Lithium is an excellent geochemical tracer in unders-tanding hydrothermal processes, island arc volcanismand crust-mantle recycling (e.g., Chan and Edmond

1988, You et al. 1996, Moriguti and Nakamura 1998a,Tomascak et al. 2002, Zack et al. 2003). This is aconsequence of a series of unique physicochemical

Determination of Lithium Contents in Silicates by Isotope Dilution ICP-MS and its Evaluation by IsotopeDilution Thermal Ionisation Mass Spectrometry

Vol. 28 — N° 3 p . 3 7 1 - 3 8 2

A precise and simple method for the determinationof lithium concentrations in small amounts of silicatesample was developed by applying isotope dilution-inductively coupled plasma-mass spectrometry (ID-ICP-MS). Samples plus a Li spikewere digested with HF-HClO4, dried and dilutedwith HNO3, and measured by ICP-MS. No matrixeffects were observed for 7Li/6Li in rock solutionswith a dilution factor (DF) of ≥ 97 at an ICP powerof 1.7 kW. By this method, the determination of 0.5µg g-1 Li in a silicate sample of 1 mg can be madewith a blank correction of < 1%. Lithium contents ofultrabasic to acidic silicate reference materials (JP-1,JB-2, JB-3, JA-1, JA-2, JA-3, JR-1 and JR-2 from theGeological Survey of Japan, and PCC-1 from the USGeological Survey) and chondrites (three differentAllende and one Murchison sample) of 8 to 81 mgwere determined. The relative standard deviation(RSD) was typically < 1.7%. Lithium contents of thesesamples were further determined by isotope dilution-thermal ionisation mass spectrometry (ID-TIMS). The relative differences between ID-ICP-MS and ID-TIMS were typically < 2%, indicating the high accuracy of ID-ICP-MS developed in this study.

Keywords: lithium, isotope dilution-ICP-MS, TIMS, silicate reference materials, meteorites.

Nous avons développé une méthode simple et précise de détermination des concentrations enlithium dans de très petits échantillons silicatés. Elleest basée sur la méthode de dilution isotopiquecouplée à l'analyse par spectrométrie de masseavec couplage induit (ID-ICP-MS). Les échantillonsauxquels est ajouté le spike de Li, sont mis en solution avec un mélange HF-HClO4, évaporés àsec, puis repris avec HNO3 et analysés à l'ICP-MS.Aucun effet de matrice n'est observé sur les rapports7Li/6Li dans les solutions quand les facteurs de dilution sont ≥ 97 et qu'elles sont analysées avecune puissance du plasma de 1.7 kW. Par cetteméthode, la détermination de 0.5 µg g-1 de Li dansun échantillon silicaté de 1 mg peut être effectuéeavec une correction de blanc < 1%. Les teneurs enlithium des matériaux de référence de compositionultrabasique à acide (JP-1, JB-2, JB-3, JA-1, JA-2,JA-3, JR-1 et JR-2 du Service Géologique du Japon,et PCC-1 du Service Géologique des USA) et de chondrites (trois échantillons différents d'Allende etun de Murchison), de poids variant entre 8 et 81mg ont été déterminées. La déviation standard relative typique était < 1.7%. Les teneurs en lithiumde ces échantillons ont été ensuite mesurées pardilution isotopique et spectrométrie de masse àthermo-ionisation (ID-TIMS). Les différences entre lesrésultats obtenus par ID-ICP-MS et ID-TIMS étaient < 2%, démontrant ainsi la grande justesse de latechnique ID-ICP-MS développée dans cette étude.

Mots-clés : Lithium, ICP-MS avec dilution isotopique,TIMS, matériaux silicatés de référence, météorites.

3 7 1

1104

Takuya Moriguti, Akio Makishima and Eizo Nakamura*

The Pheasant Memorial Laboratory for Geochemistry and Cosmochemistry, Institute for Study of the Earth’s Interior, Okayama University at Misasa, Tottori-ken 682-0193, Japan* Corresponding author. e-mail: eizonak@misasa.okayama-u.ac.jp

Received 03 Sep 03 — Accepted 18 Oct 04

GEOSTANDARDS and

RESEARCHGEOANALYTICAL

characteristics: (i) lithium commonly exists as a traceelement in rocks and minerals, and is a moderatelyincompatible element in magmatic processes (Ryanand Langmuir 1987); (ii) lithium is highly mobile influid-related processes such as sea floor water-rockinteraction and subduction zone dehydration (e.g.,Berger et al. 1988); (iii) lithium possesses two stableisotopes, 6Li and 7Li, with a large relative mass diffe-rence, and thereby suffers significant isotopic fractiona-tion at relatively low temperatures on the Earth’s surface.

Lithium is produced in big-bang nucleosynthesis,H-burning in s tars , and spal lat ion of in ters te l larmedium (e.g., Reeves et al. 1970, Reeves 1994). Its cos-mic abundance is as low as those of Be and B, all ofwhich were produced in the “x-process” (Burbidge etal. 1957). Therefore, the determination of lithium inextraterrestrial materials gives us important informationfor the nucleosynthesis of these “x-elements” and theformation of the solar system when combined withother isotope systematics such as B, O and Mg (e.g.,Chaussidon and Robert 1998).

