ABSTRACT
Chapter 3 has described the dependence of FAIMS separation parameter (compensation field, EC) and its range (the separation space) on the properties of asymmetric waveform (amplitude or dispersion field, ED, and form F(t)) and the ion-gas molecule potential embedded in the function a(E=N). While EC defines the position of FAIMS spectral peak, it is silent about its width or height in absolute terms or relative to those of other features. Those quantities are controlled by the resolving power R (1.3.4), sensitivity, and dynamic range-the key metrics of any separation method. As FAIMS passes filtered ions to a detector, the ultimate number of counts depends on its characteristics and the interfaces in front of and behind FAIMS that will be reviewed in a future companion volume. With respect to FAIMS per se, the relevant figure is the ion utilization or transmission efficiency s-the ratio of currents after and prior to filtering (Iout=I0) for ions sought: removing others is the separation objective. For any ion, s depends on the applied EC and is defined by the maximum Iout at the peak apex where that EC matches proper EC of the ion. As any ion handling device (1.3.5), FAIMS has space-charge limitations that set the maximum Iout (the saturation current Isat) while the minimum detectable Iout depends on the detector noise. The ratio of those values determines the dynamic range.
