Investigating the fragmentation pathways of β-naphthol pigments using liquid chromatography/electrospray ionization quadrupole time-of-flight mass spectrometry

Rationale: Today, β-naphthol pigments are among the largest and most widely used classes of synthetic organic pigments. Their application fields range from textiles, food and beverages, printing inks, plastics to paint formulations. Most of the research dealing with their study using mass spectrometry focuses on developing sensitive methods for their quantification or their removal from industrial wastewater. Their qualitative recognition in formulations, whose composition is undisclosed by manufacturers, has not been tackled yet. Methods: The collision-induced dissociation fragmentation pathways of 10 red and orange widely used pigments of the late 19th to the early 20th centuries, belonging to the β-naphthol and Naphthol AS classes, were characterized using liquid chromatography/electrospray ionization quadrupole time-of-flight mass spectrometry in negative-ion mode. Results: The fragmentation pathways of these pigments were determined, and the pigments are discussed in relation to their chemical structures. The results were used to determine the composition in terms of the secondary components of one of our reference commercial formulations, characterized by a complex mixture of synthesis by-products. Conclusions: By coupling the high sensitivity of high-resolution mass spectrometry with an efficient chromatographic separation optimized for the analysis of β-naphthol dyes and pigments, we provided a method for the qualitative recognition of this class of molecules in unknown formulations possibly used in different kinds of industrial and synthetic products.