1. Headspace Analysis of Volatile Compounds Using Segmented Chirped-Pulse Fourier Transform mm-wave Spectroscopy
Brent J. Harris, Amanda L. Steber, and Brooks H. Pate
Department of Chemistry
University of Virginia

2. Spectroscopy and its applications
Instruments for Spectroscopy
Spectroscopy and its applications
Frequency
Molecular Energy Levels
Commercial Techniques
RF (<1 GHz)
Nuclear Spin in Magnetic Field
NMR, MRI
MW (9-90 GHz)
Electron Spin in Magnetic Field
EPR/ESR
MW-THz ( GHz)
Overall Molecular Rotation
NONE
IR ( cm-1)
Molecular Vibration
FTIR, NDIR
Diode laser
UV-VIS ( nm)
Electronic Excitation
Fluorimeters, Imaging,
LIBS, Raman microscopy
X-rays (< 100 nm)
Inner Core Electron Excitation
X-ray emission for chemical analysis
The terahertz gap… the analytical chemist misses some of the spectrum
Growing area for development by DOD and communications industry.
Also imaging applications.

3. Rotational Spectroscopy as an Analytical Technique
“The Routine Rotational Spectrometer”
H. W. Harrington, J. R. Hearn, R. F. Rauskolb, Hewlett-Packard Journal, 1971, 22, 1
1) Competitive performance metrics
2) Benchtop operation, low sample consumption, no consumables or cryogens
3) Multispecies analysis (broadband spectrum)
For basic science research and for wide-spread adoption for analytical chemistry.
So we have seen an attempt before
We have seen

4. Chirped-Pulse Fourier Transform for Room-Temperature Gases
Why millimeter-wave?
Effect of temperature
Effect of molecule size/mass
300 K
Room temperature volaitiles, toxins favor millimeter-wave spectroscopy
Solid state, high bandwidth AMCs can reach into 2 THz, however, there is a limitation in power – 320 (WR3) the best match
GHz best window for high power sources and
3-5 heavy atom, room-temperature, volatiles

5. CP-FTmmW Spectrometer
Block schematic and segmented waveform
1) 12GS/s, 12 bit Arbitrary Waveform Generator
for custom excitation, local oscillator waveforms
2) 45 mWatt Peak Power, High Bandwidth Active Multiplier Chain
3) 4GS/s High Speed Digitizer with FPGA Signal Accumulation (16M averages)
Explain how spurs influence in LO detection

6. Segmented CP-FTmmW Near 100% duty cycle Segmented
(lowers performance requirements for digitizers)
Chirped Pulse
(add instantaneous bandwidth to utilize entire source power
Fourier Transform
(relieve pressure broadening in digital signal processing)
(enhance analysis with time resolved analytical tools)
Explain timing
The AWG and digitizer combination make it easy for software controlled customization. Trade time for analysis goal.
Near 100% duty cycle
Noise Level: mV

7. Sensitivity and Sample Matrix
Sampling Directly from Air
Max Cell Pressure: 100 mTorr (collisional broadening)
Detection Limit: 30 ppb (good, similar to CRDS)
2) Gas Processing to Isolate Volatiles
Gas Sample: 1 L-atm
Total Volatiles less than 100 mTorr
Detection Limit: 4 ppq (excellent)
1L Chamber Volume
160 fmol detection
Partial Pressure: 3 nTorr
8 minutes
18OC36S (1 : 3.4 million)
Modulate one of the connected transitions to confirm the presence of the manifold…and hence, the molecule.
Highlight selectivity
J is not really a pure quantum number (mJ degeneracy and influence to the transition dipole moment)
Still modulation is unmistakable.
Highly selective.
For a large number of volatiles, segmented CP-FT offers sub-ppb detection limits in a 10 minute broadband measurement with gas processing.

