The fragmentations of a number of cinnamic acids substituted at the phenyl ring have been studied with the aid of 70eV mass spectra and mass analysed ion kinetic energy spectra. Evidence is presented that the formation of [GH,O,]+ ions occurs by intramolecular aromatic substitution reactions. A mechanism is proposed for the energetically favourable loss of the substituents from meta and para positions of the phenyl ring. The analytical use of intramolecular aromatic substitution reactions is briefly discussed.

meta and para substituted benzalacetones lose the substituents after electron impact in a multi-step intramolecular aromatic substitution. The differences in the relative abundances of the benzopyrylium ions thus formed are not determined by the activation energies for the substituent losses but depend on a delicate balance between the thermodynamic stability of the intermediates involved and the rates of several H-shifts within the intermediates (kinetic stability). The consequences for the analytical utility of intramolecular aromatic substitution are discussed brietly.

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Ast et al.' reported on interference peaks in mass analysed ion kinetic energy (MIKE) spectra. Referring to mixture analysis with MIKE spectra the authors conduded that '. . . . artifact peaks . . . . never appear when performing MIKE analysis of [M]", [M-11' or [M-2]+' ions in pure samples'.' This statement is somewhat misleading. It is shown in this note that there are indeed cases in which 'artifact' peaks (better: interference peaks) do appear in MIKE analysis of [MI'. and [M-11' ions. Consider the general case of two ions, [XI" and [X + 2]+', which are separated by two mass units. The ions of the higher mass, [X+2]+', lose a H atom and the ions of the lower mass, [XI+', are tuned in with the magnet for MIKE analysis. The apparent mass of metastable [X + 2]+' ions losing H in the first field free region will then beThese ions will therefore be transmitted through the magnetic sector tuned to m/z [XI. If now the electric sector is scanned the [(X+2)-1]+ ions will be transmitted at a value of Accordingly, the [(X+ 2) -13' ions will appear in the MIKE spectrum of [XI' very close to the position for [X-71' ions. This situation will frequently occur, if chemically pure compounds containing suitable isotopes (e.g. C1, Br) lose a hydrogen atom in an energetically favourable fragmentation.' For illustration Fig. 1 shows the partial MIKE spectra of [M("Cl)]+' (peak l), [M("Cl)]" (peak 2) and [M(3'Cl)-H]c ions (peak 3 ) generated from p-chlorocinnamic acid.3 The 7 0 e V mass spectrum of this compound contains the [M(35Cl)]" peak as the base peak and the [M(35Cl)-H]i peak is the second most abundant peak (68% of base peak). The MIKE spectrum of the [M3'Cl]+. ions ( m / z 182) clearly shows a signal for the loss of a H atom; however, the peak is composite (peak 1). According to Eqns (1 and 2) the narrow component IS assigned to metastable [M(37Cl)]+. ions which lose a H atom in the 1st field free I I -' f0 0.995f0 f0 0.995f0 Eo 0.995f0 I I -d I 182 I81 -m/z 184 183 -m / z 181 180--m/r Figure 1. Partial MIKE spectra of (a) [M(35Cl)l" (peak l), tb) M(37C1)]+.(peak 2) and (c) [M(35CI)-H]+ ions (peak 3) obtained from p-chlorocinnamic acid (m/z 182, 184, 183, respectively). Peaks 1 and 2 are distorted due to H losses from [MI+' ions within the electric sector of the mass spectrometer. region (FFR) of the mass spectrometer and the broad component of peak 1 to metastable [M(35Cl)]+. ions which lose a H atom in the 2nd FFR. This last assignment can be checked by recording the MIKE spectrum of [M(37Cl)]+. ions ( m / z 184), where interference peaks must be absent. Peak 2 thus obtained is no longer composite and the narrow component of peak 1 has vanished.This experiment shows that the composite nature of peak 1 is not due to different mechanisms for the H loss from molecular ions of p-chlorocinnamic acid, but is due to the physics of ion paths through electric and magnetic fields as given by Eqns (1 and 2). This situation will also arise with all chloro and bromo compounds losing a hydrogen atom from the molecular ions. Peak 3 (F...