Effects of Field Strength and Ion Residence Time

5 EFFECTS OF FIELD STRENGTH AND ION RESIDENCE TIME 11.5.1 Ion Mobility Spectrometry 

Ion residence inside an analyzer is perhaps the most overlooked parameter in all of IMS methods and fundamentally governs the appearance of a mobility spectrum. The principle is reasonably simple Response to a substance will be favorable if the lifetime of an ion is greater than the residence time of the same ion in a mobility analyzer. In contrast, response will be poor or nil when the lifetime of an ion is shorter 

Mobility spectra will exhibit protonated monomers for most polar or strongly polarizable molecules when the reactant ions are hydrated protons and vapor levels of analyte are more than 10 to 100 ppb, the detection limits for most such compounds. Mobility spectra may contain a proton-bound dimer when vapor levels are increased to 0.5 to 1 ppm, yet a proton-bound trimer or tetramer is never observed, even if vapor concentrations exceed those needed to form these higher cluster ions according to equilibrium calculations. In mobility spectrometers today, ions are formed and then drawn into purified air or gases excluding neutrals of sample. Thus, equilibrium does not exist in analytical mobility spectrometers, and ion passage through purified gas should be seen as a kinetic experiment. 

Ion lifetimes for protonated monomers are commonly much greater than 20 ms at temperatures up to lOO C and beyond. In contrast, some proton-bound dimers have lifetimes under a few milliseconds and thus are not observed unless temperatures are comparatively low (e.g., -20 C) molecules with strong dipoles have proton-bound dimers that are comparatively long lived and can drift for 20 ms or more. In contrast, the lifetimes for proton-bound trimers in a purified gas atmosphere at ambient pressure and temperatures are under 1 to 5 ms and undergo rapid decomposition these are never seen in analytical IMS drift tubes unless control of sample vapors is lost and ion-neutral reactions in the drift region occur. Higher clustered ions have even shorter lifetimes than proton-bound trimers and thus are not ever observed in mobility spectra.  

A large record exists on the mobility of small ions often in nonclustering atmospheres, and in analytical DMS or FAIMS, the emphasis has been on molecules of industrial, environmental, or medical importance, which tend to be large, complex structures of organic molecules with dipoles, and measurements are made in purified 

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