Membrane Interface Probe (MIP) Diagram
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 MIP coupled with an electrical conductivity sensor will provide continuous stratigraphic information of the soil as well as semi-quantitative concentrations of volatile organic compounds (VOCs). The MIP can be used in both saturated and unsaturated materials to detect VOCs in the gaseous, sorbed, dissolved or free phases. The “Membrane” acts as an interface between the VOCs in the subsurface and gas phase detectors at the surface. The membrane is semi-permeable and is comprised of a thin film polymer impregnated into a stainless steel screen for support. The membrane is approximately 6.35mm in diameter and may be easily replaced if damaged. The membrane is placed in a heated block attached to the probe. This block is heated to approximately 120 degrees C and is raised at the leading edge to help protect the membrane from damage when being pushed through the geologic matrix. Heating the block helps accelerate diffusion of the VOCs through the membrane. Diffusion occurs due to a concentration gradient between the impacted 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 diffused VOCs to the gas phase detectors at the surface. Travel time from the membrane interface to the detector(s) is approximately 60-75 seconds (depending on the length of trunkline, flow rate, and ambient air temperature). S2C2 will be using a MP 6520 or equivalent MIP probe, FI 6000 Field Instrument, MP 6500 MIP controller, coupled with a HP GC. S2C2 will use a combination of the three following detectors: PID/FID/XSD. THE XSD detector is a halogen (chlorinated) specific detector, which is similar to an ECD but has a larger working range. Detection limits for typical MIP configurations are generally between 1ppm and 150ppb or less, depending upon contaminant concentrations and soil matrix.