High-precision cadmium stable isotope measurements by double spike thermal ionisation mass...

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

High-precision cadmium stable isotope measurements by double spike thermalionisation mass spectrometry

Anne-D�esir�ee Schmitt,†* Stephen J. G. Galer and Wafa Abouchami

Received 5th December 2008, Accepted 28th April 2009

First published as an Advance Article on the web 26th May 2009

DOI: 10.1039/b821576f

Natural mass-dependent fractionation (MDF) of cadmium isotopes is a promising new tool for

investigating Cd pathways and cycling in geological and biological materials. One interesting new

application is as chemical tracer of deep water circulation and nutrient distribution in the oceans. But

since natural isotope fractionation of Cd appears to be extremely limited and Cd abundances low,

excellent external precision and sensitivity are needed to make full use of its potential. Here, we

describe a newly-developed double spike (DS) method for determining the MDF of Cd isotopes. For

inorganic matrices, samples are spiked with a mixed 106Cd-108Cd tracer prior to high-purity anion

exchange-based separation of Cd. Isotope measurements are performed by TIMS using a silica gel

activator. Overall, the DS-TIMS technique offers benefits in terms of superior precision and

sensitivity compared to MC-ICPMS methods currently in use. External precisions on 100-ng-sized Cd

standard loads, double spiked as unknowns, are �14 ppm on the 112Cd/110Cd ratio (2SD), while

ionization efficiencies (ions per atom loaded) are�0.3%. Using this technique, we calibrated the relative

difference in MDF between several Cd standard materials used in various laboratories as ‘‘zero

reference standards’’. We also show that the 112Cd/110Cd ratio can be fractionated by as much as

�0.2% by ion-exchange elution, which is potentially problematic for Cd isotope studies not using

a double spike.

Introduction

Over the past decade there has been increasing interest in

investigating natural mass-dependent stable isotope fraction-

ation (MDF) of transition metals, such as Cu, Zn, Fe, Cr, Mo

and Tl.1 This interest has been spawned, in part, by the advent of

MC-ICPMS technology which allows simple ionization of most

elements, including many of those virtually impossible to ionize

beforehand by TIMS due to refractory chemistry and/or high

first-ionization potential.

In the case of the transition metal Cd, however, the first studies

of MDF were made by TIMS as far back as 1975.2 In a long

series of papers by Rosman, de Laeter and co-workers it has been

shown that large MDF effects can be found in meteorites3–5 and

in lunar samples.6,7 The magnitude of these effects in some

ordinary chondrites is up to �0.5%/Da (i.e. half-percent per

dalton), and so pronounced that it dominates over instrumental

mass bias. The large MDF in extraterrestrial samples has been

verified in more recent MC-ICPMS work, and is thought to be

related to evaporation-condensation or thermal metamorphism

on the meteorite parent bodies.8,9

MC-ICPMS has proven to be the popular method of choice

for Cd isotopes in recent times, instigated by the work of

Wombacher et al.8 In this and subsequent studies, some other

Max-Planck-Institut f€ur Chemie, Postfach 3060, D-55020 Mainz,Germany

† Present address: Universit�e de Franche-Comt�e-CNRS/UMR 6249Chrono-environnement, 16, Route de Gray, F-25030 Besancon Cedex,France. E-mail: [email protected]; Fax: +33 (0)3 81 66 65 58;Tel: +33 (0)3 81 66 65 61.

This journal is ª The Royal Society of Chemistry 2009

useful applications of Cd isotopes have been identified in addi-

tion to those pertaining to extraterrestrial materials. For

example, it has been found that industrially-processed cadmium

is highly isotopically fractionated compared to ‘‘natural’’

terrestrial cadmium. Under such circumstances, the MDF of Cd

is an interesting new tracer for anthropogenic cadmium, and as

such can be used to monitor heavy metal pollution and cycling of

Cd in the environment.10 The isotopic fractionation presumably

occurs during smelting of Cd from Zn ores and/or its purification

by electrodeposition.

Lastly, Cd abundance is a well-known chemical oceanographic

tracer of deep water circulation and a proxy for nutrient distri-

bution.11 For example, the temporal distribution of Cd/Ca ratios

in foraminifera shells has been used to reconstruct past biological

productivity in the oceans. An obvious extension of these ideas is

to search for corresponding Cd isotope effects, and the first

studies have appeared examining MDF of Cd isotopes in

seawater12,13 and ocean Fe–Mn deposits.14

By far the largest Cd isotope variations found thus far are in

extraterrestrial samples and in industrially-processed cadmium

(i.e. man-made). By contrast, two recent, comprehensive surveys

have shown that cadmium MDF in ‘‘natural’’ terrestrial samples

is quite limited, and only of the order of 0.01%/Da.8,14 For this

reason, it is absolutely paramount to acquire Cd isotope data

with the utmost in analytical precision if cadmium MDF is to be

of much practical use. An additional complication is the

extremely low abundance of cadmium in most common mate-

rials, which calls for an analytical method of high sensitivity. The

aim of the present contribution is to address both the issues

of measurement precision and sensitivity. We present a new

J. Anal. At. Spectrom., 2009, 24, 1079–1088 | 1079

double-spike method for Cd stable isotope of cadmium, in which

isotope measurements are performed by TIMS; chemical

protocols for the pre-separation of sample Cd are also described

in detail.