For these terrestrial and extraterrestrial materialstudies, a precise and accurate quantitative analyticaltechnique for lithium is required. External and/or inter-nal standardisation has been adopted for the determi-nation of lithium and other trace elements by ICP-MSusing ≥ 100 mg of sample (e.g., Jenner et al. 1990,Longerich 1990, Yoshida et al. 1992). However, externalstandardisation suffers from instrumental instabilitycaused by machine dri f t and matr ix ef fec ts (e .g. ,Cheatham et al. 1993). Internal standardisation is analternative option, but can give erroneous correctionsbecause variations in sensitivity are large and complexin the low-mass region (< 80 amu) for ICP-MS (e.g.,Eggins et al. 1997). In order to overcome this problem,Eggins et al. (1997) used enriched isotopes of 6Li, 84Sr,147Sm and 235U as internal standards but they did notuse isotope dilution (ID) for Li, which should give higherprecision and accuracy.

In this study, ID was employed in order to overco-me these problems inherent in external and internalstandardisation. A flow injection system was used toreduce the amount of sample solution consumed in themeasurement. Using this method, lithium in 8 to 80 mgtest portions of ultrabasic to acidic silicate referencematerials of the Geological Survey of Japan (GSJ) andUnited States Geological Survey (USGS), and in 10 mgamounts of three different Allende powders includingthe Smithsonian reference material and the Murchison

powder was determined. Furthermore, higher accuracyID-TIMS (after Moriguti and Nakamura 1998b) wasalso applied to these silicate reference materials inorder to evaluate the accuracy of the ID-ICP-MS tech-nique developed in this study. In addition, the lithiumisotope composition of these extraterrestrial materialswas determined by TIMS because the Li isotope ratioof these samples is generally unknown.

Experimental

Reagents

Water: Water was prepared by de-ionising with amixed-bed resin (> 18.0 MΩ) with subsequent 0.22 µmfinal filtration (MILLIPORE).

Hydrofluoric acid: Analytical grade 46% (m/v) HF(Wako) was sub-boiled twice using a two-bottle Teflonstill, resulting in a 30 mol l-1 solution.

Hydrochloric acid: EL grade (highly purified chemi-cals for the electronic industry) 36% (m/v) HCl (KantoChemical) was diluted to 6 mol l-1 with water andsub-boiled once in a two-bottle Teflon still.

Nitric acid: EL grade 69% (m/v) HNO3 (KantoChemical) was sub-boiled once in a two-bottle Teflonstill, resulting in a 16 mol l-1 solution.

Perchloric acid: Highly purified 70% (m/v) HClO4

(TAMAPURE-AA-100) was used without further purification.

Ion-exchange res in : Cat ion-exchange res in ,Muromac AG 50W-X12 (200-400 mesh), was soakedwith 3 mol l-1 HF for 1 hour. The resin was then succes-sively cleaned by packing into a polypropylene columnwith water and 3 mol l-1 HCl, prior to final washing byalternate addition of 6 mol l-1 HCl and distilled water.The resin was subsequently preserved in water.

Ethanol: Analytical 99.5% (v/v) ethanol was dis-tilled once using a two-bottle Teflon still.

Phosphoric acid: Analytical grade 85% (m/v)phosphoric acid solution was diluted to 40% (v/v) withwater and then passed through 1 ml of a AG 50W-X12 resin bed column. The result ing solution wasdiluted to 0.017 mol l-1 with water.

L i th ium s tandard so lu t ion and sp ike : U.S .National Institute of Standards and Technology (NIST)

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standard material, LSVEC lithium carbonate was dissol-ved with water to yield a stock solution of 1000 µg g-1,and diluted to 50 ng g-1 with 0.5 mol l-1 HNO3. Thissolution was used as a calibrator for instrumental massdiscrimination correction. Isotopic reference materiallithium carbonate, CBNM-IRM 015 (Joint ResearchCentre), in which the 6Li enrichment is 96%, was usedas a spike. This lithium carbonate spike was dissolvedin water to form a 1 µg g-1 solut ion. The l i thiumcontent of the spike solution was calibrated by ID-TIMSusing commercial 1000 µg ml-1 lithium standard solu-tion (Kanto Chemical). Prior to this calibration, the7Li/6Li ratio of the commercial standard solution wasdetermined by TIMS because the lithium isotope ratiowas quite variable and showed > 15% difference fromthe natural isotope ratio.

Silicate reference materials andsample preparation for ICP-MS

Geological Survey of Japan silicate reference mate-rials, JP-1 (peridotite), JB-2, JB-3 (basalts), JA-1, JA-2,JA-3 (andesites), JR-1, JR-2 (rhyolites), and USGS silicatere fe rence mater ia l , PCC-1 (per ido t i te ) , and theSmithsonian reference Allende powder (USNM-3529,Split 1, Pos. 23, denoted as SM-Allende) plus twodifferent Allende powders prepared at the PML (deno-

ted as PML-Allende 1 and PML-Allende 2) and aMurchison powder were analysed. These GSJ andUSGS silicate reference materials were further ground inorder to reduce sample heterogeneity (Makishima andNakamura 1997).

