Monitoring Polar Compounds Using Membrane Extraction and High-Speed Gas Chromatography Authors: Jonathan Maurer, Dr. Anthony J. Borgerding* Department of Chemistry, University of St. Thomas, St. Paul, MN 55105

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Monitoring Polar Compounds Using Membrane
Extraction and High-Speed Gas ChromatographyAutho
rs Jonathan Maurer, Dr. Anthony J.
BorgerdingDepartment of Chemistry, University
of St. Thomas, St. Paul, MN 55105
Results Discussion
Introduction

• Gas Chromatography System
• Hewlett Packard 5890A Chromatograph with an
  FID
• Used a 6-port, 2-position diaphragm valve
  secured to the ceiling of the oven (Figure 1)
• Actuated by a 3-port solenoid valve
• Solenoid driven by a pulse generated by
  computer program
• Positive flow system connected to sample port
  of valve so controllable flow could continuously
  flush sample into valve to be injected (Figure 2)

• Overview
• Volatile organic compounds (VOCs) are found
  throughout our environment and can be toxic to
  humans.
• The dangers of VOCs make their monitoring
  important, and many methods have been devised to
  do so.
• Much of this research has focused mostly on
  analyzing nonpolar compounds, largely ignoring
  polar analytes.
• Solid phase microextraction (SPME) is one
  technique that has been explored in the analysis
  of polar compounds.
• However, there is great room for improvement
  in this area of monitoring.
• Goal
• Devise a method that can be used to monitor
  polar analytes in living systems
• Main compounds to monitor methanol, ethanol,
  acetaldehyde
• Acetaldehyde, a toxic metabolite of ethanol,
  has yet to be monitored
• Present Study
• Therefore, a process to develop previous and
  new techniques into a usable method was
  investigated.
• Using high-speed gas chromatography (HSGC),
  various techniques were attempted to monitor
  polar compounds.
• HSGC enables rapid separation of compounds,
  which means it is possible to monitor a system.
• Semi-permeable membranes
• Nafion tubing
• Custom-made SPME fibers coated with liquid
  Nafion

• Diaphragm Valve Installation
• Diaphragm valve more physically efficient (See
  Figure 1)
• Shortest possible injection time 6 ms
• 75 ms was shortest attained in previous
  testing with 2-position rotor valve
• Faster separation, more reproducible, more
  effective monitoring
• Silicon Membrane Systems
• System not used to monitor polar compounds,
  but as a model for later systems
• Data gave representation of how analytes
  permeate through a membrane (Figure 3)

Figure 3 10 ppm sample of toluene (dissolved in
methanol) in water. First peak methanol.
Second peak toluene
Load Position
Inject Position
Figure 1 Diagram of physical operation of
diaphragm valve Images taken from Valco catalog

• Sample Preparation
• Gas samples made in 1-liter sample bags with
  septum
• Aqueous samples prepared in 20 mL vials and
  250 mL flasks
• Typical concentrations between 1 and 20 parts
  per million (1,000-20,000 ng/mL)
• Silicon Membrane Systems
• Systems constructed so that a flow could be
  obtained through membrane and into sample port of
  diaphragm valve (Figure 2)
• Analytes permeated through membrane and were
  carried by helium flow into GC to be monitored
• Nafion Tubing Systems
• Systems constructed much like silicon membrane
  systems
• Nafion tubing integrated into flow system
  using compression fittings and epoxy
• Nafion-coated SPME Fibers
• Initially, fused-silica rods from commercial
  SPME fibers were used
• These rods were extremely fragile, so custom
  fibers were made from syringe cleaning wire (.25
  mm diameter)
• Fibers then dipped in liquid Nafion until
  coating was visible

• Nafion Tubing Systems
• Only high concentrations of methanol in water
  (gt 1 part per thousand or 1,000,000 ng/mL) could
  be detected with system placed in headspace
• System submerged in solution was more
  sensitive (Figure 4), but detection limits still
  too high for effective monitoring (gt 10 part per
  million or 10,000 ng/mL)
• Permeation times much greater than for silicon
  membranes
• Adsorption 3-5 minutes vs. 1-2 minutes for
  silicon membranes
• Desorption gt 1 hour vs. 10 minutes for
  silicon membranes

• Nafion-coated SPME Fibers
• Commercial SPME fibers are typically used for
  analysis of nonpolar compounds
• Nafion, a polar substance, was coated on a
  SPME fiber and used to detect polar analytes
• Nafion fibers performed 10 times better than
  commercial fibers in the detection of methanol
  (Figure 5)
• To be useful in natural systems the fiber must
  be able to monitor in a liquid environment
• Nafion coating swelled and was stripped off
  metal fiber when placed in solution, so no
  results could be obtained -- more work needed

Figure 2 Diagrams of GC and silicon membrane
systems
Figure 4 1 methanol in water solutions used in
both trials. Top Nafion tubing placed in
headspace of flask. Bottom Nafion
tubing submerged in solution.
Implications and Future Work

• Future work will continue the development of
  techniques to monitor polar compounds,
  specifically methanol, ethanol, and
  acetaldehyde.
• Nafion-coated SPME fibers are promising, but
  more work is needed to enable monitoring in
  solution
• Rougher, more porous fiber surface so that
  coating does not scrape off so easily
• More uniform coatings, perhaps glued onto
  fiber, again to keep coating intact in solution
• Another custom SPME fiber, made of anodized
  aluminum, showed some capability for monitoring
  polar compounds
• Studies done by other scientists left room for
  exploration into the monitoring ability of the
  fiber

Figure 5 20 ppm methanol in water solutions
used in both trials. Left Poly(dimethylsiloxane
) SPME fiber. Right Nafion SPME fiber