Modeling the Ion Funnel Trap for Ion Mobility Spectrometry Aleksey V. Tolmachev, Yehia Ibrahim, Richard D. Smith, and Mikhail E. BelovPacific Northwest National Laboratory, Richland, WA

Introduction

Overview

Methods

Results

References1. Y. M. Ibrahim, M. E. Belov, A. V. Tolmachev, D. C. Prior, R. D. Smith. Ion Funnel Trap

Interface for Orthogonal Time-of-Flight Mass Spectrometry. Anal. Chem., 2007, 79(20), 7845-7852.

2. B. H. Clowers, Y. M. Ibrahim, D. C. Prior, W. F. Danielson, III, M. E. Belov, and R. D. Smith. Enhanced Ion Utilization Efficiency Using an Electrodynamic Ion Funnel Trap as an Injection Mechanism for Ion Mobility Spectrometry. Anal. Chem. 2008, 80, 612-623.

3. E. Baker, B. H. Clowers, F. Li, K. Tang, A. V. Tolmachev, D. C. Prior, M. E. Belov, and R. D. Smith. Ion Mobility Spectrometry–Mass Spectrometry Performance Using Electrodynamic Ion Funnels and Elevated Drift Gas Pressures. JASMS 2007, 18, 1176–1187.

4. A.V. Tolmachev, Taeman Kim, H. Udseth, R.D. Smith, T. H. Bailey, and J. H. Futrell. Simulation based optimization of the Electrodynamic Ion Funnel for high sensitivity electrospray ionization - mass spectrometry. Int. J. Mass Spec., 2000, 203, 31-47.

5. A. V. Tolmachev; B. H. Clowers; M. E. Belov; R. D. Smith. Coulombic Effects in Ion Mobility Spectrometry. Anal Chem. 2009 Jun 15; 81(12):4778-87.

6. B. H. Clowers, W. F. Siems, H. H. Hill, S. M. Massick. Hadamard Transform Ion Mobility Spectrometry. Anal. Chem. 2006, 78, 44–51.

7. M. E. Belov, M. A. Buschbach, D. C. Prior, K. Tang, R. D. Smith. Multiplexed ion mobility spectrometry-orthogonal time-of-flight mass spectrometry. Anal. Chem. 2007, 79, 2451–2462.

Conclusions

Configuration 1

CONTACT: Aleksey Tolmachev, Ph.D.Biological Sciences Division, K8-98Pacific Northwest National LaboratoryP.O. Box 999, Richland, WA 99352E-mail: tolmachev@pnl.gov

• The ion funnel trap accumulates ions and forms a compact ion packet, which can then be injected, e.g., into the drift region of an IMS

• An improved computer model of the ion funnel trap includes realistic simulations of ion-neutral and Coulomb interactions, and provides flexibility in specifying various RF and DC electrode configurations

• We report on new ion funnel trap designs that provide potential further improvements in IMS resolution, sensitivity and dynamic range

• The new ion funnel trap model is capable of realistic simulations of ion accumulation and ejection; the program has an interactive user interface, allowing rapid analysis of various configurations

• Improved ion funnel trap configurations have been designed with the aim of more efficient operation over a wide range of ion charge and pressure

• Both conventional and multiplexed IMS modes modeled with the new ion funnel trap have demonstrated improved performance in an extended range of pressure

The electrodynamic ion funnel greatly improves performance of the ion mobility spectrometer (IMS) [1-3]. The ion funnel trap accumulates ions and forms a compact ion packet, which is then injected into the IMS drift region. We recently developed a new generation IMS platform that is characterized by improved resolution, sensitivity and dynamic range. A critical component of the new IMS platform is an improved IF trap, which operates efficiently in a wide range of m/z, ion populations and gas pressures, and ejects ions fast enough to enable high resolution IMS. Here, we report results from a computer modeling of the ion funnel traps of various configurations.

The computer model was based upon theoretical approaches and algorithms developed previously for the simulation-based design and optimization of the ion funnel and the IF trap [4]. The model was extended to facilitate consideration of various profiles of RF ring electrodes and designs that incorporate RF and DC grids and DC-focusing electrodes. The ion trajectory calculations take into account time-resolved RF and DC electric fields and the bath gas influence. The model is capable of realistically describing the random (i.e., diffusion)component of the ion motion, collisional damping of the ion kinetic energy, effective focusing of ions in the RF fields, ionmotion in the DC fields, and drag forces due to gas flows.

The model was used to review various configurations of the ion funnel trap, to study operation under a range of conditions. The ion capacity of the trap was evaluated using the multi-particle simulation approach. A primary objective was to achieve efficient operation under conditions of the bath gas pressure increased to>~10 Torr, where RF focusing efficiency is reduced and higher ejection fields are needed to provide fast enough ejection. Ion accumulation, storage, and ejection into the IMS drift region were studied for three types of trap configuration.

