Min Li, Yufei Chen, David Hinks and Nelson R Vinueza
A tandem mass spectrometric method using a commercial quadrupole–time-of-flight (QTOF) mass spectrometer is described for the identification of sulfonated anthraquinone type dyes, having a 1-amino anthraquinone-2- sulfonate backbone. A total of 9 anthraquinone dye model compounds were evaporated and ionized via negative-ion electrospray ionization (ESI). Ionization of the sulfonated anthraquinone compounds primarily results in the formation of deprotonated molecules, [M-H]-. Once ionized, the ions were subjected to collision-activated dissociation (CAD). The type of neutral molecules or ions cleaved during CAD facilitates identification of the original compound. In most cases, a loss of 64 amu was observed for all dyes and was confirmed to be SO2 by high resolution mass spectrometry analysis. A unimolecular rearrangement of the sulfonate (SO3) group was triggered by CAD that allowed loss of SO2. Also, it was found that different group functionalities attached to the anthraquinone backbone (e.g., secondary aromatic amines and secondary alkyl amines) have specific fragmentation pathways that can be used to distinguish them under similar CAD conditions. For example, an anthraquinone having a secondary amine with an aromatic group attached to it (e.g., Acid Blue 25) can be differentiated from an anthraquinone having a secondary alkyl amine (e.g., Acid Blue 62) based on the product ions. The resultant fragmentation patterns could contribute to the identification of unknown dyes with similar chemical structures. The method was also successfully used in concert with targeted CAD for quantification purposes. The methodology presented here is the first stage in building a high resolution mass spectrometry dye database from the extensive uncatalogued Max Weaver Dye Library at North Carolina State University.
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