A proposal that a VCDT scale be established was originally made in 1993, at the Consultants' Meeting on Stable Isotope Standards and Intercomparison Materials, in Vienna, sponsored by the International Atomic Energy Agency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 SO234S = -0.25 ± 0.31‰(2σ) for 17 laboratories3) and SF634S = -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: