A set of three heparin-derived disaccharide deprotonated ions was isolated in a linear ion trap and subjected to UV laser irradiation in the 220 -290 nm wavelength range. The dissociation yields of the deprotonated molecular ions were recorded as a function of laser wavelength. They revealed maximum absorption at 220 nm for the nonsulfated disaccharide, but centered at 240 nm for the sulfated species. The comparison of the fragmentation patterns between ultraviolet photodissociation (UVPD) at 240 nm and CID modes showed roughly the same distribution of fragment ions resulting from glycosidic bond cleavages. Interestingly, UVPD favored additional cross ring cleavages of A and X type ion series enabling easier sulfate group location. It also reduced small neutral losses ( T he major advances achieved in mass spectrometry have revolutionized the structural analysis of proteins and led to the emergence of proteomic strategies. Although software-assisted de novo sequencing of peptides resulting from unknown proteins has become a routine task, the structural assignment of complex N-and O-glycans is much less trivial. Unlike proteins accurately translated from a unique template that follows a universal code, a similar set of glycosyl transferases may, across different species, build oligosaccharides of different structure according to an a priori unpredictable chronological catalysis activity.Compared with N-and O-linked glycans, the issue of structural complexity is continuing to increase in the world of glycosaminoglycans. Glycosaminoglycans are linear polymerized disaccharidic repeating units containing hexuronic acids and hexosamines. Their essential roles in governing numerous biologic functions [1] have led many groups to focus their efforts on defining generic strategies based on mass spectrometry, to obtain the structural definition of these acidic oligosaccharides. In the case of oligosaccharides derived from heparin or heparane sulfate, both glucuronic and iduronic acids and glucosamine may be sulfated at their OH and NH 2 groups. The fragmentation behavior of heparin-derived oligosaccharides has proven to be difficult to predict. Sulfate groups often impede access to exhaustive structural information by collision induced dissociation (CID)-MS/MS. Indeed, when the charge state of the selected molecular ion is low, neutral SO 3 losses most often predominate in comparison with glycosidic backbone cleavages [2]. This situation can be improved by combining MS/MS data resulting from different charge states.Very recently, Amster and coworkers published outstanding results showing how promising the electron detachment dissociation (EDD) of deprotonated sulfated oligosaccharides can be in providing informative fragment ions [3]. In particular, they documented the discrimination between the C5 epimeric glucuronic and iduronic acids based on specific fragment ions [4]. As in CID mode, they also observed that SO 3 loss can be dramatically reduced when EDD is performed on the charge states where the carboxylate group is ...

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