The technology is based on the fact that organic contaminants in the ground will partially evaporate, and the vapors produced will tend to move upward through the ground toward the atmosphere. Sensors placed in the ground near the contaminated areas can potentially come in contact with the vapors. The sensors are designed to hold on to organic vapor molecules that come in contact with them. The more vapors that are present, the more molecules that will be attached to the sensors.
Sensors may consist of absorbent materials, which take the contaminant molecules and hold them inside, or adsorbent surfaces, to which the contaminant molecules can attach. These sensors are called "passive" because they essentially do nothing except sit and wait for the vapors to come to them. A related technology is the "active soil gas" techniques, which pull soil contaminant vapors out of the ground by a vacuum or pump system for analysis by instrumentation at the surface. The PETREX Technique, commercially developed by Northeast Research Institute LLC, is a technologically advanced passive soil gas method originally targeted to petroleum exploration but now being applied to organic contaminant investigations.
Each PETREX soil gas sampler consists of two or three activated carbon adsorption elements (heavy gauge metal wires called "collectors") housed in a re-sealable glass container (in the shape of a test tube) in an inert atmosphere.
A shallow hole is dug in the ground, and one of the samplers is placed with it open end down into the hole. The samplers are then covered, their location marked, and then are allowed to sit undisturbed in the ground for the desired length of time, usually one to two weeks. Soil gases diffuse through the opening of the uncapped sampler and adsorb onto the activated carbon. (Click here for movie.) After an appropriate period of time, the sampler is retrieved from the borehole, resealed, and submitted for analysis.
Analysis is performed on the individual collectors using thermal desorption (TD) and a quadrupole mass spectrometer (MS). Data are reported as ion counts from the mass spectral peak which indicates the presence of a particular compound. Second collectors from selected samplers are also analyzed by TD-GC (gas chromatography)/MS to confirm and identify more completely the compounds presented in the TD-MS analysis.
The method is highly sensitive for the detection of very small concentrations of organics. For this reason, extreme care is required in the handling and placement of the samplers so that organic molecules in the open atmosphere or on a person's skin do not attach to any significant degree to the collectors.
