Ion Mobility Spectrometry

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.

Experimentally had been see that the mobility K (cm2/V·s) of ions at constant temperature and pressure through a drift gas with density (m-3), subject to an high electric field (V/cm) doesn’t remain constant, and that its dependence with the electric filed (E) can be expressed as[1]:

K(E/N) K0 [1 + α(E/N)]

Molecules ions behaviour in high and low electric fields. Alpha-function is dependent upon the applied electric field.
Molecules ions behaviour in high and low electric fields. Alpha-function is dependent upon the applied electric field.

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[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.

Alpha_taylor_serie

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[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 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.

Different ion mobility spectrometers exist. But in each IMS instrument, four main regions can be identified: sample introduction system; ionization area; drift tube (where separation or selection occurs) and detection area.

Sections IMS_v4
Needed elements to operate an Ion Mobility Spectrometer.

 

IMS contributions:

  • Coupling a branch enclosure with differential Mobility Spectrometry to isolate and measure plant volatiles in contained greenhouse settings. M.M.McCartney, S.L.Spitulski, A.Pasamontes, D.J.Peirano, M.J.Schirle, R.Cumeras, J.D.Simmons, J.L.Ware, J.F.Brown, A.J.Y.Poh, S.C.Dike, E.K.Foster, K.E.Godfrey, C.E.Davis. Talanta, 2016, 146: 148-54. http://dx.doi.org/10.1016/j.talanta.2015.08.039.
  • Review on Ion Mobility Spectrometry. Part 1: current instrumentation. R.Cumeras, E.Figueras, C.E.Davis, J.I.Baumbach, I.Gràcia. Analyst, 2015, DOI: 10.1039/c4an01100g.
  • A Gas Sensor: the Ion Mobility Spectrometer. R. Cumeras, ORAL PRESENTATION. In Abstracts of Papers Presented to the First Interdisciplinary phD Student Conference (“Primera joranda d’Investigadors Predoctorals Interdisciplinària”), Pages 11-12. Barcelona, Spain. February, 7th 2013.
  • Influence of Operational Background Emissions on Breath Analysis using MCC/IMS devices. R. Cumeras, P. Favrod, K. Rupp, E. Figueras, I. Gràcia, S. Maddula, J.I.Baumbach.,  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. R.Cumeras, T.Schneider, P.Favrod, E.Figueras, I.Gràcia, S.Maddula, J.I.Baumbach., 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. R.Cumeras, I.Gràcia, E.Figueras, L.Fonseca, J.Santander, M.Salleras, C.Calaza, N.Sabaté, C.Cané., 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. R.Cumeras, I.Gràcia, E.Figueras, L.Fonseca, J.Santander, M.Salleras, C.Calaza, N.Sabaté, C.Cané., 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., R.Cumeras, I.Gràcia, E.Figueras, L.Fonseca, J.Santander, M.Salleras, C.Calaza, N.Sabaté, C.Cané. Procedia Engineering, 2010, 5: 1236-1239. DOI:10.1016/j.proeng.2010.09.336.

 

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.

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