Membrane Interface Probe
The Membrane Interface Probe (MIP) is a rapid in-field soil and groundwater analysis system that can detect volatile organic compounds (VOCs) and petroleum products to depths of up to a hundred feet or more. It is deployed on a cone penetrometer system that pushes a probe into the ground. Portable analytical equipment allows analysis in nearly real-time.
The MIP can be driven or pushed to the depth of interest and operated to extract and collect a VOC vapor sample. This sample is then transported to the surface for analysis via a carrier gas within a transfer line. The MIP can then be advanced to the next depth of interest to repeat the process.
The MIP consists of a membrane that is permeable to gas but impermeable to liquid. It is affixed to a port that contains a resistive heater coil. As the device is pushed to the required depth, it is heated, volatilizing and capturing gases. Increasing the heater temperature increases the rate of diffusion through the membrane. The gas is then carried to the surface by a carrier gas. The sample is then injected directly into a measuring device (e.g., flame ionization detector, photo ionization detector, ion trap mass spectrometer, or electron capture detector), or captured on an adsorbent, and desorbed and injected into a GC/MS.
Limitations and Concerns
The technology is not intended to be a replacement for traditional soil bores and monitoring wells. Rather, it is designed to optimize the placement of a reduced number of bores and monitoring wells to achieve site characterization and long-term monitoring during remedial actions.
The MIP samples VOCs in direct contact with its heated membrane surface (approximately 37 square millimeters). The sample size or area influenced by the heated membrane has not been studied well, but it is affected by the temperature of the membrane, the type of subsurface media (vadose zone soil or saturated soil), and the contact time between membrane and soil. Because of these constraints, the data are considered to be estimates.
Some sites may not be accessible because of the size of the cone penetrometer vehicle.
The cone penetrometer may be difficult to advance in certain subsurface areas (e.g., cemented sands and clays, buried debris, gravel units, cobbles, boulders, and shallow bedrock).
It is essential that all underground utilities and structures be located using reliable geophysical equipment before the MIP is advanced below surface.
Speciation with the ion trap mass spectrometer (ITMS) can be problematic when the gas stream contains several different chemicals of concern.
Determining the depth at which the sample was taken when the sampler is in a near-continuous operating mode is more difficult the deeper the instrument is driven.
There has been an attempt to use this tool to verify or rule out the presence of old sources at a contaminated site. Because of uncertainty about the radius of influence in soil, diffusion rates through the permeable membrane, and a host of other issues that are unrelated to this technology, it is difficult to imagine that this method could rule out the existence of old sources.
This characterization technique can be used to detect VOCs and petroleum products in the subsurface. Data obtained can be used to optimize the placement of conventional soil bores and monitoring wells.
Technology Development Status
The MIP was developed for logging VOCs in the subsurface. It has been commercially available since 1998.
Other Resources and Demonstrations
See related Technology Descriptions of the Cone Penetrometer, and Site Characterization and Analysis Penetrometer System (SCAPs).
See http://www.clu-in.org/download/contaminantfocus/dnapl/Detection_and_Site_Characterization/MIP_Savannah_Rivers2003050.pdf for a description of characterization using MIP at Savannah River.