Lithium has two stable isotopes with masses 6and 7. This large relative mass difference results inlarge natural isotopic fractionations ~ 40‰ (Chan andEdmond 1988, Chan 1992) and Li isotopic measure-ments are becoming increasingly used as tracers ofmany different geological processes (Chan et al. 1992,You et al. 1995, Hoefs and Sywall 1997, Huh et al.1998, Moriguti and Nakamura 1998b, Chaussidonand Robert 1998, 1999, Tomascak et al . 1999b,2002, Decitre et al. 2002, Chan and Frey 2003, Zacket al. 2003). Lithium isotope ratios have been measu-red in geological materials using several analyticaltechniques, namely TIMS (e.g., Svec and Anderson1965, Michiels and De Bièvre 1983, Chan 1987, Xiaoand Berry 1989, Moriguti and Nakamura 1993),

quadrupole ICP-MS (Gregoire et al. 1996, Kosler et al.2001), AAS (Meier 1982), SIMS (Chaussidon andRobert 1998, Decitre et al. 2002) and more recentlymulti-collector inductively coupled plasma-mass spec-trometry (MC-ICP-MS) (Tomascak et al. 1999a, Nishioand Nakai 2002, Pistiner and Henderson 2003).

TIMS has been the most common method forprecisely measuring Li isotopes. A number of differentionisation schemes have been employed (Flesch et al.1973, Chan 1987, Green et al. 1988, Xiao and Berry1989, Datta et al . 1992, Hoefs and Sywall 1997,Moriguti and Nakamura 1993, Bickle et al. 2000),but analysing Li+ ions emitted from a Li3PO4 loadhas evolved as the most commonly used technique

Precise, Small Sample Size Determinations of Lithium Isotopic Compositions of Geological ReferenceMaterials and Modern Seawater by MC-ICP-MS

Vol. 28 — N° 1 p . 1 6 1 - 1 7 2

The Li isotope ratios of four international rock reference materials, USGS BHVO-2, GSJ JB-2, JG-2,JA-1 and modern seawater (Mediterranean, Pacificand North Atlantic) were determined using multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). These reference materials of natural samples were chosen to span a considerable range in Li isotope ratios and coverseveral different matrices in order to provide a useful benchmark for future studies. Our new analytical technique achieves significantly higherprecision and reproducibility (< ± 0.3‰ 2s) than previous methods, with the additional advantage ofrequiring very low sample masses of ca. 2 ng of Li.

Keywords: Li isotopic composition, silicate reference materials, seawater, MC-ICP-MS, Li standard.

Les rapports isotopiques du Li de 4 matériaux deréférence, de provenance internationale, BHVO-2,JB-2, JG-2, JA-1 et d’eau de mer (Méditerranée,Pacifique et Atlantique Nord) ont été déterminéspar MC-ICP-MS (spectrométrie de masse avec sourceà plasma induit à multicollection). Ces matériaux deréférence naturels ont été choisis car ils balaient unlarge champ des rapports isotopiques du Lithium etcouvrent différentes matrices afin de fournir un point de repère utile pour les études futures. Notrenouvelle technique analytique permet d’atteindreune précision et une reproductibilité (< ± 0.3‰ 2s)nettement supérieures à celles des méthodes précédemment utilisées et présente l’avantage depouvoir travailler avec des échantillons de petitemasse, ~ 2 ng de Li.

Mots-clés : composition isotopique de Li, matériaux de référence silicates, eau de mer, MC-ICP-MS, Li standard.

1 6 1

0504

Alistair B. Jeffcoate (1)*, Tim Elliott (1), Alex Thomas (2) and Claudia Bouman (3)

(1) Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK(2) Department of Earth Sciences, Parks Road, Oxford OX1 3PR, UK(3) ThermoFinnigan MAT, Bremen, Germany* Corresponding author. e-mail: A.Jeffcoate@bristol.ac.uk

Received 05 Dec 03 — Accepted 11 Mar 04

GEOSTANDARDS and

RESEARCHGEOANALYTICAL

(Moriguti and Nakamura 1993, Qi et al. 1997, Huh etal . 1998, Sahoo and Masuda 1998, James andPalmer 2000). This Li3PO4 method has been used inthe published TIMS studies that report the most repro-ducible data (± 0.5‰, Chan and Frey 2003) andanalyses of the smallest samples (< 10 ng Li, You andChan 1996). However, 100-250 ng of Li has typicallybeen used to obtain results with an accuracy ~ ± 1‰2s (see Mor igu t i and Nakamura 1998a) . T IMSanalyses are also time consuming and only a fewmeasurements can be made per day (see Sahoo andMasuda 1998). More rapid and precise measure-ments would be highly advantageous for a number ofgeological applications. The possibility to analyseprecisely smaller amounts of Li would be useful whenworking with low [Li] material (< 1 µg g-1), which iseither rare or requires painstaking preparation (e.g.,meteorites, single species foraminifera, mineral sepa-rates). SIMS analyses (Chaussidon and Robert 1998,Decitre et al. 2002) allow isotope determinations onas little as 10’s of femtograms of Li. The precisionreached by SIMS analysis however, is typically a fewper mil and thorough calibrat ion of instrumentalfractionations for different sample matrices is required(Chaussidon and Robert 1998, Decitre et al. 2002).The advent of MC-ICP-MS has increased speed ofanaly s i s (~ 30 min per sample ) and somewhatdecreased the sample size required (40-45 ng Li) forLi isotope measurements. However, the precision (±0.8-1.1‰ 2s) reported by MC-ICP-MS procedures(Tomascak et al. 1999a, Nishio and Nakai 2002,Pistiner and Henderson 2003) has not bettered recentTIMS measurements (Chan and Frey 2003).

In this study we report a new analytical techniquefor the determination of Li isotopes by MC-ICP-MS thathas achieved significantly better precision and repro-ducibility (< ± 0.3‰ 2s) than previous methods. Toassess the accuracy of our technique we have made adetailed study of several natural rock reference mate-rials and seawater, which span a wide range of [Li],δ7Li and sample matrices.

