Raman spectroscopy is the measurement of the wavelength and intensity of scattered light from molecules. When electromagnetic radiation passes through matter, most of the radiation continues in its original direction. However, a small fraction is scattered in other directions. C.V. Raman, an Indian physicist, discovered this effect in 1928. Using Raman spectroscopy, the Raman probe detects many organic and inorganic chemicals in the media surrounding the probe. The probe uses laser light beamed through a sapphire window. When the light hits the sample, it causes molecules to vibrate in a distinctive way, creating a "fingerprint." The fingerprint is captured and transmitted via fiber optic cables to an analyzer, where it is compared to known signals.
The Raman probe is often used to characterize the liquids found in underground high-level radioactive waste tanks. It is inserted with a cone penetrometer (CPT). All of the hardware is radiation hardened, designed for and tested in the high-radiation, highly caustic chemical environment of the Department of Energy's (DOE's) waste storage tanks. When deployed in tanks, the system is useful for rapidly assessing the concentrations of various organic compounds and oxidizers found in tank wastes. The chemical compositions of waste need to be known to assess chemical compatibility hazards. Organic chemicals and oxidizers are of concern because of flammability. Nitrate and nitrite levels affect corrosion. Phosphate levels need to be known before wastes can be allowed to mix in transfer lines because sodium phosphate crystals may form a viscous gel that reduces the ability to pump and move wastes.
Open-path Raman spectroscopy is capable of detecting a wide range of chemicals in the vapor, liquid, or solid phase. The detection limits depend upon the length of the path (i.e., the distance between light source and the sapphire detector) and , the excitation wavelength used, and the individual chemical. Typical detection limits range from low ppm to percent levels.
Limitations and Concerns
Data interpretation can be complex.
The data produced by a probe to identify TCE does not always agree with sampling results, possibly due to variations between the two locations or length of time allowed for spectra collection.. The technology is normally used to detect gross (large amounts of) contamination.
To address in-tank flammable gas concerns, it is necessary to be able to detach the Raman probe from the rest of the equipment. This requires quick-disconnect connectors between the probe and the CPT, resulting in loss of probe sensitivity and reliability. In some demonstrations, misaligned and poor connections have caused the probe's signal strength to drop considerably. The quick-disconnect connections in fiber optics have the potential for misalignment and foreign material in the connections.
When used as an open path system for measuring pollutants in the atmosphere or ambient air, detection limits can be quite high.
ApplicabilityThis system has been designed for two main applications: to collect characterization data directly inside the harsh radioactive and chemical environment of DOE waste storage tanks and to collect data on the extent of soil contamination. It can detect a host of chemicals including volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs), polynuclear aromatic hydrocarbons, total petroleum hydrocarbons, inorganic compounds, and some metals. In addition, open-path systems can be used to detect contaminants in air, although detection limits are usually high.
Technology Development Status
This technology is commercialized.
Other Resources and Demonstrations
This instrument has been tested with tank waste in hot cells at Hanford to provide data to resolve tank safety concerns. The Raman probe can also be used to characterize the vadose zone surrounding tanks to estimate leaks and determine remedial action and site closure plans.
In February and June, 1998, DOE tested this instrument on soil at the Savannah River Site that is contaminated with TCE and tetrachloroethylene (PCE). Use of the Raman probe for measuring soil contamination provided proof of concept for the in-tank deployment system.