Optimisation of ESI-MS detection for the HPLC of anthraquinone dyes

HPLC analysis of dyes from ancient objects is often difficult because of the small quantity of colorant extracted; their identification, usually based on DAD, is limited to commercially available compounds. It was shown that the structural characterisation of unknown compounds could be achieved using HPLC–MS and could be finely tuned to enable both detection and characterisation. Optimisation was carried out using source parameter step-adjustment, mobile phase composition and post-column additive testing; dye characterisation trial was carried out using MS3 pattern study. The results clearly show advantages in comparison to DAD namely improved molecular identification and better selectivity. Unfortunately, as several compounds had the same molecular mass and MS3 fragmentation, their retention parameters were very useful for adequate structure recognition. The optimised conditions were successfully applied to the identification of components from a small sample of wool thread dyed with madder (Rubia tinctorum) and Our Lady's bedstraw (Galium verum).