All experiments were carried out in a clean roomat the PML (Nakamura et al. 2003). The acid digestionmethod in this study is essentially the same as thatdescribed in Yokoyama et al. (1999). Rock powder (8-32 mg) was weighed into a 7 ml Savillex Teflon PFAjar and a known amount of the lithium spike wasadded to the rock powder. Perchloric acid (0.5 ml of 7mol l-1 HClO4), 0.15 ml of 16 mol l-1 HNO3, and 0.3ml of 30 mol l-1 HF were added to the sample. Themixture was decomposed in an ultrasonic bath forseveral hours in a tightly sealed beaker until maficminerals decolourised. Afterwards, it was heated forseveral hours at 120 °C on a hot plate in a fumechamber equipped with a HEPA filter. The decompo-sed sample was progressively evaporated at 120 °Cfor 12 hours, 165 °C for 12 hours and 195 °C untildryness. Then, 0.5 ml of 7 mol l-1 HClO4 was addedagain to the dried sample and the sample was hea-ted in the tightly sealed beaker at 120 °C for severalhours. The dissolved sample was then progressivelyevaporated at 120 °C, 165 °C and 195 °C unti l

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Table 1.ICP-MS operating conditions

1. ICP operating conditionsPower 1.7 kWFrequency 27.12 MHzTorch Quartz glass torchPlasma Ar flow rate 14 l min-1

Auxiliary Ar flow rate 1.2 l min-1

Nebuliser Ar flow rate 0.93 l min-1

2. Nebuliser and spray chamberNebuliser Microconcentric nebuliser, MCN-100 (CETAC Technologies Inc., Omaha, Nebraska, USA)Spray chamber Cooled by Peltier cooling device (0 °C), made of polyethyleneSolution uptake rate 0.18 ml min-1 (pumped)

3. InterfaceSampling orifice 1.0 mm (made of Pt)Skimming orifice 0.5 mm (made of Pt)

4. Flow injection systemValve A manual Reodyme 3-way Teflon rotary valve (0.8 mm bore) with 0.1 ml sample loop (0.8 mm bore)

5. Mass spectrometer operating conditionsData acquisition Time integration method with peak jumping, 5 points per unit, 10 scans in 1 sIntegration time From 15 s to 155 s after sample injectionBackground Measured in each run, calculated by the average count of 0-15 s and 155-170 sIon counting time per 1 s

6Li 0.48 s7Li 0.48 s

dryness. The dried sample was dissolved using 1 ml of6 mol l-1 HCl, and dried again at 120 °C. The samplewas finally diluted to an appropriate concentrationwith 0.5 mol l-1 HNO3.

Peridotite samples (PCC-1 and JP-1) of 22 to 81 mgwere decomposed completely with 0.3 ml of HF at205 °C in Teflon bombs (Krogh 1973) to dissolve acidresistant minerals. After this treatment, 0.5 ml of 7 mol l-1

HClO4 and 0.15 ml of 16 mol l-1 HNO3 were added,and acid digestion was undertaken successively inthe same manner as that for other samples. Total pro-cedural blanks for both the ultrasonic and the bombdecompositions were < 5 pg.

ICP-MS

The ICP-MS used in this study at the PML was aPMS 2000 in s t r umen t ( YOKOGAWA Ana l y t i ca lSystems, Japan) . ICP operat ing condit ions, spraychamber and interface were the same as those ofMakishima and Nakamura (1997). A flow injectionsample introduction system with a sampling loopsize of 0.1 ml was used, and 0.5 mol l -1 HNO3

solution was kept nebulised as a carrier solution.Data acquis i t ion t ime was 170 s fo r a sample .Details of the operating condit ions are shown inTable 1.

Sample preparation for TIMS

Lithium concentrations for all samples were alsomeasured by ID-TIMS in this study. In addition, lithiumisotope ratios of the Allende and Murchison powderswere determined by TIMS. The acid digestion proce-dure was the same as that used for ICP-MS. The Liseparation procedure of Moriguti and Nakamura(1998b), which consists of four steps, was employedfor basalt, andesite and rhyolite. For the ultrabasicand chondritic samples, the lithium separation methodwas modified because the higher Mg and Cr abun-dances of these samples significantly disturb isolationof Li by column chromatography in comparison tothose in basalt, andesite and rhyolite (e.g., Andersand Grevesse 1989, Govindaraju 1994, Imai et al.1995, Makishima et al. 2001). Therefore, the secondand the th i rd separat ion s teps in Mor igut i andNakamura (1998b) were repeated twice for removalof Mg. The fourth step was also repeated twice toensure removal of Cr, which is incorporated in the Lifraction in the first to third column steps for Cr-richsamples.

