Fig. 1. Spectrum of dyes in different solvents.

Fig. 2. Chromatograms obtained with water (A)/methanol (B) as mobile phase in C₈, C₁₈ and NH₂ as stationary phase and programed gradient (5% B to 95% B).

Fig. 3. Chromatograms obtained with water (A)/acetonitrile (B) in C₈, C₁₈ and NH₂ as stationary phase and programed gradient (5% B to 95% B) (peaks: (1) carminic acid; (4) crocetin and (6) gambogic acid).

Fig. 4. Chromatograms obtained with SDS/acetonitrile in C₈, C₁₈ and NH₂ as stationary phase and programed gradient (5% B to 95% B) (peak 1: carminic acid; peak 2: alizarin; peak 3: indigotin; peak 4: crocetin; peak 5: purpurin and peak 6: gambogic acid).

Fig. 5. Influence of pH on the retention factor (A) and plate number (B) of natural dyes: (◊) carminic acid, (■) indigotin, (Δ) gambogic acid, (□) crocetin, ( ) alizarin and (○) purpurin.

Fig. 6. Influence of SDS concentration on the retention factor (A) and plate number (B) of natural dyes: (◊) carminic acid, (■) indigotin, (Δ) gambogic acid, (□) crocetin, ( ) alizarin and (○) purpurin.

Fig. 7. Influence of different organic solvents on the retention factor of natural dyes using SDS as mobil phase ((◊) carminic acid, (■) indigotin, (Δ) gambogic acid, (□) crocetin, ( ) alizarin and (○) purpurin).

Fig. 8. Chromatograms obtained for different concentrations of TFA.

Fig. 9. Chromatograms obtained for solutions of natural dyes extracted from natural sources.

Fig. 10. Chromatogram of a mixture of the different natural dyes studied obtained under the optimised conditions (peaks: (1) carminic acid; (2) alizarin; (3) indigotin; (4) crocetin; (5) purpurin and (6) gambogic acid).

Fig. 11. Chromatogram obtained for an extract from two maps: (A) map 94 and (B) map 11.

Parameters for gradient elution

Solubility of dyes in different solvents

(−) Non-soluble; (+) low solubility, coloured solution with precipitate; (++) soluble.

Chromatographic retention time and absorption maxima for studied dyes

Identification of dyes in maps