Asymmetric electric field waveform

Fig. 6. Schematic of a differential mobility spectrometer showing the principles of ion separation in a differential mobility spectrometry (DMS) drift tube. Ion paths are governed by both the asymmetric electric field and field dependence of mobility for an ion. The inset displays the asymmetric waveform of separation electric field used in the DMS drift tube. The waveforms shown are theoretical (top part) and actual or experimental (bottom part) used in these experiments.

This chapter has described the use of asymmetric waveforms to separate and identify ion species based on the dependence of mobility on electric field intensity. We have reviewed the mechanism of differential mobility effect, the ways to quantify and maximize its magnitude, and the associated processes and limitations for various ion types. Understanding those issues allows us to start exploring and optimizing the performance of FAIMS systems depending on the instmmental parameters and ion properties. 

Guevremont, R., Purves, R.W., Atmospheric pressure ion focusing in a high-field asymmetric waveform ion mobility spectrometer. Rev. Set Instrum. 1999, 70, 1370. Kudryavtsev, A., Makas, A., Ion focusing in an ion mobiUty increment spectrometer (IMIS) with non-uniform electric fields fundamental considerations. Int. J. Ion Mobility... 

Over the last decade, scientific and engineering interests have been shifting from canventional ion mobility spectrometry (IMS) to field asymmetric waveform ion mobility spectrometry (FAIMS). Differential Ion Mobility Spectrometry Nonlinear Ion Transport and Fundamentals of FAIMS explores this new analytical technology that separates and characterizes ions by the difference between their mobility in gases at high and low electric fields. It also covers the novel topics of higher-order differential IMS and IMS with alignment of dipole direction. 

This is the first book on differential ion mobility spectrometry (IMS), an analytical technique also called field asymmetric waveform ion mobility spectrometry (FAIMS) and, on occasion, several of the altemafive names mentioned in the Introduction. These terms refer to the evolving methods for separation and characterization of ions based on the nonlinearity of their motion in gases under the influence of a strong electric field. 

In FAMS, the gas-phase mobility separation of ions in an electric field is achieved at atmospheric pressure [145-147], In its simplest design, the FAIMS device comprises two parallel rectangular electrodes at a uniform distance (Fig. 4.9). One of the electrodes is grounded, while at the other an asymmetrical waveform is applied. The asymmetric waveform is characterized by a significant difference in voltage in the positive and negative polarities of the waveform. FAMS nti-... 

Miniaturized ion Mobiiity Spectrometry, Figure 3 (a) Schematic view of a differentiai ion mobiiity spectrometer. For ions of positive the effective ion mobiiity increases with increase in electric field, while for ions with Of < 0, the mobility decreases with increase in electric field. It is important to note that at low electric field strength (E < 10,700 V/cm at atmospheric pressure) the value of a is zero, (b) Cyclic waveform electric field used to generate asymmetric field in the drift tube... 

Whereas high-field asymmetric waveform ion mobility spectrometry (FAIMS) is able to separate ions in a manner similar to IMS, the method cannot be considered an alternative to IMS in the context (elucidation of ion structure) discussed here, since electric fields employed in FAIMS are way above the low-field limit. " - However, two other newer developments, traveling-wave IMS and overtone ion mobility spectrometry (OMS), are worth mentioning here briefly. 

---