Solvent effects on isotope effects: methyl cation as a model system.

The isotopic sensitivity (CH3(+) vs CD3(+)) of the equilibrium between the methyl cation in vacuum and in solution has been investigated. Two alternative options for describing the shape of the solute cavity within the widely used polarized continuum model for implicit solvation were compared; the UFF and UA0 methods give equilibrium isotope effects (EIEs) that vary as a function of the dielectric constant in opposite directions. The same isotope effect was also obtained as the average over 40 structures from a hybrid quantum mechanical/molecular mechanical molecular dynamics simulation for the methyl cation explicitly solvated by many water molecules; the inverse value of the EIE agrees with UFF but not UA0. The opposing trends may be satisfactorily explained in terms of the different degrees of exposure of the atomic charges to the dielectric continuum in cavities of different shapes.

Investigation of the advanced functionalities of a hybrid quadrupole orthogonal acceleration time-of-flight mass spectrometer.

Time-of-flight mass spectrometry (ToF-MS) has gained wide acceptance in many fields of chemistry, proteomics, metabolomics and small molecule analysis. ToF-MS, however, has some inherent advantages and drawbacks. Numerous developments have been made to hybrid ToF instruments to improve their capabilities. We have used a quadrupole orthogonal acceleration ToF (Q-oa-ToF) instrument to assess developments made to improve resolution, dynamic range and signal-to-noise (S/N) ratios (i.e. sensitivity). Higher mass resolution can improve the analysis of mixtures containing compounds with similar m/z values and improved mass accuracy gives greater confidence for structural elucidation applications. Wide dynamic ranges are necessary for the analysis of unknown samples or samples that vary widely in analyte concentrations. The performance of the advanced functionalities for routine structural elucidation in terms of resolution, dynamic range and S/N ratios was investigated using test compounds. The results presented in this work demonstrate and validate the use of these new enhancements for Q-ToF instruments and also show their limitations.