Experimental

Chemical separation protocol for natural samples

The column chemistry separation procedure for Cd was modified

from that discussed for Pb by Lugmair and Galer.15 Cadmium

was eluted using 0.25 N HNO3, once matrix elements and Pb had

been selectively removed using two different dilute hydrobromic-

nitric acid mixtures by anion exchange (column: 100 mL of AG1-

X8 resin, 100–200 mesh). First, the sample is loaded and virtually

all the matrix eluted using 0.2 M HBr–0.5 M HNO3 (Soln. A);

then Pb is selectively eluted using 0.03 M HBr–0.5 M HNO3

(Soln. B). The subsequent dilute HNO3 elution effectively strips

the column of most retained elements, including Cd.16 Partition

coefficients for Cd are �9100 in Soln. A and �33 in Soln. B.17 A

more selective Cd elution can be obtained using a 0.03 M HBr–

2.0 M HNO3 mixture based upon the partitioning data of Stre-

low.18 However, this eluent was found to exhibit too much peak

tailing of Cd to yield a useful elution; by contrast, elution of Cd

with 0.25 N HNO3 does not tail appreciably. Other relevant

partition coefficients can be found elsewhere.17–19

The ‘‘pure’’ Cd fraction was dried down and passed a second time

through exactly the same chemical protocol to ensure complete

purification. Prior to the first anion exchange separation, iron was

removed from the sample by solvent extraction into MIBK (methyl

isobutyl ketone, a.k.a. 4-methylpentan-2-one) from 6 N HCl, which

is essentially quantitative under these conditions.20 This step was

found to be necessary, since traces of iron fully inhibit the thermal

ionization of cadmium. The Cd chemistry blank ranged between 54

and 160 pg over the course of this study, and was negligible

compared to the amount of Cd processed.

Analysis of cadmium isotopes by TIMS

The isotopic composition of cadmium was determined by

thermal ionization mass spectrometry using a Triton instrument

(ThermoFisher) operating in static multicollection mode. This

instrument is fitted with nine Faraday collectors. However, the

collectors cannot be positioned to allow all eight isotopes of Cd

to be measured simultaneously.

Rhenium single filaments (thickness 0.04 mm, width 0.7 mm)

of 99.99% purity were used, that were outgassed in vacuo prior to

use. Samples of cadmium were loaded onto the filaments in dilute

HNO3 followed by 1 mL silica gel activator added in a step-wise

fashion on top. The silica gel activator was prepared from

commercial colloidal silicic acid and orthophosphoric acid.21 For

runs of our Cd isotope standard, 100 ng of cadmium was nor-

mally loaded.

Isotopes of Sn, Pd and In are potentially important isobaric

interferences on Cd, but none were observed for standard or

sample runs. There are several reasons for this. First, Sn, Pd and

In should be quantitatively separated from Cd by our chemical

separation protocol, while second, Sn does not ionize at all by

TIMS under Cd running conditions. Although we did not notice

any isobaric interference from indium, this would only affect

1080 | J. Anal. At. Spectrom., 2009, 24, 1079–1088

113Cd which is problematic during TIMS analysis in any case (see

below). We tested doping Cd standards with palladium, from

which we observed that the Pd emission occurred at far lower

temperatures than those optimal for Cd, consistent with the

findings of others.2,6 Here, even large amounts of doped Pd (e.g.

Pd/Cd¼ 1) were gone before peak Cd emission. However, we did

find that undegassed Re filaments did emit Pd beams significant

enough to warrant isobaric correction on the low Cd masses.

Such interferences are presumably coming from Pd present in the

Re filament, and were absent in filaments degassed normally

before use.

In the mass spectrometer, peak emission of Cd ions occurs at

�1150 �C, about 50 �C lower than that for Pb ions with this acti-

vator. These conditions are similar to those previously noted by

other workers.5,22 Sample sizes of �100 ng yielded slowly decaying

ion beams, averaging �30 pA 112Cd over a half-hour analysis.

Though not run to exhaustion, such ion beams correspond to

ionization efficiencies (i.e. ions registered per atom loaded) of

�0.3% overall. We also observed that the thermal ionization of Cd

is seriously hindered by trace amounts of iron. Quantitative elimi-

nation of residual iron from the sample is thus imperative, and was

achieved here by solvent extraction as noted above.

Odd-even isotope effects. During analysis of cadmium isotopes

by TIMS using a silica gel activator, fractionation of isotope

ratios involving just even-mass isotopes of cadmium appears to

obey a simple mass dependence within the limits of precision. By

contrast, ratios with odd-mass isotopes 111Cd and 113Cd do not

fractionate in a mass-dependent way with respect to even-mass-

only isotope ratios (Fig. 1). This mass-independent fractionation

of odd isotopes of Cd – which is not subtle – has been observed

previously,14,23 and appears to be exhibited by odd-mass isotopes

of Pb and Zn as well, when measured by TIMS using a silica gel

activator.23,24

The origin of the anomalous behaviour of odd isotopes of

cadmium will be discussed in detail elsewhere, but appears to be

due to the magnetic isotope effect25 rather than the nuclear

volume, a.k.a. nuclear field shift of Bigeleisen.26 Since the

isotopes 111Cd and 113Cd do not follow a normal mass-dependent

law during instrumental fractionation by TIMS, they cannot be

used in the determination of the natural fractionation of sample

cadmium.

Choice of TIMS versus MC-ICPMS

The use of TIMS in cadmium isotope studies has had a long

history starting with the initial work by Rosman and de Laeter.2

However, since almost all recent studies have used MC-ICPMS

technology,8–10,12,13,27 it seems pertinent to address the question of

why we decided to pursue TIMS methodology in this work.

First, it should be made clear that our intention was to develop

specifically a double spike (DS) method (cf. Dodson28) for

cadmium isotopes. The most important role the instrument must

play in double spike work is to generate accurate isotope ratio

data with a high degree of long-term stability. While it is possible

in normal isotope ratio work to compensate for shorter-term

variation by normalization to standard runs, drift and inaccur-

acies are anathema to the double spike method. This is so, since

the aim is to try to reconstruct the mixing line once formed by the


Fig. 1 Contrasting behaviour of even-even versus odd-even mass

isotopes during TIMS measurement with a silica gel-H3PO4 activator.