TIMS

A thermal ionisation mass spectrometer (a modi-f ied Finnigan MAT-261; Nakano and Nakamura1998) at PML was used. Li+ ions were generated usingLi phosphate as an ion-source material by the Redouble filament technique (0.025 mm thickness x 0.75mm width). The purified sample was dissolved in 1 µlof water and then loaded at a spot on the filamentwith a filament current of 0.9 A. The filament currentwas then slowly raised to 1.7 A and held there untilphosphoric acid fumes disappeared. The sample wasthen introduced into the mass spectrometer. This spotloading technique was used in order to minimise theamount of sample (Nakamura et al. 2003). Using thisloading technique, a mean 7Li/6Li ratio of 12.1131 ±0.0121 (2s; n = 8) was determined for 100 ng of Li;this ratio is consistent within analytical error with theresults utilising the spread-loading technique of 250 -1000 ng of Li from Moriguti and Nakamura (1998b;7Li/6Li = 12.1163 ± 0.0098 (2s; n = 22). Fifteen sepa-rate analyses of NIST LSVEC undertaken during thisstudy gave a mean 7Li/6Li ratio of 12.1117 ± 0.0115(2s).

Isotope dilution method

The principles of ID are described elsewhere (e.g.,Faure 1986). The amount of lithium in a sample iscalculated by the following equation:

p = [(C-R)/(R-A)](Dspike/Dsample)(Msample/Mspike)P (1)

where p and P are the amounts of li thium in thesample and the spike; A,C and R are 7Li/6Li ratiosof the sample, the spike and the sample-spike mix-ture respectively; Dsample and Dspike are the isotopicabundances of 6Li in the sample and the spike;and Msample and Mspike are the atomic weights ofthe sample and the spike, respectively. The 7Li/6Lirat io of NIST LSVEC was adopted as the naturallithium isotope ratio to calculate lithium content byequation (1). Lithium isotope analysis by TIMS afterM o r i g u t i a n d N a ka m u ra ( 19 9 8 b ) g a v e C =0.04545 ± 0.00003 (2s; n = 5), Dspike = 95.65%,and Mspike = 6.043 for the spike, and A = 12.1163± 0.0098 (2s; n = 22), Dsample = 7.62% and Msample

= 6.924 for NIST LSVEC. In order to minimise errormagnif ication in ID (Makishima et al . 1990), thespike solution was added to the samples to producel i th ium isotope rat ios in the range between 0.3and 2.

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Instrumental mass discrimination correction

Significant instrumental mass discrimination wasapparent in the lithium isotope ratio measurement byICP-MS. The mass discrimination was variable, beingaffected by tuning of ion lenses and torch position indifferent runs on the ICP-MS. In our method, the massdiscrimination was corrected by evaluating a massdiscrimination factor (F) as defined in the followingequation:

F = [7Li/6Li]TIMS/[7Li/6Li]ICP-MS (2)

where [7Li/6Li]TIMS is a 7Li/6Li ratio of NIST LSVECobtained by TIMS (7Li/6Li = 12.1163) after Morigutiand Nakamura (1998b); [7Li/6Li ] ICP-MS is a 7Li/6Liratio of NIST LSVEC obtained by ICP-MS, which wasdetermined from the average of six measurementsof 7Li/6Li ratio using a 50 ng g-1 solution of NISTLSVEC at the beginning of the analytical run. 7Li/6Liva lues o f t he sp i ked samp le s we re measu redsu c cess i ve l y, and mul t ip l ied by F. T ime dr i f t o f[7Li/6Li]ICP-MS was < 0.3% per hour for over two hoursand negl ig ib le , and the range of [ 7L i/6L i ] ICP-MS

in each day was 11.43 - 13.25 throughout this study(~ 1 year).

Results and discussion

Basic performance of ICP-MSfor determination of lithium

The typical 3s detection limits for 6Li and 7Li were4.9 and 5.8 pg g-1, respectively. Linearity in ICP-MSwas assessed by analysing NIST LSVEC solutions to bein the range 2 - 800 ng g-1. The stability of signals for6Li and 7Li were evaluated by measuring the NISTLSVEC solution of 50 ng g-1 for 140 minutes at aninterval of 170 seconds, 60 minutes after ignition of theplasma. Irregular shifts in signals of both 6Li and 7Liduring this measurement were observed. We assessedinstability of the instrument from the relative percentdifference (RPD) defined by the following equation(Makishima and Nakamura 1997):

RPD (%) = |Ci-Ci+1|/[(Ci + Ci+1)/2] x 100 (3)

where Ci and Ci+1 are the signal counts obtained inthe standard measurement before and after the i-thrun, respectively. The RPD was quite variable; 0.05 -12.7% (2.9% on average) and 0.05 - 13.0% (2.9% onaverage) for 6Li and 7Li, respectively.

In contrast to the signal stability, the 7Li/6Li ratioobtained by the same experiment was more stable byan order of magnitude over a period of 140 minutes.The stability of 7Li/6Li (RSD) was one order magnitudebetter than those of 6Li and 7Li. Therefore, the greaterprecision of ID than that of external standardisationwas confirmed.

