ION SEPARATION IN IMPERFECT FIELDS OF THE QUADRUPOLE MASS ANALYZER .3. TRANSMISSION AND OPTIMAL ION INJECTION

Theoretical aspects of ion separation in imperfect fields of the quadrupole mass analyser are applied to simulate transmission using an analytical approach. Mathematical simulation based on the principles of statistical mechanics reveals in analytical form that the region of beam capture, i.e. the transmission, is inversely proportional to the relative value of the mass analyser distortions to the 1.18th power in log scale. Since the beam at the mass analyser exit has a pulsing profile and the ion current is amplitude modulated by the frequency of the alternating component of the field, transmission losses because of a parasitic modulation increase more than tenfold as the resolution and distortion increase. These losses are reduced to a minimum by a heterogeneous standing wave voltage applied to the mass analyser with a linear distribution of amplitude along the ion transit axis. Then, the transmission increases exponentially 10-100-fold depending on the resolution and relative value of the distortions. This procedure has the advantages of the ion injection at zero phase in each cycle of an r.f. field.