page_title: Δ34S
section_title: Reference standard
hierarchical_section_title: Δ34S / Calculation / Reference standard
caption_attribution_description: Diablo Canyon meteorite.
context_page_description: The δ³⁴S value is a standardized method for reporting measurements of the ratio of two stable isotopes of sulfur, ³⁴S:³²S, in a sample against the equivalent ratio in a known reference standard. Presently, the most commonly used standard is Vienna-Canyon Diablo Troilite. Results are reported as variations from the standard ratio in parts per thousand, per mil or per mille, using the ‰ symbol. Heavy and light sulfur isotopes fractionate at different rates and the resulting δ³⁴S values, recorded in marine sulfate or sedimentary sulfides, have been studied and interpreted as records of the changing sulfur cycle throughout the earth's history.
context_section_description: Sulfur from meteorites was determined in the early 1950s to be an adequate reference standard because it exhibited a small variability in isotopic ratios. It was also believed that because of their extraterrestrial provenances, meteors represented primordial terrestrial isotopic conditions.  During a meeting of the National Science Foundation in April 1962, troilite from the Canyon Diablo meteorite found in Arizona, US, was established as the standard with which δ³⁴S values (and other sulfur stable isotopic ratios) could be calculated. Known as Canyon Diablo Troilite (CDT), the standard was established as having a ³²S:³⁴S ratio of 22.220 and was used for around three decades. In 1993, the International Atomic Energy Agency (IAEA) established a new standard, Vienna-CDT (VCDT), based on artificially prepared silver sulfide (IAEA-S-1) that was defined to have a δ³⁴SVCDT value of −0.3‰. In 1994, the original CDT material was found not to be isotopically homogeneous, with internal variations as great as 0.4‰, confirming its unsuitability as a reference standard.
caption: Troilite from the Canyon Diablo meteorite was the first reference standard for δ34S.
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