Presentation on theme: "Analytical Chemical Sensing using High Resolution Terahertz/Sub-millimeter Wave Spectroscopy Benjamin L. Moran, Alyssa M. Fosnight, Ivan R. Medvedev Department."— Presentation transcript:
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 SpectrometerAdditional Details Microwave SynthesizerCustom Built VDI Diode MultipliersVirginia Diodes ( GHz) PreconcentratorEntech 7100A Absorption Cell2m 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 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. 66 th 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 USA; GRANT M. PLUMMER, Enthalpy Analytical, Inc., 2202 Ellis Rd., Durham, NC USA; CHRISTOPHER D. BALL, AARON J. FRANK, Battelle Memorial Institute, 505 King Ave., Columbus, OH USA. IEEE SENSORS JOURNAL, VOL. 12, NO. 8, AUGUST 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
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) *Benzene1,2 Dichlorobenzene *Bromomethane1,3 Dichlorobenzene *Carbon Tetrachloride1,4 Dichlorobenzene *Chlorobenzene*1,1 Dichloroethane *Chloroethane1,2 Dichloroethane *Chloroform1,1 Dichloroethene *Chloromethane*1,2 Dichloropropane 1,2 Dibromoethane*Styrene *Methylene Chloride1,1,1 Trichloroethane *1,2,4 Trichlorobenzene1,3,5 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 1131,2,4 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 (O 2, N 2, H 2 O, Ar, CO 2 ) 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 GHz 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
Preconcentrator Removes major atmospheric constituents CO 2, H 2 O, N 2, O 2, 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
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 Volumetric Dilution Preconcentration Efficiency Dilution of an analyte in mixture
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 VOCRelevance Concentration /ppb IsopreneLung injury, Cholesterol metabolism 150 MercaptansLiver disease Dimethyl sulfide2-14 MethanethiolMethionine metabolism AmmoniaRenal failure AminesRenal failure MethylamineProtein metabolism AcetoneDiabetes MethanolMetabolism of fruit EthanolIntestinal bacterial flora propanolEnzyme mediated reduction of acetone AcetaldehydeOxidation of endogenous ethanol 10 OCSAcute marker of organ rejection, gut bacteria 10 NO, COAirway inflammation Toluene 4 Ethylbenzene 2 H2O2H2O2 Airway inflammation 0.1-2