an isotope dilution icpms method for the determination of mgca

An Isotope Dilution ICP-MS Method for the Determinationof Mg ⁄ Ca and Sr ⁄ Ca Ratios in Calcium Carbonate

Diego P. Fernandez (1)*, Alex C. Gagnon and Jess F. Adkins

Geology and Planetary Science, California Institute of Technology, MS 100-23, 1200 E California Blvd, Pasadena, CA 91125, USA* Corresponding author. e-mail: [email protected](1) Present address: Geology and Geophysics, University of Utah, 115 South 1460 East Room 383, Salt Lake City, UT 84112, USA

Mg ⁄ Ca and Sr ⁄ Ca ratios in calcium carbonate areimportant components of many palaeoclimatestudies. We present an isotope dilution methodrelying on a single mixed spike containing 25Mg,43Ca and 87Sr. Dozens of samples per day, as smallas 10 lg of carbonate, could be dissolved, spikedand run in an ICP-MS with a precision of 0.8% (2RSD). Two instruments types, a sector field and aquadrupole ICP-MS, were compared. The best longterm precision found was 0.4% (2 RSD), althoughthis increased by up to a factor of two when samplesof very different Mg or Sr content were run togetherin the same sequence. Long term averages for thetwo instruments concurred. No matrix effects weredetected for a range of Ca concentrations between0.2 and 2 mmol l-1. Accuracy, tested by measuringsynthetic standard solutions, was 0.8% with somesystematic trends. We demonstrate the strength ofthis isotope dilution method for (a) obtainingaccurate results for sample sets that present a broadMg and Sr range and (b) testing solid carbonates ascandidate reference materials for interlaboratoryconsistency. Mg ⁄ Ca and Sr ⁄ Ca results for referencematerials were in good agreement with values fromthe literature.

Keywords: carbonate, ICP-MS, quadrupole ICP-MS, sectorfield ICP-MS, isotope dilution, minor elements.

Received 05 Jan 09 – Accepted 07 Aug 09

Les rapports Mg ⁄ Ca et Sr ⁄ Ca des carbonates decalcium sont importants pour de nombreuses étudespaléoclimatiques. Nous présentons une méthode dedilution isotopique s’appuyant sur un simple ajout(« spike ») constitué d’un mélange contenant 25Mg,43Ca et 87Sr. Des dizaines d’échantillons par jour,aussi petit que 10 lg of carbonate, pourraient êtredissous, enrichis (« spiked ») et analysés dans unICP-MS avec une précision de 0.8% (2 RSD). Deuxtypes d’instruments, ICP-MS à secteur magnétique etICP-MS quadripolaire, ont été comparés. Lameilleure précision à long terme constaté était de0.4% (2 RSD), bien que cette valeur ait augmentéjusqu’à un facteur deux lorsque des échantillonscaractérisés par des teneurs en Mg ou Sr trèsdifférentes ont été analysés ensemble dans unemême séquence. Les moyennes à long terme pourles deux instruments sont du même ordre. Aucuneffet de matrice n’a été détecté pour une gamme deconcentrations de Ca comprise entre 0.2 et2 mmol l-1. La précision, testée par la mesure desolutions standards de synthèse, était de 0.8%, aveccertaines tendances systématiques. Nous démontronsla robustesse de cette méthode de dilutionisotopique pour: (1) l’obtention de résultats précispour des ensembles d’échantillons qui présententune large gamme de teneurs en Mg et Sr, et (2)tester des carbonates solides comme matériaux deréférence candidats à des études de cohérence entrelaboratoires. Les rapports Mg ⁄ Ca et Sr ⁄ Ca obtenuspour les matériaux de référence sont en bon accordavec les valeurs de la littérature.

Mots-clés : carbonate, ICP-MS quadripolaire, ICP-MS àsecteur magnétique, dilution isotopique, éléments mineurs.

Naturally occurring calcium carbonate in corals, sclero-sponges, foraminifera, otoliths, speleothems, etc. containsmagnesium and strontium as minor components. Notwith-

standing the complex mechanisms involved in the incorpo-ration of these elements in calcitic or aragonitic matrices, anumber of studies indicate that their spatial distribution

Vol. 35 – N� 10311 p . 2 3 – 3 7

doi: 10.1111/j.1751-908X.2010.0031.xª 2010 The Authors. Geostandards and Geoanalytical Research ª 2010 International Association of Geoanalysts 2 3

reflects simple environmental changes. The best knownexamples, the reconstruction of past sea surface tempera-tures from foraminiferal Mg ⁄ Ca data (Nurnberg et al.1996) and coralline Sr ⁄ Ca values (Smith et al. 1979, Becket al. 1992) have opened up a wealth of opportunities inpalaeoclimate research.

Analytical advances in the 1990s were in great partresponsible for this development. On the one hand, thewidespread use of inductively coupled plasma (ICP) intro-duction systems allowed a high throughput of samples withvery effective ionisation efficiency. In addition, new detectorspossessing unsurpassed dynamic ranges made it feasibleto acquire large signals from major elements together withsmaller signals from minor or trace elements. Finally, sam-ples as small as 25 lg could be analysed owing to thehigh sensitivity of these new instruments. The first methodspresented used quadrupole mass spectrometry (Q-ICP-MS)(Lea and Martin 1996, LeCornec and Correge 1997), sec-tor field mass spectrometry (SF-ICP-MS) (Rosenthal et al.1999) and atomic emission spectroscopy (ICP-AES) (Schrag1999). The determination of trace elements in addition toMg ⁄ Ca and Sr ⁄ Ca ratios using the same method was alsoinvestigated (Lear et al. 2002, Yu et al. 2005, Marchitto2006), and a careful comparison between emission andmass spectrometry methods was considered (Andreasenet al. 2006).

Despite the high precision attained by some of thesemethods, specific problems hampering their accuracywere evident. Matrix effects for ICP-AES methods, beinglarge and complex (Lehn et al. 2003), required specialcalibrations (de Villiers et al. 2002, Wara et al. 2003).However, for those methods using bracketing calibrators,the uncertainty associated with differences in matrixeffects between the natural samples and the calibratorswas not assessed. On the other hand, ICP-MS methodsrelying on internal standards assume the selected spikeelement, for example Sc or Y, is subject to the samebehaviour in the plasma as Mg, Ca and Sr. Moreover,the linearity of the broad dynamic range detectorsdepends on the cross-calibration between differentmodes of operation, a requirement that cannot alwaysbe easily fulfilled (Marchitto 2006). These shortcomings,in addition to the uncertainties in composition of the cali-brators used, became evident in an inter-laboratory cali-bration study (Rosenthal et al. 2004) revealing precisionsof 3.4% and 1.8% for Mg ⁄ Ca and Sr ⁄ Ca ratios, respec-tively, in determinations of the synthetic standard solutiondistributed for the study. The need was also clear for wellcharacterised matrix-matched reference materials (RMs),with Mg and ⁄ or Sr contents in the appropriate range,

and some effort has been devoted to this goal (Greaveset al. 2005, 2008, Sturgeon et al. 2005).

In this work, we present an isotope dilution mass spec-trometry (ID-ICP-MS) method to determine Mg ⁄ Ca andSr ⁄ Ca ratios, and we assess the range of concentrations forwhich it can be applied as well as the long term precision.Isotope dilution-ICP-MS relies on the addition (spiking) ofan enriched isotope (tracer) of the element of interest fol-lowed by the measurement of the ratio between this iso-tope and a different one from the sample (Klingbeil et al.2001, Beauchemin 2006). In the present study, we used amixed tracer containing mainly the isotopes 25Mg, 43Caand 87Sr and measured the isotopic ratios 24Mg ⁄ 25Mg,48Ca ⁄ 43Ca and 88Sr ⁄ 87Sr using ICP-MS. The use of an iso-tope of the same element to be determined minimised thedifferences in plasma behaviour. Since isotope measure-ments are all made in the same sample matrix and regularcalibration curves prepared from external calibrators arenot used, the isotope dilution method has been recognisedas an analytical tool capable of the highest accuracy. Theuse of a mixed tracer has an additional advantagebecause the elemental ratio calculation does not dependon the amount of tracer added. The dissolution of micro-gram amounts of the calcium carbonate sample understudy and the addition and mixing of the tracer could bequickly accomplished in sub-millilitre auto-sampler vialssince there was no need to measure their masses.

