Improving Ion Mobility Measurement Sensitivity by Utilizing Helium in an Ion Funnel Trap Yehia M. Ibrahim, Sandilya V.B. Garimella, Aleksey V. Tolmachev, Erin S. Baker and Richard D. Smith Biological Sciences Division, Pacific Northwest National Laboratory, Richland WA 99352

Introduction

Overview Methods Results

Acknowledgements Portions of this work were supported by the NIH National Institute of General Medical Sciences grants (8 P41 GM103493-10, R21 GM103497, and R01 ES022190). This project was supported by the U.S. Department of Energy (DOE) Office of Biological and Environmental Research (OBER) Pan-omics program at Pacific Northwest National Laboratory (PNNL) and performed in the Environmental Molecular Sciences Laboratory, a DOE OBER national scientific user facility on the PNNL campus. PNNL is a multiprogram national laboratory operated by Battelle for the DOE under contract DE-AC05-76RL01830.

Conclusions

No trapping

CONTACT: Yehia M. Ibrahim, Ph.D. Biological Sciences Division Pacific Northwest National Laboratory E-mail: yehia.ibrahim@pnnl.gov

• A significant increase of IMS-MS sensitivity is realized by utilizing He-enriched gas in an ion funnel trap.

• Gas composition has minimal effects on ion funnel performance.

• Improvements are attributed to faster trap ejection.

• IMS resolving power did not degrade by introducing He.

• SIMION® simulations support experimental observations.

• Large improvements in sensitivity were observed upon adding He to the ion funnel trap as compared to pure N2

.

• No degradation of IMS resolving power was observed.

• The improvement in sensitivity was attributed mainly to faster ion ejection.

• At 4 torr the trap efficiency is slightly better in He than in N2

.

• Ion simulations using SIMION are consistent with experimental observations.

• Ion mobility spectrometry (IMS) is a gas phase separation technique in which ions are distinguished according to their collision cross sections.

• Ion packets are pulsed into a drift cell that contains a buffer gas (usually He or N2) and pulled by a uniform weak electric field.

• Traditionally, ion traps that precede the drift cell utilize the same buffer gas composition as the drift cell.

• In this work we show that introducing helium into the trap while operating the drift cell with nitrogen further improves the sensitivity of the platform, in some cases by more than an order of magnitude.

• This improvement is attributed to the faster ejection of ions from the trap.

• Buffer gas composition has no significant effect on the performance of the ion funnels.

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• Trypsin digested Bovine Serum Albumin (BSA) solution

• 0.2 μg/μL in a 49.5:49.5:1 methanol/water/acetic acid buffer

• 94 cm long drift tube • 50 mm i.d. drift rings made of PCB lenses • 4 Torr pressure in IFT and in drift cell • IMS resolving power (for 1+): 75 • Optional high efficiency fragmentation (up to 70 % CID efficiency) in the segmented quadrupole

• Curtain plate in front of inlet capillary • 0.5 mm i.d. inlet capillary • Ion funnel trap lenses are made of printed circuit boards (PCB) • Efficient trapping at 4 torr in the ion funnel trap

Source

Drift tube

• TOF resolution ~25,000 at 1500 m/z • Agilent 8 bit ADC • 3 s IMS-QTOF data frame

Acquisition system

Sample

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Trapping

Significant intensity improvement (>10) upon adding He to trap.

Improvement is pronounced for low mobility/high m/z ions.

High mobility ions are not affected.

• More ions exit the trap when He was used. • Sensitivity increase correlates with mobility. • The lower the mobility, the larger the

improvement in sensitivity.

Top panel: Normalized intensities of three peaks as a function of gate opening time at trap injection time of 3.24 ms. Solid symbols are for 30% N2 / 70% He while open symbols are for 100% N2 / 0% He. For clarity intensities are normalized to the highest value for each ion

Bottom panel: Arrival time distribution of m/z 571, 831 and 784 peaks.

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IMS-TOF of BSA digest 100% N2 30% N2 / 70% He

Faster trap ejection SIMION Simulation

• Ion simulations were performed using SIMION 8.1.

• Calibrated SDS code was used to model ion-neutral collision.

• m/z 784 (2+ ) ions was trapped fr 2 ms and then released using 480 µs pulse.

• In pure nitrogen, ~44% of ions failed to exit the trap in 480 µs.

• In pure helium, all ions exited the trap in the same time of 480 µs.

• 23% of ions were lost to the converging part of the ion funnel trap.

• Changing the geometry of the converging section drastically reduced ions losses to 1%.

Simulations results

Ion confinement1

• ω is RF frequency (angular units), ω = 2π f; the ion velocity collisional relaxation time τ, K is ion mobility, m is ion mass and ze is charge.

• For P = 4 torr and f = 1.0 MHz, γ(He) = 0.96 and for γ(N2) = 0.65 which is a slight reduction (~30%) in the effective confinement.

22

22

1 τωτωγ

+= K

zem

=τ

References 1. Tolmachev A.V. et al. Nucl. Instrum. Methods Phys. Res. Sect. B 1997, 124(1), 112-119

γ is the ratio of the effective RF field in the presence of gas collisions to that in vacuum

lost transmitted retained0

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IMS-TOF platform

Screenshot of SIMION simulation in N2. Notice the red dots indicating ions failed to exit the trap.

exit gate

trap converging part

2D IMS-TOF data of BSA digest in 100% N2

2D IMS-TOF data of BSA digest in 30% N2 / 70% He