Differential IMS

The science and technology of IMS has been developing rapidly over the last decade and now branches into two subfields conventional and differential IMS. The fundamental distinction between them is in the physical quantity underlying the separation (the separation parameter). 

Differential IMS comprises methods dependent on a change of some ion transport property as a function of electric field and thus requiring a time-dependent field that substantially varies during the measurement. 

This section summarizes the ion motion in IMS in general and applies to both conventional and differential IMS. The dynamics in high electric fields specific to the latter is detailed in Chapter 2. Here we show how the salient features, advantages, and drawbacks of IMS ensue from the fundamentals of ion mobility and diffusion in gases. 

IMS is within a limited range for a great diversity of ions, varying by just one order of magnitude between small atomic species and proteins (3.2.1). Nonetheless, increase of frx for extremely large macroions and nanoparticles eventually constrains the mass range of differential IMS instruments (3.2.1). 

The dehnition of R by Equation 1.20 presumes that wi/2 scales with u, else R would be a function of separation parameter. That is true in conventional IMS where R does not depend on K, but generally false in differential IMS, making the dehnition of R debatable. Anyhow, the resolving power of differential IMS is determined by different formulae and depends on the mobility of specihc ion, but is limited by same phenomena—mostly diffusion, with contributions of Coulomb repulsion and inihal packet dimensions (Chapter 4). Despite recent instramental and operational ... 

Often the most relevant characteristics of a separation method is peak capacity (pc)—the number of separable species for a particular sample. The pc is proportional to R, but also to the width of separation space—the range of separation parameters possible for a certain analyte type. As an illustration, an MS system with R = 1000 would produce pc 10 for a mixture of ions uniformly distributed between m/z = 500 and 1300 (a complex proteolytic digest) but only 10 for a mixture comprising m/z = 500-505 only (an isotopic envelope of a hypothetical compound). Hence broader separation space may compensate for lower R, which is typically the case for differential IMS in comparison with conventional IMS (Chapter 4). 

By Equation 1.23, the values of R and thus pc in IMS increase as a square root of analysis time, which is standard for separations in both gas and liquid media, including CE and chromatographic methods such as liquid chromatography (LC). This happens because the distance between separated species is proportional to t while the diffusional spread of each scales as Differential IMS separations are subject to the same fundamental scaling (4.2.1). 

Such gas-dependent shifts of relative K for two ions reflect differences in size, mass, and polarizability of gas molecules, though the effect on isomeric separations cannot be due to different M. To quantify these phenomena and predict the optimum gas for separation of any two ions, one needs to calculate mobilities of polyatomic ions in any gas. So far, that has been demonstrated only with He, in which attractive ion-gas interactions are weak and even crude models produce accurate fl (1.4.4). The choice of gas is much more important in differential IMS, where relative separation parameters of ions in different gases often differ dramatically (3.4) and not by a few percent as in conventional IMS. 

The capability to control separations by adjusting the temperature and, in some cases, pressure of buffer gas adds to the flexibility of IMS provided by variability of gas composition. Those effects become much more pronounced in differential IMS (3.3.4 and 4.2.6). 

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... 

Beyond a much higher speed, the major distinction of IMS from condensed-phase separations is that it is also a stmctural characterization tool of broad utility. This capability, central to the analytical profile and potential of IMS, arises from the possibility to compute the mobility (under some conditions) for any hypothetical geometry reasonably accurately. In this section, we review the approaches to calculation of ion mobilities in gases and point out the challenges of extending those methods to differential IMS. 

Figure 2.2). (Such data are commonly collected in IMS smdies to determine K more precisely and verify that the measured K equals Klf)) needed to derive H using Equation 1.10.) A lower measurement accuracy obviously increases the apparent (E/AOc- In Ihs result, the high-field behavior is seen in differential IMS at much lower E/N than in conventional IMS (3.2.4). The distortion of Maxwell-Boltzmann distribution also causes ion heating by Equation 1.27. Hence a negligible deviation of K from K(relative measurement accuracy) is achieved when ATh < yT, where y is a coefficient dependent on x. Using Equation 1.27, that can be expressed as... 

A profound significance of that fact for high-field and differential IMS makes understanding its origin important. The quantity tp in Equation 1.12 can be expressed as " ... 

Hence of primary relevance to differential IMS at and around STP are the intermediate E/N where neither Equation 2.8 nor 2.11 applies. Various interpolations between the (E/N) 0 and (E/N) oo regimes were devised to cover the whole... 

Clustering has a particular bearing on mobilities in gas mixtures, especially when one or more components (e.g., strongly polar molecules) bind to ions much tighter than other(s) (e.g., small nonpolar molecules). This effect, considered in the following section, is increasingly employed to improve the resolution of differential IMS (3.4). 

E/N = 173 Td. Hence the hard-shell model for non-Blanc phenomena is misleading at E/N relevant to differential IMS. ... 

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