Experimental procedures

Instrumentation

A Finnigan Element ICP-MS, with standard Ni cones,quartz injector and torch without a capacitive decoupling(CD) system, was used to develop the experimentalmethod. We also used cooled dual pass quartz (PC3 SSIElemental Scientific; Omaha, Nebraska, USA) or PTFE Scott-type spray chambers, PTFE nebulisers with flow rates of 22,35 and 50 ll min-1 (Elemental Scientific PFA-20 and PFA-50), and an auto-sampler (ASX-100 CETAC; Omaha,Nebraska, USA). Low resolution was selected and the intro-duction system was tuned in the usual way (sample gasflow, torch x, y, z position and lenses) with observed sensitiv-ities up to 300 kcps (for 88Sr) for a 1 lg l-1 Sr solution. AThermo Scientific; West Palm Beach, Florida, USA, Neptunemulti-collector ICP-MS with Ni cones and a dual pass spraychamber was used to check the isotopic composition of theCa analytical reference solution. The method, with minorchanges, was also implemented using an Agilent 7500cequadrupole ICP-MS with a CD system (at the University ofUtah) together with a CETAC ASX-520 autosampler,

2 4 ª 2010 The Authors. Geostandards and Geoanalytical Research ª 2010 International Association of Geoanalysts

Pt cones, cooled dual pass quartz spray chamber (Elemen-tal Scientific PC3-SSI) and PTFE nebuliser with a100 ll min-1 flow rate (Elemental Scientific ST-100). Sensi-tivities up to 140 kcps for a 1 lg l-1 Sr solution wereobtained. The dynamic reaction cell usually employed toreduce the presence of molecular interferences installed inthis instrument was not used. A micro-sampling device(Micro Mill; New Wave Research, Portland, Oregon, USA)was used to obtain microgram amounts of samples alongbands in corals, stalagmites or soil carbonates.

All solutions and samples were handled in laminarflow benches. Standard and tracer solutions were preparedgravimetrically in fluorinated ethylene propylene (FEP) bot-tles previously leached sequentially with hot 50% v ⁄ vHNO3, 50% v ⁄ v HCl and water, and solutions were keptin a closed humid environment in order to minimise evapo-ration losses. Polypropylene or FEP auto-sampler vials wereleached in hot 10% v ⁄ v HCl. Pipette tips were acidwashed followed by three rinses with water. Proper anti-sta-tic procedures were used when weighing FEP containerson calibrated scales, and buoyancy corrections wereapplied. All solid reference samples were transferred orrinsed from the weighing container into the bottle withwater and dissolved by slow addition of nitric acid in orderto prevent losses. Reference samples were dried or han-dled under argon. Samples were weighed in a stainlesssteel boat using a microbalance, transferred into previouslycleaned polypropylene vials and dissolved with 5% v ⁄ vHNO3 (Baseline; Seastar, Sidney, British Columbia,Canada). No centrifugation was applied to samples.

Method

For our full isotope dilution method we prepared twotypes of solution containing enriched isotopic tracers. Forthe first, a ‘spike’ was added to each natural sample(labelled ‘XS’). This solution required a very different isotopicratio from the natural samples for the three elements ofinterest. Spiked standard solutions were used to monitorbias and drift of measured isotopic ratios in the ICP-MS dur-ing a run. This solution was labelled ‘XG’ and it was pre-pared to have isotopic ratios close to those present in thespiked samples, to minimise bias inherent in measurementby ICP-MS. An accurate characterisation of the isotopicratios in XG and both the isotopic ratios and concentrationsin XS was crucial to implementing the method.

A minimum of � 10 lg of CaCO3 was weighed usinga microbalance into a stainless steel container and thentransferred into an autosampler vial, where it was dissolvedin 200 ll of 5% v ⁄ v HNO3 and spiked with 1 ll of XS tra-

cer for each lg of CaCO3. The mass of the sample wasdetermined only to aid the rough calculation of therequired amount of tracer and therefore � 30% uncer-tainty in mass could be tolerated. Total Ca concentrationsbetween 0.5 mmol l-1 and 1 mmol l-1 were normallyobtained. Alternatively, larger samples were dissolved in avial with enough nitric acid to obtain a 4 mmol l-1 total Casolution and an aliquot of this solution was mixed in anautosampler vial with the mixed spike, and diluted with5% v ⁄ v HNO3 to a total Ca concentration between0.5 mmol l-1 and 1 mmol l-1. The isotopic ratios attained inthis way for coral samples were: 24Mg ⁄ 25Mg � 0.4,48Ca ⁄ 43Ca � 0.12 and 88Sr ⁄ 87Sr � 1.2, close to the opti-mal ratios predicted by isotope dilution error equations of0.34, 0.04 and 1 respectively. In the case of Ca, a ratiothree times larger was used in order to minimise the use ofthis costly isotope, although this does not entail a significantdeterioration of the method precision (Hearn et al. 2005).Solutions were aspirated in aliquots of about 50 ll (for thelowest flow rate nebuliser), at least three times during theanalysis sequence. The same volume of solution XG,diluted to match the chosen total Ca concentration for thesamples, was run between sample test portions, and itsmeasured ratios used to correct for mass bias. Typicalwash-out times of 1 min or less for the cooled dual spraychamber were enough to reach intensity levels comparableto the background signal, usually below 5 kcps. After about18 hr, the precision declined because of the deposition ofsolids on the cones, allowing a maximum throughput ofabout fifty samples per day.

43Ca2+, 24Mg+, 25Mg+, 43Ca+, 87Sr2+, 48Ca+, 86Sr+,87Sr+ and 88Sr+ beams were collected in low resolutionmode in the sector field mass spectrometer using the fol-lowing conditions: 10% mass window, 200 masses perpeak, and 10 ms acquisition time per sample. The settlingtime for the magnet was selected automatically by theinstrument’s software, although some experiments were per-formed with different values for this parameter. The stan-dard deviation (1s) for fifty replicates was usually below1.5% and the minimum intensity allowed for any of the iso-topes, excluding doubly-charged ions, was 3 Mcps. Usu-ally, this minimum intensity was achieved by using 10 lgof carbonate, although in occasions when the instrumentalsensitivity was lower than usual or when the Mg content ofthe sample was below � 2 mmol mol-1, a larger masswas required to fulfil this requirement. The largest back-ground correction, around 0.2%, was applied to the24Mg+ ⁄ 25Mg+ intensity ratio, since 24Mg+ usually had thesmallest intensity when analysing corals. For sclerospongesand some stalagmites, having much higher Mg contentthan corals, the lowest intensity corresponded to 48Ca+, in

ª 2010 The Authors. Geostandards and Geoanalytical Research ª 2010 International Association of Geoanalysts 2 5

which case the highest background correction wasapplied, usually � 0.1%. The RM BAM RS3 (Federal Insti-tute for Materials Research and Testing, Germany) repre-sents a special case, where Mg ⁄ Ca is 0.8 mmol mol-1. Inthis case, it was necessary to dissolve a larger amount ofmaterial (� 30 lg) in order to maintain the backgroundcorrection for 24Mg+ lower than 0.2%.

