materials & engineering sciences center atoms to continuum external second gate, fourier...

Materials & EngineeringSciences Center

Atoms to Continuum

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

Materials & EngineeringSciences CenterAtoms to Continuum

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.


Atoms to Continuum

Sample Inlet

Ion Drift Region

Drift Gas Exhaust Air Drift Gas Inlet

High VoltageRepeller

63NiIonization Region

EntranceGate

FocusingRings

ApertureGrid

Faraday Collector

Signal Out

Commercial/Military IMS Drift Tube

Drif

t Gas

Flo

w----


Atoms to Continuum

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.


Atoms to Continuum

0

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10

12

0 5 10 15 20 25

Signal Averaging IMSReactant Ion and Calibrant Peaks

Inte

nsity

Drift Time (ms)

Signal intensity and spectral resolution generated by conventional (signal averaging) IMS.

The observed peak tailing is due to ion-molecule reactions occurring during time-of-flight and further compounded by the signal averaging process.


Atoms to Continuum

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.


Atoms to Continuum

Entrance gate pulse

Exit gate pulse

open

open

closed

closed

CYCLE REPEATED(IF DESIRED)

LOW FREQUENCY

HIGHFREQUENCY

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.

Ion Gating in FT-IMS

8Materials & EngineeringSciences Center

Atoms to Continuum

Fourier Transform of theIon Mobility Interferogram

0 1 2 3 4

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Frequency / kHz

Ion Mobility Interferogram Ion Mobility Spectrum

Fourier Transform

0

20

40

60

80

100

0 5 10 15 20 25

1b. 10 kHz FT-IMS Spectrum

Drift Time (ms)

0

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60

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1a. Signal Averaged IMS Spectrum

Drift Time (ms)

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1b. 10 kHz FT-IMS Spectrum

Drift Time (ms)


Conventional IMS vs. FT-IMS

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1c. 20 kHz FT-IMS Spectrum

Drift Time (ms)

0

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60

80

100

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1d. 40 kHz FT-IMS Spectrum

Drift Time (ms)


FT-IMS Allows Tunable Resolution


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Fourier Transform IMSOpen

Closed

Open

Closed

Signal Averaging IMS

****** ***** ***** ***** ***** **** * ** * ** * ** ** ** *

*

************

*******

*

************


TNT Response as a Function of Scanning Time


PETN Response as a Function of Scanning Frequency


HNS Response at 10kHz and 20kHz Scanning Frequency


HMX Response: Frequency Range and Scan Time

20


RDX Response as a Function of Frequency Range Scanned

Resolution vs. Aspect Ratio as Indicator of Peak Quality


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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.

Resolution Number vs. Aspect Ratio (Drift Time/w1/2) (Peak Height/wb)


Atoms to Continuum

0 5 10 15 20 25 30 35 40 45Drift Time (ms)

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

Resolution in IMS


Atoms to Continuum

Selected Bench-top IMS Instruments

IMS 5000 UVIMS-MCC Itemiser AirSentry IonScan 400B

Draeger G.A.S. G.E./Ion Track SAES/Molecular Smiths DetectionSafety Co. Technol. AnalyticsGermany Germany U.S.A. Italy U.K.

Tritium 63Ni or UV 63Ni 63Ni 63Ni50 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. CorporationGermany Germany Germany U.S.A. U.S.A. U.K.

63Ni Tritium 63Ni 63Ni Laser Corona30+ 50 30-60 NA 50 NA

Reference: Analytical Chemistry, Product Review. October 1, 2003. Pages 435-438A

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 ------

Peak Quality Determines

False Alarm Rate


Atoms to Continuum


Atoms to Continuum

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Signal Averaging IMS100ppb RDX in acetoneIn

ten

sity

Drift Time (ms)

RDX

acetone

reactant ionpeak

8.5 ms


Atoms to Continuum

-1

0

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Comparison of FT-IMS and Signal Averaging IMSSample 100 ppb RDX

Signal Averaged IMS

Fourier Transform IMS

Inte

nsi

ty

Drift Time (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.

8.5 ms


Atoms to Continuum

Handheld FT-IMS


FT-IMS: Rear View


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 WAProfessor David Harris, Harvey Mudd College, Claremont CA

United States Department of the Navy: Contract N4175603GO14803

materials & engineering sciences center atoms to continuum external second gate, fourier...

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