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 δS values (and other sulfur stable isotopic ratios) could be calculated. Known as Canyon Diablo Troilite (CDT), the standard was established as having a S:S 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 δS 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.