In a previous ICP-MS method using an Element instru-ment (Marchitto 2006), the drift in the cross-calibrationparameter between the two modes of the detector (ana-logue and counting) prevented precise results when acquir-ing ratios by using mixed detection, i.e., the less abundantisotope measured using counting mode and the moreabundant using analogue mode. An example of theamount of drift in the case of our sector field instrument isshown in Figure 1, where it can be observed that the valuefor a ratio obtained using mixed detection varied consider-ably more than for a pure analogue or pure counting ratio.Even when it was possible to include a cross-calibrationprocedure during a sequence by measuring an isotope inthe appropriate intensity range and the detector set to‘both’ mode, it was observed that this procedure yielded alower precision, in addition to slowing down the wholemeasurement. The preferred detection mode was ana-logue for all the isotopes measured for the samples, RMsand blanks.

The contribution of the doubly-charged ion 48Ca2+ tothe 24Mg+ beam was estimated by measuring43Ca2+ ⁄ 43Ca+ in the sector field instrument. The valueobtained for this ratio was about 0.7%, and assuming thatthis also holds for 48Ca2+ ⁄ 48Ca+, a correction of about 2%on 24Mg+ ⁄ 25Mg+ was required for a sample with Mg ⁄ Cavalues close to those found in corals (around 3 mmolmol-1). Because of the magnitude of this correction, themeasurement of the ratio 43Ca2+ ⁄ 43Ca+ was added to themethod, and the correction applied offline. In the samefashion, the impact of 86Sr2+ on 43Ca+ was considered.The ratio 87Sr2+ ⁄ 87Sr+ was consistently 2.5% and the cor-rection that this value entailed for 43Ca+ ⁄ 48Ca+ was about0.5%. 87Sr2+ and 86Sr+ were thus measured and used,together with the 86Sr+ intensity, to correct 48Ca+ ⁄ 43Ca+. Itwas also important to monitor the existence of Kr as a con-taminant of argon. Usually the correction due to its pres-ence was unimportant because the ratio 86Kr2+ ⁄ 86Kr+ isfifty times smaller than the one for Sr and the Kr is onlypresent in very low concentrations, although occasionallyhigh levels were observed.

A number of possible interferences other than thedoubly-charged ions were identified by acquiring spectrain medium and high resolution modes when using theElement ICP-MS. The interferences [14N(16O)2H]+,[14N18O16O]+ and possibly [32S16O]+ on 48Ca+ wereobserved both in HNO3 and Ca solutions. Their combinedintensity represented, at most, 0.2% of the signal obtainedfor 48Ca+ in a 1 mmol l-1 total Ca solution, so it wasassumed that their contribution was stable and could becorrected with the background measurements. Weobserved an even smaller effect of [(14N)3H]+ on 43Ca+.The presence of 48Ti+ was also studied at the highest reso-lution available (about 10000, where 48Ti+ and 48Ca+

could be resolved) and its presence could not be detectedfor a surface coral solution. The presence of Ti was notstudied for sclerosponges or stalagmites. Similarly, the pres-ence of [48Ca40Ar]+, a possible interference on 88Sr+, wasnot observed.

Over one year, the average mass bias (and 1s uncer-tainties) for 24Mg+ ⁄ 25Mg+, 43Ca+ ⁄ 48Ca+ and 88Sr+ ⁄ 87Sr+

using the sector field ICP-MS were 18 ± 6%, 13 ± 4%and 2 ± 0.6%, respectively. The mass bias drift within a runwas variable from day to day and its magnitudedepended on the mass calibration stability as well as theconditions of the introduction system, especially of thecones. It was observed that the peak shapes of 24Mg+ and48Ca+ played a crucial role in the amount of drift during arun. In general, these two isotopes presented peak topsthat were less horizontal than those of the other isotopes,

7.2

7.4

7.6

24M

g+ /

25M

g+

7.8

8.0

0 250 500Time (minutes)

750 1000

Figure 1. Mixed detection mode drift of raw ratios.24Mg+ ⁄ 25Mg+ measured for the same solution at

three different concentrations and corresponding

detection modes: open squares, both 24Mg+ and25Mg+ in analogue mode; open triangles, both24Mg+ and 25Mg+ in counting mode, filled-in circles,24Mg+ in analogue mode and 25Mg+ in counting

mode.


and thus the average intensities were more sensitive tosmall changes in the mass calibration. These characteristicpeak shapes for the different isotopes remained consistentfor most of the runs and did not change substantially whenthe settling time for the magnet or the focus offset voltagewas modified. Due to this behaviour, the amount of drift for24Mg+ ⁄ 25Mg+ and 43Ca+ ⁄ 48Ca+ was usually larger thanthat corresponding to 88Sr+ ⁄ 87Sr+ and 88Sr+ ⁄ 86Sr+. Theaverage relative difference between consecutive ratioswhen comparing different aliquots of solution XG wasaround 0.4% for Mg and Ca ratios and 0.2% for Sr ratios.Consequently, the drift correction for Mg and Ca was animportant component of the precision of the method.

In order to monitor this behaviour, peak shapes werequantified by the relative standard deviation (RSD) of thetwenty intensity values collected for each peak. Since, ingeneral, these intensities changed monotonically with mass,the RSD was an indicator of the slope of the peak top.Figure 2 shows the correlation between the peak top RSDcorresponding to 24Mg+ and the relative differencebetween consecutive values of 24Mg ⁄ 25Mg for a run withan exceptionally high amount of drift. Monitoring of thepeak shape was used as a quality control tool for themethod: a sample was rejected when the relative differ-ence between the bracketing ratios corresponding to XGsolutions was > 0.75%. Due to this peak shape drift, it wasobserved that the precision improved when the sampleswere run in three or more aliquots, with an XG solution runin between, rather than by using a longer single collectiontime interval. In spite of some deterioration of the replicate

statistics for the shorter sample time (2 min per aliquot), theaverage of three or more drift corrected replicates wasfound to be a more reliable value.

Blank, interference and mass bias corrections, using lin-ear interpolation, were applied offline to obtain the cor-rected ratios RMg = 24Mg ⁄ 25Mg, RCa = 48Ca ⁄ 43Ca andRSr = 88Sr ⁄ 87Sr. The mass bias correction factor for eachelement was calculated as the ratio between the calibratedand the measured value of each of those isotopic ratios forthe bracketing solution XG. The isotope dilution equationwas then used to calculate Mg ⁄ Ca and Sr ⁄ Ca present inthe sample as follows:

MgCa¼ðRMg � CMgÞðRCa �HCaÞC25 A48

ðRCa � CCaÞðRMg �HMgÞ C43 A24ð1Þ

SrCa¼ ðRSr � CSrÞðRCa �HCaÞC87 A48

ðRCa � CCaÞðRSr �HSrÞ C43 A88ð2Þ

where CMg = 24Mg ⁄ 25Mg, CCa = 48Ca ⁄ 43Ca andCSr = 88Sr ⁄ 87Sr are the isotopic ratios in the mixed spike;C25, C43 and C87 are the molar concentration of isotopes25Mg, 43Ca and 87Sr in the mixed spike respectively;QMg = 25Mg ⁄ 24Mg, QCa = 43Ca ⁄ 48Ca and QSr =87Sr ⁄ 88Sr are the isotopic ratios in the sample, assumed tobe natural; A24, A48, A88 are the natural isotopic abun-dances of 24Mg, 48Ca and 88Sr respectively. See below(Calibration) for the values used for the calculation.

