FAIMS Analyses at Reduced Gas Pressure

The mobility and diffusion coefficient are proportional to l/P (1.3.1), and the best gap width gopt by Equation 4.29 depends on the pressure even when the dispersion field is adjusted such that E /N is fixed. In that case, the value of Ad by Equation 3.43 is independent of P, but Dn scales as l/P because D scales as l/P while Dadd is a function of Ejy/N by Equation 3.22 and thus stays constant. In the limit of high waveform frequency where Ad can be ignored, g pt would scale as P . In reality. Ad is not negligible, leading to a smaller increase of gopt at lower pressure. With the exemplary of 2 and 100 ms (4.2.4), the gop, values for medimn-size ions rise from respectively 0.36 and 2.0 mm at P = 1 atm to 1.3 and 8.7 mm at 38 Torr. Then the Dd needed for constant Ej)/N at reduced pressure would scale slower than P but somewhat faster than P. In the above examples, the Dd values for equal Ejy/N = 140 Td at P= 300 K would decrease fivefold from SSO V for 2 ms and 4900 V for 100 ms at 1 atm to lOO V and llOO V, respectively, at 38 Torr. Even with Ej)/N lifted (at low P) to 360 Td to deliver far greater specificity, we would stiU have t/o of 410 and 2800 V, respectively, or half of the values for 1 atm. 

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. 

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