Nomenclature

Lithium isotopes are commonly reported as δ7Li,such that:

δ7Li = ([7Li/6Li]sample / [7Li/6Li]NIST L-SVEC - 1) x 1000 (1)

In this formulation, higher δ7Li values are also“heavier” (higher 7Li/6Li). Some data in the literature

are reported as δ6Li. Multiplying δ6Li values by -1yields a good approximation of δ7Li for most samples(i.e., those that have a Li isotope composition close tothe reference standard). A rigorous formulation for theconversion of δ6Li to δ7Li can be expressed as:

δ7Li = -δ6Li / (1 + (δ6Li / 1000)) (2)

Note that the reference material L-SVEC was for-merly distributed by NIST (RM 8545) but is now out ofstock. It can however be obtained from the IAEA as a13C isotope RM.

Chemical preparation

Our chemical separation procedure is based onthe efficient separation of Li from other cations usingcationic resin with mixed inorganic acid and methanoleluents (Strelow et al. 1974). Other workers (Morigutiand Nakamura 1998, Tomascak et al. 1999a, Nishioand Nakai 2002, Pistiner and Henderson 2003,) havesimilarly used mixed inorganic acid-methanol solutionsfor purifying Li. We propose a method simpler than thefour column approach of Moriguti and Nakamura(1998), but with an additional cleaning stage compa-red to Tomascak et al. (1999a). Our two-step columnprocedure uses a first stage to separate Li from thesample matrix, and a second column to further purifythe Li (dominantly from the results of resin degradationin the initial column step, see below). Strelow et al.(1974) noted mixed methanol-HNO3 eluents providethe most effective separation of Li from the full range ofcations found in silicate matrices, and this is the basisof our first column. We used a column filled with 6 mlBio-Rad AG50W X8 (200-400 mesh) resin and useda 1 mol l -1 HNO3:80% v/v methanol eluent ( i .e . ,a mixture that is both 80% by volume methanol and1 mol l-1 HNO3). All acids and eluents used in thisstudy are 1 x sub-boiling distilled, unless otherwise sta-ted. Full details of the first column procedure are repor-ted in Table 1a and elution curves shown in Figure 1a.

It became apparent, however, that the 1 mol l-1

HNO3:80% v/v methanol eluent degrades the resin.This can be seen from the prominent dark (organic?)residue obtained after drying down the Li fraction,which should contain scant other cations (Strelow et al.1974). It was also inferred that degradation of the resinreleased sodium into the Li fraction. Some 200 ng Nawas found in the Li fraction after the first columnprocedure regardless of whether samples (typicallywith ~ 150 µg Na) had been loaded or not. This

1 6 2

GEOSTANDARDS and

RESEARCHGEOANALYTICAL

1 6 3

Table 1.Protocols used for chromatographic Li separation procedure: (a) column 1 and (b) column 2

Step Reagent Amount (ml) Comments

a. Column 11 6 mol l-1 HCl 60-80 Clean column

2 6 mol l-1 HCl 6-12 Backwash clean column

3 Milli-Q water 12 Wash through acid

4 1 mol l-1 HNO3:80% v/v methanol 12 Treat with eluent

5 6 mol l-1 HCl 30 Remove products of this degradation

6 Milli-Q water 12 Wash through acid

7 1 mol l-1 HNO3:80% v/v methanol 12 Pre-condition resin

8 1 mol l-1 HNO3:80% v/v methanol 6-12 Backwash to remove bubbles formed

9 Sample in 1 mol l-1 HNO3:80% v/v methanol 5 Add sample to column

11 1 mol l-1 HNO3:80% v/v methanol 1 Wash down the sample

11 1 mol l-1 HNO3:80% v/v methanol 3 Collect pre fraction yield check 1

12 1 mol l-1 HNO3:80% v/v methanol 54 Collect Li fraction

13 1 mol l-1 HNO3:80% v/v methanol 3 Collect post fraction yield check 2

Step Reagent Amount (ml) Comments

b. Column 21 6 mol l-1 HCl 10-12 Clean column

2 6 mol l-1 HCl 1-2 Backwash clean column

3 Milli-Q water 1-2 Wash through acid

4 0.5 mol l-1 HCl:80% v/v methanol 2 Treat with eluent

5 6 mol l-1 HCl 10 Clean resin

6 Milli-Q water 1-2 Wash through acid

7 0.5 mol l-1 HCl:80% v/v methanol 2 Pre-condition resin

8 0.5 mol l-1 HCl:80% v/v methanol 1 Backwash to remove bubbles formed

9 Sample in 0.5 mol l-1 HCl:80% v/v methanol 0.5 Add sample to column

10 0.5 mol l-1 HCl:80% v/v methanol 0.5 Wash down the sample

11 0.5 mol l-1 HCl:80% v/v methanol 1 Pre fraction removed (lose organics)

12 0.5 mol l-1 HCl:80% v/v methanol 0.25 Collect pre fraction yield check 1

13 1 mol l-1 HCl:80% v/v methanol 12.5 Collect Li fraction

14 1 mol l-1 HCl:80% v/v methanol 0.25 Collect post fraction yield check 2

The two half column volume checks, either side of the Li fraction, were collected together and analysed to verify that they contained no Li.

Figure 1. Elution curves of Li and Na and the Li isotopic composition of eluted Li fractions in (a) column 1 and (b) column 2.

The first column was calibrated using 0.015 g of BHVO-2, and the second column was calibrated using 15 ng of Li (NIST L-SVEC)

and 500 ng of Li-free Na. The light-grey shaded region shows the elution volume collected during normal sample processing.

The darker grey regions show the half column volume fraction collected either side of the main peak as yield checks. Note

different y-axes: left hand side shows mass of Li and Na measured in each calibration fraction; right hand side gives isotope

ratios of selected calibration fractions.

Volume eluted (ml) Volume eluted (ml)

δ7Li

δ7Li

ng ng

δ7Li

a

Main Li fraction

LiNa

0 10 20 30 40 50 60 700

10

20

30

40

50

60

0

20

40

60

-20

-40

cv = 6 ml

1/2 cv

split

1/2cvsplit

δ7Li

b

0

20

40

60

80

-20

-40

-600

10

20

30

40

0 102 4 86 12 14 16

Main Li fraction

cv = 0.5 ml

LiNa

1/2 cv

split

1/2cvsplit

GEOSTANDARDS and

RESEARCHGEOANALYTICAL

amount of Na is well in excess of ~ 10 ng Na that wasmeasured in 5 column volumes of 6 mol l-1 HCl runthrough a cleaned column, which should efficientlystrip any resin-bound Na. Therefore, the first columncleans Li from the rest of the rock matrix very well(F igure 1a) but adds substant ia l addi t ional Na.Fortunately the Li blank imparted by column degrada-tion is low (see Table 2).