Minor changes in the method were needed when theQ-ICP-MS Agilent 7500ce was used to implement themethod. The measured beams for spiked samples andRMs were in this case 24Mg+, 25Mg+, 43Ca+, 48Ca+, 86Sr+,87Sr+ and 88Sr+, with the following conditions: 0.12 s totalintegration time (0.04 s at three masses), analogue detec-tion and hundred replicates. Instead of considering theintensity of doubly-charged ions at half masses (43Ca2+and87Sr2+), a different approach was implemented in thequadrupole instrument to correct the intensities of 24Mg+

and 43Ca+ respectively. Natural Ca and Sr standard solu-tions were included in the sequence and the fractions ofdoubly-charged ions for each of these two elementswere obtained through the ratios 44Ca2+ ⁄ 44Ca+ and88Sr2+ ⁄ 88Sr+. The values obtained were close to 0.5%and 1.2% respectively, and good agreement was foundwhen other ratios were considered to estimate the fractionof doubly-charged ions, e.g., 48Ca2+ ⁄ 48Ca+ and86Sr2+ ⁄ 88Sr+. The average mass bias (and 1s uncertainties)for 24Mg+ ⁄ 25Mg+, 43Ca+ ⁄ 48Ca+ and 88Sr+ ⁄ 87Sr+ were inthis case 4 ± 1%, 15 ± 1% and 2 ± 0.3% respectively,while the average relative difference between consecutiveratios when comparing different aliquots of solution XG

-5

-3

0

D(24

Mg

/25 M

g)

(%)

3

-4 -2 024Mg peak top (% RSD)

2 4

Figure 2. Correlation between isotopic ratio drift and

peak shape. Relative 24Mg+ ⁄ 25Mg+ difference

between two consecutive measurements of solution

XG [D(24Mg+ ⁄ 25Mg+)] versus the relative standard

deviation of the peak top intensity of 24Mg+ (N = 20)

(24Mg+ peak top RSD).


was 0.2% for Mg and Ca ratios and 0.1% for Sr ratios.These values, about half of those observed for the Elementinstrument, played a lesser role in the error associated withdrift. Finally, analyses of sample solutions were run in onlyone test portion.

Reference materials and tracer

To calibrate the solutions for a given element, both isoto-pic and concentration calibrators (normally RMs) wererequired. Ideally, a single RM could be used for both pur-poses, but we were only able to do this for strontium. NISTSRM 987 (from the National Institute of Standards andTechnology) is an isotopic certified RM (CRM). Its Ca andMg levels reported (5 lg g-1 and < 2 lg g-1, respectively)had a very small effect on the Mg-Ca-Sr synthetic mixedstandard solutions used for the accuracy test or calibrationpurposes (see below). The analytical and isotopic Sr stan-dard solution was prepared by weighing this material afterdrying at 110 �C for 2 hr and dissolving it in 5% v ⁄ vHNO3. A magnesium metal CRM (NIST SRM 980) wasobtained and dissolved in 5% v ⁄ v HNO3 to prepare an iso-topic standard solution. Because this material was partiallyoxidised and its purity not reported, distilled magnesium99.999% (47, 475-4 Aldrich; St. Louis, Missouri, USA) wasused as an analytical calibrator. Trace contamination of Caand Sr had no effect on synthetic mixed standard solutions.The lustrous metal was weighed under argon and dissolvedin dilute nitric acid. Finally, the isotopic calcium referencesolution was prepared from CaF2 used previously by Russellet al. (1978). A commercial CaCO3 99.999% (Aldrich48,180-7), dried at 125 �C for 2 hr, was the source for theanalytical Ca standard solution. The Sr content of this stan-dard solution was estimated as 12 lg g-1 and this valuewas used to correct the Sr concentration when this solutionwas used to prepare synthetic mixed standard solutions.Gravimetric dilutions were prepared from all the isotopicand analytical standard solutions in 5% v ⁄ v HNO3.

25Mg enriched MgO (Series RN Batch 217201), 43Caenriched CaCO3 (NX Batch 169191) and 87Sr enrichedSrCO3 (Series LH Batch 136990) were purchased from theOak Ridge National Laboratory and used to prepare themixed tracer solution XS. These materials were separatelydissolved in 5% v ⁄ v HNO3 and then aliquots of the solu-tions mixed gravimetrically to obtain the spike solution. Ourgoal was to make a mixed spike that when added to natu-ral samples yielded ratios that were optimised for the iso-tope dilution method assuming natural Mg ⁄ Ca and Sr ⁄ Caratios of 3 mmol mol-1 and 10 mmol mol-1 respectively.Finally, a solution made from a coral specimen (Porites lu-tea) was mixed with an aliquot of the mixed tracer XS in

order to obtain the spiked mass bias calibrator XG, withisotopic ratios 24Mg ⁄ 25Mg, 48Ca ⁄ 43Ca and 88Sr ⁄ 87Srclose to those of spiked samples. This solution was used tocorrect for mass bias during ICP-MS runs by sample brac-keting. The coral was cleaned repeatedly with distilledwater under ultrasound treatment, leached with diluteHNO3 and dried at 110 �C for 2 hr.

Calibration

Both the concentration and isotopic characterisation ofthe mixed tracer XS were required to implement the iso-tope dilution method. On the other hand, the isotopic com-position of solution XG was needed to correct for massbias. This procedure relied on the knowledge of the isoto-pic abundances and concentrations of individual Mg, Caand Sr natural analytical standard solutions. Once thesewere known, it was possible to calibrate (a) isotopic ratiosof solutions XS and XG by comparing intensity ratios mea-sured for those solutions and for the standard solution cali-brators and (b) concentrations of mixed tracer XS byobtaining the intensity ratios for gravimetric mixtures pre-pared from XS and the individual analytical standard solu-tions (reverse isotope dilution). Table 1 reports the results ofthe calibration. It should be noted that the isotopic ratios forthe mixed tracer XS, needed in Equations (1, 2) cannot beassumed to be equal to the values reported by Oak RidgeNational Laboratory since cross-contamination is importantin one case (Sr, see below).

To obtain the isotopic abundances of the Mg analyticalstandard solution, 24Mg+, 25Mg+ and 26Mg+ intensitieswere measured in the sector field instrument for an alter-nate sequence of isotopic and analytical standard solutionspossessing similar total Mg concentration. The reported iso-topic ratios for the NIST CRM (see Table 1) were used tocorrect for mass bias and drift. The range of three meansfor 24Mg ⁄ 25Mg and 24Mg ⁄ 26Mg spanning three differentdays were 0.15% and the intra-run precision was about1% RSD. The abundances calculated from these two ratiosare reported in Table 1, with errors propagated from errorsin the ratios estimated as three times the range obtained.

For the Ca calibration, 48Ca ⁄ 43Ca intensity ratio wasinitially compared for both standard solutions (isotopic andanalytical) using the Element ICP-MS. In this case, the preci-sion of the 48Ca+ ⁄ 43Ca+ ratio on ten separate days over1 year was about twenty times larger than the Mg case,close to 2%, and on several occasions a drift over timewas observed. We speculate that the difference in thematrix of the two Ca solutions (the isotopic standardsolution was prepared before the start of this research in


1% v ⁄ v HCl) could have been in part responsible for thisamount of variability by affecting some of the processes atthe spectrometer introduction system, such as deposition onthe cones or adsorption on the spray chamber (Andrenet al. 2004). Thus, given this poor precision we could notrule out an isotopic difference between the analytical andisotopic Ca standard solutions. Moreover, commercial syn-thetic CaCO3 have previously been found to be isotopi-cally distinct from natural samples (Russell et al. 1978).

In order to circumvent this shortcoming, the Ca isotopicand analytical standard solutions together with two morenatural abundance Ca solutions (Durango Apatite anddeep sea coral) were analysed using a multi-collector ICP-MS (Neptune). In addition, an aliquot of the isotopic Castandard solution (prepared in 1% v ⁄ v HCl) was evapo-rated and dissolved in 5% v ⁄ v HNO3, thus matching thematrix of the other standard solutions. The intensities of43Ca+, 87Sr2+, 44Ca+, 48Ca+, 86Sr+, 87Sr+ and 88Sr+ weremeasured in medium resolution (to optically resolve interfer-ences on 44Ca+ and 48Ca+). The non-resolvable interfer-ences of 86Sr2+ on 43Ca+ and 88Sr2+ on 44Ca+ werecorrected offline using the intensity of 87Sr2+ and the mea-sured 86Sr+ ⁄ 87Sr+ and 88Sr+ ⁄ 87Sr+ ratios, as was explainedin the previous section. Each sample was preceded by apre-wash, wash and blank, and an offline blank subtrac-tion was applied. Solution XG was measured every thirdsample; peak centring on 87Sr+ and 43Ca+ was performedjust prior to measuring each standard solution. Becausesolution XG was designed for ID analysis, its isotopic abun-dance was very different from that found in natural Casamples. To minimise a systematic memory bias due to themeasurement of very different ratios, samples were

analysed in random order, with different samples followingsolution XG throughout the run. During the 13-hr run, solu-tion XG was measured nine times with an overall drift inthe measured 43Ca+ ⁄ 48Ca+ and 44Ca+ ⁄ 48Ca+ intensityratios of 0.22% and 0.15%, respectively. To correct for driftin instrumental mass fractionation, a linear model was usedto interpolate between the bracketing XG measurements.The 43Ca+ ⁄ 48Ca+ and 44Ca+ ⁄ 48Ca+ ratios for all thematerials were the same after drift correction within 0.1%,and the values were in good agreement with the averagesreported by Russell et al. (1978).

