Preparation of (.+-.)-[1,2-13C2]Abscisic Acid for Use as a Stable and Pure Internal Standard

[1,2-13C2]ABA, which is designed as a stable and pure internal standard for GC/MS analysis, is synthesized through the Witting reaction of 1-hydroxy-4-keto-α-ionone with carbomethoxymethylenetriphenylphosphorane prepared from [1,2-13C2]bromoacetic acid followed by saponification of the product methyl [1,2-13C2]ABA.