Tandem mass spectrometry of laser-reduced anthraquinones for painted works and dyed cultural artifacts

Abstract

The peculiar ionization of anthraquinones, including the artistic dyes alizarin and purpurin, is examined. When alizarin (MW=240) is ionized by either LDI or MALDI it exhibits a dominant ion of m/z 242 [M+2H]+ with a far greater abundance than expected from the 13C isotopic contribution from the [M+H]+ ion at m/z 241. For the first time, MS/MS analysis of these anomalous [M+2H]+ ions is presented, which indicates that they arise from a laser-induced photoreduction of one of the anthraquinone’s carbonyl groups. MS/MS daughter ions produced from neutral losses of either water or ammonia from the [M+2H]+ ions exhibit different relative abundances, depending upon their respective functional groups’ proximity to the reduced carbonyl. LDI-MS/MS was used for the in situ detection of alizarin in a painting cross section and a swatch of dyed silk.