The results for the Mg and Ca standard solutions,together with reported Sr reference sample abundances,are shown in Table 1. The Mg analytical reference samplewas enriched in 25Mg as compared with the isotopic refer-ence, and the difference is consistent with the distillationprocess used in the purification (Esat et al. 1986). Consid-ering the narrow range of Ca fractionation in nature (DeLa Rocha and DePaolo 2000, Wieser et al. 2004, Steuberand Buhl 2006) and the agreement (within the experimen-tal error of 0.1%) of isotopic ratios 48Ca ⁄ 43Ca and44Ca ⁄ 48Ca between analytical and isotopic Ca referencesamples, their isotopic abundances were assumed to beequal for all isotopes. The uncertainty reported in Table 1for 43Ca and 48Ca abundances were calculated assuminga 0.1% difference in the 43Ca ⁄ 48Ca isotopic ratio betweenthe analytical and isotopic Ca reference samples andusing an exponential fractionation law.

The isotopic ratio calibrator XG was itself calibratedusing the Mg, Ca and Sr isotopic reference samples in thesame fashion. In this case, the substantial difference

Table 1.Isotopic abundancesa of reference samples

Isotope Mg isotopicb Mg analyticalc Ca isotopicd Ca analyticale Srf

24Mg 78.99(3) 78.7(4)25Mg 10.00(1) 10.1(3)26Mg 11.01(2) 11.2(3)40Ca 96.982(9)42Ca 0.64214(6)43Ca 0.13340(2) 0.1334(2)44Ca 2.0568(1)48Ca 0.18245(5) 0.1825(6)84Sr 0.5574(15)86Sr 9.8566(34)87Sr 7.0015(26)88Sr 82.5845(66)

a Abundances are in mole percent. The number in parentheses represents the uncertainty, which corresponds to the last figure shown. The uncertaintyreported for Mg analytical reference samples was propagated from the uncertainty of isotopic ratios estimated as three times the range(N = 3). b NIST SRM 980, Isotopic Mg (oxide) CRM. c Mg, distilled, dendritic pieces, 99.999%, Aldrich 47,475-4 Lot No16734LO. d Solution obtained from CaF2 (Russell et al. 1978). e CaCO3 99.999 +% Aldrich 48,180-7 Lot No 11511AO.f NIST SRM 987, isotopic and analytical SrCO3 CRM.


between the ratios of the reference samples (close to natu-ral abundances) and those of XG required special condi-tions, including wash-out times of at least 10 min and theuse of HF, with concentrations � 1 lmol l-1 for about aminute as an additional rinsing step. Intensity ratios of24Mg+ ⁄ 25Mg+, 24Mg+ ⁄ 26Mg+, 48Ca+ ⁄ 43Ca+, 88Sr+ ⁄ 86Sr+

and 88Sr+ ⁄ 87Sr+ were compared, and 48Ca2+ and 86Sr2+

interferences on 24Mg+ and 43Ca+ respectively were cor-rected by measuring the intensities of ions 43Ca2+ and87Sr2+ (see above). With the calibration of XG in hand, theisotopic ratios of the mixed spike XS were calibrated bycomparison with the XG solution, which was also used tocorrect for mass bias and drift. Finally, gravimetric mixturesof analytical standard solutions and mixed tracer XS wereprepared and their isotopic ratios measured in the sectorfield mass spectrometer and corrected offline in a similarway as that reported above (Method). Using the isotopicratios for the mixed tracer and reverse isotope dilution, theconcentrations for the mixed spike were obtained.

Table 2 shows the isotopic ratios for solutions XG andXS and Table 3 compares the abundances of individualtracers, as reported by Oak Ridge National Laboratory,with the abundances for the mixed tracer calculated fromthe measured ratios. In the case of Mg, the abundances inXS were calculated from the measured 24Mg ⁄ 25Mg and24Mg ⁄ 26Mg ratios. Since all three Mg isotopes were col-lected, the corresponding abundances could be calculatedwithout any assumption. Although the abundance obtainedfor 24Mg agreed within uncertainty with that reported by

the Oak Ridge National Laboratory, there was a possibleincrease of the 24Mg and the 26Mg abundance for themixed tracer with respect to the Oak Ridge National Labo-ratory values. This difference is consistent with cross-contam-ination, as reported Mg impurities were < 100 lg g-1 forboth 43Ca and 87Sr tracers.

In the case of Ca, only the ratio 48Ca ⁄ 43Ca was mea-sured, which agreed within experimental error with thatreported by Oak Ridge National Laboratory. The cross-con-tamination was estimated considering the reported Cacontent in 25Mg and 87Sr individual tracers and themasses and concentrations of individual tracer solutionsused to make up the mixed tracer solution XS. Assuming anatural isotopic composition for the contaminant Ca andusing the measured 48Ca ⁄ 43Ca value, it is possible to cal-culate the abundances for all Ca isotopes in the mixed

Table 3.Isotopic abundances of individual and mixed (XS) standard solutionsa and concentration of mixedstandard solutions

Isotope Mgb (%) Cac (%) Srd (%) XS

(%) (nmol g-1)

24Mg 0.963(10) 0.972(2) 0.512(6)25Mg 98.814(20) 98.8(4) 52.07(17)26Mg 0.223(5) 0.227(1) 0.120(3)Mg 52.70(9)40Ca 10.13(8) 10.15(8) 16.47(13)42Ca 0.780(6) 0.78(1) 1.27(1)43Ca 83.93(10) 83.91(11) 136.2(2)44Ca 5.06(3) 5.06(3) 8.21(5)48Ca 0.090(1) 0.089(1) 0.145(1)Ca 162.3(1)84Sr 0.06(3) 0.010(5) 0.010(6) < 0.0186Sr 88(4) 0.82(2) 1.90(8) 1.19(5)87Sr 3(2) 91.26(10) 90.12(12) 61.0(1)88Sr 9(3) 7.91(10) 7.98(10) 5.40(1)Sr 67.58 (5)

a Abundances are in mole percent. The number between parentheses represents the uncertainty which correspond to the last figuresshown. b 25Mg enriched MgO from Oak Ridge National Laboratory Series RN Batch 217201. c 43Ca enriched CaCO3 from OakRidge National Laboratory Series NX Batch 169191. d 87Sr enriched SrCO3 from Oak Ridge National Laboratory Series LH Batch 136990.

Table 2.Isotopic ratios of spiked stock solution XG andtracer XS

XG XS

Average SE (2s ) N Average SE (2s) N

24Mg ⁄ 25Mg 0.515 0.001 13 0.00984 0.00004 524Mg ⁄ 26Mg 6.99 0.04 3 4.27 0.18 348Ca ⁄ 43Ca 0.1152 0.0004 14 0.001063 0.000008 588Sr ⁄ 86Sr 7.9 0.04 3 4.52 0.16 388Sr ⁄ 87Sr 1.1211 0.0008 11 0.08849 0.00004 3

SE, standard error or standard deviation of the mean.


spike. As can be observed in Table 3 by comparing thevalues for the individual Ca tracer (reported by ORNL) andthose calculated for XS, the change due to the contamina-tion was smaller than the uncertainty for all the isotopeswith the exception of 48Ca, for which the difference wascomparable to the calculated uncertainty.