This is illustrated in plots of (a) 114Cd/112Cd against 110Cd/112Cd, and (b)111Cd/112Cd against 110Cd/112Cd, where each point is the mean of all raw,

unnormalized data collected over a single run. The even-even data in (a)

lie along a well-defined line, consistent with mass-dependent fraction-

ation (FL). The data in (b) cannot be explained by isobaric interference,

but suggest, instead, mass-independent fractionation of 111Cd with

respect to even isotopes of cadmium. Similar anomalous behaviour is

seen for 113Cd.

double spike and sample in the beaker, which necessitates

knowing three isotopic compositions precisely and absolutely

from separate measurements.28–30

To this end, we performed an extensive series of tests over

a two-year period on a Nu Plasma MC-ICPMS instrument fitted

with a Cetac Aridus desolvating nebulizer. Cadmium isotopic

compositions of standard solutions were measured and internally

normalized using the exponential law31 in a variety of cup

configurations, and using OPZ (On-Peak Zero) baselines. From

this, we found that the Cd isotopic compositions (internally

normalized) were irreproducible between sessions, and often

drifted alarmingly within a single session. These effects could not

be ascribed, for example, to simple isobaric interferences.

Cadmium seems to be uniquely affected in this way, as internally

normalized data for Nd and Hf isotopes reproduce remarkably

well using the same set-up and are almost of the same quality as

TIMS data.

Overall, we were unable to obtain a satisfactory degree of

reliability and accuracy from our MC-ICPMS instrument to

allow straightforward application of a double spike for Cd.

While it is certainly feasible to determine the MDF of sample Cd

relative to that of a standard by simple sample-standard brack-

eting, as used by the majority of workers,8,10 a double spike

method offers many advantages. We note that Ripperger and


co-workers13 have been successful in implementing such

a method with MC-ICPMS.

A further sticking point with the MC-ICPMS analyses was the

ionization efficiency. Here, it should be borne in mind that

cadmium, as a ‘‘light’’ element, does not have a particularly good

sensitivity on a MC-ICPMS instrument compared to that of

heavier elements. This would not be so much of an issue, were

cadmium an abundant element in most materials. But on the

contrary, cadmium has an abundance less than a ppm in most

samples, with the exception of zinc ores, and for this reason,

instrument sensitivity is a matter to be taken extremely seriously.

During our MC-ICPMS sessions we achieved sensitivities

ranging from �50 to 150 V/ppm Cd on the Nu Plasma machine

operating with the Aridus desolvating nebulizer. Based on the

sample uptake rate of �0.07 mL/min, this corresponds to an

‘‘ionization efficiency’’ (i.e. ions collected per atoms aspirated) of

�0.05 to 0.15% for cadmium. These values compare well with

those found by other workers using desolvating nebulizers.

Corresponding values are 125 V/ppm and �0.15% (estimated) in

the study by Wombacher et al.,27 and 75–150 V/ppm and

0.05–0.10% efficiency, in the case of Ripperger and co-workers.13

Without the desolvating nebulizer, sensitivities are several times

worse.10,27 On the other hand, higher sensitivity of �240 V/ppm

and ionization efficiency (�0.28%, calculated) has been reported

by Lacan et al.12 using a Neptune (ThermoFisher) MC-ICPMS

fitted with an Aridus nebulizer.

These ionization efficiencies should be compared with those

obtained using our TIMS technique (see above). Here, 100-ng-

sized loads of standard cadmium yielded beams averaging �120

pA total Cd for a half-hour run and were not run to exhaustion.

These beams correspond to minimum ionization efficiencies of

�0.3% for cadmium.

Thus, for a fixed number of Cd atoms, it appears – in most

cases – that around two- to six-times as many ions more can be

registered by TIMS than by MC-ICPMS techniques. In terms of

pure ion statistics, this translates into error bars around a factor

of two smaller with TIMS, and is sufficient reason alone to

favour TIMS over MC-ICPMS. Consequently, we abandoned

further attempts at running Cd by MC-ICPMS and concentrated

on using TIMS.

Double spike optimization methodology

A useful way of envisioning the double spike problem is via a 3D

isotope space, formed from three independent isotope ratios

lying along the mutually orthogonal axes.29,30 Use of a common

denominator isotope ensures that mixing is linear, meaning that

four isotopes are required to define such an isotope space.

Nomenclature below follows closely that of Galer.30 The mixture

M to be measured lies between normal (unspiked) N, and double

spike S compositions, forming a straight line in 3D. Here, N ¼(n0, n1, n2) in components, and likewise for M and S. The so-

called fractionation vectors through the points N and M are fN

and fM respectively. These are defined as fN ¼ (D0n0, D1n1, D2n2),

for example, where Di are the components of the mass differences

(in daltons) between the numerator and denominator isotopes of

the isotope ratios forming the three axes. The locus of natural

MDF can then be written as N + a. fN for scalar a, and is accurate

in the region close to N where a is small.


The optimization of the double spike composition, and

mixture, was done numerically with a penalty function approach.

This yields (1) an estimate of the ‘‘best’’ mixture (M) to use within

a given 3D isotope space (and its corresponding spike, S), along

with (2) a ‘‘figure of merit’’ w (the penalty function) enabling

comparison between different isotope spaces of each element.

The methodology contrasts with that of previous studies in two

ways: first, the ‘‘optimal’’ mixture is explicitly solved for rather

than the spike composition, and second, more rigorous criteria

are used.

Four parameters are built into the penalty function w, each of

which reflect a desirable property of the spiked mixture. These

parameters take on a value of 1 in the ideal or ‘‘perfect’’ case and

are larger otherwise. First, the ‘‘lever’’ parameter g

g ¼ si � ni

si �mi

(1)

stipulates that M should lie far from S and close to N. Second,

q is the angle between the fractionation vector through N and the

mixing line, and is derived via the dot-product

cosq ¼ m$f N (2)

where m ¼ S � N is the mixing vector and ‘‘hats’’ signify unit

vectors. Angle q should be large, and ideally close to 90�. Third,

the so-called ‘‘intersection angle’’ f plays a key and intuitive role

in deciding the worthiness of a double spike composition, as

discussed elsewhere.30 Angle f is the angle between the normal

and fractionation vectors (fN and fM) as viewed along the

direction of the mixing line. One simple way (but not the only

one) of obtaining angle f is via

cosf ¼ (m � f N)$(m � f M) (3)

and its magnitude should be large, while 90� would be perfect.

