Thin-Layer Chromatography of Inks— Efficiency of Separation of Ink Components

There are numerous publications relating to the use of thin-layer chromatography (TLC) in the analysis of inks on documents. This paper compares the efficiencies of TLC separation of ink components when using several ‘developing’ solvents (solvent systems) commonly used for ink analysis in the forensic laboratory. The parameters chosen for this evaluation are the shape of and distance between chromatographic zones of separated dye components of ink, including components of Methyl Violet, a dye mixture often used in the manufacture of black, violet and blue ballpoint, fountain pen, and stamp pad inks. (This mixture typically consists of the dye Crystal Violet and its two or three homologues, which are triphenylmethane dyes of a similar chemical structure and hence similar physical and chemical properties, whose chemical structures differ only by one methylene [CH₂] group). The results obtained show that at least three solvent systems, one of which (ethyl acetate  isopropanol  water  acetic acid  30:15:10:1) was developed and reported by this author in the 1980s, provide significantly improved TLC separation efficiency compared with the “Solvent System I” recommended in the current SWGDOC Standard for Test Methods for Forensic Writing Ink Comparison.