Validity of the instrumentalmass discrimination correction

In order to verify the method of mass discriminationcorrection based on a single mass discriminationcorrection factor, lithium isotope ratios of the 6Li-enri-ched spike and six NIST LSVEC - 6Li-enriched spikemixtures with 7Li/6Li ratios of 0.5622 to 9.915 weremeasured by ICP-MS and TIMS, and the mass discrimi-nation corrected 7Li/6Li ratios between ICP-MS andTIMS were compared. In this experiment, the lithiumstandard - spike mixtures of 50 ng g-1 were measuredfive times by ICP-MS. Each aliquot, containing 300 ngLi, was measured by TIMS. The relative differences (%)between the mass discrimination corrected 7Li/6Liratios in ICP-MS and those in TIMS are plotted inFigure 1 and were < 0.9% and within analytical errors(2s). It is, therefore, concluded that the mass discrimina-tion can be properly corrected by multiplying a singlecorrection factor determined at the start of the analytical

3 7 5

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Figure 1. Relative difference of 7Li/6Li ratios of ICP-MS from

those of TIMS. The horizontal axis is the 7Li/6Li ratio obtained

by TIMS. Error bars are 2s values from five measurements

that include the error of the correction factor. The mass

discrimination correction factor in ICP-MS was 0.9443 ±

0.0070 (2s). The 2s errors were 0.2 to 1.4%. The analytical

uncertainty in TIMS was 0.08% (2s).

7Li/6Li (TIMS)

Rela

tive

diff

eren

ce (%

)

0.01 0.1 1 10

4

2

0

-2

-4

run. These results also imply that possible molecularinterferences such as 12C2+ and 14N2+ do not affect Liisotope ratios at the concentration level of 50 ng g-1.

Lithium concentration vs. analytical reproducibility

The analytical reproducibility was examined bylithium standard-spike mixtures of 2 to 200 ng g-1. Themixed solution of 200 ng g-1 with an isotope ratioof ~ 0.94 was further diluted to give lithium contents of2 and 20 ng g-1 in the solutions. Analytical results forthese three solutions are shown in Table 2. All 7Li/6Liratios were identical with each other within analyticalerror. This also verifies the linearity of ICP-MS in theconcentration range from 2 to 200 ng g-1.

Analytical reproducibility, including the error in thecorrection factor for instrumental mass discrimination, wasbetter than 1.5% (RSD) at ≥ 2 ng g-1. The expandederror in ID (Makishima et al. 1990) was < 1.8% (RSD) inthe range of spiked 7Li/6Li ratios between 0.3 and 2. Thiserror is considered to be the largest uncertainty relevantto determination of lithium contents in the range between2 ng g-1 and 200 ng g-1 in this study. It is suggested thatthese data indicate that molecular interferences are notsignificant down to Li concentrations of 2 ng g-1.

Effects of fluorides

In previous studies (e.g., Jenner et al. 1990, Eggins etal. 1997), little attention has been paid to insolublefluorides such as ralstonite formed by acid digestionof sil icate samples using HF (e.g., Croudace 1980,Yokoyama et al. 1999). These fluorides disturb flows ofsample solution into the nebuliser and also cause memoryeffects. It is well known that large amounts of HClO4 areeffective in the decomposition of fluorides and ensureisotopic equilibrium during sample decomposition with

ID (Ryan and Langmuir 1987, Moriguti and Nakamura1998b). To examine the effect of fluorides, we decompo-sed JB-2 without HClO4 and obtained an RSD of 9.5%(n = 5), which is significantly larger than that of 1.3% withthe HClO4 treatment, although the average content ofJB-2 of the former was 8.13 µg g-1 and similar to the lat-ter (8.01 µg g-1). Therefore, it was concluded that HClO4

treatment is required for the determination of lithium.

Matrix effects and dilution factor (DF)

Larger matrix effects in the determination of lithiumwere observed than has been reported in the measu-rement of isotope ratios of higher atomic numberelements such as Sr, Ce and U (Sun et al . 1987,Kawaguchi et al. 1987, Makishima and Nakamura1997). Makishima and Nakamura (1997) demonstratethat the signal suppression on 7Li is significant ata d i l u t i o n f a c t o r ( D F ; [we i gh t o f t h e s amp l esolution]/[weight of the sample]) of < 800, and was> 15% for the solution of DF = 113 using the same ana-lytical conditions employed in this study. Therefore, thecalibration curve method can give erroneous results ifcorrection of the signal suppression is not undertaken.

