Gas-phase ion dynamics in a periodic-focusing DC ion guide (Part II): Discrete transport modes

The purpose of this work is to expand on the theory presented by Silveira et al. [Silveira et al., International Journal of Mass Spectrometry 296 (2010) 36–42], to include a detailed discussion of discrete ion transport properties in the periodic-focusing DC ion guide (PDC IG) that result in radial ion focusing and ion mobility. We previously noted that although the PDC IG utilizes only electrostatic fields, ions are subjected to an effective RF as they traverse the device in the axial (z) direction. Here, the radial electric field (Er) oscillations generating the effective RF are investigated in detail. Equations of motion are derived to explain ion movement in the radial (r) direction. The results suggest that a collisionally dampened effective potential (V*) model can explain the observed radial ion confinement. Furthermore, a mathematical explanation regarding the effects of the non-uniform axial electric field and periodic collisional cooling phenomena generated in the PDC IG is presented in the context of ion mobility spectrometry (IMS). Included is a detailed discussion of the ion mobility coefficient (K), ion mobility resolution (R), and subsequent determination of the ion-neutral collision cross section (Ω) using the PDC IG. The results indicate that the PDC IG affords straightforward and accurate determination of K and Ω via incorporation of a mobility damping coefficient (α) which is easily derived based upon the operating conditions and the electrode geometry.

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► Discrete ion transport modes in a PDC IG (axial drift, radial ripple, and central drift motion) are deconvoluted and discussed.
► Equations of motion are derived to mathematically explain ion motion in the axial and radial directions.
► The results support the radial focusing model based on a collisionally dampened effective potential.
► Derivation of ion-neutral collision cross sections using first-order IMS principles is discussed.