Structural analysis of sulfonated monoazo dyestuff intermediates by electrospray tandem mass spectrometry and matrix-assisted laser desorption/ionization post-source decay mass spectrometry

Sulfonated azo molecules containing hydroxy groups ortho- or para- to the azo linkage exhibit azo/hydrazone tautomerism, which affects their coloristic properties and also the fragmentation pathways observed in tandem mass spectrometry. Electrospray tandem MS and matrix-assisted laser desorption/ionization post-source decay are used in this work to study the fragmentation mechanisms of such molecules. The two techniques show closely matching product ion distributions, the predominant fragmentation pathways depending on the extent of tautomerism in each molecule. Evidence of tautomerism is afforded by comparing the product ions of the hydroxyazo molecules with their methoxyazo analogues where tautomerism is not possible. Keto-tautomers show cleavage of the single N–N bond whereas the azo forms dissociate via the azo C–N bonds, followed in both cases by neutral losses of SO2. In addition, the simple introduction of a nitro group on a naphthyl group in an azo dye is shown to have a marked influence on its observed fragmentation.