Sulphur isotope data consistency improved

Abstract

The standard used for reporting relative sulphur isotope-abundance data has, historically, been troilite (FeS) derived from the Cañon Diablo meteorite, CDT. However, the isotopic inhomogeneity of this material — the variability in its sulphur-34/sulphur-32 isotope ratio is at least 0.4‰ — greatly exceeds the achievable analytical uncertainty of 0.05‰ (ref. 1). Accordingly, the International Union of Pure and Applied Chemistry (IUPAC) has recommended the setting up of a VCDT scale, which is expected to improve agreement between laboratories in measuring this ratio, δ³⁴S.

Main

A proposal that a VCDT scale be established was originally made in 1993, at the Consultants' Meeting on Stable Isotope Standards and Intercomparison Materials, in Vienna, sponsored by the International Atomic Energy Agency (IAEA)². The scale is defined by assigning a δ³⁴S value of -0.3‰ exactly (relative to VCDT) to the internationally distributed silver sulphide reference material IAEA-S-1 (formerly called NZ1). This decision was based upon measurements of IAEA-S-1 in several laboratories using SO₂(δ³⁴S = -0.25 ± 0.31‰(2σ) for 17 laboratories³) and SF₆(δ³⁴S = -0.3 ± 0.1‰ (2σ) for three laboratories³).

This recommendation by the IAEA consultants found favour with the Commission on Atomic Weights and Isotopic Abundances of IUPAC at a meeting in Guildford in the United Kingdom, in August 1995 (ref. 4). Accordingly, IUPAC recommended that:

• the use of meteoritic troilite and the reporting of δ³⁴S data relative to CDT be discontinued;

• relative sulphur isotopic abundance data be reported relative to VCDT; and

• the VCDT scale be established in laboratories through the use of IAEA-S-1 silver sulphide.

In addition, authors are encouraged to report δ³⁴S values for internationally distributed sulphur isotopic reference materials (Table 1) if they have been analysed. Sources of isotopic reference materials include:

Table 1 Examples of reference material

• the National Institute of Standards and Technology, Standard Reference Materials Program, Room 204, Building 202, Gaithersburg, Maryland 20899-0001, United States. Tel: +1 301 975 6776; fax: +1 301 948 3730; e-mail: SRMINFO@enh.nist.govand

• the International Atomic Energy Agency, Section of Isotope Hydrology, Wagramerstrasse 5, PO Box 100, A-1400 Vienna, Austria. Tel: +43 1 206021740; fax: +43 1 20607; e-mail: RIALIHL@iaea.org