FAIMS Filtering Using Compensation Field

As d is quite small compared to [X(0) brc], usually 0.1%-1% (3.2.2), Ec should be similarly small compared to Eo and the K(E) variation small compared to absolute K. Hence we can approximate Equation 3.48 as 

Equation 3.50 does not contain KiQ) the Eq value is independent of absolute mobility and hence FAIMS and DT IMS can, in principle, be orthogonal (5.2.3). The leading term of Equation 3.50 is 

Of practical interest is the inverse problem of extracting a from measured Ec(Ed) dependences. This involves regression analysis to fit the series 

Ttie values in parentheses are for the bisinusoidal F(t) by Equation 3.26 with/=2. The values in Ref. [2.54] include 6-8 digits, the present values with three digits should suffice for practical calculations. 

FIGURE 3.21 The K E/N) function for Cl in air extracted from FAIMS measurements (solid line) and fit using Equation 3.55 up to 1, 2, or 3—dashed lines with n labeled. Squares are DT IMS data. (From Viehland, L.A., Guevremont, R., Purves, R.W., Barnett, D.A., Int. J. Mass Spectrom., 197, 123, 2000.) 

---