The presence of sodium and organic material inthe Li fraction has potential implications for the accuracyof isotope ratio measurements. In their TIMS procedure,Moriguti and Nakamura (1998) documented a signifi-cant effect of impurities in their loaded Li on measuredLi isotope ratios. This led them to employ a complica-ted four-column separation procedure to ensure aclean Li fraction. Likewise for MC-ICP-MS, Nishio andNakai (2002) and Tomascak et al. (1999a) reported a“matrix effect” caused by impurities in their aspiratedsample solutions. Both Nishio and Nakai (2002) andTomascak et al. (1999a) concluded that Na had themost significant effect (compared to Mg and Fe) on themeasured Li isotope ratios. Nevertheless, both the MC-ICP-MS studies reported that their one step columnprocedures were sufficient, at their level of reproduci-bility (± 1‰), to eliminate discernible matrix bias.

Given the higher precision of our MC-ICP-MS mea-surements compared to previous studies (see below),we decided to quantify further the effect of variableNa concentrations in sample solutions on measured Liisotope ratios. We ran an experiment where samplesof 10 ng Li (NIST L-SVEC) solutions were doped withvarying amounts of Li-free Na (produced by removingLi from an analytical grade Na solution using a modi-fied procedure of our first column), such that they hadNa/Li ratios ranging from 0.2 to 10. The Li isotopic

ratios determined on these solutions are shown inFigure 2. The Li isotopic signatures of the Na-Li mixesare significantly lower than pure L-SVEC in solutionswith Na/Li > 0.5 (i.e. > 5 ng g-1 in Figure 2) and syste-matically decrease with increasing Na/Li. Figure 2 alsoshows the isotopic analysis of a 10 ng load of thepure L-SVEC solut ion processed through the f i rs tcolumn. The ~ 200 ng of Na delivered from the resinto the Li fraction resulted in a Na/Li ratio ~ 20 and ina measured Li isotopic ratio -2.5‰ lower than that ofpure L-SVEC. This second experiment yielded a matrixeffect compatible with an extrapolation of the firststudy on mixed Na-Li solutions and highlights the needfor a further separation procedure.

To clean-up the Li fraction, we used a shrink-fitTeflon column filled with 0.5 ml of cationic resin (Bio-Rad AG50W X12, 200-400 mesh). A combination of0.5 mol l -1 HCl:80% v/v methanol and 1 mol l-1

HCl:80% v/v methanol were employed as eluents.Dilute HCl: methanol mixes provide more efficient Li-Na separation than dilute HNO3:methanol (Strelow etal. 1974) and are less aggressive on the resin. Bothorganic material and Na gained from the first columncan thus be readily separated without significant fur-ther additions. The Na/Li ratio of 10 ng of pure NIST L-SVEC processed through the separation procedure wasreduced from 20 after the first column only to 0.3 afterthe second. A dilute HCl:methanol eluent was not usedin the first column, as it was less efficient than 1 mol l-1

HNO3:80% v/v methanol in separating Li from Fe,which occurred at the 10% m/m level in many samplesof interest. Details of the second column procedure areshown in Table 1b and elution curves in Figure 1b.

As has been extensively documented by others,there is substantial fractionation of lithium isotopes

1 6 4

0 2 4 6 8 10 12 14 16 18 20

0.4

0

-0.4

-0.8

-1.2

-1.6

-2.0

-2.4

Figure 2. The effect of Na on the

measured Li isotope ratio of NIST

L-SVEC reference solution. The data

shown by black filled circles are

determinations on solutions of 10

ng g-1 Li NIST L-SVEC to which

variable amounts of Li-free Na had

been added. Also plotted is the Li

isotopic ratio of 10 ng Li of pure NIST

L-SVEC solution that was processed

through the first column only (grey

filled circle). This process added ~ 200 ng

Na to the collected Li fraction (see text).Na/Li ratio

δ7Li

GEOSTANDARDS and

RESEARCHGEOANALYTICAL

during ion-exchange chromatography (Taylor andUrey 1938, Chan 1987, James and Palmer 2000).Unsurprisingly, there is significant isotopic fractionationduring the elut ion of L i in our column procedure(Figure 1a and b). Failure to collect all of the extended“tail” of the Li elution peak (Figure 1a, b) can potentiallylead to problems. For example, the last 1% of eluted Lihas an extremely light isotopic ratio (δ7Li ~ -40‰) andif this last 1% were not collected, the purified Li frac-tion would consequently be 0.4‰ heavier than thetrue sample value. NIST L-SVEC was run through the fullcolumn procedure to check if there was any “chemistryinduced” change in the Li ratio. The column processedNIST L-SVEC gave δ7Li of -0.04‰ ± 0.2‰ (2s), whichis clearly within error of the standard solution.

It is also important to consider the effect of differentmatrices in effecting the elution behaviour of Li inreference materials and various samples (James andPalmer 2000, Chan et al. 2002). The mixed inorganicacid-methanol approach, however, provides a muchgreater Li-Na separation than the weak HCl procedureused by Chan (1987) and James and Palmer (2000).Thus, a generous collection cut around the Li fractioncan be made (Figure 1a and b) such that a drift in itsexact position due to variable matrices should not becritical. Furthermore, to verify sufficiently high yield che-mistry, an independent check was carried out on allsamples. A 0.5 column volume fraction was collectedon either side of the main Li split for both columnprocedures (Figure 1a and b). These fractions werecombined and analysed. If Li was measured above thenormal blank levels in these pooled pre- and post-

collection fractions, then the yield was inferred to betoo low and the associated sample measurementswere rejected. Elevated Li concentrations in these “halfsplits” were rarely observed.

