columbus spec

Report
Analytical Chemical Sensing using High Resolution
Terahertz/Sub-millimeter Wave Spectroscopy
Benjamin L. Moran, Alyssa M. Fosnight, Ivan R. Medvedev
Department of Physics
Wright State University
Christopher F. Neese
Department of Physics
Ohio State University
The Experiment
•
•
A THz gas phase chemical sensor was created which is capable of analyzing complex
atmospheric mixtures of volatile organic compounds(VOC’s).
A chemical preconcentrator was coupled to a custom THz spectrometer. Using this sensor
we can analyze complex mixtures. This experiment is a proof of principle for the long term
goal of analyzing environmental gas mixtures and exhaled human breath.
The System
Continuous Wave THz Spectrometer
Additional Details
Microwave Synthesizer
Custom Built
VDI Diode Multipliers
Virginia Diodes
(210-270 GHz)
Preconcentrator
Entech 7100A
Absorption Cell
2m long by 4” wide (14L)
Scientific Advantages of Rotational
Spectroscopy for Chemical Detection
•
Advantages for Chemical Detection
– Spectral signature is extremely sensitive to conformational and isotopic changes of
molecular structure.
– Energy level separations are much less than kT, resulting in a large number of thermally
populated energy levels.
– Applicable to polar neutral and reactive species.
– High Accuracy of the Measured Frequency of Molecular Transitions.
– High Number of Resolution Elements (Determined by Doppler limited line width) for
Analysis of Complex Mixtures.
– Total amount of sample needed for detection is small.
• Samples are generally static.
– Highly-Sensitive Spectrometer Design.
– Detection based on spectroscopic signatures requires no calibration.
Related Work:
Mission Adaptable Chemical Sensor Developed at Ohio State University
•
Project Goals:
– Entire sensor must fit inside a 1 CF box including:
• Vacuum system capable of providing 10-5 atm ideal
sample pressure
• Preconcentrator for removing atmospheric gases
• X-band synthesizer
• SMM TX/RX and Folded Absorption Cell
• Data acquisition hardware
• Computer for data analysis
• Power and conditioning
– No consumables (cryogens / carrier gases)
– Sensitivity goal of < 100 ppt for one analyte
• Tests preconcentrator and spectrometer
– Selectivity goal of analyzing mixtures from a library of >31 analytes
• Test for spectrometer only.
IEEE SENSORS JOURNAL, VOL. 12, NO. 8, AUGUST 2012 2565
Compact Submillimeter/Terahertz Gas Sensor With Efficient Gas
Collection, Preconcentration, and ppt Sensitivity
Christopher F. Neese, Ivan R. Medvedev, Grant M. Plummer, Aaron J.
Frank,Christopher D. Ball, and Frank C. De Lucia, Member, IEEE
66th International Symposium on Molecular Spectroscopy in 2011
A SUBMILLIMETER CHEMICAL SENSOR
CHRISTOPHER F. NEESE, IVAN R. MEDVEDEV, FRANK C. DE LUCIA, Department of Physics, 191
W. Woodruff Ave., Ohio State University, Columbus, OH 43210 USA; GRANT M. PLUMMER, Enthalpy
Analytical, Inc., 2202 Ellis Rd., Durham, NC 27703 USA; CHRISTOPHER D. BALL, AARON J. FRANK,
Battelle Memorial Institute, 505 King Ave., Columbus, OH 43201 USA.
Chemical Selection
•
•
•
•
Method TO-14A certified
mixture sold by Scott Specialty
Gases
Selection Process:
– Only polar molecules exhibit
rotational spectra.
– Two ab-initio software
programs, GAMESS and
Gaussian, were used to
calculate electric dipole
moments and molecular
structures.
26 of the 39 chemicals were
identified to be suitable for THz
spectroscopic detection.
