Assigning Structures to Ions in Mass Spectrometry by Paul M. Mayer, Christiane Aubry, John Holmes

By Paul M. Mayer, Christiane Aubry, John Holmes

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Mostly determined experimentally by measuring the minimum energy required for their observation in the mass spectrum of a suitable precursor molecule, AB. This measurement is referred to as an appearance energy (AE) measurement. Preferably Aþ is generated directly from ionized AB by a simple bond cleavage: AB ! (ABþ ) ! Aþ þ B (1:6) The AE of Aþ is related to DfH(Aþ) by the following equation: AE(Aþ ) ¼ Df H(Aþ ) þ Df H(B ) À Df H(AB) (1:7) Thus in principle, DfH(Aþ) can be obtained from an AE measurement, provided that the ancillary thermochemical data are known or can be estimated.

8 PROTON AFFINITIES AND THE HEATS OF PROTONATED MOLECULES OF FORMATION Before we conclude the discussion on ion enthalpies, it is necessary to consider the heats of formation of protonated molecules. The PA of a species is defined as the negative of the enthalpy change accompanying the reaction (DrH): M þ Hþ ! 56 As might be expected, PA values also vary linearly with the reciprocal of molecular size (1=n) for homologous series. The slopes of such plots are negative because the PA increases with increase in molecular size.

The residence time of molecular and fragment ions in the ion source of a typical sector mass spectrometer is generally of the order of microseconds. ABþ ions of longer lifetime will therefore not dissociate in the ion source but may do so en route to the detector, or not at all. The limiting smallest first-order rate constant for the observation of Aþ in an AE experiment with ion lifetimes of microseconds will be approximately 103 sÀ1. , higher rate constants are accessed by ions with higher ß 2006 by Taylor & Francis Group, LLC.

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