Basic substances are chemical elements or chemical combinations as they naturally occur or are produced. This includes all agents necessary to maintain stability. This excludes solvents which can be separated from the material without reducing its stability or changing its composition. In IMDS, every MDS “path” from the top of the tree structure must terminate in a basic substance or substances for the datasheet to be valid and allowed to be released. Most basic substances are identified by a Chemical Abstract Service (CAS) number.