Ion Mobility-Mass Spectrometry Systems

Mass spectrometry obviously is an ideal detection method for gaseous ions eluting from an ion mobihty device. Therefore, the ion mobility-mass spectrometry (IM-MS) coupling has received considerable attention since the 1970s [238-240]. 

Note A detailed treatment of ion mobihty spectrometry is beyond the scope of this book, nonetheless, a brief description of the method is required. While IM-MS systems can in principle be regarded as hyphenated instrumentation (treated in more detail in Chap. 14), the pecuharities of ion mobility demand for an integrated instrumental approach rather than gas or hquid chromatography couplings that can rapidly be attached and removed from a mass spectrometer as required [235,241], Therefore, IM-MS is dealt with here as a special case under the heading of instrumentation. 

Ion mobility effects separation of ions based on their different velocities when accelerated by a constant electric field along a drift tube with a countercurrent inert gas, typically helium or nitrogen. The average velocity, Vd, of a drifting ion is then governed by the number of soft collisions it experiences within the drift tube. This number of collisions with the neutrals is directly proportional to the electric field strength, E, and the ion mobility constant K (in cm s ) 

The value of K depends on the ion and the drift gas. Equation 4.49 is only valid up to field strengths of about 1000 V cm whereas above that electric field strength the proportionality between K and E is canceled [235], 

the reduced mobility, Kq, is calculated for comparing IMS data between different instruments, by using the relation 

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