RAPID MEASUREMENT OF GASEOUS VOC MIXTURES WITH A NEW MASS SPECTROMETRY TECHNIQUE Robert Walters Moll

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RAPID MEASUREMENT OF GASEOUS VOC MIXTURES WITH A
NEW MASS SPECTROMETRY TECHNIQUERobert
WaltersMolly GribbJerome ImonigieBoise State
University Geoenvironmental Research
Labcoen.boisestate.edu/gerlPresented at the
2004 INRA Environmental Subsurface Science
Symposium
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Rapid VOC Measurement

• The Big Picture
• Our Approach
• The Payoff
• Future Directions

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Rapid VOC Measurement

• Our team is developing a subsurface probe that
• detects and identifies gaseous volatile organic
  compounds (VOCs) in the unsaturated zone,
• is inexpensive and minimally-invasive,
• provides real-time and long-term sampling,
• is deployed underground in networked arrays,
• performs wireless reporting and allows remote
  access.

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Rapid VOC Measurement

• The high-resolution ion mobility spectrometer
  (HRIMS) probe

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Rapid VOC Measurement
The HRIMS probe is first validated in the lab in
a large soil column using comparison measurements
with the GC/MS.
Once fully validated, the HRIMS probe will be
deployed in the field using a direct-push truck.
Approximately 65 cm in length, the HRIMS probe
can be deployed to a depth of up to 100 feet for
over six months
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Rapid VOC Measurement

• The Approach
• Cyclic Valving
• Continuous sampling from soil column effluent
• Maintain packet integrity from soil column to
  detector
• Limit peak spreading in GC column
• GC/MS in Selective Ion Monitoring (SIM) mode
• Isothermal chromatographic column conditions
• Resolve co-eluting peaks
• Compare mixtures with single components

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Rapid VOC Measurement
An external timer is programmed to cycle the
sampling valve at a specific interval. The
sample passes through the GC column and into the
MS for quantitation.
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The flow controller and a small soil column
during a test.
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Rapid VOC Measurement
Traditional GC/MS method
The new SIM-CV method
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Rapid VOC Measurement
INTENSITY
RETENTION TIME min
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Rapid VOC Measurement

• CYCLIC VALVING
• Continuous sampling using external timer
• Packet integrity maintained
• Timer interval and measured interval are equal
• No peak spreading
• No skewness or flattening of peaks was observed
  when comparing traditional to SIM-CV

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Rapid VOC Measurement

• GC/MS in SIM MODE
• Thermal effects
• Pronounced skewing at temps below 300 ºC
• GC column temp gt 300 ºC
• Co-eluting peaks
• Look at secondary and tertiary ions
• Mixtures vs. single components
• Examine toluene in mixture and as single specie

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Rapid VOC Measurement

• BTEX mixtures produce six ions of interest
• m/z 77.05 Benzene 78.05, 77.05
• 78.05 Toluene 91.10, 92.05
• 91.10 E.benzene 91.10, 106.1
• 92.05 m-Xylene 91.10, 106.1, 105.1
• 105.1 o-Xylene 91.10, 106.1, 105.1
• 106.1 p-Xylene 91.10, 106.1, 105.1

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Rapid VOC Measurement
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Rapid VOC Measurement

• Why Use SIM-CV?
• Utilizes existing equipment and configuration
• A high measurement rate allowing breakthrough
  tests at higher interstitial velocities
• A lower detection limit than GC/FID for low
  concentration/residual studies
• High selectivity, allowing simultaneous, multiple
  compound testing

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Rapid VOC Measurement

• SIM-CV is suitable for gaseous transport studies.
  However, the SIM-CV method can be further
  refined by
• Extend the deconvolution method to capture all
  components
• Characterization of a wider range of volatiles
  (aromatics, alcohols, etc.)
• Investigation of multiple, competitive
  interactions (sorption/desorption and reactions)

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Partners

• Funded by the United States EPA under Contract
  no. X97031101-0
• Shimadzu Scientific Instruments