2009

DOI: 10.1039/b821403b

A blind comparison on Pb-isotope d-scale measurements by MC-ICPMS of 0.01& to 0.1& level was

organised, involving five laboratories. Test samples were obtained from the series of candidate

ERM-3810 d-isotopic Certified Reference Materials (d-iCRMs), and comprise four pairs of a material

with �natural Pb-isotopic composition (‘delta zero’ or ‘d-0’) and the same natural Pb progressively

enriched in 207Pb (with d207Pb values certified to �0.1% relative uncertainty, k ¼ 2). Participants were

free to apply the measurement strategy of their choice. A result was considered ‘acceptable’ only when,

simultaneously, there was agreement within stated uncertainties with the corresponding reference value

and the relative uncertainty stated by the participant was < 100%. This study illustrates the high degree

of difficulty inherent to these d-scale measurements by ‘routine’ MC-ICPMS methodologies (in this

case, three participants reported 55% of their results which were deemed accurate, and the other two

reported none). The closer to unity the isotope ratio value the better the results became (‘acceptable’

results mostly for d7/6& and d7/8& measurements). This first experiment of its kind demonstrates that

Pb d-scale isotopic measurements by MC-ICPMS can be reliably carried out down to 0.05& levels (two

participants delivered accurate results above this threshold systematically for d7/6&, d7/8& and

d7/4&). Below this limit, at �0.01& and � 0.03& levels, results are no longer consistent or

reproducible and appear to be susceptible to a number of effects introducing error (such as short term

changes in mass discrimination) which are either not well understood, or not controlled and/or not

corrected for at a sufficiently low level of uncertainty. These results also suggest that ‘routine’ methods

for absolute (calibrated) Pb-isotope ratio determination by MC-ICPMS produce relative combined

uncertainties on results which are unlikely to be better than 0.05& (k ¼ 2).

Introduction

Multiple collector-inductively coupled plasma mass spectrometry

(MC-ICPMS) is increasingly used for d-scale isotopic measure-

ments to study and characterize pollution and environmental

issues,1–3 geological materials,4–10 human and animal and vegetal

metabolisms,11–15 chemical reactions16 etc. Owing to the flexibility

of the plasma ionization source, a wide range of samples matrices

and sizes can be accommodated and, ideally, users wish to be able to

investigate isotopic variations at the sub-& level. Working on

a relative isotope ratio scale is advantageous when symmetric and

similar analytical biases (i.e., those of a multiplicative nature and/or

aEC-JRC-IRMM, European Commission – Joint Research Centre –Institute for Reference Materials and Measurements, Retieseweg 111,B-2440 Geel, Belgium. E-mail: Christophe.Quetel@ec.europa.eubIFREMER, Z.I. Pointe du diable, BP 70 – 29280 Plouzan�e, FrancecALS Scandinavia, Aurorum 10, 97775 Lule�a, SwedendInstitut fuer Geowissenschaften, JW Goethe University, Altenhoeferallee1, D-60438 Frankfurt am Main, GermanyeLawrence Livermore National Laboratory, 7000 East Avenue, CA 94550,Livermore, USAfSchool of Earth Sciences, the University of Melbourne, 3010 Victoria,Australia

† This article is part of a themed issue dedicated to Professor Jean-MichelMermet, in recognition of his contributions to the field of atomicspectrometry.

This journal is ª The Royal Society of Chemistry 2009

influencing all ratios the same way) are cancelled in the measured

isotope ratio values of the d expression.17 However, MC-ICPMS

isotope ratio measurements are also susceptible to a range of

asymmetric effects that are significant at the & level, such as short

term variations in mass discrimination effects,18,19 matrix

effects,20,21 instrumental background (sample to sample contami-

nation),22 and isobaric interferences23 etc. Failing to implement

appropriate corrections may lead to errors similar to or larger than

the size of the isotopic variations under study, and indeed imple-

menting poorly understood corrections may also introduce

uncertainty to the d-scale result at a level that is not compatible with

the size of the isotopic variation to be measured. As a result some of

the apparent & level isotopic variations described in the literature

may be strongly influenced by analytical artefacts.

IRMM has produced by gravimetric mixing a series of a new

type of Pb isotopic Certified Reference Materials, described as

d-iCRMs, which may be used to validate methods developed for

d-scale isotopic measurements.24 These comprise four pairs of

ampoules containing natural-like Pb (candidate ERM-3800) in

one and the same natural Pb material progressively enriched in207Pb (candidate ERM-3810 series) in the others, producing

d207Pb values ranging from �0.01& to �0.1&, certified to

� 0.1% relative uncertainty (k ¼ 2).

