Raman
Probe
Description
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.
Applicability
Technology
Development Status
This
technology is commercialized.
Web Links
http://costperformance.org/monitoring/pdf/itsr1544.pdf
http://www.frtr.gov/site/6_2_18.html
http://www.clu-in.org/programs/21m2/openpath/raman/
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.