84Sr abundance was assumed equal to that of theOak Ridge tracer, a reasonable approximation consideringthe low abundance of this isotope. The substantial differ-ence between individual and mixed tracer 86Sr abun-dance was due to the Sr content in the 43CaCO3 tracer,reported as 5 mg g-1. The isotopic composition of thisimpurity was measured confirming the expected enrich-ment in 86Sr due to the electromagnetic separation methodfor 43Ca tracer production, which cannot separate com-pletely 43Ca+ ions from the 86Sr2+ ions. The correctionfor Sr contamination in the 25Mg tracer (reported< 100 lg g-1) was negligible.

The total Mg, Ca and Sr concentrations for the mixedtracer XS and their standard errors (standard deviation ofthe mean) (SE 1s, N ‡ 4) are also reported in Table 3.Using the total concentration for a given element and theabundances for the different isotopes in the tracer, it waspossible to calculate the isotopic concentrations, reportedin nmol g-1. The larger relative uncertainties for the concen-tration of individual isotopes as compared with the preci-sion of the total element concentration reflect the combineduncertainties obtained from the uncertainties in the abun-dances and the total concentrations.

Matrix effect and accuracy

The matrix effect was studied for a solution from adeep sea coral as a function of the total Ca concentrationand is shown in Figure 3. The error bar assigned to eachpoint is the standard deviation of three 50 ll aliquots runfrom the same solution. The precision for the twelve differentCa concentrations measured was 0.8% (2s), for bothMg ⁄ Ca and Sr ⁄ Ca ratios, and no trend was detected. Boththe Mg ⁄ Ca and the Sr ⁄ Ca values were randomly scatteredaround the mean over about a factor of ten of the total Caconcentration. We conclude that there was no matrix effectin our method at the 1% level.

In addition, the matrix effect can also be a function ofMg and Sr concentration. In order to assess the range ofapplication for the method and as a test for its accuracy,nine synthetic solutions were prepared gravimetrically usingthe analytical Mg, Ca and Sr standard solutions. The calcu-lated Mg ⁄ Ca and Sr ⁄ Ca values spanned a decade

around the average values for deep sea corals. The pres-ence of contaminant Sr in the Ca analytical standard solu-tion, estimated as 12 lg g-1, entailed a correction for thecalculated Sr ⁄ Ca value in the synthetic solution of at most0.4%. This number can be contrasted with the correctionestimated just from uncertainties in the masses and concen-trations of under 0.1%.

Figure 4 shows the difference between the measuredand calculated Mg ⁄ Ca and Sr ⁄ Ca values for nine syntheticsolutions. Measured Mg ⁄ Ca values were on average 0.5%smaller than those calculated and an improvement in theagreement could be observed for the low Mg values. Theeffect of Sr on the Mg ⁄ Ca values showed no cleartrend. Synthetic samples with Mg ⁄ Ca contents up to� 15 mmol mol-1 could be balanced within � 0.8%,regardless of Sr content. However, the error for the abun-dance of 24Mg in the analytical Mg standard solution(0.5%) could shift the points horizontally; for instance, usingand abundance of 78.3% for 24Mg the points would bescattered around the origin within 0.2% accuracy.

Strontium measurements showed, on the contrary, amarked trend with the Mg content of the sample, decreas-

3.01

3.06

Mg

/Ca

(mm

ol m

ol-1

)S

r/C

a (m

mo

l mo

l-1)

3.11

10.1

10.3

10.5

0 0.5 1 1.5

[Ca]TOTAL (mmol l-1)2

0 0.5 1 1.5

[Ca]TOTAL (mmol l-1)2

(a)

(b)

Figure 3. Matrix effect. (a) Mg ⁄ Ca and (b) Sr ⁄ Ca

ratios as a function of total Ca concentration. The

uncertainties represent the standard deviation of

three replicates.


ing by almost 1% when Mg ⁄ Ca increased from 1 to15 mmol mol-1. The change of the measured Sr ⁄ Ca ratiowith the amount of Mg present in the sample limited theexpected accuracy to about 0.8%, but for low Mg contentsolutions, that number fell under 0.3% when Sr ⁄ Ca wasabove 9 mmol mol-1. In summary, the bias of Sr measure-ments depended on the Mg content: an overall value of0.8% could be established for the whole range of Sr andMg values considered, but for solutions with low Mg con-tent and high Sr, the bias was below 0.3%. In this case, theuncertainty for the Sr concentration in the synthetic solutions,due to the Sr contamination in the Ca standard solution,could modify this picture to some degree. The presence ofSr in the solid CaCO3 used as the analytical calibratorwas estimated as 12 ± 8 lg g-1 using a standard addi-tion procedure. For the higher limit of this range, the threepoints with a value of Sr ⁄ Ca = 3 mmol mol-1 would shiftdownward by 0.3%, while the points at higher Sr contents

would not be modified. In this case, the agreementbetween measured and calculated Sr ⁄ Ca would improvefor the two solutions with lower Mg content, while the pointcorresponding to the solution with Mg ⁄ Ca � 15 mmolmol-1 would resemble the behaviour for the two solutionswith higher Mg and Sr.

Results and discussion

A number of carbonate materials were considered inthis study: corals (Desmophylum dianthus and Porites lutea),a calcitic sclerosponge from the North Atlantic, a stalagmitefrom Borneo, a soil carbonate from Johnson Mesa, Utah,and a tufa from Red Butte Canyon, Utah. The Mg ⁄ Ca ratiovaried from 3 mmol mol-1 for the coral to about200 mmol mol-1 for the sponge. This higher value fell out-side the application range of our method, while the Sr ⁄ Carange was more modest, from 0.4 mmol mol-1 in the soilcarbonate to 10 mmol mol-1 in the corals. In addition tothese, five RMs were studied: NIST SRM 8544 (Friedmanet al. 1982); UN AK (Institute of Mineral Raw Materials,Czech Republic); ECRM 752-1 (Bureau of Analysed Sam-ples Ltd, UK); BAM RS3 and CM 1767 (Metallurgical Stan-dardisation Research Institute, China).

Figure 5 shows the results obtained for a stalagmiteand a soil carbonate, which were sampled with a 25 lmresolution along the axis of growth using a mill (MicroMill,New Wave Research) with a tungsten carbide tool(H21.11.009 Brasseler; Savannah, Georgia, USA.). For thestalagmite, both Mg and Sr content displayed a pro-nounced decrease around 0.4 mm from the origin of sam-pling, down to values as small as one-third of the average.Although the Mg ⁄ Ca values around 30 mmol mol-1

obtained for the stalagmite were outside the range whereaccuracy was tested, the increase of the uncertainty due tothe isotope dilution equation was only 0.1% for thisamount of departure and we expect the accuracy to bealso around 0.8%, as was shown in the previous section.The soil carbonate total variation for Sr ⁄ Ca, of only0.2 mmol mol-1, illustrates the precision of the method inthe low Sr region and also suggests a lower limit of appli-cation in the lmol mol-1 range. The variation found in thestalagmite (a factor of three) was also observed for thedeep sea corals Mg ⁄ Ca variation around the centralbands found in their septae (Gagnon et al. 2007).

