Exploring the optimal HPLC-DAD-HRMS parameters for acid dye-based artistic materials: An analytical challenge

Acid synthetic organic pigments (SOPs), namely water-soluble anionic compounds containing acidic groups (e.g. −SO3H, –NO2, etc), were introduced into artistic materials in the late 19th century. SOPs were often combined in highly complex mixtures to achieve the desired colours, and therefore, chemical analysis requires ultra-selective and ultra-sensitive methods such as reversed-phase liquid chromatography coupled with mass spectrometric detectors. However, acid SOPs produce chromatographic tailing, leading to loss of resolution and sensitivity. To improve their detection, in this work an approach based on liquid chromatography coupled to a diode array detector and high-resolution mass spectrometry (HPLC-DAD-HRMS) was optimized. The study was carried out using standard acid SOPs (Amido naphthol red G, Cotton scarlet, and Naphthol yellow S), also assessing the effect of chromatographic parameter modification on a selection of natural dyes (Carminic acid, Alizarin, and Purpurin) and basic SOPs (Methylene blue and Methyl violet). Starting from a chromatographic gradient optimized for the analysis of tannins in iron gall inks, different flow rates and mobile phase modifier (formic acid) concentration were tested. The chromatographic efficiency (number of theoretical plates, NTP), resolution (R), symmetry factor (SF), ionisation efficiency (IE), signal-to-noise ratio (SNR), limit of detection (LOD) and limit of quantification (LOQ) were evaluated for each analyte. This enabled us to determine the optimal chromatographic conditions for the analysis of acid dye-based artistic materials (gradient with water with 1% v/v formic acid (FA); acetonitrile with 0.3% v/v FA; flow rate = 0.6 mL/min). Finally, the optimized method was applied for the investigation of organic dyes in a Persian carpet, and proved effective in improving both sensitivity and selectivity of the target analytes in a complex matrix. This study not only provides us with a chromatographic method that could be universally valid for the analysis of historical and contemporary artistic materials, but also caters to the difficult chromatographic separation of acid dyes in food, environmental, and forensic chemistry.