Dimerization reaction of protonated and deprotonated amino acids in atmospheric pressure chemical ionization

► Formation of dimerized product ions of amino acids by APCI was investigated. ► Formation of the [2M−H]− ions was more favorable than that of the [2M+H]+ ions. ► The [2M+H]+ ions of proline and tryptophan were observed in abundance. ► The [2M−H]− ions of glycine, histidine, methionine, and tryptophan were observed in abundance.