Analytical Chemical Sensing using High Resolution Terahertz/Sub-millimeter Wave Spectroscopy

Benjamin L. Moran, Alyssa M. Fosnight, Ivan R. MedvedevDepartment of PhysicsWright State University

Christopher F. NeeseDepartment of PhysicsOhio 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.

66th International Symposium on Molecular Spectroscopy in 2011A SUBMILLIMETER CHEMICAL SENSORCHRISTOPHER F. NEESE, IVAN R. MEDVEDEV, FRANK C. DE LUCIA, Department of Physics, 191W. Woodruff Ave., Ohio State University, Columbus, OH 43210 USA; GRANT M. PLUMMER, EnthalpyAnalytical, Inc., 2202 Ellis Rd., Durham, NC 27703 USA; CHRISTOPHER D. BALL, AARON J. FRANK,Battelle Memorial Institute, 505 King Ave., Columbus, OH 43201 USA.

IEEE SENSORS JOURNAL, VOL. 12, NO. 8, AUGUST 2012 2565Compact Submillimeter/Terahertz Gas Sensor With Efficient Gas Collection, Preconcentration, and ppt SensitivityChristopher F. Neese, Ivan R. Medvedev, Grant M. Plummer, Aaron J. Frank,Christopher D. Ball, and Frank C. De Lucia, Member, IEEE

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 1,2 Dichlorobenzene

*Bromomethane 1,3 Dichlorobenzene

*Carbon Tetrachloride 1,4 Dichlorobenzene

*Chlorobenzene *1,1 Dichloroethane

*Chloroethane 1,2 Dichloroethane

*Chloroform 1,1 Dichloroethene

*Chloromethane *1,2 Dichloropropane

1,2 Dibromoethane *Styrene

*Methylene Chloride 1,1,1 Trichloroethane

*1,2,4 Trichlorobenzene 1,3,5 Trimethylbenzene

1,2,4 Trimethylbenzene *Trichloroethylene

*Toluene *o-Xylene

*m-Xylene *Hexachloro-1,3 Butadiene

*Cis 1,3 Dichloropropene *1,1,2,2 Tertachloroethane

Cis-1,2 Dichloroethene *1,1,2 Trichloroethane

*Ethylbenzene *Tetrachloroethene

*Freon 11 *Vinyl Chloride

*Freon 12 *p-Xylene

*Freon 113 1,2,4 Trimethylbenzene

*Freon 114 *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

330320310300290GHz210 225 240GHz

255 270

Overview Spectrum of Chloroethane

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

• 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

ENTECH 7100A Preconcentrator

Dual Stage Cryo-Sorbent Device

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=MixtureBLACK=Least Squares FitBlue=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 Volumetric Dilution

Preconcentration Efficiency

Dilution of an analyte in mixture

𝑃𝑎𝑟𝑡𝑖𝑎𝑙 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒𝑖𝑛𝐶𝑒𝑙𝑙=(𝑝𝑝𝑚)(10− 6)(760𝑇𝑜𝑟𝑟1𝑎𝑡𝑚 )( 4.5𝐿14 𝐿 )(𝑝)

Results(Glass Beads, Glass Beads)

ChemicalLeast Squares partial

pressure (mTorr)Expected Partial Pressure (mTorr)

Preconcentration Efficiency

Chloromethane S2 0.326 0.254 127%Bromomethane S3 0.271 0.232 116%Vinyl Chloride S4 0.225 0.244 92%Chloroethane S5 0.197 0.237 83%Methylene Chloride S6 0.177 0.235 75%cis-1,2-Dichloroethene S7 0.173 0.249 69%1,1-Dichloroethane S8 0.178 0.244 73%1,1,1 Trichloroethane S9 0.180 0.244 74%Chloroform S10 0.067 0.247 27%Chlorobenzene S11 0.013 0.244 5%Freon 12 S12 0.23 0.249 94%1,2 Dichloroethane S13 0.156 0.251 62%Trichloroethylene S14 0.246 0.251 98%1,2 Dichlorobenzene S15 0.027 0.251 12%1,1 Dichloroethene S16 0.173 0.244 71%Freon 11 S17 0.193 0.244 77%1,2 Dibromoethane S19 0.221 0.252 88%1,1,2,2Tetrachloroethane S25 0.333 0.254 133%

Results: Preconcentration is 100% Efficient(Glass Beads, Glass Beads)

ChemicalDilution if 100% Preconcentration

Efficiency(ppm) Expected Dilution (ppm)Percentage Recovery if

preconcentration is 100%

Chloromethane S2 1.33 1.04 127%Bromomethane S3 1.107 0.95 116%Vinyl Chloride S4 0.919 1.00 92%Chloroethane S5 0.807 0.97 83%Methylene Chloride S6 0.723 0.96 75%cis-1,2-Dichloroethene S7 0.708 1.02 69%1,1-Dichloroethane S8 0.728 1.00 73%1,1,1 Trichloroethane S9 0.738 1.00 74%Chloroform S10 0.272 1.01 27%Chlorobenzene S11 0.052 1.00 5%Freon 12 S12 0.935 1.02 94%1,2 Dichloroethane S13 0.638 1.03 62%Trichloroethylene S14 1.01 1.03 98%1,2 Dichlorobenzene S15 0.122 1.03 12%1,1 Dichloroethene S16 0.708 1.00 71%Freon 11 S17 0.790 1.03 77%1,2 Dibromoethane S19 0.905 1.03 88%1,1,2,2Tetrachloroethane S25 1.362 1.02 133%

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 RelevanceConcentration

/ppb

Isoprene Lung injury, Cholesterol metabolism 150Mercaptans Liver disease

Dimethyl sulfide 2-14Methanethiol Methionine metabolism

Ammonia Renal failure 400-1200Amines Renal failureMethylamine Protein metabolism

Acetone Diabetes 400-1200Methanol Metabolism of fruit 100-400Ethanol Intestinal bacterial flora 20-3002-propanol Enzyme mediated reduction of acetone 50-400Acetaldehyde Oxidation of endogenous ethanol 10OCS Acute marker of organ rejection, gut bacteria 10NO, CO Airway inflammationToluene 4Ethylbenzene 2H2O2 Airway inflammation 0.1-2

Questions?

Sensitivity

Chemical

Least Squares Partial Pressure/Least Squares Partial

Pressure Error

Chloromethane S2 470Bromomethane S3 371Vinyl Chloride S4 300Chloroethane S5 133Methylene Chloride S6 105cis-1,2-Dichloroethene S7 1041,1-Dichloroethane S8 59.21,1,1 Trichloroethane S9 12.3Chloroform S10 15.8Chlorobenzene S11 1.41Freon 12 S12 7.671,2 Dichloroethane S13 11.2Trichloroethylene S14 3.411,2 Dichlorobenzene S15 2.231,1 Dichloroethene S16 68.7Freon 11 S17 3.061,2 Dibromoethane S19 6.831,1,2,2Tetrachloroethane S25 2.08