Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

296 DETECTION AND ESTIMATION OF INTERMEDIATESif a parent of the intermediate is also present in the reaction mixture, then electronimpact may produce ions of the intermediate from this source as well as from theintermediate itself. Thus suppose the species of interest is a radical R, and its precursorin a reaction is RX, then we may write the processesR+e + R++2eRX+e + R++X+2etogether with similar reactions for the fragment X. Fortunately, if the electronbeam is produced with sufficiently closely defined energy, then it is possible todistinguish between R+ produced in (20) and (21). The appearance potential ofR+ in (21) will be greater, by an amount depending on the dissociation energy ofR-X, than the appearance potential of R+ in (20). Fig. 4 represents the variationVlv2Electron energy (€1Fig. 4. Variation of ion current with electron energy for a radical (R) and for its precursor (RX).of ion current with electron energy (E) for-the two processes (20) and (21).V1 is the ionization potential of R in (20) while V, is the appearance potential ofR+ from RX in (21). If E lies between V, and V,, then any ion current observedwill be a result of the radical R entering the spectrometer, while if E is greater thanV, both ionization processes (20) and (21) contribute to the ion current. The sensitivityof the apparatus to radicals depends on the shape and on the separationof the two curves in Fig. 4, and the most favourable electron energy is not immediatelyobvious. Robertson96 has shown that the minimum concentration of R(CJthat can be detected in a concentration C,, of RX is given by the expression