Dehmelt force

Reducing the pressure in curved FAIMS may materially change separation parameters because of Dehmelt force, which may be a benefit or a problem (4.3.8). This effect may necessitate limiting reduced-pressure FAIMS to planar gaps. 

The Foe value is always proportional to by Equation 4.51 and thus to the inverse square of gas pressure (P). If E (i.e., Uj)/g) in FAIMS is scaled with P to keep E /N constant, Foe by Equations 4.53 and 4.56 and thus Foe remain fixed because K normally scales as 1/V (unless in the clustering or dipole ahgnment regimes, 2.3 and 2.7). In contrast, conservation of Ec/N means that Eq scales with N (4.2.6). Hence the relative c change due to Dehmelt force (Foe/Fc) increases linearly with pressure, which should notably affect the spectra in any curved FAIMS below some pressure (Foe) depending on the curvature and instrumental resolution. For example, at F=38 Torr (4.2.6), the values of Foe 0.1 V/cm calculated above would exceed the measurement uncertainty of 0.03 V/cm. Then the magnitude of Foe (defined as the point where that uncertainty compares with Foe) for typical cylindrical FAIMS systems can be crudely gauged as 100 Torr. This does not mean that FAIMS separation would be impossible at lower pressure, but that the results would depend on the gap curvature and differ from those with planar FAIMS. 

As the Dehmelt force actually varies across the gap (decreasing toward the external electrode), it affects not only the location of the bottom of pseudopotential due to ion drift nonlinearity (which determines c). but also its profile that controls ion focusing properties and thus the peak shape and height (4.3.1). This matter remains to be explored, as well as the effect in planar gaps with uneven temperature across (4.3.9). Overall, the issue of Dehmelt force in curved gaps should become topical with ongoing efforts to reduce FAIMS pressure (4.2.6). 

Equation 4.53) decrease. Therefore the total force due to field gradient in curved gaps might oppose the Dehmelt force, pointing to the internal electrode. 

Unlike the Dehmelt force, Fjh depends on pressure only weakly via W( ) and does not increase at reduced pressure. Hence the force on dipoles in inhomogeneous field should not be important for FAIMS, unless for exceptionally large macrodipoles such as found for DNA (2.7.3) or in extremely curved gaps. 

In fact, if Ed is scaled to keep Ed/N constant at lower P as we exemplified for the Dehmelt force, must drop relative to Ec- While Ifinrl maximum by Equation 4.59 would be proportional to f/o/g d and thus to Ec, the actual would increasingly fall below that value because dipole alignment weakens with decreasing E (2.7.2). 

Force upon an ion in the Dehmelt pseudopotential Force upon a dipole in an inhomogeneous electric field Fractions of B flowing into translational ion motion parallel and perpendicular to the collision axis Fractions of s flowing into rotational ion motion parallel and perpendicular to the collision axis... 

---