Identification of unknown colorants in pre-Columbian textiles dyed with American cochineal (Dactylopius coccus Costa) using high-performance liquid chromatography and tandem mass spectrometry

Abstract

The present study concerns the identification of nine thus-far unknown derivatives of carminic acid extracted from pre-Columbian Peruvian textiles dyed with American cochineal-these derivatives are not found in commercially available preparations of the dye. These compounds probably represent a unique fingerprint of dyed textiles from this region, as they have never been reported to occur in other fabrics of historical value. They were separated by reversed-phase high-performance liquid chromatography (phenyl column) and detected using a UV/vis spectrophotometer and two tandem mass spectrometers. Peaks observed in chromatograms registered at 450 and 500 nm were further identified by ESI QqQ MS (mainly in the negative ion mode), supported by high-resolution ESI QIT/ToF MS data. The characteristic fragmentation pathways of isolated carminic acid and its derivatives provided additional information concerning lost neutrals and thus the functional groups and substituents present in the parent molecules. This information mainly related to multiple cleavages of the hexoside moiety (initially cross-ring cleavage), which are characteristic of C-glucosides (loss of 90, 120, and 148 Da). This is accompanied by the elimination of H2O as well as the further loss of 60 Da from the hexoside moiety. Moreover, other losses from the carbonyl groups (44 Da from CO2 loss, 62 Da from ethylene glycol loss, 32 Da from O2 loss, 138 Da from hydroxybenzoic acid, and 120 Da from oxomethylene cyclohexadienone) provided more specific information about structures of the identified derivatives of carminic acid.

Fig. 1a–c

ESI QqQ MS product ion spectra of a carminic acid, b dcIV, and c dcVII (parent ion at m/z 491, CE 20 V)

Scheme 1

Postulated fragmentation pathways of carminic acid

Fig. 2a–b

Chromatograms of the extract from fiber KPT8G (the same compounds are present in fiber 15885) registered using a a UV-vis detector at 500 nm, and b an ESI MS detector operated in negative ion mode (reconstructed for quasi-molecular ions, cf. Table 2)

Fig. 3a–j

ESI QqQ MS product ion spectra (mother ions [M − H]⁻, CE 20 V) of a dc1, b dc2, c dcII, d dc3, e dc4, f dc5, g dc6, h dc7, i dc8, and j dc9

Fig. 4

Proposed structures of carminic acid derivatives found in pre-Columbian threads

Scheme 2

Proposed mechanism for the fragmentation of the m/z 611 ion from dc7