The total chemical procedural blank was 0.050-0.180 ng Li, determined by isotope dilution and relativebeam intensity measurements (Table 2). The largestcomponent of the blanks came from the reagents used(Table 2). The δ7Li of the blank was around 20‰ ±5‰ (2s), although this was hard to measure accuratelydue to its low abundance. Our column procedure wasbased around separat ing L i f rom ~ 10 mg rockpowder. For low Li abundance samples (~ 1 µg g-1

yielding ~ 10 ng Li) the blank contribution could possi-bly bias measured δ7Li measurements by ~ 0.3‰,which is typically just above analytical error. Notably,such a blank contribution was not observed for the 10ng of L-SVEC processed through both columns (seeabove), which gave a δ7Li isotope value within error ofthe L-SVEC standard solution. Moreover, for commonsilicate samples with [Li] ~ 5 µg g-1, the blank wouldbe insignificant. As discussed below, the mass spectro-metry required only ~ 2 ng Li for analyses and so thesmallest sample size that could be analysed accuratelyis currently blank limited.

Mass spectrometry

Lithium isotope ratios were measured on a Thermo-F inn igan Neptune MC- ICP-MS ( see Weyer andSchwieters 2003). The Neptune is a forward geometrydouble-focusing mass spectrometer with an effective

1 6 5

Table 2.Lithium and sodium blanks in reagents and chemical procedures

Reagent Li in reagent Na in reagent

Water (Milli-Q) 0.010 pg g-1 1.2 ng g-1

HF (Aristar) 0.005 pg g-1 1.5 ng g-1

HCl (1x sub-boiling distilled) 0.020 pg g-1 1.4 ng g-1

HNO3 (1x sub-boiling distilled) 0.030 pg g-1 1.6 ng g-1

Methanol (1x sub-boiling distilled) 0.003 pg g-1 0.6 ng g-1

HClO4 (Aristar) 0.047 pg g-1 2.4 ng g-1

30 ml of 6 mol l-1 HCl 1st column*1 0.062 pg 10.9 ng

54 ml of 1 mol l-1 HNO3:80% v/v methanol 1st column*2 0.073 pg 208 ng

10 ml of 6 mol l-1 HCl 2nd column*3 0.050 pg 3.6 ng

12.5 ml of 1 mol l-1 HCl:80% v/v methanol 2nd column*4 0.010 pg 4.2 ng

Total procedural blank (digestion acids and full column chemistry) 50-180 pg 4.3 ng

*1 30 ml of 6 mol l-1 HCl run through a first column, with no sample added. *2 54 ml of 1 mol l-1 HNO3:80% v/v methanol eluent run through the same first column. *3 10 ml of 6 mol l-1 HCl run through a second column, with no sample added. *4 After 10 ml of 6 mol l-1 HCl run through, the blank from the 1st column *2 was loaded and run through the second column procedure, and measured for its blank.

GEOSTANDARDS and

RESEARCHGEOANALYTICAL

81 cm radius magnet and 10 kV acceleration voltage.The samples were run at “low resolution” (nominalmass resolution of 500 M/∆M), using a 1200 Wargon plasma. The purified Li solutions were introdu-ced into the MC-ICP-MS through an Aridus de-solva-ting system (Aridus Cetac, Omaha, Nebraska, USA)with a micro-concentric PTFE nebuliser (nominal 100 µlmin-1, effective 90 µl min-1). Aridus sweep gas settingsof ~ 4.8 l min-1 argon and ~ 6 ml min-1 nitrogen wereused with a sample gas flow rate of 1 l min-1. Highsensitivity skimmer cones, “x-cones” (Thermo-Finnigan)were used. Lithium isotope ratios were determined bysimultaneous collection of 7Li+ and 6Li+ on the highestand lowest mass Faradays of a nine Faraday cuparray. Good, flat-topped peak shapes are obtained onboth masses despite the large mass dispersion.

The Li fraction from the two-column separation wasdissolved in 2% m/m HNO3 and diluted to 10 ng g-1

(the same concentration as our NIST L-SVEC standardsolution). The measured Li solutions should have similarconcentrations to minimise errors in background cor-rection (see below). An initial, rapid measurement ofsample intensity allowed appropriate sample dilutionsto be made for subsequent isotope measurements.

The samples were run using a rapid referencesample bracketing technique to correct for instrumentalmass bias. The measured 7Li/6Li ratio of the samplewas normalised to the mean 7Li/6Li ratios of the NISTL-SVEC CRM, which was run before and af ter i t .Acquisition comprised ten ratios with 4 second integra-tion times. Peak centring was performed before the firstacquisition was started and drifted by no more than0.0005 mass units in a whole day of operation. Thetotal amount of sample (i.e., including sample introduc-tion) typically used to make a Li isotope measurement

was ~ 2 ng Li. The total time required to obtain oneδ7Li measurement on a sample was around 14 minutes(including reference samples and backgrounds). The 10ng g-1 Li standard solution typically yielded 7Li intensi-ties of ~ 100 pA, significantly higher than reported inthe literature for other MC-ICP-MS machines. In ourmeasurement routine a 100 pA beam typically gave“internal” errors of around 0.05 to 0.1‰ (2s).

The instrument parameters, such as gas flow rates,source lens parameters and torch position were notchanged during reference sample-unknown-referencesample measurement sets. However, between thesesets, there could be a need to adjust some of the focusparameters, in particular the torch position. Revisitingthe lens parameters and sometimes the Aridus gasflow rates also helped to keep the 7Li beam to a maxi-mum over a prolonged analytical session. As withother MC-ICP-MS procedures (e.g., Tomascak et al.1999a), substantial instrumental mass bias resulted ina measured 7Li/6Li of L-SVEC ~ 15 rather than a truevalue of 12.15 (Qi et al. 1997). The correction of stablemass bias during analysis is thus critical for accurateand precise measurements. The difference in isotoperatios between the sample-bracketing NIST L-SVECreference samples rarely varied by more than 0.01(~ 0.5‰) and averaged around 0.005 (~ 0.1‰).Figure 3 shows a plot of the 7Li/6Li of NIST L-SVECmeasured by the MC-ICP-MS over a period of around12 hours (other solutions were run between every otherNIST L-SVEC measurement, but the values were notplotted). On the rare occasions that there was a diffe-rence greater than 0.01 (= 0.5‰) in the ratios of thebracketing reference samples, the sample data wererejec ted. The combined errors associated in thesample-reference sample bracketing technique weremost readily assessed from replicate analyses. We

1 6 6

14.98

14.99

15.00

15.01

15.02

0 100 200 300 400 500 600 700 800 900

Figure 3. 7Li/6Li measurements

(n = 102) of NIST L-SVEC

performed by MC-ICP-MS,

made over a period of around

12 hours. Another reference

solution was run between

every second NIST L-SVEC

measurement, but these data

are omitted for clarity.

