Ion dynamics in a trapped ion mobility spectrometer

Abstract

In the present paper, theoretical simulations and experimental observations are used to describe the ion dynamics in a trapped ion mobility spectrometer. In particular, the ion motion, ion transmission and mobility separation are discussed as a function of the bath gas velocity, radial confinement, analysis time and speed. Mobility analysis and calibration procedure are reported for the case of sphere-like molecules for positive and negative ion modes. Results showed that a maximal mobility resolution can be achieved by optimizing the gas velocity, radial confinement (RF amplitude) and ramp speed (voltage range and ramp time). The mobility resolution scales with the electric field and gas velocity and R = 100-250 can be routinely obtained at room temperature.

Fig. 1

Cross-sectional view of the TIMS entrance funnel, analyser section and exit funnel.

Fig. 2

Time sequence of a mobility analysis in a TIMS analyser for positive ion mode. Notice that fill and trap times can be adjusted for the maximum mobility resolution and sensitivity and for kinetic measurements, respectively.

Fig. 3

Total pressure, gas velocity and velocity vector profiles for nitrogen in the TIMS analyser section. P₁ and P₂ were used as boundary conditions (P₁ = 2.6 and P₂ = 1.0 mbar). Scale: red: high and blue low.

Fig. 4

Comparison of the experimental and theoretical relative ion abundance (m/z = 622) as a function RF amplitude. Theoretical simulations were performed using one (v_x = 70 m s⁻¹) and three (v_xyz, parabolic) dimensional gas profiles for the same conditions as shown in Fig. 3.

Fig. 5

Ion trajectories as a function the RF amplitude theoretical relative on abundance (m/z = 622) as a function RF in the TIMS analyser. Simulations were performed at the same conditions as Fig. 3 and 4 for m/z = 322 and using an RF frequency of 890 kHz.

Fig. 6

(Top) Elution voltage (V_elut) dependence on the fill/ramp time (20/150–20/450 ms) for m/z = 622, 922 and 1222 at RF 250 V_pp. (Bottom) Dependence of the elution voltage peak width (FWHM) as a function of RF amplitude.

Fig. 7

Dependence of the total IMS time on the ramp time as a function of m/z.

Fig. 8

(Top) Mobility resolution as a function of the ramp time for the sphere-like ESI Tuning Mix standards for P₁ = 1.9 and 2.6 mbar and P₂ = 1.0 mbar, respectively. (Bottom) IMS profile for m/z = 2722 with RF = 300 V_pp, T_ramp = 450 ms and P₁ = 2.6 and P₂ = 1.0 mbar.