DYE ANALYSIS OF PRE-COLUMBIAN PERUVIAN TEXTILES WITH HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY AND DIODE-ARRAY DETECTION
JAN WOUTERS, & NOEMI ROSARIO-CHIRINOS
3 DYE ANALYSIS
Several factors may contribute to the actual hue of old textiles. Among the most important may be the chemical composition of the mordant and the aging processes, due to the action of electromagnetic radiation, cleaning activities, or the acidity of the atmosphere. Since these factors influence the hue observed over time, it will be difficult, if not impossible, to determine the natural source from observing the actual hue only.
No natural dye is a pure product, and often the exact natural source of a given dye can only be derived from the presence of minor dye components. The most refined analytical result will be obtained if it is possible to consider all the dye components present on a dyed yarn and if their relative abundances may be calculated. Therefore, high-performance liquid chromatography (HPLC) was our method of choice. Great care was taken to perform any preliminary manipulation or the chromatography itself in a reproducible manner, so that the eventual transformations would be the same in reference and unknown materials. Only then it is relevant to compare analyses by considering relative ratios of dye components.
3.1 EXPERIMENTAL METHOD
About 1 mg of yarn was treated in 400 μl of H2O/methanol/37% HCl (1/1/2, v/v/v) for 10 min at 100°C in an open Pyrex tube. The extract was rapidly cooled down and filtered by centrifugal force in a Vac-Elut tube (Analytichem, U.S.A.). The filtrate was dried in a desiccator over NaOH and in vacuum. The residue was redissolved in an appropriate volume of methanol/water (1/1, v/v), and 20 μl of this preparation was used for HPLC analysis.
The HPLC equipment consisted of: a high-pressure pump and a four-channel electromagnetic mixing valve, programmable for flow-rate, times, and composition of the eluting solvent (Series 4 HPLC pump, Perkin-Elmer, USA); a column with renewable cartridges of 4.6 × 100 mm Spherisorb ODS2, 3 μm (RSL, Eke, Belgium); a photodiode array detector (model 990, Waters, USA); and a system for data storage, manipulation, and retrieval (NEC APCIII computer with 20 Mb hard disk). Three solvents were used: (A) water, (B) methanol, and (C) 5% (w/v) phosphoric acid in water. The elution program was: 66A/24B/10C: 2 minutes; linear gradient to 0A/90B/10C: 27 minutes; 0A/90B/10C: 3 minutes; flow rate: 1.2 ml/minute, creating a system back-pressure of 17 to 24 MPa. The temperature in the chromatography laboratory was stabilized between 20°C and 22°C. The detection wavelength was selected according to the chemical nature of the peaks present. In general, animal dyes were best analyzed at 275 nm, whereas 255 nm was the optimal detection wavelength for vegetal mordant dyes and 288 nm for indigoids.
A test for indigo was performed using the method of reductive bleaching in alkaline medium (Na2S2O5/OH−) that allows reversible reoxidation to indigotin in the air (Hofenk-DeGraaff 1974). Whether or not indigotin was suspected to be present on a yarn, the usual HPLC procedure for mordant dyes was always applied. In some cases, both indigotin and indirubin were detected in this manner. Recently, a whole new chromatographic system was developedfor the separation and characterization of blue arid purple indigoid dyes (Wouters and Verhecken 1991). Unfortunately, this technique was not yet available for the analysis of the early Peruvian yarns.
Any dye component was identified according to two criteria: the retention time and the UV-VIS spectrum. The following commercially available products were used as reference products: alizarin, apigenin, carminic acid, ellagic acid, fisetin, indigotin, kampferol, luteolin, morin, purpurin, quercetin, rhamnetin. Pseudopupurin, rubiadin,and xanthopurpurin were given by R. H. Thompson (University of Aberdeen, Scotland). The characterization of dcII, dcIV, dcVII, flavokemesic acid, and kermesic acid, is described by Wouters and Verhecken (1989a, 1989b). Munjistin was identified according to its predominant presence in Rubia munjista and to spectral data in Thomson (1971). Indirubin was synthesized at our Iaboratory by Verhecken.