This paper presents the initial results of a blind comparison

on d-scale isotopic measurements using these materials. Five

J. Anal. At. Spectrom., 2009, 24, 407–412 | 407

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MC-ICPMS laboratories were invited to participate in this

exercise and were free to apply the measurement strategy of their

choice.

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Experimental

Participants and measurement methods

The participants are the co-authors of this paper, representing at

the time of measurements 5 laboratories from 5 countries, all of

which reported MC-ICPMS measurement results. There were no

stipulations on the analytical method, and Table 1 summarises

the methodology utilised by each laboratory. Participants 2, 4

and 5 declared that they had applied ‘routine’ analytical proto-

cols ‘which are usually adequate for most geological problems’.

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Test samples

Materials of the candidate ERM-3810 series of d-iCRMs served

as test samples for this inter-laboratory comparison and were

distributed blindly, i.e. not sorted according to nominal reference

d207Pb values (and of course without preliminary disclosure of

these reference values). A detailed description of the way the

candidate ERM-3810 series was produced can be found else-

where.24 In brief, these materials are available in quartz ampoules

as 2 mL of 100 mg g�1 Pb solutions. Fortification with 207Pb was

achieved gravimetrically from � 10 mg Pb g�1 solutions, origi-

nating from the dissolution of weighed amounts of pure solid Pb

materials. The mixing scheme was designed to ensure that

uncertainties on weighing would represent the essential part

(80%) of the combined uncertainties estimated for the d207Pb

certified values. Weighing were always performed using substi-

tution measurements (no impact from potential non linearity

effects of the balance) against operational mass standards

traceable to the Kg in the shortest possible comparative way.

Humidity and temperature were controlled to correct for air

buoyancy effects. Relative uncertainties (k ¼ 2) obtained for the

solid and liquid weighing were 5.5 to 6 � 10�5 and 1.2 to 1.7 �10�4 respectively, leading to relative combined uncertainties

on d207Pb values �0.03–0.04% extended conservatively to 0.1%

(k ¼ 2). Reference values of the candidate ERM-3810 series of

4 d-iCRMs are reported in Table 2. Prepared under ultra-clean

conditions in 3% HNO3 these solutions taken individually do not

present particular analytical challenges and do not contain

compounds/elements producing interferences during Pb

measurements by ICPMS.

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Results reporting, including uncertainty statements

The only pre-requisite was on data format and participants were

asked to report their d7/4&, d7/6& and d7/8& results according

to eqn (1) below.

d7=x ¼" �207

Pb=20xPb�a�207

Pb=20xPb

�PbNat

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#� 103 (1)

with ‘a’ corresponding to any of the four isotopic mixtures (I, II,

III or IV), ‘x’ corresponding to 4 or 6 or 8, and ‘PbNat’ corre-

sponding to the natural Pb solution provided as the delta zero

(d-0) material for these measurements.

408 | J. Anal. At. Spectrom., 2009, 24, 407–412 This journal is ª The Royal Society of Chemistry 2009

Table 2 Results reported by participants and reference values (incl. expanded uncertainty statements, k ¼ 2) for the 4 pairs of the candidate ERM-3810series of d-iCRMs. All 5 participants are those indicated in Table 1

Mixture I versus PbNat Mixture II versus PbNat Mixture III versus PbNat Mixture IV versus PbNat

Results U (k ¼ 2) Results U (k ¼ 2) Results U (k ¼ 2) Results U (k ¼ 2)

Participant 1 d(7/4) 0.046 0.077 0.04 0.10 0.088 0.074 0.146 0.078d(7/6) 0.003 0.024 0.032 0.026 0.059 0.021 0.125 0.020d(7/8) 0.014 0.025 0.029 0.031 0.054 0.021 0.110 0.019

Participant 2 d(7/4) 0.203 0.030 0.262 0.028 0.276 0.027 0.293 0.029d(7/6) 0.081 0.022 0.101 0.021 0.126 0.021 0.176 0.022d(7/8) �0.064 0.034 �0.055 0.026 �0.038 0.026 0.019 0.026

Participant 3 d(7/4) 0.059 0.065 0.065 0.089 0.094 0.068 0.149 0.049d(7/6) 0.016 0.009 0.012 0.012 0.069 0.032 0.131 0.028d(7/8) 0.029 0.023 0.034 0.024 0.055 0.042 0.127 0.036

Participant 4 d(7/4) �0.36 0.17 �0.34 0.49 �0.16 0.41 0.26 0.65d(7/6) �0.04 0.17 �0.03 0.24 0.03 0.25 0.24 0.25d(7/8) 0.13 0.13 0.15 0.15 0.10 0.17 0.03 0.24

