Calculations have been carried out, using the Quasi-Equilibrium Theory of Mass Spectra to show that a decrease in penetration of the accelerating field into the ion source of a mass spectrometer may significantly increase the number of primary daughter ions [A]+ produced from a molecular ion [MI+ in the source. The calculations show that this effect appears to be more important than the decay of [MI+ between source and collector in implementing an increase in [A]+/[M]+ at the collector with decreasing accelerator potential. W E HAVE recently presented results1 to show that a reduction in accelerator potential V in the mass spectrometer will in general cause an increase in the ion ratio [A]+/[M]+ for the electron-impact induced reaction [MI+ + [A]+ at low eV. This increase in [A]+/[M]+ at the collector was explained in terms of a decrease in [MI+ due to decomposition in the region source to collector as time of flight t, was increased. The possibility of penetration of the accelerating field into the source was recognised but was not regarded as a major contributing factor to this change in [A]+/[M]+ on changing from 8 to 2 kV.
Recent calculations,2 however, have indicated that field penetration (into an AEI MS-9 source) may bring about a considerable change in source residence time t, which will in turn influence [A]+ produced in the source and surviving at the collector. (In the original publication,l a variation in V was considered to have a negligible effect on [A]+ produced in the source). It is emphasised that a reduction in V means a slower drawout time for ions in the s o ~r c e ~' ~ (i.e. a longer t,). Hence [MI+ ions will have longer to decompose to [A]+ in the source and so [A]+/[M]+ is predicted to increase also if there is source penetration. The problem is to evaluate the contribution caused by source penetration to the 'kV effect'.
In the original publication1 the 'kV effect' was evaluated initially for the primary decomposition from 9 aromatic compounds at various low electron beam energies (low eV's were employed to minimise secondary decompositions). [A]+/ [MI+ ratios were listed at 1 kV intervals between 8 and 2 kV and were seen to increase with decreasing V in all cases (see Table 2 in reference 1).
It has recently been shown4s5 that reasonable calculations of daughter/molecular * See equations in reference 4.
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* The concentration of [A]+ at the exit slit to the source.