The Membrane Interface Probe (MIP) is a direct-push screening tool with semi-quantitative analytical capabilities that is used to detect and log the relative concentration of volatile organic compounds (VOCs) and some semi-VOCs with depth in soil. The MIP can also detect and log electrical conductivity that can be used to infer soil texture (i.e., clay content typically exhibits a higher conductivity). The MIP was developed by Geoprobe Systems® and has been used extensively in the U.S., Europe and Canada for mapping the extent of VOC contamination in the soil and underground. As a screening tool, the MIP offers many benefits to site investigators:
- Useful for detecting and logging both chlorinated (PCE, TCE, Carbon Tet, etc.) and non-chlorinated VOC contaminants (BTEX, PHCs, etc.).
- Able to detect contaminants in both coarse and fine-grained soils.
- Works in both saturated and unsaturated soils of varying types (ie. clay/silt to sand/gravel).
- Real time contaminant screening information is generated, allowing field adjustment of the site investigation program as required.
- Cost-effective in terms of the amount and level of detail of in-situ site characterization data as compared to traditional Phase II Environmental Site Assessment techniques.
VOC diffusion across the membrane is driven by both the pressure and concentration gradients between the contaminated soil and the clean carrier gas behind the membrane. A constant gas flow sweeps behind the membrane and carries the VOC contaminants to the gas phase detectors housed in an onboard gas chromatograph (GC) at the surface.
The standard detectors within the GC include a photoionization detector (PID), a flame ionization detector (FID), and a halogen-specific detector (XSD). These detectors are employed in series with each detector providing sensitivity to a particular group or type of VOC contaminant. The XSD is highly specific to halogenated compounds and is the best detector when the MIP is used for logging chlorinated solvent plumes or source areas (e.g. TCE, PCE, Carbon Tet, etc.). The PID provides sensitivity to aromatic compounds (e.g. BTEX compounds) as well as confirmation of chlorinated ethylene compounds detected by the XSD. The FID is a general detector useful for petroleum hydrocarbon detection as well as confirmation of high concentrations of all compounds recorded by the other two detectors.