Are Dispersive FAIMS Separators Feasible

FIGURE 3.17 Schematic motion of ions of species X, Y, and Z in hypothetical dispersive FAIMS device. Separated ions are registered by array detector. 

The dispersion of ions by any mechanism requires a substantial separation region for reasonable R. That is practical in TOF where =0 or DT IMS where E is moderate and constant, but not in FAIMS that needs strong oscillatory E. With R defined by Equation 1.20, for dispersive FAIMS R = djwy where w /i characterizes the ion packet broadening (1.3.4). Substimting by Equation 3.8 and 

FIGURE 3.18 Ion oscillation amplitude for three common F t) t)fpes, in lt(0) btc units. Dotted bars mark/= 2 for bisinusoidal or rectangular and/= 2.51 for clipped Fit). 

All differential IMS waveforms must be periodic for same reason. Otherwise, the chamber length would have to exceed Ad by Equations 3.42 and 3.43 with / = fres-For the above Ed = 20 kV/cm and K(0) = 1.2 cm (V s), that translates into Ad 7-24 m, which is of course unrealistic. 

By Equation 3.39, at fixed E/N the needed gap is proportional to 1/Ed and thus to l/P. However, /d = tfmax X Ed and thus does not depend on P. Therefore dispersive FAIMS with reasonable resolution would require inordinately high voltages at any gas pressure, and, to date, FAIMS has been practiced in the filtering mode only. 

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