Participant 5 d(7/4) 0.090 0.130 0.146 0.156 0.122 0.122 0.105 0.167d(7/6) 0.043 0.029 0.026 0.013 0.083 0.014 0.117 0.037d(7/8) �0.012 0.066 �0.037 0.073 0.038 0.072 0.108 0.066

Certified referencevalues24

d(7/4) 0.012 827 0.000 013 0.028 535 0.000 029 0.054 005 0.000 055 0.107 75 0.000 11d(7/6) 0.013 090 0.000 015 0.029 126 0.000 030 0.055 126 0.000 056 0.109 99 0.000 11d(7/8) 0.013 108 0.000 013 0.029 162 0.000 029 0.055 195 0.000 056 0.110 13 0.000 11

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Uncertainty statements on results were requested according to

the expression U ¼ kuc(y) from the ISO guide on measurement

uncertainty,27 where uc(y) is a fully evaluated combined standard

uncertainty and k is a coverage factor equal to 2. It was explained

that ‘‘in practice (when the probability distribution characterized

by y and uc(y) is approximately normal and the effective number

of degrees of freedom of uc(y) is of significant size), one can

assume that taking k ¼ 2 produces an interval having a level of

confidence of approximately 95%, and that taking k ¼ 3

Fig. 1 a–d. Results reported by participants and reference values (incl. expand

Table 1.

This journal is ª The Royal Society of Chemistry 2009

produces an interval having a level of confidence of approxi-

mately 99%’’.

Results and discussion

Results of all participants are reported in Table 2 and Fig. 1a–d.

Performance evaluation per participant and per pair of

d-iCRMs (Table 3) was carried out according to the following

convention:

ed uncertainty statements, k ¼ 2). All 5 participants are those indicated in

J. Anal. At. Spectrom., 2009, 24, 407–412 | 409

Table 3 Performance per participant and per pair of the candidateERM-3810 series of d-iCRMs. All 5 participants are those indicated inTable 1

MixtureI versusPbNat

MixtureII versusPbNat

MixtureIII versusPbNat

MixtureIV versusPbNat OKs

Participant 1 d(7/4) — — OK OK 2d(7/6) — OK OK OK 3d(7/8) — — OK OK 2OKs 0 1 3 3 7

Participant 2 d(7/4) — — — — 0d(7/6) — — — — 0d(7/8) — — — — 0OKs 0 0 0 0 0

Participant 3 d(7/4) — — OK OK 2d(7/6) OK — OK OK 3d(7/8) OK OK OK OK 4OKs 2 1 3 3 9

Participant 4 d(7/4) — — — — 0d(7/6) — — — — 0d(7/8) — — — — 0OKs 0 0 0 0 0

Participant 5 d(7/4) — — — — 0d(7/6) OK OK — OK 3d(7/8) — — — OK 1OKs 1 1 0 2 4

OKs 3 3 6 8 20

Fig. 2 a–c. Results reported by participants 1, 3 and 5 (identical to those

reported in Fig. 1) and reference values (incl. expanded uncertainty

statements, k ¼ 2) sorted according to Pb ratio values.

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—a result was considered ‘acceptable’ when, simultaneously,

there was agreement within stated uncertainties with the corre-

sponding reference value and the relative uncertainty stated by

the participant was < 100%;

—a result was considered ‘not acceptable’ (i.e. not exploitable

for the measurement purpose) in any other case (no agreement

within stated uncertainties with the corresponding reference

value and/or the relative uncertainty stated by the participant

was $ 100%).

As can be seen from Table 3 and as expected, the lower the

d207Pb values the more complicated the measurements and the

worse the results. For pairs ‘Mixt I’ and ‘Mixt II’ (i.e. d207Pb

values � 0.01& and � 0.03&, respectively) accurate results were

obtained only in 3 cases (out of a total of 15 each time). The

outcome improves for pairs ‘Mixt III’ (6/15 accurate results) and

‘Mixt IV’ (8/15 accurate results), i.e. d207Pb values � 0.05& and

� 0.1&, respectively.

Fig. 2a–c show that, overall, the closer to unity the isotope

ratio value, the better the capacity of participants to measure

small size isotopic differences: most ‘acceptable’ results were

obtained for d7/6& measurements (9 out of a total of 20) and for

d7/8& measurements (7/20), while only 4/20 ‘acceptable’ results

were obtained for d7/4& measurements.

The fact that only 20 accurate results were obtained, out of 60

possible, is a clear demonstration of the high degree of difficulty

inherent to these d-scale measurements by ‘routine’ MC-ICPMS

methodologies, at least at such low levels of isotopic variation.

When results from participants 2 and 4 are removed (none are

‘acceptable’), the success rate rises only to � 55%.

