improving ion mobility measurement sensitivity by ... · improving ion mobility measurement...
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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: [email protected]
• 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
400 600 800 1000 1200-2.5-2.0-1.5-1.0-0.50.00.51.01.52.0
m/z
100% N2, 0% He
Inte
nsity
(x10
6 )
30% N2, 70% He
400 500 600 700 800 900 1000 1100 1200
-4
-2
0
2
4
6
8
100% N2, 0% He
Inte
nsity
(x10
4 )
m/z
30% N2, 70% He
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.
0 200 400 600 800 1000 1200 1400 16000.0
0.2
0.4
0.6
0.8
1.0
20 21 22 23 24 25 26 27 28 290
1
2
3
4
5
6
100% N2/ 0% He
30% N2/ 70% He
571, 2+ 571, 2+ 784, 2+ 784, 2+ 831, 3+ 831, 3+
Norm
alize
d In
tens
ity
Gate opening time (µs)
831 784
Coun
ts (x
104 )
Time (ms)
571
1200
1000
800
600
400
0
20k
40k
60k
80k
100k
120k
140k
160k
14 16 18 20 22 24 26
Inte
nsity
Time (ms)
m/z
1200
1000
800
600
400
0
20k
40k
60k
80k
16 18 20 22 24 26 28 30
Inte
nsity
Time (ms)
m/z
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
20
40
60
80
100
% Io
ns
100 % N2
100 % He
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