External Second Gate, Fourier Transform Ion Mobility Spectrometry:

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External Second Gate, Fourier Transform Ion
Mobility Spectrometry FT-IMS Next
Generation Ion Mobility Spectrometer
Edward E. Tarver, Ph.D. Analytical Material
Sciences Department Sandia National
Laboratories-Livermore, California
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Ion Mobility Spectrometry

• Real-time response few seconds analysis time.
• Sensitivity low part-per-billion detection
  without pre-concentration.
• Versatility simultaneous/universal response.
• Simplicity of electronics no vacuum
  pumps/chromatographs.
• Field portability low power, size and weight.
• Battery powered military and commercial units
  available.
• Unattended monitoring perimeter and network
  defense.

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Drift Gas Exhaust
Air Drift Gas Inlet
Faraday Collector Signal Out
-
Ion Drift Region
Sample Inlet
-
63Ni Ionization Region
Drift Gas Flow
-
-
High Voltage Repeller
Entrance Gate
Focusing Rings
Aperture Grid
Commercial/Military IMS Drift Tube
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The observed peak tailing is due to ion-molecule
reactions occurring during time-of-flight and
further compounded by the signal averaging
process.
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Fourier Transform Ion Mobility Spectrometry

• Increased Sensitivity, Lower Detection Limits
  Sensitivity depends on the duty cycle.
• FT-IMS operates with 50 ion gating efficiency
  compared to 1 with conventional IMS.
• Fifty times more ions transmitted and detected
  than conventional IMS.
• Improved Resolution, Fewer False Alarms FT-IMS
  dual-gate design eliminates broadening due to
  ion-molecule reactions and averaging process.
• Conventional IMS sums all variations in ion
  velocity, broadening peaks and reducing
  resolution. No need to average with FT-IMS.
• Suited for Miniaturization FT-IMS performance
  allows miniaturization of detectors.
• Adaptable to Current IMS Systems No hardware
  modifications to drift tube.

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Ion Gating in FT-IMS
CYCLE REPEATED (IF DESIRED)
LOW FREQUENCY
HIGH FREQUENCY
open
Entrance gate pulse
closed
open
Exit gate pulse
closed
1. Gates are open and closed for equal amounts of
time no matter how frequently they are pulsed. 2.
Ion collection during half of the analytical
cycle time, i.e., 50 duty cycle. 3. Low
frequency greater Signal/Noise, High frequency
better Resolution.
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Fourier Transform of the Ion Mobility
Interferogram
Fourier Transform
Ion Mobility Interferogram
Ion Mobility Spectrum
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Conventional IMS vs. FT-IMS
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FT-IMS Allows Tunable Resolution
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TNT Response as a Function of Scanning Time
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PETN Response as a Function of Scanning Frequency
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HNS Response at 10kHz and 20kHz Scanning Frequency
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HMX Response Frequency Range and Scan Time
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RDX Response as a Function of Frequency Range
Scanned
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Resolution vs. Aspect Ratio as Indicator of Peak
Quality

• RESOLUTION (R) R Drift Time (ms) / Peak Width
  at Half Height (ms)
• Resolution calculation ignores peak broadening
  below Half Height
• where peak tailing and overlap limits
  ability to separate adjacent peaks.
• Drift time dependent broad, low intensity peaks
  with long drift times can
• give higher Resolution (R) than strong, sharp
  peaks with short drift times.
• Misleading indicator of instrumental resolving
  power.
• ASPECT RATIO AR Peak Height (h) / Peak Width
  at Base (w)
• Unbiased indicator of peak quality, includes peak
  width below Half Height.
• Aspect Ratio is Independent of drift time and
  describes actual peak shape.

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Resolution Number vs. Aspect Ratio
(Drift Time/w1/2) (Peak
Height/wb)
R 5/2 2.5
R 20/2 10
R 32/2 16 R 40/2.5
16 AR 3.25/.375 8.6
AR 8.6
AR 8.6 AR 0.235
0 5 10 15 20 25 30 35 40 45 Drift Time (ms)
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Resolution in IMS

• Selected Bench-top IMS Instruments
• IMS 5000 UVIMS-MCC Itemiser
  AirSentry IonScan 400B
• Draeger G.A.S.
  G.E./Ion Track SAES/Molecular Smiths
  Detection
• Safety Co. Technol.
  Analytics
• Germany Germany U.S.A.
  Italy U.K.
• Tritium 63Ni or UV 63Ni
  63Ni
  63Ni
• 30-60 NA
  25 44
• Selected Handheld IMS Instruments
• RAID-M IMS Mobile µIMS VaporTracer
  Quantum Sniffer LCD3.2
• Bruker Draeger G.A.S.
  G.E./Ion Track Implant Sciences Smiths
  Detection
• Daltonics Safety Co. Technol.
  Corporation
• Germany Germany Germany U.S.A.
  U.S.A. U.K.

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Peak Quality Determines False Alarm Rate
Peak Resolution R td/w1/2 Aspect Ratio AR
h/wb PEAK IMS X2G-FT-IMS IMS
X2G-FT-IMS Ko 1.84 SA 10K 20K 40K
SA 10K 20K 40K TNT 40.97 30.27
36.59 10.74 156.8 101.6 PETN 41.23 28.74
39.56 13.68 209.8 18.88
HNS 41.94 28.74 34.31 5.98 188.4 130.2
HMX 41.35 28.57 40.98 3.02 185.6 36.56
RDX ------ 28.84 50.92 ------ 113.4
31.89 Averages 41.37 29.03 37.72
8.35 170.8 63.82 Ko 1.54
TNT 45.59 30.41 30.75 42.47 9.12 156.8
134.0 56.87 PETN 38.20 37.42 41.40
------ 5.68 47.14 75.90 ------
HNS 45.70 26.86 40.67 ------ 12.8 51.70
77.13 ------ HMX 42.04 31.76 41.49
65.99 7.52 147.4 56.84 29.81
RDX 46.33 ------ 34.11 75.27 9.32 ------
17.86 ------ Averages 43.57 31.61 37.68
61.24 8.88 100.8 72.34 ------
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acetone
RDX
reactant ion peak 8.5 ms
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8.5 ms
RDX
Note the comparative resolution of the peak a 8.5
ms. FT-IMS is able to resolve both species
Present whereas signal averaging cannot. The
peak at 12 ms is residual acetone.
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Handheld FT-IMS
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FT-IMS Rear View
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FT-IMS 9-Volt Batteries in Parallel
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FT-IMS Interior View
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FT-IMS Vertical Battery Arrangement
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Acknowledgements Sandia National Laboratories,
Research Foundations Laboratory Directed
Research and Development Grants Sandia National
Laboratories, Livermore CA Analytical Material
Sciences Department Dr. Jim Wang, Mr. Anh Phan,
Dr. Kent Pfeiffer, Mr. John Warmouth Professor
Herbert Hill, Washington State University,
Pullman WA Professor David Harris, Harvey Mudd
College, Claremont CA United States Department
of the Navy Contract N4175603GO14803