Time (min)

δ7Li

GEOSTANDARDS and

RESEARCHGEOANALYTICAL

routinely ran sample solutions in duplicate and typicalreproducibility was 0.2‰, and always better than0.3‰. Full duplicate sample dissolutions were consis-tently 0.3‰ or better.

An important consideration in our mass spectrome-tric procedure is Li memory in the mass spectrometer,represented by a persistent background of ~ 0.2-2 pAat mass 7. This “machine blank” washed out veryslowly, 0.1 pA per hour, and was never completely remo-ved. The background rose gradually during normal run-ning conditions, but over the short time scale of a singleanalysis the background was stable. The persistent Libackground had an extremely light isotope ratio, δ7Li ~-200, lighter than any naturally occurring Li. We thus inferthat this “mass spectrometer blank” was caused byfractionated Li deposited after the skimmer cone, possiblyre-ionised by stray Ar+ ions. Replacing the skimmer conedid not substantially affect the background signal and soit was not simply material deposited on this cone. It isimportant to note that the background was a result of

discrete peaks at masses 6 and 7 and the extreme rationot an artefact of a general sloping background.

Thus, the “mass spectrometer blank” had to be sub-tracted accurately in the measurement routine. Wemeasured “on-peak baselines” at the beginning andend of a reference sample–unknown–reference samplesequence and subtracted the mean intensi t ies atmasses 6 and 7 from the reference samples andunknowns. This process clearly added an additionalerror to our measurements. The intensity of the “massspectrometer blank” was no more than 1% of thesample intensity, and its intensity never varied by morethan 10%. The ratio of the “mass spectrometer blank”never varied by more than 2‰, within a referencesample–unknown–reference sample routine. Nevertheless,correction for the “mass spectrometer blank” was pro-bably the greatest cause of inaccuracy in our measure-ments and there is the concern that the measurement ofthe “mass spectrometer blank” was not necessarilyrepresentative of its contribution during sample analysis.

1 6 7

Table 3.International rock reference materials showing their δ7Li values and concentrations measured in this study

RM name Number disolutions Number of runs δ7Li average Uncertainty (2s) [Li] µg g-1 ±2s

Seawater (Mediterranean(2) 9 31 31.1 0.20 0.189 0.05+ Pacific(4) + N. Atlantic(3))BHVO-2 8 24 4.7 0.22 4.81 0.25JB-2 8 26 4.3 0.26 8.15 0.40JG-2 3 12 0.2 0.15 42.9 2.15JA-1 1 8 5.0 0.19 5.76 0.12

Rock reference materials BHVO-2 from the United States Geological Survey (USGS) and JB-2, JG-2, and JA-1 from the Geological Survey ofJapan (GSJ). Open seawater samples are from an International Oceanographic Commission baseline survey (Hall and Measures 1998, Aliboet al. 1999). The N. Pacific samples are from water depths of 25 m and station T26, and 800 m and station P26. The N Atlantic samples arefrom water depths, 298 m and station 3,1, and 1240 m and station 3,2 (just north of Newfoundland). Mediterranean samples represent a mixof various samples obtained from Southampton Oceanography Centre.

Figure 4. The δ7Li of IRMM-016

(filled grey squares) measured

over a period of ~ 800 minutes.

Also shown are bracketing NIST

L-SVEC solution measurements

(open diamonds). The NIST L-SVEC

results are reported in a delta

notation calculated for each

measurement relative to

proceeding and subsequent NIST

L-SVEC analyses.

Time (min)

δ7Li

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 100 200 300 400 500 600

NIST L-SVEC mean = 0.0 +/- 0.03 (2 std err mean)

IRMM-016 mean = 0.14+/- 0.04 (2 std err mean)

700 800 900

NIST L-SVEC

IRMM-016

GEOSTANDARDS and

RESEARCHGEOANALYTICAL

The agreement of our absolute values for referencematerials with other less precise techniques, however,suggests that this is not a significant issue (see below).We checked for the possible interference of doublycharged carbon by scanning with the secondary elec-tron multiplier to measure the intensity of 13C++ aroundmass 6.5. We never detected more than 20 counts per

second of 13C++, which implies an insignificant contri-bution of 12C++ on mass 6.

Since our chemical separation procedure requiredand was checked for near 100% (> 99.5%) columnyields, the Li concentrations of samples could be deter-mined from a comparison of the relative beam intensities

1 6 8

Table 4.Comparison of the results of this study with published work

Reference material Reference Method δ7Li average Uncertainty (2s) [Li] µg g-1 ±2s

Seawater This study MC-ICP-MS 31.14 0.2 0.189 0.05Pistiner and Henderson (2003) (1) MC-ICP-MS 29.60 0.9 - -Nishio and Nakai (2002) (3) MC-ICP-MS 29.30 0.9 - -James and Palmer (2000) (4) TIMS-Li3PO4 31.50 1.6 - -Tomascak et al. (1999) (5) MC-ICP-MS 31.80 1.9 - -Moriguti and Nakamura (1998) (6) TIMS-Li3PO4 30.00 0.7 - -Chan and Edmond (1998) (7) TIMS-Li3B4O7 33.30 1.2 - -Sahoo and Masuda (1998) (8) TIMS-Li3PO4 28.80 2.0 - -You and Chan (1996) (10) TIMS-Li3PO4 32.40 2.6 - -Chen and Chan (1996) (11) TIMS-Li3PO4 31.40 1.0 - -

JB-2 This study MC-ICP-MS 4.30 0.3 8.150 0.40Pistiner and Henderson (2003) (1) MC-ICP-MS 3.90 0.9 6.700 -Chan and Frey (2003) (2) TIMS-Li3PO4 5.10 0.4 - -Nishio and Nakai (2002) (3) MC-ICP-MS 4.29 0.3 8.340 0.25James and Palmer (2000) (4) TIMS-Li3PO4 6.80 0.3 8.000 -Tomascak et al. (1999) (5) MC-ICP-MS 5.10 1.1 - -Moriguti and Nakamura (1998) (6) TIMS-Li3PO4 4.90 0.7 - -Govindaraju (1994) (12) - - - 8.000 -