Lastly, the ‘‘magnitude’’ parameter r expresses the desire that the

sizes of the 3 isotope ratios of M lie close to unity; this is

expressed in terms of the penalty

r ¼ 1

6

Xi

�mi þ

1

mi

�(4)

which takes a minimum value of 1 for M ¼ (1.0, 1.0, 1.0) and is

larger otherwise.

All four parameters are incorporated into the single ‘‘figure of

merit’’ function w given by

w ¼ rg��sinq , sinf�� (5)

Clearly, another functional form for w could also have been

chosen, leading to different ‘‘weightings’’ of the four factors.

However, for the time being, Eqn. (5) is sufficient for our

purposes.

The ‘‘optimization’’ of the double spike was performed by

solving for M in a given defined isotope space and N that

resulted in a minimum value of w. Normal compositions (N)

were taken from IUPAC.32 The multi-dimensional minimization

of the objective function w was done numerically using

the slow-but-sure Nelder-Mead simplex method. Once prom-

ising compositions M and S have been identified, the


overall performance is examined as a function of spike-to-

sample ratio.

Cd isotope notation

We decided to express our isotopic results in terms of the relative

deviation of the sample 112Cd/110Cd ratio from that of the stan-

dard:

3112=110Cd ¼ �

112Cd=110Cd�

spl�112Cd=110Cd

�std

� 1

!� 104 (6)

using the notation outlined by Wombacher and Rehk€amper.33

The most important consideration in this choice is that there is

a two-dalton offset between numerator and denominator for the

reference 112Cd/110Cd ratio. This offset is consistent with the

notation for nearly all stable isotope systems of other elements

(where possible), thus facilitating easy comparison.

We chose not to use 114Cd/110Cd as the reference ratio, as

advocated by Wombacher and Rehk€amper33 and others, since

this involves a four-dalton offset. Further, there is no ultimate

improvement in the relative error in using this ratio. This is so,

since while the natural variation in 114Cd/110Cd is, of course,

double that of the 112Cd/110Cd ratio, the measurement error is

also double.

More specifically, we chose first to avoid odd-mass isotopes of

cadmium because of the problems they pose during TIMS

analyses. Second, isotopes 106Cd and 108Cd are of low natural

abundance and are therefore unsuitable in the reference ratio. Of

the remaining possibilities, it is clear that 116Cd should be avoided

due to a large potential isobaric interference from Sn, which can

be problematic.8 Similarly, a good reason not to use 114Cd is that

it might be affected by thermal neutron capture, according to113Cd (n, g) 114Cd, which is worthy of study itself as a measure of

neutron fluence.3 This leaves the only reasonable two-dalton

reference ratio as 112Cd/110Cd, which we have used here and

elsewhere.14 A further advantage is that the 110Cd/112Cd ratio of

the sample is determined directly during double spike data

reduction (see below).

Results and discussion

Cadmium double spike

The double-spike method was first formulated by Dodson,28 and

can be applied to determine the naturally-occurring MDF of the

thirty-one elements having four or more stable isotopes. Four

isotopes are needed in order to form three independent isotope

ratios.29,30 Three simultaneous equations arise from isotope mass

balance in the mixture of the three isotope ratios, which then can

be used to solve for three unknowns: (1) the mass dependent

fractionation of the sample at the time of adding the double

spike, (2) the instrumental mass bias during measurement, and

(3) the spike-to-sample proportions in the mixture.

Choice and optimization. There is far more to arriving at

a suitable ‘‘double spike’’ than simply picking a tracer compo-

sition, and a considerable amount of optimization is involved.30

First, there is the choice of the three equation set to use, con-

sisting of a trio of independent isotope ratios sharing a common


denominator isotope. Second, the double spike tracer composi-

tion must be picked within this isotope space; third, once the

tracer composition is finalized, the best proportions of spike to

sample in the mixture need to be determined. For this work, we

attempted to optimize the propagated error on the MDF of the

sample using methodology described in the Experimental

section. This envisions the minimization of a penalty function, w;

as a rule of thumb, a value of w < 2 would qualify as a ‘‘good’’

double spike to use, while spike-sample mixtures can be varied

until w is up to 5 and still provide useful results.

Cadmium consists of eight isotopes – two odd and six even –

which implies that there are 280 possible combinations of 3D

isotope spaces to consider. If we exclude the two odd-mass

isotopes 111Cd and 113Cd from consideration, for reasons of the

mass-independent fractionation effects discussed earlier, we are

left with the six even isotopes and sixty potential isotope spaces

to evaluate.

Potential spike compositions were examined in all sixty 3D

isotope spaces in which the six even isotopes – 106Cd, 108Cd, 110Cd,112Cd, 114Cd, 116Cd – in turn act as the common denominator on

the isotope ratios on the axes. From this analysis, we decided to

use a mixed 106Cd-108Cd double spike, working in the isotope

space {106Cd/112Cd, 108Cd/112Cd, 110Cd/112Cd}. This combination

appears to provide a low value of w at the optimal mixture, and

a broad ‘‘valley’’ of low w-values as the spike-to-sample ratio is

varied from the optimal one. The respective isotopic composi-

tions of our Cd standard and the double spike are summarized in

Table 1.

