1-(2-Ethoxy-2-oxoethyl)pyridin-1-ium-d5 Chloride is an intermediate used in the synthesis of Girard’s Reagent P-d5 (G388502), which is a labeled girard’s reagent used in the identification and quantification of protein carbonylation as well as in the modification of nucleosides and nucleotides. Used in the preparation of potent photoactive agents.