We examined 7Li/6Li ratios at various rock concen-trations in solution as a function of ICP power. The GSJs i l i ca te re fe rence mater ia l JB -2 was used a f te rMakishima and Nakamura (1997). The 7Li/6Li ratios ofrock solutions with dilution factor (DF) of 999, 391,234, 167 and 97 were measured at ICP powers of1.1, 1.4 and 1.7 kW. The results are shown in Figure 2.Matrix effects on the 7Li/6Li ratio were clearly observedat 1.1 kW. The 7Li/6Li ratio decreased with decreasingDF at this power. At higher ICP power settings of 1.4and 1.7 kW, 7Li/6Li ratios were almost constant withinanalytical error (2s). Thus, matrix effects are negligibleas long as the DF was > 97 at both power settings. Inaddition, the isotope ratios at 1.4 and 1.7 kW after

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Table 2.Lithium concentration vs. reproducibility for 7Li/6Li

Lithium concentration in the spike-standard mixture 7Li/6Li * % RSD(ng g-1) avg ± 2s (n = 5)

2 0.9448 ± 0.0277 1.5

20 0.9431 ± 0.0078 0.4

200 0.9443 ± 0.0119 0.6

* After instrumental mass discrimination correction. Errors were estimated from the precision and the error of the discrimination correction factor. The correction factor in this experiment was 0.9965 ± 0.0083 (2s; n = 6).

mass discrimination correction were identical to the7Li/6Li rat ios of JB-2 obtained by TIMS (7Li/6Li =12.1758 ± 0.0090 (2s) in Moriguti and Nakamura1998b) , wi th in analy t ica l er ror (2s ) . Th is againindicates that interferences on signals of 6Li and 7Liare negligible and the lithium isotope ratio can bedetermined correctly even in the presence of matrixelements. In other words, lithium in natural silicatesamples can be determined without lithium separationand pre-concentration.

The analytical precision of the 7Li/6Li ratio using alldata was calculated to be 1.8, 0.26 and 0.18% at 1.1,1.4 and 1.7 kW, respectively, and was dependent onICP power. The higher power operation yielded the

better precision in determination of 7Li/6Li. Thus, 1.7kW of ICP power was employed.

7Li/6Li ratios of terrestrialand extraterrestrial samples

The range of 7Li/6Li ratios caused by natural isoto-pic fractionation is from δ7Li = -11 to +33‰ (e.g., Chan1987, Chan et al. 1992, You and Chan 1996, Morigutiand Nakamura 1998b, Tomascak et al. 1999, 2002,Zack et al . 2003) , where δ7L i (‰) is def ined as[(7Li/6Li)sample/(7Li/6Li)NIST LSVEC - 1] x 1000. Naturalvariations in isotopic composition contribute an error of< 0.8% in the determination of lithium concentrationswhen the 7Li/6Li ratio of the natural sample is assumedto be the same as that of NIST LSVEC and the 7Li/6Liratio of the spiked sample is kept in the range 0.3 to2. This difference in δ7Li is significantly smaller than theanalytical uncertainty of 1.8% in the determination oflithium concentration. Therefore, the variation in naturalisotopic compositions in terrestrial samples can beneglected for Li concentration analysis by ICP-MS.

The range of lithium isotope ratios of extraterrestrialmaterials is not clear because few data are available.Early lithium isotopic studies in meteorites showed thatthe isotopic compositions of stony meteorites wereconsistent with terrestrial materials to within 2% analyt-ical error (e .g . , Krankovsky and Müller 1967) . Incontrast, anomalous lithium isotopic compositions werefound in iron meteorites that have 7Li/6Li ratios between

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Figure 2. 7Li/6Li ratios after mass discrimination correction

at various dilution factors of JB-2 solution at ICP powers of

(a) 1.1 kW, (b) 1.4 kW and (c) 1.7 kW. Error bars are 2s (n = 5).

The grand average of all 7Li/6Li ratios was 12.13 ± 0.06

(2s) and 12.15 ± 0.06 (2s) for (b) and (c), respectively.

Relative standard deviations for the NIST LSVEC of 50 ng g-1

were 1.8, 0.26 and 0.18% for ICP powers of 1.1, 1.4 and

1.7 kW of ICP power, respectively. The horizontal dashed

lines show the 7Li/6Li ratios of JB-2 determined by TIMS

(7Li/6Li = 12.1758, Moriguti and Nakamura 1998b).

Dilution factor

7Li

/6Li

afte

r m

ass

dis

crim

ina

tion

corr

ectio

n

Table 3.Li isotope ratios of meteorites by TIMS

Sample name Run δ7Li (‰) 2s mean

AllendeSM-Allende

1 3.82 0.072 3.20 0.04

Avg. 3.51

PML-Allende 1 1 2.39 0.042 1.95 0.04

Avg. 2.17

PML-Allende 2 1 2.49 0.052 2.04 0.043 1.97 0.06

Avg. 2.17

Murchison1 2.74 0.052 1.79 0.063 3.21 0.05

Avg. 2.50

11.4

11.6

11.8

12.0

12.2

12.4

11.4

11.6

11.8

12.0

12.2

12.4

100 1000

(c) 1.7 kW

(b) 1.4 kW

11.4

11.6

11.8

12.0

12.2

12.4 (a) 1.1 kW

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Table 4.Analytical results and reference values for lithium in silicate reference materials by ICP-MS and TIMS

Sample Sample weight in ICP-MS TIMS Reference value Difference betweenICP-MS (mg) (µg g-1) (µg g-1) (µg g-1) ICP-MS and TIMS (%)