BHVO-2 This study MC-ICP-MS 4.66 0.2 4.806 0.25BHVO-1 Pistiner and Henderson (2003) (1) MC-ICP-MS 5.10 0.9 4.000 -BHVO-1 Chan and Frey (2003) (2) TIMS-Li3PO4 5.20 0.5 - -BHVO-1 James and Palmer (2000) (4) TIMS-Li3PO4 5.80 0.8 4.900 -JG-2 This study MC-ICP-MS 0.24 0.2 42.973 2.15

Pistiner and Henderson (2003) (1) MC-ICP-MS -0.70 0.9 39.000 -James and Palmer (2000) (4) TIMS-Li3PO4 -0.40 0.2 41.000 -Oi et al. (1997) (9) TIMS-LiI 0.30 1.7 - -Govindaraju (1994) - - - 43.400 -

JA-1 This study MC-ICP-MS 5.00 0.2 1.500 0.07Oi et al. (1997) (9) TIMS-LiI 3.50 0.7 - -

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BHVO-(1/2)JB-2

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Figure 5. Comparison of the Liisotopic composition of seawaterand the rock reference materialswith previously reported values.References: (1) Pistiner andHenderson (2003). (2) Chan andFrey (2003). (3) Nishio andNakai (2002). (4) James andPalmer (2002). (5) Tomascak etal. (1999). (6) Moriguti andNakamura (1998a). (7) Chanand Edmond (1996). (8) Sahooand Masuda (1998). (9) Oi et al.(1997). (10) You and Chan(1996). (11) Chen and Chan(1996). Error bars are 2s.

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of the sample solution and NIST L-SVEC calibrant solu-tion. The rock reference material and seawater concen-trations were reproducible to ∼ 10% (2s), whereas therepeatability on each concentration measurement was~ 1-2%. The repeatability was a function of beaminstability over the relatively long time between sampleand reference sample measurement in a routine opti-mised for isotope ratio measurement. The poorer repro-ducibility of reference material concentrations in fullduplicate measurements presumably relates to weighingerrors (in the initial 10 mg sample and the several subse-quent dilutions) and variable losses in transferringsamples between beakers and columns. Given thehighly reproducible isotope ratios of replicated referencesample measurements, it is clear that variable Li lossesduring the chemical separation procedure did notcause isotopic fractionation of the sample. Pistiner andHenderson (2003) reached a similar conclusion.

NIST L-SVEC and IRMM-016

Flesch et al. (1973) obtained an isotopic ratio 7Li/6Liof 12.0192 ± 0.030 (2s) on a sample taken from 13 kgof highly purified Li2CO3. This material, formerly distri-buted by NIST as L-SVEC, is used by most workers asthe reference standard for calculating δ7Li values.Interestingly, TIMS studies by Chan (1987), Chan et al.(1992, 2002a, b), Lamberty et al. (1987), Green et al.(1988) , X iao and Beary (1989) , Mor igu t i andNakamura (1993), Sahoo and Masuda (1995), You andChan (1996) , Huh e t a l . (1998) , Mor igu t i andNakamura (1998a, b) and James and Palmer (2000),all show higher 7Li/6Li than that of Flesch et al. (1973).Since instrumental mass bias should decrease themagnitude of the measured 7Li/6Li ratio relative toits true value during the measurement phase of TIMSanalyses, it is likely that the original value of Flesch et al.(1973) is too low. Michiels and De Bièvre (1983)obtained an isotope ratio of 12.289 ± 0.050 (2s) on asample of highly purified Li2CO3, which was preparedand distributed by IRMM as IRMM-016. Qi et al. (1997)revisited the isotopic composition of IRMM-016 and alsocompared it to NIST L-SVEC. These authors conclude thatthe Li isotope ratios of NIST L-SVEC and IRMM-016 werethe same within the uncertainties of measurement(~ 2‰). The measured isotope ratios were notablyhigher than the determinations of Flesch et al. (1973),namely NIST L-SVEC 7Li/6Li = 12.1728 ± 0.034 (2s), andIRMM-016 7Li/6Li = 12.1773 ± 0.041 (2s).

We can now produce relative Li isotopic data toa higher precision and have therefore measured

IRMM-016 against NIST L-SVEC. NIST L-SVEC andIRMM-016 ratios were measured sequentially overtime, and converted into δ7Li values relative to NIST L-SVEC (Figure 4). Measurements of NIST L-SVEC, bracke-ted by measurements of NIST L-SVEC yielded a δ7Livalue of 0.00 ± 0.03‰ (2 standard errormean) whilstIRMM bracketed by NIST L-SVEC gave a mean δ7Li of0.14 ± 0.04‰ (2 standard errormean) (Figure 4). Thesereference materials are very similar but the differencein their means of ~ 0.13‰ is outside the combined 2standard errorsmean of the measurements. This impliesthat IRMM-016 is slightly heavier than NIST L-SVEC.

Results and discussion

Reproducibility

The long term precision of this technique, includingfull repeat measurements, has been determined frommeasurements on reference materials and seawaterduring a period of over a year. Reproducibility was0.3‰ (2s) or better (Table 3). It is also notable that ourmeasurements used only ~ 2 ng of Li . This means thatfor typical silicate samples with [Li] ~ 5 µg g-1, the 10mg of sample processed through column chromatogra-phy yielded enough Li for several isotope measure-ments. This is highly favourable when compared tomost TIMS analyses (Oi et al. 1997, Chan and Frey2003, Moriguti and Nakamura 1998, Sahoo andMasuda 1998, James and Palmer 2000), which requirelarger amounts of Li (typically 10 to 100 ng) to obtain asingle Li isotope ratio measurement. Moreover, thereproducibility of TIMS measurements is around ± 1‰(2s), although Chan and Frey (2003) recently cite0.5‰. Our technique also compares favourably toother MC-ICP-MS work (Tomascak et al. 1999a, Nishioand Nakai 2002, Pistiner and Henderson 2003), whichreport reproducibility values in the range of ± 0.8-1.1‰(2s), and use between 30 and 45 ng of lithium for asingle analysis.