For the pair ‘Mixt I’ the best outcome is from participant 3,

with ‘acceptable’ results for d7/6& and d7/8&. But participant 3

fails to repeat this performance for ‘Mixt II’ (agreement with the

reference value only for its d7/8& result). Similarly, d7/6&

410 | J. Anal. At. Spectrom., 2009, 24, 407–412

results from participant 5 are in agreement with corresponding

reference values for pairs ‘Mixt I’, ‘Mixt II’ and ‘Mixt IV’ but not

for the pair ‘Mixt III’. This lack of consistency indicates that if

accurate MC-ICPMS based results can be obtained on occasion

below 0.05&, they cannot necessarily be reproduced and are

easily subjects to small size effects and errors which may not

always be understood, controlled and/or corrected for at a suffi-

ciently low uncertainty level.

This study does indicate, however, that coherent results can

consistently be obtained above a threshold value around

�0.05&, as demonstrated by participants 1 and 3 with 6/6

‘acceptable’ results for pairs ‘Mixt III’ and ‘Mixt IV’, each.

A systematic examination of what may explain the difference

in performance between participants is outside the scope of this

study. It is however evident from Table 1 that most ‘routine’

This journal is ª The Royal Society of Chemistry 2009

Fig. 3 Relative expanded uncertainties (%) reported by participants 1, 3 and 5 (identical to those reported in Fig. 1) for the 4 pairs of the candidate

ERM-3810 series of d-iCRMs.

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measurement protocols do not work reliably at or below �0.1&

level variations, at least for Pb measurements. A key issue is

obviously the way mass discrimination effects are controlled and,

possibly, corrected for short-term fluctuations. During isotope

ratio measurements by MC-ICPMS these effects are, compared

to what is observed with other mass spectrometry techniques,

large (% per m/z level, or more) and easily subject to �0.05&–

0.1& shifts during measurement sessions19 including on a short-

term basis (i.e. between two consecutive samples). It is

noteworthy that these effects may even be amplified in case

matrix characteristics do not match enough between the d-0 and

target samples. While many geological problems can be easily

addressed with �0.1& data, for others it may be critical to

correct for these fluctuations, and the most efficient way is

internally using an isotope ratio from the same element of

interest and for which the value is considered constant. For

instance, using such internal normalization MC-ICPMS are able

to distinguish routinely 0.1& differences in Nd isotope ratios for

‘real world’ samples. Since Pb has no stable isotope pair to enable

internal normalization such an approach is rarely applicable to

Pb-isotope measurements (see Getty and DePaolo, 1995 for

a notable exception using very young samples28) but internal

standardization with Tl is a powerful alternative as promoted by

numerous literature studies and confirmed by the present

experiment. The best and most consistent results were obtained

by participants who had applied a Tl normalization technique

according to the method described by Woodhead26 (participant

5) or to a revised version described by Baxter et al.25 (participants

1 and 3). Results from participants 1 and 3 seem also to indicate

that the smaller the number of samples bracketed by two

consecutive measurements of the d-0 material the better.

Measurements uncertainties estimated by participants of this

comparison are systematically 2 to 4 orders of magnitude higher

than uncertainties associated to corresponding reference values,

which indicates that the candidate ERM-3810 series of d-iCRMs

is particularly suited for this method validation exercise. While

uncertainties quoted by participants 2 and 4 are systematically

either too low or far too large, those estimated by participants 1,

This journal is ª The Royal Society of Chemistry 2009

3 and 5 are globally more homogeneous and appropriate. For

instance and as expected, the higher the d207Pb target value

(typically, the pair ‘Mixt IV’) the smaller the stated relative

uncertainties on results (roughly though, because this trend is

not systematically observed). However, as illustrated in Fig. 3,

there are also important differences between statements made by

these 3 participants that, most likely, do not come so much from

differences between measurement methods deployed, but from

wide differences of concepts in uncertainty estimation.

Although crucial this issue is insufficiently tackled and dis-

cussed in the literature. It is not confined to delta scale isotopic

measurements by MC-ICPMS, but a more general problem.

Similar observations have been made in regard to results from

two recent comparisons on absolute isotope ratio measurements

involving also other mass spectrometry techniques, also organ-

ised by IRMM, under the auspices of CIPM (Comit�e Interna-

tional des Poids et Mesures): the first, CCQM-P48, about

uranium isotope ratios in biological/environmental materials29

and the second, CCQM-P105, completed in 2008, about the

n(87Sr)/n(86Sr) in food/biological matrices.30

Finally, results from the current study also suggest that

‘routine’ methods for absolute (calibrated) Pb-isotope ratio

determination by MC-ICPMS produce relative combined

uncertainties on results which are unlikely to be better than

0.05& (k ¼ 2) considering that accurate delta scale measurement

results cannot be reproducibly obtained below this threshold.

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