8. 4(5)-Methylimidadazole
Unknown impurity – Prediction does not match
5 mTorr total pressure
Same direct sampling
No match, calculations are not wrong….we have an impurity
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9. 4(5)-Methylimidadazole
Identification of unknown
A search for low molecular weight, common synthesis solvents
“Submillimeter wave spectrum of acetic acid”
V.V. Illyushin, C.P. Endres, F. Lewen, S. Schlemmer, B.J. Drouin, J. Mol. Spec., 2013, 290, 31
5 mTorr total pressure
Glacial acetic acid residual solvent identified with pure reference spectrum
Look at common synthesis solvents and consult library databases (not reference spectra, but calculated from rotational parameters)
Exact match by spectral catalogue
Measure pure reference to determine partial pressure (well over 20% acetic acid)
Need to purify
Melt solid to evaporate solvents and analyze purified crystals
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10. Mixture analysis - Solutions
EPA VOC mix 6: 2000 μg/mL each component in methanol
Analytical standard
Table 3.1: VOC mix #6 Components and Properties
Chemical
Formula
Mass
(amu)
Henry's Law Constant
(atm m3/mol)
Net Dipole Moment
S/N 100mg/L
Chloromethane
ClCH3
50.49
8.82E-03 b
1.9 e
26,000:1
Chloroethane
ClCH3CH3
64.51
1.11E-02 b
2.1 e
6000:1
Bromomethane
BrCH3
94.94
6.24E-03 c
1.8 f
Vinyl Chloride
ClCHCH3
62.50
2.70E-02 c
1.5 e
5000:1
Dichlorodifluoromethane
Cl2F2C
120.91
2.25E-01 c
0.5 e
25:1
Trichlorofluoromethane
Cl3FC
137.37
9.70E-02 d
35:1
Methanol a
CH3OH
32.04
4.55E-06 c
1.7 e
7000:1
- Describe the purpose of the standard.
Highlight the components (CFCs….range of expected partition into headspace and range of dipole moments)
There is an established EPA method for this
a) Diluent
b) J. M. Gosset, Environ. Sci. Technol., 1987, 21, 202
c) USEPA, Industrial Environmental Research Laboratory, Cincinnati, OH, USA, 1982.
d) R. D. Nelson Jr., D. R. Lide, A. A. Maryott, NSRDS-NBS10, 1967
e) C. G. Gray, K. E. Gubbins, Theory of molecular fluids. Volume 1: Fundamentals, Clarendon Press, Oxford 1984
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11. Mixture analysis - Solutions
Unbiased Search
Components well resolved
Spectral redundancy
Isotopic ratios
2 minutes to acquire
Direct sampling….removed air, allowed equilibration and sampled a 10 mTorr aliquot.
Spectral redundancy for simple library matching…no complex chemometrics.
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12. Linearity and LDL Table 3.4: Detection Limits in Water
Broadband (5min, μg/L )
Target Mix (10min, μg/L)
EPA 524.2
60 min (μg/L)
Chloromethane
5.0
0.1
Bromomethane 22
0.8
Chloroethane 26
0.5
Vinyl Chloride 42
0.2
Dichlorodifluoromethane
100,000
1000
Trifluorochloromethane
Without any heating, purge/trap preconcentration, or salts…get some pretty competitive results to the EPA method for volatiles in water.
Environmental applications for air, soils, and waters…but also solvent analysis in manufacturing.
Isotopic analysis in environmental samples. There is a particular challenge with chlorine (and a particular advantage because the ratio is high).
Regulation Limits for Residual Solvents: mg/L

13. Conclusions
Segmented CP-FT mmW provides high sensitivity broadband detection of volatiles.
In favorable cases, headspace detection has a sensitivity that rivals GC/MS.
Acknowledgments:
This work was support by:
NSF I-Corps
NSF Chemistry
Brent Harris received an NSF Graduate Fellowship.
University of Virginia
Don’t settle on a spectral region because of technology availability.
brightspec.com
9/19/2018