In order to compare meaningfully environmental sig-nals from different laboratories, accurate methods over abroad range of Mg and Sr content are required. For thoserelying on external calibration curves, a matrix effect studyis essential. This was shown for an ICP-OES method with

-0.01

0.00

(Mg

/Ca)

mea

s/(M

g/C

a)tr

ue

- 1

(Sr/

Ca)

mea

s/(S

r/C

a)tr

ue

- 1

0.01

-0.01

0.00

0.01

0 5 10Mg/Ca (mmol mol-1)

Sr/Ca (mmol mol-1)

15 20

0 10 20 30

(a)

(b)

Figure 4. Accuracy test. (a) Mg ⁄ Ca ratio relative devi-

ation of measured from calculated values for mixed

synthetic standard solutions with three different Sr

content: 3 mmol mol-1 (diamonds); 9 mmol mol-1

(circles); 28 mmol mol-1 (triangles). (b) Sr ⁄ Ca relative

deviation of measured from calculated values for

mixed synthetic standard solutions with three differ-

ent Mg contents: 1 mmol mol-1 (diamonds); 4 mmol

mol-1 (circles); 15 mmol mol-1 (triangles).


sub-percent accuracy over Mg ⁄ Ca and Sr ⁄ Ca ranges simi-lar to the one shown in the present work (de Villiers et al.2002). The matrix effect for this method, due to the Ca self-absorption and the increased background on Mg and Sremission intensities, was as large as 15% and correctedusing an intensity ratio calibration. In this way, it was possi-ble to obtain precise results by utilising a number of RMsor standard solutions with the same Ca concentrations asthe samples and a broad range of Mg and Sr concentra-tions. However, the impact of Mg content on the Sr ⁄ Caratio was not dealt with by these authors.

In order to study the long term behaviour of the presentID-ICP-MS method and to monitor the trustworthiness forbatches of samples run on different days, three solutionswere prepared by dissolving � 50 mg of deep sea coral(Desmophylum dianthus), calcitic sclerosponge and stalag-mite from Borneo; each had a total Ca content of4 mmol l-1. The long term reproducibility of the methodwas studied by measuring the consistency of the standardsolutions over a period of at least 1 year. Figure 6 showsMg ⁄ Ca and Sr ⁄ Ca values obtained for the deep sea coralsolution over 5 years with two different instruments: sector

field and quadrupole ICP-MS. Each point represents theaverage of at least six measurements of the coral solutiondiluted to a Ca concentration of about 1 mmol l-1 and theuncertainty shown is two standard deviations (2s). In addi-tion, two points are shown associated with runs in whichsamples with very different Mg ⁄ Ca and Sr ⁄ Ca ratios werealso included: NIST SRM 8544 (limestone) (Mg ⁄ Ca �18 mmol mol-1, Sr ⁄ Ca � 0.2 mmol mol-1) and UN AK(Mg ⁄ Ca � 2.4 mmol mol-1, Sr ⁄ Ca � 3 mmol mol-1).Excluding these two points, the inter-run precision (2s) forboth Mg ⁄ Ca and Sr ⁄ Ca was close to 0.4% for the sectorfield instrument values, while the quadrupole instrumentvalues were 2.6% and 0.8%, respectively. Intra-run preci-sion was as high as 4.6% for the sector field instrument,while the quadrupole instrument’s worst case was 1.4%.The averages obtained for the two sets corresponding tothe different instruments (excluding the two points men-tioned previously) agreed within 0.1% for Mg ⁄ Ca and0.2% for Sr ⁄ Ca.

0

10

20

Mg

/Ca

(mm

ol m

ol-1

)S

r/C

a (m

mo

l mo

l-1)

30

40

0

5

10

0 0.5Distance (mm)

Distance (mm)

1

0 0.5 10.2

0.6

1.0

(a)

(b)

Figure 5. (a) Mg ⁄ Ca and (b) Sr ⁄ Ca ratios for a stalag-

mite (circles) and a soil carbonate (diamond) sampled

from an arbitrary origin along the axis of growth.

2.9

3.0

3.1

Mg

/Ca

(mm

ol m

ol-1

)S

r/C

a (m

mo

l mo

l-1)

3.2

Jun 04 Mar 07 Dec 09

10.0

10.2

10.4

10.6

Jun 04 Mar 07 Dec 09

(a)

(b)

Figure 6. (a) Mg ⁄ Ca and (b) Sr ⁄ Ca ratio long-term

precision tests for a deep sea coral solution. Values

obtained with the sector field ICP-MS are shown by

diamond symbols, and values obtained with the

quadrupole ICP-MS are shown by triangles. Values

obtained with the quadrupole ICP-MS for runs in

which samples with very different Mg ⁄ Ca and Sr ⁄ Ca

ratios were also measured are shown by circles.


The effect of running samples possessing Mg ⁄ Ca andSr ⁄ Ca ratios with marked differences in the same batch isillustrated by the two points shown in Figure 6 correspond-ing to the measurements of RMs NIST SRM 8544 and UNAK. In this case, in the face of a normal drift in the massbias for the isotopic ratios for solution XG, similar to thevalue routinely obtained, a clear trend with run time wasobserved for the Mg ⁄ Ca and Sr ⁄ Ca replicate values. Infact, the increased intra-run precision compared to the casewhen only the deep sea coral solution was measured isassociated with this trend. The effect is also exemplified inFigure 7, which displays the Sr ⁄ Ca reproducibility found forthe sclerosponge reference sample using the sector fieldICP-MS. Again, two different types of runs were considered:those for which only the sclerosponge reference samplewas included, showed an inter-run precision of 0.4% (2s),comparable to that found for the deep sea coral solution.When both reference samples and unknown samples werepresent in the batch, the inter-run precision increased to0.8%, and despite a deterioration of the intra-run precision,the average agreed well with the first case (just referencesample measured). We believe this effect could be relatedto the fact that the mass of test portion dissolved was lessaccurately known in the sclerosponge case, and conse-quently the isotopic ratios obtained after spiking differedmarkedly among samples. Regardless of the rinsing timeused, long enough to reduce the background intensities toless than 0.2% of the values measured for the samples, thereproducibility experienced a significant deterioration. Prob-ably, the intensity ratio values obtained for each referencesample depended on the ratio of the previous sample run,deteriorating the precision. This signals the importance of

both intensity and ratio matching between spiked samplesand solution XG in order to obtain the best possible preci-sion. Consequently, in cases such as those shown for thestalagmite in Figure 5, with marked differences betweensubsamples, batches of samples with a narrow range ofMg ⁄ Ca and Sr ⁄ Ca values should be considered in orderto achieve the best possible result.

In addition to the memory effect on ratios affecting theprecision of the method, it is important to assess the influ-ence of inadequately characterised isotopic abundancesand impurities of the RMs used. This is especially true formass spectrometry methods using calcium isotopes as inter-nal standards (Rosenthal et al. 1999, Marchitto 2006,Shen et al. 2007). In these cases, high purity materialswere used as calibrators, although no mention was madeof their isotopic composition. Russell et al. (1978) found alarge fractionation for a commercial high-purity Ca RM. Dif-ferences in isotopic abundance between samples and cali-brators can affect the outcome of the method in asignificant way. For instance, a calibrator prepared fromcommercially available high purity metal calcium, probablypurified by distillation, having a d(40Ca ⁄ 44Ca) of � 10&

(Russell et al. 1978) would have 40Ca ⁄ 43Ca, 40Ca ⁄ 46Caand 40Ca ⁄ 48Ca ratios differing by about 1%, 1.5% and2% respectively from a sample with natural abundance.Assuming that the Ca calibrator has natural abundancewould then entail a systematic error. The same holds forcommercial Mg RMs and for our case, it is conceivablethat the accuracy test for Mg is mostly limited by the uncer-tainty in the abundance of the distilled material used toprepare the synthetic solution. Furthermore, the use ofmixed synthetic standard solutions to assess the accuracy ofa method is questionable unless the impurities are carefullyevaluated. For example, as was mentioned in the previoussection, the uncertainty in the content of Sr in the Ca refer-ence sample used to prepare the synthetic solutionsobscures the evaluation of the accuracy for the low Sr con-tent range.

