Identification of Lac Dye Components by Electrospray High Performance Liquid Chromatography-Tandem Mass Spectrometry

Abstract

A reliable identification method for laccaic acids which are the main components in lac dye has been established using ESI LC/MS/MS with a volatile mobile phase containing acetylacetone. Addition of acetylacetone to the mobile phase enables us to separate laccaic acids without reduction of peak resolution. The volatile mobile phase, acetonitrile-0.05% aqueous TFA solution (1-4) containing 0.005 M acetylacetone, is applicable to directly interfaced ESI LC/MS/MS without clogging problems. In all ES tandem mass spectra of laccaic acids, [M-H-CO₂]^- and [M-H-2CO₂]^- appeared in considerable abundance, together with the weak ions of [M-H]^-, [M-H-H₂O]^-, [M-H-CO₂-H₂O]^-, and [M-H-2CO₂-H₂O]^- when [M-H]^- was selected as the precursor ion. Laccaic acids can be easily identified by these product ions. The method has been successfully applied to identification of laccaic acids in a jelly and in “Shi-kou,” which is a stick lac imported 1,200 years ago.