Triarylmethine dyes: Characterization of isomers using integrated mass spectrometry

Triarylmethine dyes, such as Methyl and Crystal Violets, Diamond green and Magentas, produced since the late nineteenth century, consist of complex mixtures of homologous compounds, often differing only for the presence or position of the same substituent on the aromatic rings. In this paper, Liquid Chromatography-High Resolution Mass Spectrometry (HPLC-HRMS) was used for the characterization of the profile of three dyes: Methyl Blue, Methyl Violet and Fuchsine. The comprehension of dye synthesis and production strategies was often achieved by analyzing the single components by diode array detector. Nevertheless, the nature of the individual components needs to be confirmed by tandem mass spectrometric techniques through the solving of the structural issues. The analysis of standards and reference materials by tandem high resolution mass spectrometry allowed us to identify peculiar fragmentation pathways for the components of magentas and methyl violet dyes. The results highlighted the importance to undertake the measurements both in negative and positive ionization mode. The elucidation of specific fragmentation patterns allows the discrimination between different classes of compounds, while the identification of specific fragment ions allows one to distinguish isomers belonging to the same series.