Structural characterization of sulfonated azo dyes using liquid secondary ion mass spectrometry/tandem mass spectrometry

Eight monosulfonated and disulfonated azo dyes were analyzed using liquid secondary ion mass spectrometry/tandem mass spectrometry, in the negative ion mode, under low-energy collision conditions (110–150 eV). Many structurally characteristic fragment ions were obtained, several of which have not been reported previously using other mass spectrometric techniques. Among the structurally important ions observed were those due to loss of SO₂, SO₃ and various aromatic substituents, such as NO₂ and NHCOCH₃. Losses of N₂ were also proposed. In addition, product ions due to cleavage at the azo linkage were observed, and also SO₃^? and HSO₃^? ions. Several of the azo cleavage product ions detected did not contain sulfonate groups. Possible pathways for the formation of product ions are proposed. The structural information obtained should help to better characterize and identify sulfonated azo dyes in the future.