Detrital material can also be important when assessingthe method accuracy. A detailed study of the limestoneECRM 752-1 (Bureau of Analysed Samples Ltd, NewmalHall, Middlesbrough, UK) made it possible to propose thismaterial as a Mg ⁄ Ca RM (Greaves et al. 2005). Removalof aluminosilicate mineral phases by centrifugation wasdemonstrated as an essential step to improve the precisionof the measurement. As was suggested by these authors,the existence of well characterised solid RMs together witha basic protocol for dissolution and separation of detritalminerals would be ideal in order to establish the accuracyof a given method and to facilitate the interpretation of

1.40

1.43

1.46

Sr/

Ca

(mm

ol m

ol-1

)

1.49

26 Aug2005

10 Oct2006

12 Jul2004

Figure 7. Long-term precision for the sclerosponge

reference sample. Filled-in diamonds, runs for which

only the reference sample was run; open circles: runs

for which both the reference sample and unknown

samples were measured.


inter-laboratory studies. Reference materials ECRM 752-1,BAM RS3 and CM 1767, were also considered in a recentinterlaboratory study (Greaves et al. 2008). In this case,centrifuging produced up to a 3% decrease in the Mg ⁄ Caratio average between laboratories (BAM RS3), with asmaller impact on the interlaboratory standard deviation.The Mg ⁄ Ca value for ECRM 752-1 (3.75 mmol mol-1) fellwithin the range found for foraminifera and corals; how-ever the Sr ⁄ Ca value, 0.19 mmol mol-1, was an order ofmagnitude lower than that usually found in aragonitic car-bonates. This was also the case for every CRM consideredby Greaves et al. (2005), with the exception of UN AK withcalculated values close to 2.8 mmol mol-1 for both Mg ⁄ Caand Sr ⁄ Ca. This reference sample is also attractive due toits relatively low content of Al, suggesting a small contribu-tion of detrital materials to its Mg ⁄ Ca and Sr ⁄ Ca values.

Table 4 gives the Mg ⁄ Ca and Sr ⁄ Ca values for thenon-centrifuged solutions prepared from NIST SRM 8544,UN AK, BAM RS3, ECRM 752-1 and CM 1767. Ourresults for NIST SRM 8544 can be compared with literaturevalues obtained by an ICP-MS method using an in-housereference sample for external calibration (Sano et al.2005). Their value for Sr ⁄ Ca (0.199 ± 0.006 mmol mol-1)is in good agreement with our value; however, for Mg ⁄ Ca,for which Sano et al. (2005) reported 13.04 ±0.64 mmol mol-1, a disagreement is manifest. Mg ⁄ Caresults obtained for BAM RS3, ECRM 752-1 and CM1767 agree within combined uncertainty with the meanvalues reported by Greaves et al. (2008) for non-centri-fuged samples. These were: 3.82 ± 0.03 mmol mol-1 forECRM 752-1; 0.78 ± 0.04 mmol mol-1 for BAM RS3 and5.73 ± 0.06 mmol mol-1 for CM 1767. The uncertaintiesconsidered for these averages represent the interlaboratorystandard deviation of the mean (SE, 2s). Greaves et al.(2008) only report Sr ⁄ Ca for CM 1767, and their value(1.506 ± 0.016 mmol mol-1) is significantly lower than thatfound by us. As an additional check on accuracy, filtered

and diluted seawater collected from the Bermuda AtlanticTime Series (BATS) at 1200 m was measured using the IDmethod on a multi-collector ICP-MS (Neptune). The high-resolution mode was required to resolve 48Ca from a sus-pected 32S16O interference in these samples where col-umn chemistry was not conducted. The value of8.56 ± 0.001 mmol mol-1 from our analysis is about 1%lower than the 8.64 mmol mol-1 result reported by de Vil-liers (1999) for North Atlantic seawater from a similardepth but different location.

We believe that the isotope dilution method presentedhere can be used to obtain precise and accurate high-res-olution Mg ⁄ Ca and Sr ⁄ Ca ratio measurements in carbon-ates with environmental significance. Inter-instrumentprecision has been demonstrated for the method. It wouldalso be a valuable tool for testing candidate materials tobe used as RMs with an accuracy within 0.8%. Further-more, a better characterisation of impurities and isotopiccomposition of calibrators could deepen the understandingof the systematic differences found between measured andcalculated values for synthetic test solutions. In summary, wepresent a method with sub-percent accuracy in the deter-mination of Mg ⁄ Ca and Sr ⁄ Ca ratio values over the broadrange found in calcium carbonates found in nature.Although the Mg range was restricted to carbonates withrelatively low Mg values for our present method, it couldbe extended easily by preparing a second mixed tracerwith a higher 25Mg content. This new tracer would allowMg ⁄ Ca values in the range 40–300 mmol mol-1 to beobtained.

Conclusions

An isotope dilution method based on a25Mg-43Ca-87Sr mixed tracer was developed for the deter-mination of Mg and Sr in natural calcium carbonates con-taining concentrations of these elements of less than

Table 4.Mg ⁄ Ca and Sr ⁄ Ca values for reference materials (not centrifuged)

Mg ⁄ Ca Sr ⁄ Ca

Average 2s N Average 2s N

NIST SRM 8544a 17.4 0.5 6 0.201 0.002 6AKb 2.42 0.08 4 3.06 0.03 4ECRM 752-1c 3.86 0.03 6 0.186 0.003 6BAM RS3d 0.80 0.02 4 0.196 0.002 6CM 1767e 5.67 0.05 6 1.534 0.015 6

a NIST SRM 8544 (NBS 19 limestone, NIST, USA). b UN AK (Institute of Mineral Raw Materials, Czech Republic).c ECRM 752-1 (Bureau of Analysed Samples Ltd, UK). d BAM RS3 (Federal Institute for Materials Research and Testing, Germany).e CM 1767 (Metallurgical Standardisation Research Institute, China).


30 mmol mol-1. Microgram amounts of carbonate couldbe dissolved, volumetrically spiked with a mixed tracer andrun in a sequence, together with a solution of known isoto-pic ratio used to correct for mass bias. A throughput of fiftysamples per day with a precision of 0.8% (2s) or betterwas possible. Two types of ICP-MS instrument, a sector fieldand a quadrupole, gave consistent results in a long termstudy, within 0.14% for Mg ⁄ Ca and 0.11% for Sr ⁄ Ca,although the inter-run precision found for the sector fieldinstrument was significantly better than that obtained withthe quadrupole for Mg measurement. For both instruments,the precision deteriorated by a factor of two when sampleswith markedly different Mg and Sr content were runsequentially. Bias was tested for the sector field instrumentusing synthetic solutions prepared by mixing gravimetricallyselected reference samples. An overall agreement of 0.8%between measured and calculated values was found,although systematic trends for these differences were foundalso. Mg ⁄ Ca results were on average 0.5% lower than thecalculated values regardless of the Sr content. On the con-trary, the Sr ⁄ Ca values depended on both the amount ofSr and Mg present. Synthetic solutions with a Sr ⁄ Ca ratio> 9 mmol mol-1 and a Mg ⁄ Ca ratio < 4 mmol mol-1 couldbe balanced within 0.3% of their calculated values, whilefor lower Sr contents or higher Mg contents the accuracydeteriorated to 0.8%. A more precise characterisation ofthe impurities and the isotopic composition of materials tobe used as calibrators should be accomplished in order togain a better understanding of the method at the sub-per-cent level. The method is suitable to obtain Mg ⁄ Ca andSr ⁄ Ca ratios with an accuracy of 0.8% for sets of samplesdiffering by a factor of three in their Mg or Sr content.Mg ⁄ Ca and Sr ⁄ Ca ratios for five RMs were measured:Mg ⁄ Ca values were found to be in good agreement withpreviously reported results, while the agreement for Sr ⁄ Cawas marginal.

Acknowledgements

This work was supported by the National ScienceFoundation, Grants OCE-0096373 and OCE-0502642,and the Comer Science and Education Foundation, GrantCM113. We acknowledge two anonymous reviewers forcomments and suggestions than improved this manuscript.

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an isotope dilution icpms method for the determination of mgca

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