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Proton transfer from CH,' to the fluorine atom substituent of fluorobenzene has been studied as a function of temperature and pressure using a high-pressure ion source and collision-induced dissociation (CID) mass spectrometry. The behaviour at low temperatures examined (<350 K) is similar to that expected of a 'locking' ion-dipole mechanism. However, the third-order pressure and overall T -' .46 temperature dependences are consistent with a mechanism involving the formation of an intermediate complex. This complex h a s been isolated from a benzene/fluorobenzene mixture in methane at ca. 200 K and was identified a s having the

Collisionally induced decomposition mass spectrometry has shown that during the gas phase reaction of CH5+ with fluorobenzene at temperatures < 300 K, a proton transfers preferentially to the F atom even though its proton affinity is ca. 180 kJ mol-1 less than the ring.Protonated species postulated as transients in solution can often be prepared and studied by mass spectrometry (m.s.) in the gas phase.1 Usually, however, structures can be assigned only by indirect means.2 The best method for determining the thermodynamically most favourable protonation site is to match molecular orbital (M.O.) calculations on postulated structures against relative proton affinity (P. A.) measurements (provided the molecule is not too large).' P.A. correlation methods have also been used.4 None of these methods takes into account kinetic effects2 during proton transfer.Cooks et al.2 have investigated the use of collisionally induced decomposition (c.i.d.) m.s. to determine protonation sites more directly. Of special interest for substituted aromatic compounds is the competition for the proton between the ring and a substituent X. It is thought that a peak in the c.i.d. spectrum corresponding to the loss of HX from the protonated molecule ought to be a measure of the degree of protonation on the substrate itself. Unfortunately, conclusions based on this premise are usually ambiguous2 because the possibility always remains that, under the influence of collisional activation, proton migraton occurs prior to decomposition. Even deuterium labelling c.i.d. experiments can be ambivalent because of possible isotope exchange, in parallel with proton transfer, in the ion source.We report here interesting observations from a c.i.d. study of proton transfer reactions involving fluorobenzene. The

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HighlightsSuperficially porous silica particles enable high resolution separation of nucleic acids.The pore size of the C18 superficially porous silica particles significantly effects resolution.Optimum separations of small oligonucleotides obtained with 80 Å pore sizes.Optimum resolution of oligonucleotides (>19 mers) was observed with pore sizes of 150 Å.Improved resolution of larger dsDNA/RNA molecules was achieved with pore sizes of 400 Å