Theme: 10. Characterization, Fate and Transport

Topic Area: 10b. Advanced Sampling and Analysis Tools and Techniques

Formatted Title:

Factors Influencing In Situ Detection of Analytes with the Membrane Interface Probe

Background/Objectives:

The membrane interface probe (MIP) is a direct push advanced logging tool for the detection of volatile organic compounds (VOCs) in soils and unconsolidated formations. The steel MIP probe is about 60 cm in length and 5 cm in diameter and is equipped with a semipermeable membrane on one side of the probe. During pauses in advancement VOCs cross the membrane and are transported by nitrogen carrier gas through a trunkline to gas phase detectors at the surface. Detectors used in the standard MIP system include a photoionization detector (PID), flame ionization detector (FID) and halogen specific detector (XSD). Experience has shown that the physical and chemical properties of the compounds of interest influence its detection limit and whether an analyte can be detected with the MIP system at all. These properties include vapor pressure, boiling point, water solubility, molecular weight, etc. Research and bench tests were performed to assess how these properties influence analyte detection with the MIP probe and system.

Approach/Activities:

Various scatter plots of the chemical/physical properties of several categories of volatile analytes were prepared to assess the influence of these properties on detection of the analytes. These plots include vapor pressure versus boiling point, boiling point versus molecular weight and Henry’s Law Constant versus boiling point, etc. Bench tests also were performed with a standard MIP system on compounds from several of the analyte categories to assess the sensitivity and detection level of different compounds with the three detectors used with the MIP system. These bench tests consisted of a series of MIP Response tests of the various analytes at concentrations near and above typical MIP detection levels used for field investigations with the MIP logging system.

Results/Lessons Learned:

Practical experience along with a scatter plot of boiling point (BP) versus Henry’s Law Constants (HLC) for analytes of interest provided valuable insights. This plot indicated that as the BP of analytes approach and then exceeded 200°C, they become increasingly more difficult and eventually impossible to detect with the MIP system. Additionally, it was observed that as polar analytes HLC exceed 0.1 mol/M³Pa detection becomes more difficult and detection limits with the MIP system typically increase with increasing HLC values above this level. Analytes in this category include ethers, ketones and alcohols. However, it has been observed that removal of the Nafion dryer from the MIP sample line can improve detection of several of these compounds, but detection limits still are typically elevated for these polar compounds. Results of MIP response tests with several categories of analytes (chloro-ethenes/ethanes, aromatics, alkanes, ketones, ethers, etc.) generally support the observations based on the scatter plots. The response tests together with the scatter plots also demonstrated that as an analyte’s BP and MW increase its trip time from the MIP membrane to the up-hole detectors generally increases, potentially leading to carry-over. It was also observed that the presence of carbon tetrachloride in the MIP sample gas can suppress the response of the PID to analytes such as benzene and toluene by a significant percentage.

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Wesley McCall