(4-Carboxybutyl-d4)triphenylphosphonium (bromide) is intended for use as an internal standard for the quantification of (4-carboxybutyl)triphenylphosphonium by GC- or LC-MS. (4-Carboxybutyl)triphenylphosphonium is a Wittig reagent.{60418} It has been used in the synthesis of prostaglandin F2a (PGF2a; Item No. 16010) and the monounsaturated fatty acid 7-methyl-6-octadecenoic acid.{60418,60419} (4-Carboxybutyl)triphenylphosphonium has also been used to enhance mitochondrial localization of albumin nanoparticles loaded with the anticancer agent docetaxel (Item No. 11637).{60420}