The 106Cd-108Cd double spike was prepared in 2005 from

commercial, single spikes of 106Cd (oxide, 96.47% nominal

purity) and 108Cd (metal, 90.70%), mixed such that the106Cd/108Cd ratio is �1.2, as implied by our optimization. The

exact compositions are reported in Table 1 while the optimal

mixture composition is given in Table 2. The concentration of the

double spike was set at �340 ppb total Cd, which was chosen so

that �200 mL of DS would yield a near-optimal mixture when

added to �100 ng normal or sample Cd. Such a concentration

yields acceptable weighing errors on the isotope dilution

measurements whilst minimizing DS usage overall. As can be

seen from Fig. 2, this DS has a relatively wide window for

spiking – roughly a factor of 5 either way – without imparing its

performance.

Comparison with other double spikes. Since we are not the first

to use a double spike in the isotope analysis of cadmium, we

compared the efficacy of our 106Cd-108Cd double spike, as mixed

up, with those previously proposed, and used, by two other

Table 1 Cd isotopic composition of spikes and laboratory standard measur

Material 106Cd/112Cd

106Cd spike 363.9108Cd spike 246.4106Cd-108Cd double spikea 360.246 � 2JMC Cd standardb 0.052463 � 8

a Calibrated composition. b Mean and 2SD of 63 runs of our in-house JMC sfrom Rosman et al.5


groups. Rosman et al.5 made use of a 106Cd-111Cd double spike in

this and other publications, while Ripperger and co-workers13

advocated a 110Cd-111Cd double spike. First, we used the DS

compositions reported by these authors to find the optimal

mixture of DS and normal Cd in the isotope space used (i.e. w is

minimized along the mixing line); second, we plotted the value of

w as a function of the spike-to-sample ratio. The comparison is

shown in Fig. 2, where we include a plot of the all-important

‘‘intersection angle’’ f as well, which is basically the angle

between the normal and fractionation vectors as viewed along

the mixing line (see Experimental section). The minimum values

of w are �33.6 (Ripperger and co-workers13), �7.83 (Rosman

et al.5) and�1.33 for our own DS. As illustrated in Fig. 2, the DS

previously used do not fare particularly well, at least based upon

the optimization criteria outlined in the Experimental section.

The performance characteristics and best mixtures of all three

DS are summarized in Table 2.

Double spike calibration. In order to make use of the double

spike, its isotopic composition must be calibrated against a Cd

standard of known isotopic composition. The calibrated double

spike can then be used, in the reverse sense, to determine the

unknown isotopic composition of Cd in a sample. Here it is

assumed that differences in isotopic composition between cali-

brating standard (known) and sample (unknown) are solely

related to mass-dependent fractionation of the isotopes.

In practise, two problems arise immediately. First, despite

some attempts and much discussion,33 there still does not exist

a suitable isotope reference material for cadmium. Second, none

of the Cd shelf standards used in various laboratories have been

calibrated ‘‘absolutely’’ to high precision. The recent isotopic

calibration by Pritzkow et al.34 is a step in the right direction, but

is problematic in detail. For example, these authors inexplicably

did not use their gravimetrically calibrated mixed spikes as

‘‘double spikes’’ in the sense of Dodson28 and as applied in this

study. More serious, though, is that they combined datasets from

MC-ICPMS and TIMS instruments, the latter of which, as

mentioned earlier, has a serious bias of odd- versus even-mass

isotopes.

For purposes of the double spike calibration, we were forced

to make two assumptions. First, we assumed that ‘‘normal’’

cadmium has a 110Cd/112Cd ratio equal to 0.520089, which is the

mean composition measured by Rosman et al.5 for six runs of

a single reagent. Note that this value differs slightly from that of

the quoted IUPAC composition.32 Second, we assumed that our

own laboratory shelf standard has the same 110Cd/112Cd ratio as

determined by Rosman et al.5 Neither of these assumptions can

ed by TIMS

108Cd/112Cd 110Cd/112Cd

7.731 0.56423607.6 123.8295.704 � 2 10.3864 � 10.037219 � 7 0.520089c

tandard (Lot #15922032). c Value used for internal normalization, taken


Table 2 Performance characteristics of Cd double spikes

DS R0 R1 R2a wb f (�)c Ref.

110 + 111 110/111 112/111 114/111 13dmd �1 1 3Se 0.49336 0.02602 0.01218Mf 0.65558 0.65119 0.76285 33.597 11.07Ng 0.97578 1.88516 2.24453106 + 111 106/111 110/111 116/111 5dm �5 �1 5S 0.35820 0.02258 0.00679M 0.23707 0.46572 0.27567 7.830 31.24N 0.09766 0.97578 0.58516106 + 108 106/112 108/112 110/112 This studydm �6 �4 �2S 360.2464 295.7036 10.3864M 1.61078 1.31658 0.56033 1.334 61.99N 0.05180 0.03688 0.51761

a Three ratio isotope space R0, R1, R2. b Minimum value of w calculated from Eqn. (5) along the mixing line S–N. c Intersection angle f (�) at minimumw, from Eqn. (3). d Mass difference, numerator� denominator. e Double spike composition S used. f Mixture composition M at minimum w. g Normal,unspiked compositions taken from IUPAC.32

be correct, absolutely, but this will only affect the final outcome

of the double-spike calculation in a relative sense.

The isotopic composition of our JMC reference standard was

determined from our own high-precision TIMS data collected in

static multicollection mode and internally normalized using the

exponential law.31 Sixty-three runs yielded normalized106Cd/112Cd ¼ 0.052463 � 8, and 108Cd/112Cd ¼ 0.037219 � 7

(2SD; Table 1). The 106Cd-108Cd double spike was calibrated by

measuring its isotopic composition alone, and mixed with our

JMC standard. The data reduction is then operated in reverse to

solve for the instrumental fractionation-corrected composition

of the double spike, using the composition of our JMC, effec-

tively, as the ‘‘spike,’’ as described elsewhere.30 The derived

composition of the spike is listed in Table 1.