PCC-1 24.8 0.973 0.97932.9 1.00 0.99421.5 0.995 0.983

Avg. 0.990 Avg. 0.985 1.6a, 0.07 - 17

b0.5

% RSD 1.5

JP-1 81.0 1.54 1.5279.7 1.65 1.6572.6 1.57 1.57

Avg. 1.59 Avg. 1.58 1.79c, < 1 - 2.02

d0.6

% RSD 3.6

JB-2 17.8 7.9311.5 7.9431.3 8.0032.4 8.15

Avg. 8.01 8.05 ± 0.09 7.78c, 0.793 - 10

d-0.5

% RSD 1.3 (2s; n = 7)†

JB-3 20.4 7.1519.1 7.2521.2 7.1719.2 7.19 7.2519.3 7.23 7.32

Avg. 7.19 Avg. 7.29 7.21c, 4.65 - 8.75

d-1.4

% RSD 0.6

JA-1 27.1 10.117.0 10.313.4 10.417.4 10.3 10.217.5 10.3 10.2

Avg. 10.3 Avg. 10.2 10.8c, 8 - 15.0

d1.0

% RSD 1.1

JA-2 24.4 29.227.7 29.516.6 29.614.4 29.6 28.719.8 29.4 28.6

Avg. 29.5 Avg. 28.7 27.3c, 23 - 30.97

d2.8

% RSD 0.5

JA-3 19.6 13.519.1 13.511.5 13.312.5 13.4 13.218.8 13.4 13.2

Avg. 13.4 Avg. 13.2 14.5c, 12.7 - 17.2

d1.5

% RSD 0.7

JR-1 11.3 64.011.5 63.115.2 63.316.0 63.4 62.418.0 63.5 62.2

Avg. 63.5 Avg. 62.4 61.4c, 24 - 69

d1.8

% RSD 0.5

JR-2 12.7 78.011.0 77.416.8 79.016.9 79.5 78.021.1 79.4 78.2

Avg. 78.7 Avg. 78.2 79.2c, 69.68 - 88

d0.6

% RSD 1.2

a Data from Govindaraju (1994). b Data from Gladney et al. (1983). c Data from Imai et al. (1995). d Data from Geochemical Reference Samples DataBase in GSJ web site (http://www.aist.go.jp/RIODB/geostand/welcome.html).† Data from Moriguti and Nakamura (1998b).

1.17 and 5.30 (Birck and Allégre 1980, Voshage1981). In addition, large lithium isotope variations( > 4%) were obse r ved in chondru les f rom theSemarkona in a recent ion probe analysis (Chaussidonand Robert 1998). Thus, lithium isotope ratios of threeAllende powders (SM-Allende, PML-Allende 1 and 2)and Murchison powder used in this study were deter-mined by TIMS.

Results are shown in Table 3. The averages of δ7Livalues of the SM-Allende, PML-Allende 1 and PML-Allende 2 were 3.51, 2.17 and 2.17‰, respectively.These isotopic ratios are considered to be consistentwith each other to within the 0.7‰ error (2s) of TIMS.The δ7Li value of Murchison was 2.50‰. All of theselithium isotopic compositions are close to that of Orgueil(+3.9‰) obtained by recent analysis (James and Palmer2000), and similar to that of terrestrial fresh mid oceanridge basalts , +1.5 to +4.7‰ (Chan et al . 1992,Moriguti and Nakamura 1998a). Thus, the 7Li/6Li ratioof NIST LSVEC can be used in ID analysis for thesemeteorites as well as terrestrial samples.

Reproducibility and accuracy in theanalysis of silicate reference materials

Three to five separate analyses by ICP-MS andTIMS of JP-1, JB-2, JB-3, JA-1, JA-2, JA-3, JR-1, JR-2 andPCC-1 were carried out to evaluate the reproducibilityand accuracy of li thium concentration analysis byICP-MS. Our analytical results are given in Table 4,together with previously published values.

The reproducibility (RSD) of silicate analyses byICP-MS (except for JP-1) was 0.3 to 1.5%, which issmaller than the analytical uncertainty (1.8%) estimatedin the section on lithium concentration versus analyticalreproducibility. However, the RSD of JP-1 was 3.6%,significantly larger than those of other samples, eventhough larger test portions (73 to 81 mg) were used.This may be caused by sample heterogeneity, becausethe lithium contents obtained by TIMS were also scatte-red resulting in an RSD of 4.1%, which is greater thanthe analytical error of TIMS.

The differences in l i thium concentration of thereference materials determined by ICP-MS comparedwith TIMS are shown in Table 4. These results indicatethat l i thium concentrations by ICP-MS (except forJA-2) are consistent with those of TIMS when theana l y t i ca l unce r ta in t y o f ICP-MS i s taken i n toaccount . Th is indicates that the ICP-MS method

developed in this study can give accurate determi-nations of lithium contents in silicate rock samples.The difference in JA-2 between ICP-MS and TIMSseems significant (2.8%) but is still within the 2 RSDrange of uncertainty.

The difference between our results by ID-ICP-MSand reference values (Govindaraju 1994, Imai et al.1995) are from 0.3 to 11%, except for PCC-1 (38%),but all our results are in the range of variation of dataobtained in previous work taking into account thelarge uncertainty (see Table 4).

