Fragmentation of protonated dansyl-labeled amines for structural analysis of amine-containing metabolites

Dansylation of amine-containing metabolites has been shown to provide a significant enhancement in detectability, thereby allowing the generation of a more comprehensive metabolome profile of biofluids using liquid chromatography (LC) electrospray ionization (ESI) mass spectrometry (MS). The relatively stable structures of dansyl-labeled amines afford no fragmentation during the ESI and ion transport to a mass analyzer, ensuring that all peaks detected are from the metabolites, not fragment ions produced, for example, in the interface region. However, for deducing or confirming chemical structures of metabolites, generation of fragment ions of the intact molecular ions by tandem MS is required. We report a study of the fragmentation behaviors of protonated dansyl-labeled amines in comparison to those of the unlabeled counterparts. Characteristic fragment ions of unlabeled amines were observed in a quadrupole linear trap (QTrap) tandem mass spectrometer, while collision-induced dissociation (CID) of the corresponding labeled amine ions mainly produced the fragment ions that contain the dansyl moiety with neutral losses of parts of the original amine molecules. In most cases, no fragment ions from the original amine molecules were detected from a labeled amine. MS3 in the linear trap did not generate any useful second generation of fragment ions from the original amine molecules. However, it was found that by increasing the skimmer voltage to produce the fragment ions of the labeled amine in the skimmer region, followed by a priori selection of the fragment ion in the first quadrupole mass analyzer with an m/z value corresponding to the mass difference between the molecular ion of the dansyl-labeled amine and the expected mass of the dansyl group, second generation of fragment ions could be produced by CID and stored in the linear trap for improved detection. These second generation of fragment ions generally show a similar fragmentation pattern to that of the protonated unlabeled amine. In some cases, additional fragment ions were found. Thus, chemical structure information of dansyl-labeled amines could be generated using MS/MS and this pseudo-MS3 approach.

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► MS/MS and MS3 spectra of dansylated amines do not give much structural information.
► Skimmer-fragmentation can produce ions with m/z corresponding to the protonated unlabeled amines.
► MS/MS of the ions generates similar fragment ions as those of MS/MS of protonated unlabeled amines.
► These pseudo-MS3 spectra can be used for identification of dansylated amines.