Direct inlet mass spectrometry for a rapid characterization of indigo in lipidic and proteinaceous matrices

Direct inlet mass spectrometry (DI-MS) was examined for its suitability to quickly identify indigo in mixtures with lipidic and proteinaceous binders, such as those characteristic of paintings and polychromy. The technique was tested on naturally aged reference mixtures of indigo with linseed oil, rabbit skin glue, and whole egg. DI-MS with electron ionization at 70 eV served as an efficient tool for screening indigo in complex matrices and provided, at the same time, a mass spectral fingerprint of the organic binder in just a few minutes. The technique was applied to the identification of indigo in a blue sample from a mural painting dated from the 18th century and housed in an Andean church in Bolivia. At the same time, the acquired data revealed for the first time the use of an egg tempera binding medium in an Andean mural painting.