External reproducibility of standard cadmium

Once our double spike was calibrated, we tried to assess the

external reproducibility of the overall methodology. This was

done by looking at how well we could reproduce our in-house

JMC Cd reference standard when measured using the double

spike rather than by internal normalization.

Mixtures of JMC-Cd and the 106Cd-108Cd double spike were

prepared at regular intervals and run alongside samples, and

treated as ‘‘unknowns’’. The original isotopic composition of the

JMC standard was then determined post-run with the aid of our

reduction software. Here, we expect the back-corrected110Cd/112Cd ratio obtained by data reduction to be equal (or

close) to 0.520089 which was the value used in normalization of

our unspiked JMC runs initially, and for calibrating the double

spike.

The results for sixty such measured mixtures of JMC Cd and

double spike over a one-year period are illustrated in Fig. 3a in

a plot of the reconstructed 110Cd/112Cd ratio. First, it can be seen

that the overall range in 110Cd/112Cd is about half an 3-unit, which

would be reasonable even for internally-normalized data. In

detail, however, the data can been subdivided into different

periods, which clearly exhibit different mean 110Cd/112Cd ratios


(Fig. 3a). These are, in temporal order: 0.520090 � 5 (2SD, N ¼22) for August 2005 to January 2006; 0.520099 � 11 (N¼ 10) for

February to March 2006; 0.520106 � 11 (N ¼ 18) for April to

July 2006; and 0.520077 � 7 (N ¼ 10) for August 2006 onwards.

These small, but systematic, offsets in values are caused by

Faraday cup deterioration in the instrument. This is known,

since there were accompanying shifts in measured 87Sr/86Sr,

which became serious enough to warrant replacing three cups in

July 2006. This change resulted in the shift in 110Cd/112Cd

downwards by �0.00003 (triangles to circles), confirming the

origin of the offsets. Note that the reproducibility became worse

as the cups deteriorated further (period: February to July 2006).

For this reason, we applied ‘‘cup bias’’ corrections to our Cd

isotope data, according to the offsets seen in Fig. 3a, to ensure

that the ‘‘zero’’ remained zero over the whole measurement

period. In terms of our in-house JMC standard, this results in the

dataset shown in Fig. 3b. Here, the mean 110Cd/112Cd ratio is

0.520089 � 7 (2SD, N ¼ 57), wherein 3 points in Fig. 3a were

rejected at 95% confidence. This value not only illustrates that

the double spike can give the right absolute answer; more

importantly, the external reproducibility is 14 ppm on the ratio

(i.e. �0.14 3112/110Cd) which is quite satisfactory. This represents

the reproducibility achievable with the TIMS Triton instrument,

our particular choice of double spike, and ion beams obtained

from �100-ng-sized loads. This does not, however, necessarily

reflect how well we can reproduce samples, which has been dis-

cussed elsewhere.14 The reproducibilities of determining the

stable isotope fractionation of standard Cd are summarized in

Table 3 from many studies. It can be seen that our double spike

TIMS method results in an improvement in external reproduc-

ibility by about a factor of three or more compared with previous

techniques.

Intercalibration of cadmium isotope standards

A currently outstanding problem in cadmium isotope studies is

that there is no consensus about the material to use as a ‘‘zero

delta’’ standard – in other words the reference relative to which


Fig. 3 Values of 112Cd/110Cd for our JMC laboratory standard deter-

mined as unknowns using our double-spike over a period of more than

a year. The different periods are in temporal order: Aug. 2005 to Jan 2006

(diamonds), Feb to Mar 2006 (squares), Apr. to July 2006 (triangles),

Aug. 2006 onwards (circles). In (a) data are presented uncorrected for cup

bias, while in (b) bias-corrections have been applied. The data in (b) imply

a long-term external reproducibility of �14 ppm (2SD, N ¼ 57), where 3

runs in (a) were rejected at 95% confidence.

Fig. 2 Performance characteristics of our 106Cd-108Cd DS compared with

those of DS compositions proposed by Rosman et al.5 (dashed lines) and

Ripperger and co-workers13 (dotted lines). A low value of the penalty w –

calculated via Eqn. (5) – is desirable for a ‘‘good’’ DS. In (a), w is shown

plotted against r/ropt which is, effectively, the spike-to-sample ratio in the

mixture (ropt corresponds to the minimum in w), and scales linearly with

the amount of spike added (see Galer30). In (b) the intersection angle f (�)

is shown, as determined by Eqn. (3).

sample data are reported.33 This not only presents a difficult

practical issue, but also hampers direct comparison of published

datasets, which is becoming increasingly problematic as the

precision of measurement improves.

Reference in-house Cd isotopic standards currently in use

include: spectroscopically pure Cd metal from Johnson Mat-

they,3,4,7 a cadmium ICP plasma solution (Lot #502552A, a.k.a.

‘‘JMC Cd’’ or ‘‘M€unster/Z€urich JMC Cd’’) from Johnson

Matthey,8,27,33 and a Cd solution (Lot #7-29) from Spex.10

Wombacher and Rehk€amper33 have already verified that most

commercial Cd solutions have a very similar isotope composition

within the limits of their external reproducibility (�0.01%/Da).

The materials above appear to consist of cadmium largely

unfractionated from that found in most terrestrial materials.3,8,14

However, there are two additional ‘‘standards’’ that have quite

fractionated isotopic compositions. The first of these is candidate

isotope reference material BAM-I012 from the German


Bundesanstalt f€ur Materialwissenschaften und -pr€ufung (BAM),

which is isotopically ‘‘light’’ and was prepared from high purity

Cd metal. The second is an isotopically ‘‘heavy’’ sample known as

‘‘M€unster Cd’’, which was prepared as described by Wombacher

and Rehk€amper.33 These two samples of cadmium are of interest

as secondary isotopic standards for inter-laboratory comparison,

though they are unsuitable as ‘‘zero delta’’ reference materials.

