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In-Source Decay (ISD) Ions

ISD fragments are the result of very fast decomposition of the molecular ions and occur within the ion source. These ions can be the result of Y-type glycosidic cleavages that give rise to compositions indistinguishable from those of native glycans, which will distort a glycan profile. This is a common problem with FAB spectra and to the spectra of glycans that contain sialic acid.

Nomenclature introduced by Domon and Costello [261] for describing the fragmentation of carbohydrates

Figure 3.6 Nomenclature introduced by Domon and Costello [261] for describing the fragmentation of carbohydrates.

Postsource Decay (PSD) Ions

Ions that decompose more slowly can give rise to PSD fragments if decomposition occurs between the ion source and the detector as first noted by Huberty et al. in 1993 [263] for sialylated glycans. The observation has developed into a general technique for obtaining fragmentation spectra with reflectron TOF instruments [256]. However, in order to record a complete spectrum using instruments without a curved-field reflectron, the reflectron needs to be stepped across the mass range and each subspectrum stitched together to give the final result. The abundance relationship of ions in different sections of the spectra can, thus, be distorted. Ion kinetics dictates that the abundance of these PSD ions depends on the relative time that the parent ions spend in the ion source compared with the time involved in traversing the flight tube. Long in-source delay times, thus, can have an adverse effect on the abundance of PSD ions. Kaufmann et al. [264] have proposed that an additional loss of PSD fragments can be due to a reduced collisional activation in delayed-extraction sources. Unlike the production of CID fragments, PSD ion production is relatively difficult to control and generally needs reasonably large amounts (e.g., up to 100 pmoles) of carbohydrate. To some extent, judicious matrix selection can be used to control fragmentation. Thus, glycans derivatized by reductive amination have been shown to ionize by MALDI to produce mainly [M+H]+ ions, which subsequently fragment to give mainly Y-type ions. Ionization with DHB, on the other hand, produces only [M+Na]+ ions, which fragment to give a complex mixture ofB and Y ions together with some cross-ring fragments [118].

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