Dispersive FAIMS

FIGURE 3.17 Schematic motion of ions of species X, Y, and Z in hypothetical dispersive FAIMS device. Separated ions are registered by array detector. 

In 3.1, we discussed using asymmetric electric field to disperse ions in space by the difference between mobilities at high and low E. However, constructing a dispersive FAIMS (Figure 3.17) in parallel to TOF MS or DT IMS presents a daunting engineering challenge. 

The dispersion of ions by any mechanism requires a substantial separation region for reasonable R. That is practical in TOF where =0 or DT IMS where E is moderate and constant, but not in FAIMS that needs strong oscillatory E. With R defined by Equation 1.20, for dispersive FAIMS R = djwy where w /i characterizes the ion packet broadening (1.3.4). Substimting by Equation 3.8 and... 

By Equation 3.39, at fixed E/N the needed gap is proportional to 1/Ed and thus to l/P. However, /d = tfmax X Ed and thus does not depend on P. Therefore dispersive FAIMS with reasonable resolution would require inordinately high voltages at any gas pressure, and, to date, FAIMS has been practiced in the filtering mode only. 

While time-dispersive ion mobility devices of the type used for drift tube IMS require aperture grids prior to the Faraday plate to preserve the resolving power of the instrument, ion filters and scanning mobility spectrometry such as differential mobility spectrometry (DMS), field asymmetric IMS (FAIMS), differential mobility analysis (DMA), and aspiration IMS (alMS) do not require an aperture grid and can efficiently detect ions with a simple Faraday plate. In these devices, ions do not travel as a discrete swarm, and the exact arrival time of the ions is not critical. Figure 7.3 shows a schematic of a typical differential ion mobility spectrometer (DIMS) in... 

The FAIMS filtering is allowed by that Ad< d (3.2.2), so Addispersive separation yet wide enough for ion motion in the E i) cycle. If one establishes E(f) in such a gap with conductive boundaries and places ions inside, species with d = Q wUl remain balanced (oscillating around initial positions) and others will drift to one of the boundaries and be destroyed by neutralization (Figure 3.19a). Such device would filter ions only with d=0, which is not very... 

As described in 3.2.3, FAIMS separates ions by the compensation field Eq at a particular dispersion field and each ion/gas pair has a specific Pc( d) curve. In this section, we discuss how those curves depend on the ion and gas properties. A note on the nomenclature is due first. 

The FAIMS data were commonly reported in terms of voltages on electrodes creating Eq and Pq—the compensation voltage, CV, ( /c) and dispersion voltage, DV, (C/d)- For planar gaps, Eq and Pq are uniform and... 

Hence FAIMS can analyze product ions if precursors convert early in the separation. This mode resembles the annealing regime in DT IMS, where ions are rapidly isomerized or dissociated by heating upon energetic injection into the drift tube from a lower-pressure region and the unreacted precursor(s) and/or product(s) are separated or characterized by mobility.Unlike in DT IMS, the remaining precursor(s) and resolved product(s) cannot pass FAIMS under same conditions if their Eq values differ by more than the instmmental resolution, one must be eliminated. However, the precursor(s) or product(s) may be selected by FAIMS and appear as separate peaks in the Eq spectrum. This distinction reflects that DT IMS is a dispersive and FAIMS is a filtering method, and has important implications for FAIMS analyses. 

Equations 4.34 and 4.35 do not depend on the mobility or diffusion properties of ions other than via g in Equation 4.34 that can be mitigated by raising like at short fres- As the dispersion field in FAIMS is not low, scales with K super-linearly and, by Equations 4.32 and 4.33, sensitivity stiU decreases and resolution improves for species with higher K. However, the linear part of the dependence of Djx on K is cancelled per Equations 4.34 and 4.35 and the discrirnination is much less than that in flow-driven FAIMS. This may be seen by comparing Figure 4.9 versus Figure 4.2 for same ions and otherwise identical conditions. For example, the spread of values in the exponent of s(t s) at Wc = 750 kHz drops from 5.4 to 1.5 times, and the spread of Rq factors in the formula for R(t es) (Table 4.1) decreases... 

Dispersion Field Gradient and Compensation Field Shifts IN Curved FAIMS... 

This approach may be extended to FAIMS and dispersive IMS-ADD by replacing a symmetric waveform with an asymmetric one. In this mode (Figure 5.13), ions will be filtered by the difference between mobUities at high and low E and simultaneously separated by the value of The 2D IMS-ADD and... 

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