We also prepared a Cd solution from high purity Johnson

Matthey Cd metal (99.9999%, Lot #M14638) as a gravimetric

standard during this study, which turns out to be isotopically

fractionated as well.

We obtained aliquots of many of these cadmium ‘‘standards’’

in common use. Each of the standards was run five times with our

calibrated 106Cd-108Cd double spike, in order to determine their

relative isotopic fractionation. The results are summarized in

Table 4, expressed, as before, in terms of deviations from our

own Cd standard (JMC Cd plasma solution, Lot #15922032).

Data for the individual runs are shown plotted in Fig. 4.

The weighted average 3112/110Cd values and weighted errors

(2SD) for ‘‘M€unster/Z€urich JMC Cd’’ (e.g. Wombacher et al.8)

and ‘‘Spex Cd’’ (Cloquet et al.10) are �1.09 � 0.11 and �1.10 �0.19, respectively. The fact that these two standards yield the

same number is in complete agreement with the findings of

Cloquet et al.10 Nevertheless, both solutions are ‘‘lighter’’ than

our JMC Cd solution (Lot #15922032) by an amount compa-

rable to the analytical error bars reported.10,33 Thus, as the

precision of measurement improves, it becomes increasingly

clearer that JMC solutions of cadmium do have resolvably

different isotopic fractionation. For this reason, it is imperative


Table 3 External reproducibility of Cd isotope standards

Study Method Instrument 3Cd/Da, 2SDa

Rosman and de Laeter2 DS TIMS 14Rosman and de Laeter3 DS TIMS 4Schediwy et al.7 DS TIMS 2b

Wombacher et al.8 Ag or Sb doping MC-ICPMS 1.0c

Cloquet et al.10 Bracketing MC-ICPMS 0.3Lacan et al.12 Ag doping MC-ICPMS 0.2–0.8Ripperger and co-workers13 DS MC-ICPMS 0.3This study DS TIMS 0.07

a Reproducibility at 95% confidence expressed as relative deviation in parts-per-10000 per dalton. b Best reported internal error. c Single session 0.3–0.73Cd/Da (2SD).

Table 4 Cd isotopic composition of interlaboratory standards measured using the Cd double spike

Material Form Batch 3112/110Cdb �2SDb 3112/110Cdc �2SDc N Source

MPI JMCa solution #15922032 0.00 0.14 — — 57 This studyM€unster/Z€urich JMC solution #502552A �1.10 0.06 �1.09 0.11 5 ref. 8Nancy Spex solution #7-29 �1.09 0.29 �1.10 0.19 5 ref. 10M€unster Cd solution — 21.49 0.17 21.48 0.12 5 ref. 33MPI JMC metal metal #M14638 �7.49 0.22 �7.48 0.12 5 This studyBAM-I012 metal — �7.30 0.13 �7.30 0.14 5 ref. 34

a Zero reference standard in this study. b Unweighted mean and 2SD of the dataset. c Weighted mean and 2SD of the dataset.

Fig. 4 Cd isotopic fractionation in standards used by various labora-

tories, as determined using our DS and expressed in terms of 3112/110Cd.

Unweighted means and 2SD (dashed lines) are illustrated in each case.

Data for (a) the isotopically heavy M€unster Cd standard, (b) commercial

Cd solutions, (c) high purity Cd.


that a single cadmium reference material be agreed upon as soon

as possible for use as ‘‘zero delta’’ and inter-laboratory

comparison purposes.

We found BAM-I012 to have Cd isotope ratios virtually

identical to those of our own gravimetric JMC metal solution,

namely with 3112/110Cd of �7.30 � 0.14 versus �7.48 � 0.12,

respectively, which probably indicates similar sources for these

materials. Both samples are isotopically very ‘‘light’’ compared

with our standard, however. This confirms the original obser-

vation of Cloquet et al.10 that industrially-processes cadmium

metal lies far outside the range in MDF found in natural

terrestrial samples. For this reason, the cadmium MDF provides

a foolproof fingerprint of anthropogenic cadmium in the envi-

ronment.

Lastly, we found ‘‘M€unster Cd’’ to be isotopically extremely

‘‘heavy’’, with 3112/110Cd of +21.48 � 0.12 (2SD), based upon five

runs. All in all, our new data for all of the above standards are

mutually consistent within error with the relative values reported

by others.10,13,33 However, our error bars are considerably

smaller, which we hope should allow more robust interlabor-

atory comparison between datasets in future studies.

Fractionation by ion exchange

In order to assess isotope fractionation effects during ion-

exchange chromatography of cadmium, we eluted some

cadmium from our anion exchange column and measured the

MDF in the fractions collected using the double spike. The

column consisted of 100 mL of AG1-X8 strongly-basic anion

exchange resin (100–200 mesh, bromide form). Our JMC Cd


Fig. 5 Cd isotope fractionation during anion-exchange elution in dilute

HNO3. Both the cumulative Cd and 3112/110Cd are shown against the

amount eluted, expressed in terms of column volumes, which is the

volume of the resin bed. It can be seen that heavy isotopes of Cd are

eluted first, and that the total extent of variation in 112Cd/110Cd

approaches �0.2%.

shelf standard (5 mg) was loaded onto the column in Soln. B

(see Experimental), and then eluted in successive 0.25 N HNO3

fractions. A total of twenty-three 50 mL fractions were

collected, upon which Cd contents were assayed on splits

by ICP-OES (Perkin Elmer, Optima 3300XL) to yield the

elution curve. Based upon the elution curve, the 23 fractions

were pooled into twelve fractions for determining the Cd

isotopic fractionation using the double spike. Unfortunately,

of these 12 fractions, runs of only the middle eight in the

elution were successful. The 3112/110Cd of these eight fractions

and cumulative eluted cadmium are reported in Table 5 and

plotted against column volume (CV; 1 CV ¼ 100 mL) eluted

in Fig. 5.

