Consequences of Ion Reactions during FAIMS Analyses

Once more, differential IMS resides on the directed drift component shifting the ion-gas molecule velocity distribution to the right of Maxwell-Boltzmann distribution at gas temperature (2.2.2). As ions in gases are thermalized, the increase of translational ion temperature (Th) mirrors onto vibrational (2.5) and rotational (2.6) temperatures via inelastic collisions. While this inelasticity and dismption of alignment by rotational excitation affect the mobility of a fixed geometry (2.5.1, 2.6, and 2.7.2), internal heating may change the ion structure via endothermic isomerization, dissociation, or reactions with carrier gas components. 

Ions may also undergo unimolecular decay or react with buffer gas regardless of heating. The latter is a major challenge for DT IMS analyses of species (e.g.. Si cluster ions) that, at the experimental temperature, are inat to He or Na but irreversibly react with some of their ubiquitous trace contaminants such as O2 or water vapor. In FAIMS, the problem would be similar or worse because the separation is often longer 

FIGURE 3.43 Schematic trajectories of ions converting inside FAIMS from precursor X to product Y with Ec values differing by more than the instrumental resolution solid and dashed fines are for the species in and not in equilibrium, respectively. The transition does or would occur in the end (a), the middle (b, d), and the beginning (c) of separation FAIMS is set for Ec of X in (a, b) and Y in (c, d). 

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. 

One is the self-cleaning effect if any aspect of FAIMS operation 

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