Natural reference materials

It is clearly important to assess the accuracy of ourtechnique by comparison with measurements of inter-national reference materials. We have selected refe-rence materials that cover a large range of Li isotoperatios and likewise encompass the different samplematrices that are typically analysed. Given the higherreproducibility of our analytical procedure comparedto methods cited in the literature, however, it is difficultto verify fully the accuracy of our technique at present.

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Seawater

Seawater is a good test of a Li analytical tech-nique, as it has an extreme δ7Li (~ 30‰) (Chan andEdmond 1988) and contains a high concentration ofsodium, that potentially causes bias in mass spectro-metric measurements (see above) . The Li isotopiccomposition of modern seawater is considered to behomogenous, since the residence time of Li in seawateris ~ 1 Ma (Huh et al. 1998). Therefore, seawater makesa good natural Li isotope reference standard. Samplesof Mediterranean, Pacific and Atlantic seawater wereanalysed (Table 3). Multiple separations were perfor-med and analysed over a period of more than a year.Reassuringly, the different samples of modern seawaterfrom around the world showed a uniform composition:

Paci f ic seawater 31.13 ± 0.18 (2s) , ( repl icatechemistries n = 2, determinations n = 8).

Mediterranean seawater 31.12 ± 0.22 (2s), (replicatechemistries n = 16).

North Atlantic seawater 31.17 ± 0.16 (2s), (replica-te chemistries n = 8).

This uniformity is reflected in the combined seawatervalue of δ7Li 31.1‰ ± 0.2‰ (2s standard deviation).This result compares favourably with the mean ofprevious determinations (Table 4, Figure 5 althoughthere is a considerable spread of data in the literaturearound our value of δ7Li of 31.1‰.

Rock reference materials

Rock reference materials BHVO-2 from the UnitedStates Geological Survey (USGS) and JB-2, JG-2, andJA-1 from the Geological Survey of Japan (GSJ) werestudied to contrast samples with different magmaticevolution (basalt vs. granite) and tectonic settings(ocean island vs. subduction zone). These referencematerials encompass a wide range of major and traceelement composit ions and possible matrix effects.Multiple sample separations over a period of over ayear (Table 3), yielded high reproducibilities: δ7Li of4.7‰ ± 0.22‰ (2s) for BHVO-2, δ7Li 4.3‰ ± 0.25‰(2s) for JB-2, δ7Li 5.0‰ ± 0.19‰ (2s) for JA-1, andδ7Li 0.24‰ ± 0.15‰ (2s) for JG-2. These values arecomparable to reported values for these rock referencematerials, given the larger uncertainties of previousmeasurements (Table 4, Figure 5).

Conclusions

We have presented a robust, high yield (> 99.5%)method to provide efficient separation of Li from asil icate sample matrix. We have further reported atechnique for measuring 7Li/6Li isotope ratios on ~ 2 ngof Li with a precision of < 0.1‰ (2 standard errormean)using the Thermo Finnigan Neptune MC-ICP-MS. Thereproducibili ty of the full analytical procedure, forseparately processed samples, is ± 0.3‰ (2s) or better.

This technique has been used to analyse the Liisotopic compositions of modern seawater and silicatereference materials BHVO-2, JB-2, JA-1 and JG-2,yielding δ7Li of 31.1‰ ± 0.2‰, 4.7‰ ± 0.22‰,4.3‰ ± 0.25‰, 5 .0‰ ± 0.19‰ and 0 .24‰ ±0.15‰, respectively. These values compare well withprevious compositions reported in the literature but ourdeterminations have a greater reproducibil i ty. Weadditionally suggest that the Li reference materialIRMM-016 is slightly heavier (δ7Li 0.13 ± 0.05‰ 2sstandard errormean) than NIST L-SVEC.

Acknowledgements

We would like to thank Silke Severmann (Universityof Wisconsin) for supplying us with the seawater usedin this study, and Chung Choi (University of Bristol) forcontributing the powdered rock reference materials.We would l ike to thank Carolyn Taylor and ChrisCoath (University of Bristol), for help and advice in thelab, and Bruce Patterson (University of Bristol) for usefulcomments. We are also grateful to Janna Pistiner,Caedmon Marriott and Gideon Henderson (Universityof Oxford) for helpful discussions. Finally, we would liketo thank Thierry Menand (University of Bristol) forhelping with translations.

References

Alibo D.S, Nozaki Y. and Jeandel C. (1999) Indium and yttrium in North Atlantic and Mediterraneanwaters: Comparison to the Pacific data. Geochimica etCosmochimica Acta, 63, 13-14.

Bickle M.J., Chapman H.J. and You C.F. (2000)Measurement of lithium isotopic ratios as lithium tetraborate ions. International Journal of MassSpectrometry, 202, 273-282.

Chaussidon M. and Robert F. (1998)7Li/6Li and 11B/10B variations in chondrules from theSemakona unequilibrated chondrite. Earth and PlanetaryScience Letters, 164, 577-589.

1 7 0

GEOSTANDARDS and

RESEARCHGEOANALYTICAL

references

Chaussidon M. and Robert F. (1999)Lithium nucleosynthesis in the Sun inferred from the solar-wind 7Li/6Li ratio. Nature, 402, 270-273.

Chan L.H. (1987)Lithium isotope analysis by thermal ionization mass spectrometry of lithium tetraborate. Analytical Chemistry,59, 2662-2665.

Chan L.H., Edmond J.M., Thomson G. and Gills K. (1992)Lithium isotopic composition of submarine basalts:Implications for the lithium cycle in the oceans. Earth andPlanetary Science Letters, 108, 151-160.

Chan L.H. and Frey F.A. (2003)Lithium isotope geochemistry of the Hawaiian plume:Results from the Hawaii Scientific Drilling Project andKoolau Volcano. Geochemistry, Geophysics, Geosystems,4, 10.1029/2002GC000365.

Datta B., Khodada P.S., Parab A.R., Goyal A.H. andChitambar S.A. (1992)Thermal ionization mass spectrometry of Li2BO2

+ ions:Determination of the isotopic abundance ratio of lithium.International Journal of Mass Spectrometry and IonProcesses, 164, 577-589.

Decitre S., Deloule E., Reisberg L., James R.,Agrinier P. and Mevel C. (2002)Behavior of Li and its isotopes during serpentinization ofoceanic peridotites. Geochemistry, Geophysics,Geosystems, 3, 10.1029/2001GC000178.

