Speed of IMS Methods Between Liquid Separations and MS

In all electrophoretic separations, directed ion drift must be slower than stochastic motion of media molecules (1.3.9). Since molecular motion is much more rapid in gases than in liquids, electrophoresis in gases (i.e., IMS) is much faster than that in liquids. At 25 °C, the mobilities of common small ions, in cm /(V s), are 2-3 in N2 

FIGURE 1.19 Separations of amino acids by (a) CE (From Soga, T., Heiger, D.N., Anal. Chem., 72, 1236, 2000.) and (b) conventional IMS (From Beegle, L.W., Kanik, I., Matz, L.M., Hill, H.H.,Anal. Chem., 73, 3028, 2001.) The latter is faster by four orders of magnitude. 

Moreover, ions could travel much faster in vacuum than in a gas where their velocity is restricted by collisions. For example, ions fly through an evacuated tube of standard TOF MS systems in 20-100 ps, or 1000 times faster than through an IMS drift tube of similar length ( 100 cm). Of course, the feasibility of rapid ion travel in MS does not imply that any MS process must be quick analyses in FTICR, quadrupole trap, and orbitrap systems where ions are stored in circular orbits often last 100 ms, i.e., longer than typical IMS separations. However, the existence of MS techniques placing the usual duration of IMS analyses is (on a logarithmic scale) about halfway between those of MS and condensed-phase methods have crucial implications for practical utility of IMS. 

Changes of ion mobility as a function of electric field intensity are smaller than absolute K values. Hence differential IMS is slower than conventional IMS (Chapter 4), and fitting it between liquid separations and MS is not as easy. Nonetheless, high speed in comparison with chromatographic alternatives is a major advantage 

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