cis-1-[(3,4-Dimethoxyphenyl)methyl]-1,2,3,4-tetrahydro-6,7-dimethoxy-2-methyl-2-[3-oxo-3-[[5-[(1-oxo-2-propenyl)oxy]pentyl]oxy]propyl]-isoquinolinium-d3 Benzenesulfonate is the labeled analogue of cis-1-[(3,4-Dimethoxyphenyl)methyl]-1,2,3,4-tetrahydro-6,7-dimethoxy-2-methyl-2-[3-oxo-3-[[5-[(1-oxo-2-propenyl)oxy]pentyl]oxy]propyl]-isoquinolinium Benzenesulfonate (D461880), an impurity of Atracurium Besylate (A794500), a neuromuscular blocking agent.