The first configuration modeled an experimentally studied design [1, 2] that included three grids and a converging section that focused ejected ions into radially compact packets. The modeling resultswere consistent with experimental observations that showed efficient operation in a 1 - 4 Torr pressure range.

Configuration 2 was aimed at increased ion accumulation and ejection efficiency, for pressure up to 10 Torr. The “flat” trapping section consisted of 11 RF/DC ring electrodes, 19 mm ID, 11 mm long. The converging section and the three grids were removed; the stopping barrier for ion accumulation was generated by two DC-only circular lenses downstream from the ion funnel trap.

A third configuration was designed specifically for fast ion ejection under conditions of increased pressure (>10 Torr) and also considered the multiplexed IMS regime [6,7], where fast ejectionfronts and distinct boundaries of ion packets are required. The pusher grid has been added to help create higher extraction fields; the “flat” trapping section has been shortened to 7 mm, 6 RF ring electrode, followed by one DC-only ring.

AcknowledgementsThis work was funded by the National Center for Research Resources (RR018522), the National Institute of Allergy and Infectious Diseases (NIH/DHHS through interagency agreement Y1-AI-4894-01), the National Institute of General Medical Sciences (NIGMS, R01 GM063883), and the U. S. Department of Energy Office of Biological and Environmental Research (DOE/BER). The work was performed in the Environmental Molecular Science Laboratory, a DOE/BER national scientific user facility at Pacific Northwest National Laboratory (PNNL) in Richland, Washington. PNNL is operated for the DOE by Battelle under contract DE-AC05-76RLO-1830.

t = 0.200 ms t = 0.425 ms

Configuration 2 Configuration 3

Fig.1. Configuration 1, simulation screen shot. (a) The axial ion density of trapped ions is defined by the DC potentials applied to 3 grids and ion funnel ring electrodes, left axial view. (b) Coulombic repulsion results in the radial expansion of the ion packet, with ion number density maximum developing close to RF ring electrode inner radius, and the number density approaching zero at lower radii, seen in the right cross-section view.

Fig.2. Ion ejection at increased pressure, 10 Torr. Ion losses in thesteep converging section are minimized, when increased RF voltages are used, here 400 Vpp, RF 525 MHz. Decreased axial DC gradient further improved the ion transmission, but resulted in ion packets extended axially.

In summary, Configuration 2 has shown a highly efficient ion accumulation, storage, and ejection in a range of pressure up to ~10 Torr; Axially compact packets of ejected ions provided conditions for high resolution IMS in this pressure range.

Fig. 3. (a) The gridless configuration is capable of creating axially compact ion packets; because the ion losses on grids are not anissue, weaker radial DC fields can be used, which increases the trap capacity at increased pressure (10 Torr).(b) The ion ejection is triggered by negative voltages applied to the two exit DC circular lenses. (c) The ion packet is focused radially upon ejection; the axial spread is adequate for the high resolution IMS measurements. Both axial and radial dimensions are close to optimal for reduced space charge effects [5].

Fig. 4. Ion accumulation at increased pressure; accumulation time reduced to 1 ms (for multiplexed IMS). Coulombic expansion is not fully established, even for high levels of space charge, 1 Me (a), 10 Me (b).

Fig. 5. Ion ejection is triggered by raising the grid potential to 25 Vand dropping potentials on the two DC rings to -1 and -20 V (a). The ion packet is axially compact, temporal spread of 0.09 ms issufficiently small for a high resolution multiplexed IMS (b).

Ion charge 10,000,000 e

Ion charge 1,000,000 e

Vrf-dc : 25, 5, 1, 0 VVgrid : 4 VVdc : 5, 20, -80, -120 VRF : 525 MHz, 200 Vpp

Taccumulation = 1 msP = 10 Torr N2

a

b

t = 0.109 ms

t = 0.482 ms

Vrf-dc : 25, 5, 1, 0 VVgrid : 25 VVdc : -1, -20, -80, -120 VRF : 525 MHz, 200 Vpp

a

b

a bVdc: 100, 1, 1, 0, -5, -20 Vgrids: 1, 1, 10

Tacc = 100 msP = 4 Torr N2

t = 0.002 ms

t = 0.133 ms

t = 0.373 ms

a

b

c

Ejection:Vrf-dc : 21, 1, 0 VVdc : -1, -40, -80, -120 VRF : 525 MHz, 200 Vpp

Accumulation:Vrf-dc : 21, 1, 0 VVdc : 5, 20, -80, -120 VRF : 525 MHz, 200 VppTacc = 100 ms; 1 MeP = 10 Torr N2