**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^{[1]}:

where *E/N* is the normalized electric field and is expressed in Td (Townsends; 1Td=10^{-17}V·cm^{2}) being *K(E/N)* the mobility of the ions and *K _{0}* 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.

*K*and

_{0}*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

^{[2]}(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 *α _{2n}* 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

^{[1]}from higher-order collision integrals of the collision cross section using elaborated formalisms that will not be reported here. Experimental measurements have shown that

*α*is three to five orders of magnitude smaller than one and

_{2}*α*is two orders of magnitude smaller than

_{4}*α*. So, in practice, with only two factors, it is enough to calculate the dependence of the mobility with the electric field.

_{2}**References:**

[1] Mason, E.A. and E.W. McDaniel, Transport properties of ions in gases. 1988, New York: John Wiley & Sons Inc.

[2] Shvartsburg, A.A., Differential Ion Mobility Spectrometry: Nonlinear Ion Transport and Fundamentals of FAIMS. First ed. 2009, Boca Raton, FL: CRC Press.

#### IMS outcomes:

**Comprehensive volatilome and metabolome signatures of colorectal cancer in urine**: A systematic review and meta-analysis. Mallafré-Muro C†, Llambrich M†, Cumeras, R*, Pardo A, Brezmes J, Marco S, Gumà J. († joint first authors)*Cancers*, 2021, 13(11):2534. doi: 10.3390/cancers13112534.- Coupling a
**branch enclosure with Differential Mobility Spectrometry**to isolate and measure plant volatiles in contained greenhouse settings. McCartney MM, Spitulski SL, Pasamontes A, Peirano DJ, Schirle MJ, Cumeras R, Simmons JD, Ware JL, Brown JF, Poh AJY, Dike SC, Foster EK, Godfrey KE, Davis CE.*Talanta*,*2016, 146: 148-54*. doi: 10.1016/j.talanta.2015.08.039. **Review on Ion Mobility Spectrometry. Part 2: Hyphenated Methods and Effects of Experimental Parameters**. Cumeras R, Figueras E, Davis CE, Baumbach JI, Gràcia I.*Analyst*, 2015, 140:1391-1410. doi: 10.1039/c4an01101e.**Analyst HOT article:**http://blogs.rsc.org/an/2014/12/17/hot-articles-in-analyst-43/**Review on Ion Mobility Spectrometry. Part 1: current instrumentation**. Cumeras R, Figueras E, Davis CE, Baumbach JI, Gràcia I.*Analyst*, 2015, 140: 1376-1390. doi: 10.1039/c4an01100g.- Influence of Operational Background Emissions on
**Breath Analysis using MCC/IMS devices.**Cumeras R, Favrod P, Rupp K, Figueras E, Gràcia I, Maddula S, Baumbach JI.*International Journal for Ion Mobility Spectrometry*, 2012, 15 (2): 69-78. doi: 10.1007/s12127-012-0094-0. **Stability and Alignment of MCC/IMS devices.**Cumeras R, Schneider T, Favrod P, Figueras E, Gràcia I, Maddula S, Baumbach JI.*International Journal for Ion Mobility Spectrometry*, 2012, 15(1): 41-46. doi: 10.1007/s12127-012-0088-y.**Modelling a P-FAIMS with Multiphysics FEM.**Cumeras R, Gràcia I, Figueras E, Fonseca L, Santander J, Salleras M, Calaza C, Sabaté N, Cané C.*Journal of Mathematical Chemistry*, 2012, 50 (2): 359-373. doi: 10.1007/s10910-010-9772-5.**Finite-Element Analysis of a Miniaturized Ion Mobility Spectrometer for Security Applications.**Cumeras R, Gràcia I, Figueras E, Fonseca L, Santander J, Salleras M, Calaza C, Sabaté N, Cané C.*Sensors and Actuators B-Chemistry*, 2012, 170: 13-20. doi: 10.1016/j.snb.2010.11.047.- Modeling Vapor Detection in a
**Micro Ion Mobility Spectrometer for Security Applications**. Cumeras R, Gràcia I, Figueras E, Fonseca L, Santander J, Salleras M, Calaza C, Sabaté N, Cané C.*Procedia Engineering*, 2010, 5: 1236-1239. doi: 10.1016/j.proeng.2010.09.336.