19 of the 26 are on the Clean air
Act of 1990 as hazardous air
pollutants
TO-14A Mixture(≈1 ppm each)
*Benzene
*Bromomethane
*Carbon Tetrachloride
*Chlorobenzene
*Chloroethane
*Chloroform
*Chloromethane
1,2 Dibromoethane
*Methylene Chloride
*1,2,4 Trichlorobenzene
1,2,4 Trimethylbenzene
*Toluene
*m-Xylene
*Cis 1,3 Dichloropropene
Cis-1,2 Dichloroethene
*Ethylbenzene
*Freon 11
*Freon 12
*Freon 113
*Freon 114
1,2 Dichlorobenzene
1,3 Dichlorobenzene
1,4 Dichlorobenzene
*1,1 Dichloroethane
1,2 Dichloroethane
1,1 Dichloroethene
*1,2 Dichloropropane
*Styrene
1,1,1 Trichloroethane
1,3,5 Trimethylbenzene
*Trichloroethylene
*o-Xylene
*Hexachloro-1,3 Butadiene
*1,1,2,2 Tertachloroethane
*1,1,2 Trichloroethane
*Tetrachloroethene
*Vinyl Chloride
*p-Xylene
1,2,4 Trimethylbenzene
*Trans 1,3 Dichloropropene
Analytical Chemical Detection Algorithm
1. Create the spectral libraries
•
•
•
Collect spectra of the pure samples at a well defined pressures (1 mTorr, 2mTorr, 5mTorr)
Select several strongest lines within the library spectrum to use as markers for mixture
analysis (snippets)
Made a total snippet library
2. Record spectra of the analytes in the mixture
•
•
•
•
Fill a Tedlar bag with 1 ppm mixture of VOC’s
Use preconcentrator to remove major air constituents (O2, N2, H2O, Ar, CO2)
Inject preconcentrator mixture into the absorption cell
Record the snippet spectra
3. Perform spectral analysis
•
•
Calculate partial pressures of every analyte present in the absorption cell by performing
the Least Squares Fitting of the mixture spectrum to the library spectra.
Deduce the dilution of each analyte in the original mixture based on the volume of the
absorption cell and preconcentration efficiency of our preconcentrator
Overview Library Spectra
•
•
All chemicals were placed into flasks and were frozen using liquid nitrogen in an
attempt to ensure their chemical purity.
Each flask was separately connected to the vacuum port and an overview library
spectrum was taken from 210 to 270 GHz at a pressure of 1 mTorr.
1.0
0.5
0.0
-0.5
290
210
300
225
310
240
GHz
GHz
Overview Spectrum of Chloroethane
320
255
330
270
Intensity Linearity
•
•
Recorded spectra for a range of sample pressures.
Linearity was checked to ensure that the chosen spectral lines belong to the
chemical of choice, as well as to select a proper pressure range.
Snippets
•
•
•
A snippet is a scan around a single line in the spectrum.
For each chemical 5 to 7 lines were selected from within the overview spectrum of
each chemical, which showed no spectral overlaps with other chemicals.
Snippets for all 26 chemicals were combined and rescanned for each chemical to
catalog any possible spectral overlaps between chemicals.
210
255
Preconcentrator
ENTECH 7100A
Preconcentrator
Dual Stage Cryo-Sorbent Device
• Removes major atmospheric
constituents CO2, H2O, N2, O2,
and Ar.
• Increases our sensitivity and
specificity
• Certified to have high efficiency
for TO-14A Mixture.
• Microscale Purge and Trap
Sampling Method
• Trap 1: Glass Beads
• Trap 2: Glass Beads/Tenax
http://www.entechinst.com/
Least Squares Fitting Routine
•
•
•
•
Using Wavematric’s Igor Pro we developed a
fitting routine to fit for the baseline and
normalized for the gains.
Each chemical has 220 linear baseline fits (440
parameters).
Then using the libraries we fit the signals
intensity(26 parameters).
Results in 466 fit parameters for entire mixture.
