Ion Mobility Spectrometry (IMS) technique separates gas-phase ionized molecules based on their size-to-charge ratio. Separation is done by mobility differences under electric field in a flow of neutral gas. The advantages of IMS, including compactness and portability of instrumentation, short separation time (milliseconds scale), and low detection limits (ppt – ppb range), allow a wide range of applications.
Different ion mobility spectrometers exist. Up to eight different types of IMS have been identified, depending on their configuration and electric field ranges.
Ions mobility K (cm2/V·s) at constant temperature and pressure through a drift gas with density N (m-3), subject to a high electric field E (V/cm) doesn’t remain constant and that its dependence with the electric field (E) can be expressed as:
where E/N is the normalized electric field and is expressed in Td (Townsends; 1Td=10-17V·cm2) being K(E/N) the mobility of the ions and K0 the mobility coefficient under low or zero field; α(E/N) is a function that takes account of the ion mobility dependence on electric field. K0 and α are specific for each type of ion. Mobility is usually considered constant with regard to E, for those ion mobility spectrometers that operate at low E fields (E~7,500 V/cm, E/N<30Td. However, for high values of E/N, K varies become dependent on the electric field (i.e. K = K(E/N)).
The function α(E/N) takes account of the dependence of the ion mobility with the electrical field for a constant gas density, at ambient pressure and temperature. The approximation for the function α (E/N) corresponds to a Taylor’s series.
All α2n values may be positive and/or negative depending on the ion-neutral potential Φ among other factors. However, none is null and α(E/N) is never exactly zero, though it can be near-zero over a broad range of E/N. The n coefficients could, in principle, be derived from higher-order collision integrals of the collision cross section using elaborated formalisms that will not be reported here. Experimental measurements have shown that α2 is three to five orders of magnitude smaller than one and α4 is two orders of magnitude smaller than α2. So, in practice, with only two factors, it is enough to calculate the dependence of the mobility with the electric field.
 Mason, E.A. and E.W. McDaniel, Transport properties of ions in gases. 1988, New York: John Wiley & Sons Inc.
 Shvartsburg, A.A., Differential Ion Mobility Spectrometry: Nonlinear Ion Transport and Fundamentals of FAIMS. First ed. 2009, Boca Raton, FL: CRC Press.
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Analyst HOT article: http://blogs.rsc.org/an/2014/12/17/hot-articles-in-analyst-43/
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