The Membrane Interface Probe (MIP) is a screening tool with semi-qu**tative capabilities acting as an interface between the contaminates in the subsurface and gas phase detectors at the surface (Figure 1). The membrane is semi-permeable and is comprised of a thin film polymer impregnated into a stainless steel screen for support. The membrane is approximaby 6.35mm in diameter and may be easily replaced. The membrane is placed in a heated block attached to the probe. This block is heated to approximaby 100-120 degrees C and is raised at the leading edge to protect the membrane. Heating the block helps accelerate diffusion of the contaminate through the membrane.

Diffusion occurs because of a concentration gradient between the contaminated soil and the clean carrier gas behind the membrane. A constant gas flow of 35-45 mL/min sweeps behind the membrane and carries the contaminants to the gas phase detectors at the surface. Travel time from the membrane interface to the detector(s) is approximaby 30-45sec (depending on the length of trunkline and flow rate).

The MIP Principle of Operation.The downhole, permeable
membrane serves as an interface to a detector at the surface.
Volatiles in the subsurface diffuse across the membrane and
partition into a stream of carrier gas where they can be swept
to the detector. The membrane is heated so that travel by VOCs
across this thin film is almost instantaneous. MIP acquisition
software logs detector signal with depth.

The ability to detect a contaminant is determined by the type of detectors being used. Any laboratory grade gas phase detector with an analog output of 1-5V may be used. Most commonly used detectors include photoionization detector (PID), electron capture detector (ECD) and the flame ionization detector (FID). Each detector is designed for sensitivity to a group or type of contaminant. The ECD is used for chlorinated (TCE, PCE) contaminant detection, PID is best used for the detection of aromatic hydrocarbons (BTEX compounds), and the FID is best used for straight chained hydrocarbons (methane, butane). These detectors may be used in series with the least destructive detector being first and the most destructive detector coming last. The MIP system can process four detector signals at one time. These detector signals, in conjunction with the time in which a contaminant takes to return to the surface, are graphed versus depth.

The detector inbation and the electrical conductivity of the soil are graphed by the FC4000 field instrument. This allows the operator to determine the b of the contaminant, the relative concentration of the contaminant and the soil in which the contaminant is located. The MIP log can be used to determine the depth at which a monitoring well should be placed, at what depth samples need to be collected, or the interval for injection of remediation materials.

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