Gel microextraction from hydrophilic paint layers. A comparison between Agar-gel and Nanorestore Gel® HWR for spectroscopic identification of madder

While non-invasive analyses are widespread for the study of inorganic components in cultural heritage, the identification of organic substances can be very challenging when sampling is forbidden. In particular, organic dyes are hardly detectable because of their low concentration and tendency to fade. Hydrogels retention and release properties make them suitable for microextraction of dyes directly from the artifact, maintaining its integrity. After extraction, the hydrogels versatility makes them ideal for a multi-analytical study of the dyes. In this work, the gel microextraction procedure was put to the test of hydrophilic paint layers. Different Agar-gel weight concentrations were tested and compared with Nanorestore Gel® High Water Retention for the extraction of madder dyes from Arabic gum paint layers. UV–vis spectroscopy and SER spectroscopy were effective in identifying the analytes. Nonetheless, the sampling mechanism and the procedure invasiveness have proved to be dependent on the hydrogels retention power.