Cationic‒anionic dye mixtures in writing inks: A straightforward multianalytical approach for minute samples

A minimally invasive analytical protocol was developed to characterize cationic‒anionic dye mixtures in minute samples from 20th/21st-century writing inks taken from literary works. While synthetic dyes in inks are well-studied, the combined identification of cationic and anionic dyes in mixtures—and their influence on light stability, both crucial in the cultural heritage field—remain largely unaddressed. Previous approaches relied on mock-ups and large samples, incompatible with cultural heritage research where samples are minute and irreplaceable. We analyzed minute ink-on-paper samples (ballpoint and fiber pens) from literary works kept in the Austrian National Library using secondary ion mass spectrometry with MeV primary ions (MeV SIMS), µ-Raman spectroscopy, and microfading testing (MFT). Without chemical sample treatment, chromatographic separation or sample consumption, even the smallest samples were preserved for further study. MeV SIMS enabled soft ionization of organic molecules, identifying cationic dyes (e.g., Basic Blue 7) and aryl guanidines—markers for anionic dyes. µ-Raman detected anionic dyes (e.g., Solvent Blue 38). MFT revealed moderate fading in most inks; fiber pen inks containing only Solvent Blue 38 faded faster than ballpoint pen inks containing mixtures of Solvent Blue 38 with Basic Violet 1 and/or 3, and aryl guanidines. Mixtures of Basic Violets and anionic dyes in inks seem to increase light stability through dye-salt formation. The results obtained, addressing molecular identification and lightfastness of real ink samples, are of high importance for libraries, archives, etc., dealing with preservation of modern literary works in terms of lighting/exposure times during exhibitions.