Gas phase fragmentation of sudan and d6-labelled sudan azo dyes.

The gas phase behavior of the sudan dyes and their labeled homologues is important in the set-up of analytical methods. Sudan Azo dyes are widely used as synthetic dyes in many commodities. Many papers deals with their potential danger for human health, since some of them are carcinogenic . Mass spectrometry, and particularly tandem mass spectrometry, can play an important role in the analytical procedures that aim to detect and possibly quantify these substances. It is important, however, to understand the mechanism that rules the chemistry of sudan dyes in the gas phase. In this contribution the gas phase behavior of the sudan I, II, III, IV and para-red has been analyzed together with their labeled homologues deuterated in the naphtol moiety. The high resolution electrospray experiments were carried out in a hybrid Q-Star Pulsar-i mass spectrometer equipped with an ion spray ionization source. Samples were introduced by direct infusion (5 μL/min) of the solution the appropriate azo-dye at a spray voltage of 4800 V. MS2 experiments were performed in the collision cell q on the isotopically pure (12C) peak of the selected precursor scanning the time-of-flight (TOF) analyzer. The collision energy was set from 15 to 35 eV, depending on the compound. All the acquisitions were averaged over 60 scans at a TOF resolving power of 8000. The fragmentation in the gas phase of protonated sudan azo dyes and d6-labelled homologues produces two major ions that arise from separate pathways of reaction: the first one leads to a radical amine ion formed by the homo cleavage of the N=N bond, while the second leads to the formation of a stable even nitrene ion from the naphtol moiety that is due to the hetero cleavage of the same aza double bond. Other ions are produced by (i) the loss of a molecule of water, (ii) the release of an hydroxyl radical and (iii) in the case of sudan II and IV by the loss of a methyl radical. The only difference in the spectra of sudan and d6-sudan is the presence in the latter of some peaks having one uma less that accompanies the ions formed from the naphtol moiety. These species could arise from a sort of hydrogen/deuterium exchange that takes place in the collision cell during the fragmentation. Furthermore the intensity of these peaks varies with the analyte. They are more prominent in the spectrum of Sudan para-red, while the intensity decreases in the fragmentation of sudan I and much more in the case of sudan II. Therefore, an apparent substituent effect could rule the process: the more the groups in para and/or in ortho position are electron-withdrawing the more they could stabilize an alternative site of protonation, (i.e. the nitrogen in α to the naphtol moiety) that could bring to the H/D exchange.