Based upon the elution curve, we obtained 3112/110Cd data from

�94.0% of the elution, but are missing �5.2% at the start and

�0.8% of the Cd at the end (Table 5). Values of 3112/110Cd are

+12.5 in the first fraction (CV: 3.0 to 4.0) and �5.7 in the last

fraction (CV: 8.0 to 8.5). The cumulative 3112/110Cd calculated for

all the fractions combined, weighted by their Cd contents, is

�0.21 � 0.15 (propagated error). While this should, in principle,

yield ‘‘zero’’ – the value loaded onto the column – we did not

measure small fractions of Cd from both ends of the elution.

Since we are missing �5% of ‘‘heavy’’ Cd early on and only �1%

of ‘‘light’’ Cd from the tail-end, this is probably why the cumu-

lative 3112/110Cd overall is slightly negative.

The total amplitude in 3112/110Cd is �18 (i.e. �0.2% total vari-

ation in 112Cd/110Cd), which should realistically be considered

a minimum value due to the missing fractions from the elution.

This total range in variation is in broad agreement with ranges

found by other workers. Wombacher et al.8 reported�18 range in

3112/110Cd in five fractions eluted from anion resin with 2N HNO3,

while Cloquet et al.10 found only �12 variation in eight fractions

eluted using very dilute HCl from AG-MP-1 macroporous anion

exchange resin. The fact that each study left out some small early-

and late-eluting fractions can presumably account for the differ-

ences in overall fractionation found. In any case, the eluent used,

its strength and the performance of the column also influence just

how the isotopic fractionation evolves as the elution procedes (see

Table 5 Cd isotopic fractionation during anion exchange elutiona

Fraction CVb � Fraction % Cdc % C

1 0.75 0.75 0.06 0.14 0.57 0.2 2.25 0.75 0.89 1.43 2.13 4.3 3.50 0.50 3.48 5.47 8.4 4.25 0.25 8.17 8.5 4.75 0.25 9.22 9.6 5.50 0.50 11.99 12.69 24.7 6.50 0.50 11.70 13.69 25.8 7.25 0.25 9.17 9.9 7.75 0.25 5.95 5.10 8.25 0.25 2.46 2.11 9.50 1.00 0.67 0.08 0.02 0.01 0.12 11.00 0.50 0.00 0.00 0.

S 100.

a Resin: AG-1 X8, 100–200 mesh, eluted with 0.25N HNO3. b Column volumgiven. c Cd % in 23 fractions of 0.25 CV measured by ICP-OES. d Cumulativeat the start and 0.79% at the end of the elution. e R is 112Cd/110Cd measuredcontents along with the propagated error.


Glueckauf35). For this reason, the agreement between our study

and previous ones8,10 is encouraging.

More important, though, is the observation that for all three

studies in which cadmium is eluted from anion exchange resin,

the ‘‘heavy’’ isotopes of Cd are eluted first, implying that the

‘‘light’’ isotopes are preferentially retained on the resin. This

situation is quite consistent with what is known of the elution

behaviour for isotopes of other transition metals. For example,

Anbar et al.36 found that ‘‘heavy’’ isotopes of trivalent iron are

eluted first from AG-MP-1 resin using dilute HCl as eluent.

Studies of the elution of Cu and Zn from AG-MP-1 using more

concentrated HCl show similar behaviour.37 Why the ‘‘light’’

isotopes have slightly larger distribution coefficients onto the

resin remains unclear at present, given the absence of under-

standing of the underlying mechanism of isotope fractionation.

To make the situation even more unclear, an exception to the

‘‘heavy-first’’ rule is provided by lead. In the case of Pb elution off

d Cum % Cdd Re �2SE 3112/110Cd �2SE

78 0.7845 5.2295 14.17 1.001248 0.000101 12.48 1.0117 22.35 1.000330 0.000045 3.30 0.4522 31.56 1.000236 0.000028 2.36 0.2868 56.24 0.999921 0.000031 �0.79 0.3139 81.63 0.999701 0.000014 �2.99 0.1417 90.80 0.999571 0.000031 �4.29 0.3195 96.75 0.999448 0.000106 �5.52 1.0646 99.21 0.999426 0.000127 �5.74 1.2779 99.990100 100.00 �0.21 0.15f

e (CV) where 1 CV ¼ 0.1 mL; the CV at the midpoint of the fraction is% Cd eluted; 93.98% was assayed for isotopic composition, missing 5.22%divided by that in the standard. f Cumulative 3112/110Cd weighted by Cd


an anion exchange column with 6 M and 8 M HCl, Baker et al.38

showed that ‘‘light’’ isotopes of lead appear in the eluate first.

Conclusions

We present a new double spike methodology for determining the

mass-dependent fractionation of cadmium isotopes in nature

that offers advantages over previous techniques in terms of

precision and sensitivity. This methodology comprises (1) the

addition of a mixed 106Cd-108Cd double spike tracer to the sample,

(2) an efficient, high purity chemical separation procedure for Cd

from typical inorganic matrices, combined with (3) measurement

of the Cd isotopic composition of the mixture by TIMS. The

long-term reproducibility in determining the stable isotope

fractionation of cadmium with the double spike is �14 ppm on

the 112Cd/110Cd ratio (2SD), based upon 100-ng loads of our

standards treated as unknowns. The ionization efficiency is

�0.3% of the cadmium loaded.

Acknowledgements

We would like to thank three unnamed reviewers for their

comments on the manuscript. J. Carignan, C. Cloquet, K.

Mezger and F. Wombacher provided engaging discussion and

generously supplied Cd standard materials for interlaboratory

calibration. ICP-OES master H.-J. Voss aided and abetted with

cadmium measurements from the column elution. Special thanks

go to H. Feldmann and P. Jaeckel for help and keeping-up spirits

in a literal sense. For those of you at home: remember, cadmium

is toxic. During this study, ADS was funded by a Marie Curie

Fellowship from the European FP6 programme.

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