Mixture 4500cc Sampled
RED=Mixture
BLACK=Least Squares Fit
Blue=Residuals
Determining Partial Pressure of the Analyte
• Libraries were collected at 1mTorr
• Least squares returns partial pressure of an analyte in the mixture in mTorr
Result of Least Squares Fit
Partial Pressure of
sample in Tedlar bag
    = ()(10−6 )
Dilution of an analyte
in mixture
760 
1 
Volumetric Dilution
4.5
()
14
Preconcentration
Efficiency
Results(Glass Beads, Glass Beads)
Chemical
Chloromethane
Bromomethane
Vinyl Chloride
Chloroethane
Methylene Chloride
cis-1,2-Dichloroethene
1,1-Dichloroethane
1,1,1 Trichloroethane
Chloroform
Chlorobenzene
Freon 12
1,2 Dichloroethane
Trichloroethylene
1,2 Dichlorobenzene
1,1 Dichloroethene
Freon 11
1,2 Dibromoethane
1,1,2,2Tetrachloroethane
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S19
S25
Least Squares partial
pressure (mTorr)
Expected Partial
Pressure (mTorr)
Preconcentration
Efficiency
0.326
0.271
0.225
0.197
0.177
0.173
0.178
0.180
0.067
0.013
0.23
0.156
0.246
0.027
0.173
0.193
0.221
0.333
0.254
0.232
0.244
0.237
0.235
0.249
0.244
0.244
0.247
0.244
0.249
0.251
0.251
0.251
0.244
0.244
0.252
0.254
127%
116%
92%
83%
75%
69%
73%
74%
27%
5%
94%
62%
98%
12%
71%
77%
88%
133%
Results: Preconcentration is 100% Efficient
(Glass Beads, Glass Beads)
Dilution if 100% Preconcentration
Efficiency(ppm)
Expected Dilution (ppm)
Chemical
Chloromethane
Bromomethane
Vinyl Chloride
Chloroethane
Methylene Chloride
cis-1,2-Dichloroethene
1,1-Dichloroethane
1,1,1 Trichloroethane
Chloroform
Chlorobenzene
Freon 12
1,2 Dichloroethane
Trichloroethylene
1,2 Dichlorobenzene
1,1 Dichloroethene
Freon 11
1,2 Dibromoethane
1,1,2,2Tetrachloroethane
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
S14
S15
S16
S17
S19
S25
1.33
1.107
0.919
0.807
0.723
0.708
0.728
0.738
0.272
0.052
0.935
0.638
1.01
0.122
0.708
0.790
0.905
1.362
1.04
0.95
1.00
0.97
0.96
1.02
1.00
1.00
1.01
1.00
1.02
1.03
1.03
1.03
1.00
1.03
1.03
1.02
Percentage Recovery if
preconcentration is 100%
127%
116%
92%
83%
75%
69%
73%
74%
27%
5%
94%
62%
98%
12%
71%
77%
88%
133%
Results(1000cc): Glass Beads,Tenax
Chemical
Dilution if 100%
Preconcentration
Efficiency(ppm)
Expected Dilution (ppm)
Percentage Recovery if
preconcentration is 100%
Chloromethane
S2
1.09
1.1
98%
Bromomethane
S3
0.461
0.91
51%
Vinyl Chloride
S4
0.781
1.04
75%
Chloroethane
S5
0.561
0.98
57%
Methylene Chloride
S6
0.554
1.05
53%
cis-1,2-Dichloroethene S7
0.545
1.05
53%
1,1-Dichloroethane
S8
0.684
1.05
65%
1,1,1 Trichloroethane
S9
0.512
1.06
48%
Chloroform
S10
0.444
1.04
43%
Chlorobenzene
S11
0.608
1.05
58%
Freon 12
S12
0.824
1.05
79%
1,2 Dichloroethane
S13
0.579
1.04
55%
1,1 Dichloroethene
S15
0.503
1.05
47%
Conclusions
•
Through this research we have demonstrated a THz sensor capable of analytical
chemical sensing for environmental purposes.
•
The preconcentrator efficiency is generally above 60%
•
Sensor can be made more compact.
•
Opens possibilities using other chemicals and applications
– Exhaled Breath Analysis
Path Forward: Exhaled Breath analysis
VOC
Isoprene
Mercaptans
Dimethyl sulfide
Methanethiol
Ammonia
Amines
Methylamine
Acetone
Methanol
Ethanol
2-propanol
Acetaldehyde
OCS
NO, CO
Toluene
Ethylbenzene
H2O2
Relevance
Lung injury, Cholesterol metabolism
Liver disease
Concentration
/ppb
150
2-14
Methionine metabolism
Renal failure
Renal failure
Protein metabolism
Diabetes
Metabolism of fruit
Intestinal bacterial flora
Enzyme mediated reduction of acetone
Oxidation of endogenous ethanol
Acute marker of organ rejection, gut bacteria
Airway inflammation
Airway inflammation
400-1200
400-1200
100-400
20-300
50-400
10
10
4
2
0.1-2
Questions?
Sensitivity
Least Squares Partial
Pressure/Least Squares Partial
Pressure Error
Chemical
Chloromethane
S2
Bromomethane
S3
Vinyl Chloride
S4
Chloroethane
S5
Methylene Chloride
S6
cis-1,2-Dichloroethene
S7
1,1-Dichloroethane
S8
1,1,1 Trichloroethane
S9
Chloroform
S10
Chlorobenzene
S11
Freon 12
S12
1,2 Dichloroethane
S13
Trichloroethylene
S14
1,2 Dichlorobenzene
S15
1,1 Dichloroethene
S16
Freon 11
1,2 Dibromoethane
S17
S19
1,1,2,2Tetrachloroethane S25
470
371
300
133
105
104
59.2
12.3
15.8
1.41
7.67
11.2
3.41
2.23
68.7
3.06
6.83
2.08

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