Sampling and identification of natural dyes in historical maps and drawings by liquid chromatography with diode-array detection

A simple and rapid liquid chromatographic with diode-array UV–vis spectrophotometric detection (HPLC–DAD) method for identification of natural dyes has been developed. Chromatographic retention of carminic acid, indigotin, crocetin, gambogic acid, alizarin and purpurin has been studied. The mobile phase consisted of 40 mM SDS–10 mM phosphate buffer solution (pH 2.3)–0.1% TFA (eluent A) and acetonitrile (eluent B) using a programmed gradient (5% B to 95% B). Analyses were carried out on a Phenomenex, Luna 5u NH2 100a column (250 mm × 4.60 mm i.d., 5 μm particle) and the operating conditions were: 0.6 ml min−1 flow rate, 20 μl volume injection and 35 °C column temperature. Extracts of samples of natural dyes taken from historical maps belonging to The Royal Chancellery Archives in Granada were successfully analyzed using the proposed method including a new technique for sampling.