Flesch G., Anderson A.R. and Svec H.J. (1973)A secondary isotopic standard for 6Li/7Li determinations.International Journal of Mass Spectrometry and IonPhysics, 12, 265-272.

Grégoire D.C., Acheson B.M. and Taylor R.P. (1996)Measurement of lithium isotope ratios by inductively coupled plasma-mass spectrometry: Application to geological materials. Journal of Analytical AtomicSpectrometry, 11, 765-772.

Green L..W., Leppinen J.J. and Elliot N.L. (1988)Isotopic analysis of lithium as thermal dilithium fluorideions. Analytical Chemistry, 60, 34-37.

Hall I.R. and Measures C.I. (1998)The distribution of Al in the IOC stations of the NorthAtlantic and Norwegian Sea between 52° and 65°North. Marine Chemistry, 61, 69-85.

Hoefs J. and Sywall M. (1997)Lithium isotope composition of Quaternary and Tertiarybiogene carbonates and a global lithium balance.Geochimica et Cosmochimica Acta, 61, 2679-2690.

Huh Y., Chan L.H., Zhang L.H. and Edmond J.M. (1998)Lithium and its isotopes in major world rivers: Implicationsfor weathering and the budget. Geochimica etCosmochimica Acta, 62, 2039-2051.

James R.H. and Palmer M.R. (2000)The lithium isotope composition of international rock standards. Chemical Geology, 166, 319-326.

Kosler J., Kucera M. and Sylvester P. (2001)Precise measurement of Li isotopes in planktonic foraminiferal tests by quadrupole ICP-MS. ChemicalGeology, 181, 169-179.

Lamberty A., Michiels E. and De Bièvre P. (1987)On the atomic weight of lithium. International Journal ofMass Spectrometry and Ion Processes, 79, 311-313.

Michiels E. and De Bièvre P. (1983)Absolute isotopic composition and the atomic weight ofa natural sample of lithium. International Journal ofMass Spectrometry and Ion Processes, 49, 265-274.

Meier A.L. (1982)Determination of lithium isotopes at natural abundancelevels by atomic absorption spectrometry. AnalyticalChemistry, 54, 2158-2161.

Moriguti T. and Nakamura E. (1993)Precise lithium isotopic analysis by thermal ionizationmass spectrometry using lithium phosphate as ion sourcematerial. Proceedings of the Japan Academy, 69,123-128.

Moriguti T. and Nakamura E. (1998a)High-yield lithium separation and precise isotopic analysis for natural rock and aqueous samples. ChemicalGeology, 145, 91-104.

Moriguti T. and Nakamura E. (1998b)Across-arc variation of Li isotopes in lavas and implications for crust/mantle recycling at subductionzones. Earth and Planetary Science Letters, 163, 167-174.

Nishio Y. and Nakai S. (2002)Accurate and precise lithium isotopic determinations ofigneous rock samples using multi-collector inductivelycoupled plasma-mass spectrometry. Analytica ChimicaActa, 456, 271-281.

Oi T., Odagiri T. and Norman M. (1997)Extraction of lithium from GSJ rock reference samples anddetermination of their lithium isotopic composition.Analytical Chimica Acta, 340. 221-225.

Pistiner J.S. and Henderson G.M. (2003)Lithium-isotope fractionation during continental weathering processes. Earth and Planetary ScienceLetters, 214, 327-339.

Qi H.P., Taylor P.D.P., Berglumnd M. and De Bièvre P. (1997)Calibrated measurements of the isotopic compositionand atomic weight of the natural Li isotopic referencematerial IRMM-016. International Journal of MassSpectrometry and Ion Processes, 171, 263-268.

Sahoo S.K. and Masuda A. (1995)High-precision isotopic measurement of lithium by thermal ionization mass spectrometry. InternationalJournal of Mass Spectrometry and Ion Processes, 151,189-196.

1 7 1

GEOSTANDARDS and

RESEARCHGEOANALYTICAL

references

Sahoo S.K. and Masuda A. (1998)Precise determination of lithium isotopic composition bythermal ionization mass spectrometry in natural samplessuch as seawater. Analytica Chimica Acta, 370,215-220.

Strelow F., Weinert C.H.S.W. and Van der Walt T.N. (1974)Separation of lithium from sodium, beryllium and otherelements by cation-exchange chromatography in nitricacid-methanol. Analytical Chimica Acta, 71, 123-132.

Svec H.J. and Anderson Jr A.R. (1965)The absolute abundance of the lithium isotopes in natural sources. Geochimica et Cosmochimica Acta, 29,633-641.

Taylor T.I. and Urey H.C. (1938)Fractionation of the Li and K isotopes by chemicalexchange with zeolites. Journal of Chemical Physics, 6,123-438.

Tomascak P.B., Carlson R.W. and Shirley S.B. (1999a)Accurate and precise determination of Li isotopic compositions by multi-collector sector ICP-MS. ChemicalGeology, 158, 145-154.

Tomascak P.B., Helz F. and Walker R.J. (1999b)The absence of lithium isotope fractionation during basaltdifferentiation: New measurements by multicollector sector ICP-MS. Geochimica et Cosmochimica Acta, 63,907-910.

Tomascak P.B., Widom E., Benton L.D., Goldstein S.L. and Ryan J.G. (2002)The control of lithium budgets in island arcs. Earth andPlanetary Science Letters, 196, 227-238.

Weyer S. and Schwieters J. (2003)High precision Fe isotope measurements with mass resolution MC-ICP-MS. International Journal of MassSpectrometry, 226, 355-368.

Xiao Y. and Beary E.S. (1989)High-precision isotopic measurement of lithium by thermal ionization mass spectrometry. InternationalJournal of Mass Spectrometry and Ion Processes, 94,101-114.

You C.F., Chan L.H., Spivack A.J. and Gieskes J.M. (1995)Lithium, boron, and their isotopes in sediments and porewaters of Ocean Drilling Program Site 808, NankaiTrough: Implications for fluid expulsion in accretionaryprisms. Geology, 23, 37-40.

Zack T., Tomascak P.B., Rudnick R.L., Dalpe C. andMcDonough W.F. (2003)Extremely light Li in orogenic eclogites: The role of isotopefractionation during dehydration in subducted oceaniccrust. Earth and Planetary Science Letters, 208, 279-290.

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GEOSTANDARDS and

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