Evaluation of the 34S/32S ratio of Soufre de Lacq elemental sulfur isotopic reference material by continuous flow isotope-ratio mass spectrometryIntroductionSulfur isotope-ratio nomenclatureMaterials and methodsResults and discussionConclusionsAcknowledgementsReferencesShort communicationEvaluation of the34S/32S ratio of Soufre de Lacq elementalsulfur isotopic reference material by continuous flowisotope-ratio mass spectrometryH.P. Qi*, Tyler B. CoplenU.S. Geological Survey, 431 National Center, 12201 Sunrise Valley Drive, Reston, VA 20192, USAReceived 12 June 2002; accepted 14 February 2003AbstractSoufre de Lacq elemental sulfur reference material (IAEA-S-4) isotopically is homogeneous in amounts as small as 41 Agas determin ed by continuous flow isotope-ratio mass spectrometry. The d34S value for this reference material is+ 16.90 F 0.12x (1r) on a scale (Vienna Can˜on Diablo troilite, VCDT) where IAEA-S-1 Ag2Sis  0.3x and IAEA-S-2 Ag2S is + 22.67x.Published by Elsevier Science B.V.Keywords: Sulfur; Stable isotopes; S-34/S-32; Isotopic reference material1. IntroductionThe Soufre de Lacq elementa l sulfur isotopicreference material from Lacq in southwestern Francewas derived from natural gas and was prepared by E.Roth of Centre d’Etudes Nucle´aires de Saclay, France(Gonfiantini, 1984) and has served as an internation-ally distributed reference material for approximately20 years. This reference material is distributed by theU.S. Nationa l Institute of Standards and Technology(NIST) (http://ts.nist.gov/srm) as RM 8553. Distribu-tion of this material by the International AtomicEnergy Agency (IAEA), Vienna, Austria, as IAEA-S-4 was terminated about a decade ago because ofdoubts about its isotopic homogeneity.The demand for Soufre de Lacq has not been greatbecause it is not easily converted to SO2quantitativelyby conventional off-line dual-inlet isotope-ratio tech-niques. However, with continuous flow isotope-ratiomass spectrometry and elemental analyzer combustionsystems, Soufre de Lacq elemental sulfur is easilyanalyzed and can serve as a useful internationallydistributed isotopic reference material.Carmody and Seal (1999) demonstrated that Soufrede Lacq isotopically is homogeneous when amountsfrom 12 to 159 mg of elemental sulfur are analyzed.Because the amount of sample analyz ed with contin-uous flow isotope-ratio mass spectrometry can be afactor from 10 to 1000 smaller than that used forconventional dual-inlet isotope-ratio mass spectrome-try, it is important to demonstrate (and an objective ofthis study) that Soufre de Lacq isotopically is homo-geneous at the microgram amounts used with contin-0009-2541/03/$ - see front matter. Published by Elsevier Science B.V.doi:10.1016/S0009-2541(03)00075-5* Corresponding author. Fax: +1-703-648-5832.E-mail address: [email protected] (H.P. Qi).www.elsevier.com/locate/chemgeoChemical Geology 199 (2003) 183– 187uous flow isotope-ratio mass spectrometry. Anotherobjective of this study is to determine the34S/32S ratioof Soufre de Lacq elemental sulfur using continuousflow isotope-ratio mass spectrometry on a scale nor-malized with sulfur isotopic reference materials cali-brated by SF6isotope-ratio measurements. Becausethe bulk materials used to prepare Soufre de Lacqhave never been tested for isotopic homogeneity andbecause randomly selected samples from the distribu-tion reserves at IAEA and NIST have never beenanalyzed for valid ation of isotopic homogenei ty,another objective of this study was to perform thisvalidation. The purpose of this article is to present theresults of this study.2. Sulfur isotope-ratio nomenclatureBecause variations in sulfur stable isotope ratiosare small,34S/32S ratios are expressed as relativeisotope ratios in d (pronounced delta) notation in partsper thousand (xor per mill1) difference relative toVienna Can˜on Diablo troilite (VCDT) according tothe general expressiond34S ¼RARVCDT 11000x;where RAand RVCDTare the34S/32S ratios of unknownA and hypothetical VCDT respectively. The VCDTscale is defined by assigning a d34S value of 0.3xtoIAEA-S-1 Ag2S (Krouse and Coplen, 1997).3. Materials and methodsIAEA-S-1 and IAEA-S-2 Ag2S isotopic referencematerials were obtained from NIST.To test the homogeneity of Soufre de Lacq ele-mental sulfur, four random ly selected containers ofRM 8553 Soufre de Lacq were obtained from NIST.One of these is the same material analyzed by Car-mody and Seal (1999). As noted in their report, thismaterial spans a wide range of grain sizes. They splitthe material into coarse (>0.25 mm) and fine ( < 0.25mm) fractions using a 60-mesh sieve. Additionally,six containers of elemental sulfur were obtained fromIAEA. Two small bottles for distribution as IAEA-S-4were filled at IAEA between 7 and 20 years ago. Fourlarge bottles were subsamples of the bulk elementalsulfur. Samples were taken from different locations inthe containers. Due to the considerable variety ingrain size in the bulk material (diameter of as muchas 4 mm), larger grains were selected from two bottlesand finer grains were selected from the other twobottles. The large grains were crushed into finepowder prior to analysis.Samples were prepared and analyzed using a newlydeveloped method. In the study described here, thetoxic V2O5used in the general method of Giesemannet al. (1994) was replaced by nontoxic CeO2. Samplescontaining 47 F 6to56F 3 Ag sulfur were weighedinto 3 5-mm tin capsules; 600 F 100 Ag CeO2wasadded, and the capsules were closed. Samples wereintroduced into a Carlo Erba model 2500 elementalanalyzer,2reacted to SO2with an oxidation/reductioncolumn (Co stech Part No. 61110), heated to 1020 jCin a high purity oxygen (99.994%) atmosphereobtained from a 5-ml loop, transport ed by high purityhelium (99.997%) through a prepacked 0.8 m PTFEGC column (Costech Part No. 051083), and heated to90 jC to produce a SO2peak with a width of about100 s. The SO2from the sample was transported byhelium (90 ml/min) into a Thermoquest-FinniganConflo II interface, and subsequently, into a Thermo-quest-Finnigan Delta Plus continuous flow isotope-ratio mass spectrometer. The complete cycle time is450 s. Three SO2reference gas pulses were injectedfor 20 s each, beginning 20 s after the start of thecycle. The pressure of the SO2reference gas is 1 bar,which gives a 2.75-V signal intensity of the m/z =64ion beam. The SO2sample peak arrives at 272 s andfinishes at 372 s. A 300-AgAg2S sample gives a 3-Vsignal on the m/z = 64 ion