Negative ion ESI–MS analysis of natural yellow dye flavonoids—An isotopic labelling study

Flavonoids are amongst the most commonly used natural yellow colourants in paintings, as lakes, and in historical textiles as mordant dyes. In this paper, evidence from isotopically labelled substrates is used to propose negative ion electrospray collision induced decomposition mechanisms of flavones, flavonols and an isoflavone. These mechanisms include a retro-Diels-Alder fragmentation (observed for flavones and flavonols) and an M-122 fragmentation (characteristic of 3′,4′-dihydroxyflavonols). In addition, the presence of a m/z 125 fragment ion is shown to be characteristic of 2′-hydroxyflavonols and an ion at m/z 149 is shown to be characteristic of 4′-hydroxyflavones. Applications of these methods are exemplified by the identification of a minor component of Dyer's camomile (Anthemis tinctoria L.) and the identification of the dye source in green threads sampled from an 18th Century Scottish tartan fragment.