Ion Focusing by Thermal Gradient in the Gas

A thermal gradient may also be established in a curved gap, which may be mimicked by planar electrodes with the temperatures of 

FIGURE 4.25 Similarity between the electric fields in a planar gap with thermal gradient across (a) and an isothermal curved gap (h). The isolines of equal E/N are drawn. 

FIGURE 4.26 Spectra for [M — H] ions of taurochoUc acid and methotrexate measured in cylindrical FAIMS without (a) and with (b, c) thermal gradient, the internal and external electrode temperatures are marked. The FAIMS parameters are r e = 7.75 mm, g = 2.5 mm, bisinusoidal F t) with /= 2, t/o = 5 kV, and = 750 kHz. (Adapted fiom Barnett, D.A., Befibrd, M., Dunyach, J.J., Purves, R.W., J. Am. Soc. Mass Spectrom., 18, 1653, 2007.) 

However, Equations 4.60 and 4.61 are not rigorous. The equivalency between thermal gradient in the gap and its curvature based on the ideal gas law ignores the dependence of reduced KonT (3.3.4). That effect decreases the gradient needed to emulate a curved FAIMS when Kq T ) Ko T for Tj Tj and raises it otherwise. As each ion/gas pair has a unique Kq(T) form, no substitution of gap curvature by gradient of T in analyses of ion mixtures can be exact. For a single species, one may adjust T and T2 to match the mobilities at curved electrodes for any one U, e.g., U=Uu  

Adjusting thermal gradient across the gap is an elegant way to manage the FAIMS resolution/sensitivity balance without mechanical modifications. While this approach is slower than ripple scaling (4.3.6) because of thermal inertia of electrodes, it provides the capability to transition from planar to an effectively curved FAIMS that may transmit strong ion currents better, especially in targeted analyses (4.3.7). 

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