Transcript External Second Gate, Fourier Transform Ion Mobility Spectrometry: “FT-IMS” Next Generation Ion Mobility Spectrometer Edward E.

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
63Ni
Sample Inlet
Ionization Region
High Voltage
Repeller
-
Entrance
Gate
Air Drift Gas Inlet
Faraday
Collector
Signal Out
Ion Drift Region
Focusing
Rings
Aperture
Grid
Commercial/Military IMS Drift Tube
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Ion Gating in Signal Averaging IMS
open
closed
0.2 ms
20-25 ms
1. Gate is pulsed open to admit ions less than < 1% of the duty cycle.
2. Greater than 99% of the ions formed in the source are not detected.
3. Given the initial quantities, the sensitivity loss can be devastating.
Reference: United States Congress, Office of Technology Assessment.
“Technology Against Terrorism: The Federal Effort”, (1991) Page 84.
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Signal intensity and spectral resolution generated
by conventional (signal averaging) IMS.
Signal Av eraging IMS
Reactant Ion and Calibrant Peaks
12
Intensity
10
8
6
4
2
0
0
5
10
15
Drif t Time (ms)
20
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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25
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
LOW
FREQUENCY
HIGH
FREQUENCY
CYCLE REPEATED
(IF DESIRED)
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
1.0
1b. 10 kHz FT-IMS Spectrum
100
0.8
0.6
80
0.4
Fourier Transform
0.2
60
0.0
40
-0.2
-0.4
20
-0.6
-0.8
0
0
1
2
3
0
4
Ion Mobility Interferogram
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10
15
Drift Time (ms)
Frequency / kHz
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Ion Mobility Spectrum
8
20
25
Conventional IMS vs. FT-IMS
1a. Signal Averaged IMS Spectrum
1b. 10 kHz FT-IMS Spectrum
100
100
80
80
60
60
40
40
20
20
0
0
0
5
10
15
Drift Time (ms)
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20
25
0
5
10
15
Drift Time (ms)
20
25
FT-IMS Allows Tunable Resolution
1d. 40 kHz FT-IMS Spectrum
1c. 20 kHz FT-IMS Spectrum
100
100
80
80
60
60
40
40
20
20
0
0
0
5
10
15
Drift Time (ms)
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20
25
0
5
10
15
Drift Time (ms)
20
25
Signal Averaging IMS
Open
*
Closed
*
*
*
**
**
**
***
****
****
****
****
****
Fourier Transform IMS
Open
Closed
****
*****
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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
(Drift Time/w1/2)
0
R = 5/2 = 2.5
R = 20/2 = 10
AR = 3.25/.375 = 8.6
AR = 8.6
5
10
15
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vs.
20
25
Drift Time (ms)
Aspect Ratio
(Peak Height/wb)
R = 32/2 = 16
AR = 8.6
30
R = 40/2.5 = 16
AR = 0.235
35
40
45
Resolution in IMS
Selected Bench-top IMS Instruments
IMS 5000
UVIMS-MCC
Itemiser
Draeger
Safety Co.
Germany
G.A.S.
Technol.
Germany
G.E./Ion Track
Tritium
63Ni
50
30-60
U.S.A.
or UV
AirSentry
SAES/Molecular
Analytics
Italy
IonScan 400B
Smiths Detection
U.K.
63Ni
63Ni
63Ni
NA
25
44
Selected Handheld IMS Instruments
RAID-M
IMS Mobile µIMS
Bruker
Daltonics
Germany
Draeger
Safety Co.
Germany
G.A.S.
G.E./Ion Track Implant Sciences
Technol.
Corporation
Germany U.S.A.
U.S.A.
Smiths Detection
63Ni
Tritium
63Ni
Laser
Corona
50
NA
30+
50
30-60
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VaporTracer Quantum Sniffer
63Ni
NA
LCD3.2
U.K.
Reference: Analytical Chemistry, Product Review. October 1, 2003. Pages 435-438A
Peak Quality Determines
False Alarm Rate
Peak Resolution: R = td/w1/2
Aspect Ratio: AR = h/wb
PEAK
IMS
Ko =1.84
TNT
PETN
HNS
HMX
RDX
Averages:
SA
40.97
41.23
41.94
41.35
-----41.37
10K
30.27
28.74
28.74
28.57
28.84
29.03
20K 40K
36.59
39.56
34.31
40.98
50.92
37.72
SA
10.74
13.68
5.98
3.02
-----8.35
10K
156.8
209.8
188.4
185.6
113.4
170.8
20K 40K
101.6
18.88
130.2
36.56
31.89
63.82
Ko =1.54
TNT
PETN
HNS
HMX
RDX
Averages:
45.59
38.20
45.70
42.04
46.33
43.57
30.41
37.42
26.86
31.76
-----31.61
30.75
41.40
40.67
41.49
34.11
37.68
9.12
5.68
12.8
7.52
9.32
8.88
156.8
47.14
51.70
147.4
-----100.8
134.0
75.90
77.13
56.84
17.86
72.34
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X2G-FT-IMS
42.47
----------65.99
75.27
61.24
IMS
X2G-FT-IMS
56.87
----------29.81
-----------
Signal Av eraging IM S
100ppb RDX in acetone
6
acetone
5
Intensity
4
RDX
reactant ion
peak
8.5 ms
3
2
1
0
0
5
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10
15
Drif t Time (ms)
20
25
Comparison of FT-IMS and Signal Av eraging IMS
Sample 100 ppb RDX
6
8.5 ms
5
RDX
Intensity
4
Signal Averaged IMS
3
Fourier Transform IMS
2
1
0
-1
6
8
10
12
14
16
18
Drift Time (ms)
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
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