In order to evaluate the applicability of the presentmethod further, four to five separate analyses wereundertaken of SM-Allende, PML-Allende 1 and 2, andMurchison with test portions ranging from 8 to 11 mg.Results are shown in Table 5. Averaged lithium contentsof these three Allende powders were 1.63, 1.65 and1.56 µg g-1, with RSDs of 1.7 (n = 4), 0.3 (n = 5) and0.1% (n = 4), respectively, despite small test portions.Differences between ICP-MS and TIMS were < 1.8%. Thelithium content of PML-Allende 2 was about 5% lowerthan that of other Allende powders, beyond the error ofICP-MS. The lithium contents of other splits of Allendehave been determined by atomic absorption spectro-scopy, emission spectrometry, and spark source massspectrometry (e.g., Nichiporuk and Moore 1970, 1974,Jarosewich et al. 1987). These data show large varia-tions in the lithium content of bulk Allende (< 1 to 4 µgg-1, Table 5). Lithium contents in chondrules of Allendeare also reported to be 0.21 to 1.93 µg g-1 (Hanon etal . 1999). The large dif ference in l i thium contentbetween chondrule and metal-free matrix of about afactor of 2 has also been observed in other chondrites(Nichiporuk and Moore 1970). Therefore, the differencein lithium content of Allende powders observed in thisstudy could be due to the heterogeneity among chipsor a heterogeneous distribution of lithium. The Li contentof Murchison obtained by ICP-MS was 1.49 µg g-1,which is also consistent with that obtained by TIMSwithin error (Table 5). The difference of lithium contentbetween this study and previous work (Nichiporuk andMoore 1974) in the different chips of Murchison isabout 15%. This difference may be also attributed toheterogeneity although available data for the lithiumcontent of bulk Murchison are limited.

Utility of the present method

The present method enabled us to measure 0.5 µgg-1 Li in 1 mg test portions of rocks or minerals at a DF

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≥ 97 with blank corrections of < 1% using a samplesolution of 0.1 ml. Therefore, this method is usefulfor the determination of low lithium content materialsin samples where a limited amount of material is avai-lable, such as mineral separates and synthesised glasssamples prepared as reference materials for secondaryion mass spectrometry (SIMS) and LA (laser ablation)-ICP-MS (Jarvis and Williams 1993). Such referencesamples have a key role to play in the accurate quan-titative determination of lithium by SIMS and LA-ICP-MS.

Furthermore, the aliquot of the sample solutionprepared for Li determination can also be used for thedetermination of other trace elements, such as Rb, Sr, Y,Cs, Ba, REEs, Pb, Th and U, and for isotopic analysis ofSr, Nd and Ce, because the procedure for acid diges-t ion of sample is essent ial ly the same as that ofMak i sh ima and Nakamura (1991, 1997) and

Yoshikawa and Nakamura (1993). Therefore, thismethod is suitable for multi-element and multi-isotopeanalyses of precious samples.

Acknowledgements

We are grateful to R. Tanaka, T. Yokoyama, H. Takei,K. Kobayashi, C. Sakaguchi, T. Shibata, M. Yoshikawaand T. Nakano for constructive discussions and techni-cal assistance throughout this study. We thank N.Takeuchi and M. Tanaka and other members of thePheasant Memorial Laboratory for Geochemistry andCosmochemistry at ISEI for basic various technicalsupports and useful discussions. We also thank Y.Matsui and N. Imai for offering the USGS and the GSJstandard rocks, respectively, and R. King and S. Manyafor improving the manuscript. This work was supportedby the Ministry of Education, Culture, Sports, Science

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Table 5.Analytical results for lithium (µg g-1) in chondrites by ICP-MS and TIMS

Sample Sample weight in ICP-MS TIMS Difference between Reference valueICP-MS (mg) (µg g-1) (µg g-1) ICP-MS and TIMS (%) (µg g-1)

Allende 2.1a, 1.88

b, < 1 - 4

c

SM-Allende 10.1 1.61 1.6311.0 1.64 1.639.37 1.608.72 1.66

Avg. 1.63 Avg. 1.63 -0.3% RSD 1.7

PML-Allende 18.06 1.65 1.647.98 1.65 1.67

10.9 1.6510.8 1.668.24 1.65

Avg. 1.65 Avg. 1.66 -0.1% RSD 0.3

PML-Allende 2 8.85 1.56 1.588.01 1.55 1.598.51 1.56 1.599.30 1.56

Avg. 1.56 Avg. 1.58 -1.8% RSD 0.1

Murchison 7.98 1.51 1.46 1.71b

8.02 1.488.56 1.49

11.0 1.499.70 1.49

Avg. 1.49 2.3% RSD 0.6

a Data from Nichiporuk and Moore (1970). b Data from Nichiporuk and Moore (1974).c Data from Jarosewich et al. (1987).

and Technology to T.M., A.M. and E.N., and in part bythe “Center of Excellence for the 21st Century” inJapan (E.N.).

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