Endothermic Processes Control by the Average or Maximum Ion Temperature

2 Endothermic Processes Control by the Average OR Maximum Ion Temperature  

The relative variation of K(E) during the FAIMS cycle is normally negligible compared to that of E(t) and may be ignored, at least, for large ions. Then Equation 3.72 reduces to 

The quantity (F2) has been encountered in the formulas for average ion diffusion in FAIMS (3.1.2), which reflects the proportionality of diffusion coefhcient to ion temperature. Based on (F2) values for rectangular and bisinusoidal F t) with/=2 and clipped F t) with /= 2.51 that are optimum for K E/N) expansions tmncated to the first term (3.1.2 and 3.1.3), the maximum instantaneous Th at waveform peaks (Fn.max) exceeds Th by 2 times for the rectangular and 3.6 times for the harmonic-based F t). Profiles optimized for real K E/N) expansions including 1 terms (3.1.4) may lead to somewhat lower or higher T H,max/ H values, depending on/by Equations 3.25 and 3.28. 

To clarify that in experiment, one needs to correlate the transitions caused by FAIMS and by defined thermal heating using an ion with the observable state sensitive to the internal temperature—an ion thermometer. Good thermometers would be species with many distinguishable isomers that interconvert on the experimental timescale at points densely spaced over the relevant T range. 

FIGURE 3.45 Mobility spectra measured for ubiquitin ions (z = 6-10) in He gas using ESI/DT IMS with source temperature of 25-132 °C as labeled. (Adapted from Li, J., Taraszka, J.A., Counterman, A.E., Clemmer, D.E., Int. J. Mass Spectrom., 185-187, 37, 1999.) Geometries are classified as compact (c), partly folded (p), and unfolded (u). 

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