In-source CID of guanosine: Gas phase ion-molecule reactions

In-source collision induced dissociation was applied to access second generation ions of protonated guanosine. The in-source gas-phase behavior of [BH2]+-NH3 (m/z 135, C5H3N4O+) was investigated. Adduct formation and reactions with available solvent molecules (H2O and CH3OH) were demonstrated. Several addition/elimination sequences were observed for this particular ion and solvent molecules. Dissociation pathways for the newly formed ions were developed using a QqTOF mass spectrometer, permitting the assignment of elemental compositions of all product ions produced. Reaction schemes were suggested arising from the ring-opened intermediate of the protonated base moiety [BH2]+, obtained from fragmentation of guanosine. The mass spectral data revealed that the in-source CH3OH-reaction product underwent more complex fragmentations than the comparable ion following reaction with H2O. A rearrangement and a parallel radical dissociation pathway were discerned. Apart from the mass spectrometric evidence, the fragmentation schemes are supported by density functional theory calculations, in which the reaction of the ring-opened protonated guanine intermediate with CH3OH and a number of subsequent fragmentations were elaborated. Additionally, an in-source transition from the ring-opened intermediate of protonated guanine to the ring-opened intermediate of protonated xanthine was suggested. For comparison, a low-energy collision induced dissociation study of xanthosine was performed. Its dissociation pathways agreed with our assumption.     

A numerical scheme to diagnose interferences in gas chromatography-mass spectrometry quantitation of coeluting isotopically labeled and unlabeled counterparts with partially overlapping ion profiles

Quantitation of chromatographically coeluting compounds with partially overlapping mass profiles is a challenging task, especially if only a low-resolution mass spectrometer is available. To examine whether theoretical predictions can be utilized to determine the appropriate concentration ranges of the coeluting compounds that satisfy the non-interfering condition, we utilized an algorithm based upon a two-component model to compare the experimentally measured and predicted quantitation errors. Selected unlabeled and 13C-labeled polychlorinated biphenyl (PCB) congeners were investigated as model compounds. Standard solutions containing various concentration ratios of the unlabeled and 13C-labeled PCB congeners were analyzed, and the data were used to compare with theoretical predictions derived from the chlorine isotopic distributions (35Cl and 37Cl). Good agreements between experimental predictions and theoretical predictions were found on the magnitude of interferences for quantitation of 13C-labeled PCB congeners, as well as on the variability of the quantitation errors with the concentration ratio of 13C-labeled and unlabeled PCB counterparts. In addition, the magnitude of interferences considered in the present study was highly dependent upon the number of coexisting ions included for quantitation and their relative abundances in the mass spectrum. All these results suggest that the magnitude of interferences in quantifying a pair of coeluting compounds with partially overlapping mass spectral profiles can be effectively determined and minimized by carefully selecting the concentration ratio of the coeluting compounds and/or the number of quantitation ions. Finally, the selection of the experimental parameters to satisfy the non-interfering condition can be made purely on the basis of theoretical considerations.     

Pyridine and benzonitrile adducts of bis(4-nonafluorobiphenyl)zinc

Treatment of (Ar_F′)₂Zn(OEt₂)₂ (Ar_F′ = 4-C₆F₅C₆F₄) with 2 equiv. of benzonitrile, 4-(phenyl)benzonitrile, 4-(pyrrolyl)benzonitrile, pyridine, 4-(phenyl)pyridine or 4-(pyrrolyl)pyridine in dichloromethane afforded the corresponding adducts (Ar_F′)₂ZnL₂ in near quantitative yield. The 2,2′-bipyridine adduct was prepared similarly. Multinuclear NMR spectroscopy indicated that zinc′s four-coordinate character was maintained in solution. The pyridine complex crystallized from dichloromethane with a solid-state structure free of face-to-face aryl–aryl interactions. In contrast, the 4-(pyrrolyl)pyridine adduct crystallized from both dichloromethane and 1,2-difluorobenzene, with solvent of crystallization, but otherwise essentially identical supramolecular architectures assembled through aryl–aryl synthons, including a face-to-face pentafluorophenyl–pyrrole interaction.