Instrumentation and sensor development
Laser Raman Spectrometer
Project Manager/Lead Engineer: George Malby
Lead Scientist: Peter Brewer
The proposed year 2002 Laser Raman Spectrometer (LRS) program is a continuing project to design,
configure and field a Laser Raman Spectrometer for use by the MBARI science
community in furtherance of their research efforts in the deep Ocean. Initially,
the LRS will support the research initiatives of the Ocean Chemistry of the
Greenhouse Gases Project. It is however a general purpose measurement tool for
advanced ROV deployment allowing for the in situ molecular speciation of many
types and forms of chemical and biological samples.
The year 2001 effort resulted in the design, test, fabrication and
integration of a state-of-the-art LRS commercial instrument. The instrument was demonstrated to work well in its field configuration
in the laboratory. Laboratory testing of the LRS has shown that the high pressure tolerant fiber
optic interconnecting cables exhibit 5 dB more optical loss than their
conventional counterparts, resulting in an in-water system level configuration
reduction in detection sensitivity of 10 dB. These losses appear to be due to
the pressure-tolerant in-line connectors. Additional work to obtain lower loss
pressure tolerant fiber optic cables, connectors and penetrators is required.
The spot size of the probing laser beam is on the order of a fraction of a
square millimeter (when using a 2.75 inch focal length set of relay optics) and
J. Freeman at Washington University (WU) has determined that the depth of field for maximum sensitivity
is on the order of one millimeter. Thus in order for the LRS to be truly usable
in deep water research applications, a "through the lens"
visualization capability and a precision XYZ positioning capability are
required. As the LRS methods
utilization and development tasks in science proceed, it is anticipated that new
optical probe head requirements (different focal lengths and spot sizes) and
different sampling techniques (a sampling configuration for determining the
molecular composition of pore waters in sediments for example) will need to be
designed, fabricated and integrated with the basic LRS.
Obtaining spectral data quality control information is an important part of
the evolution of the LRS instrument capabilities if the resulting science data
is to be ported/shared between MBARI and other scientific institutions. In situ
instrument temperature and humidity measurements will be appended to the
measured spectral data obtained from the LRS. Our WU collaborators have
suggested that a diamond (insensitive to temperature and pressure) local
standard (Zheng et al., 2001) be incorporated in the LRS optical head sensing
path so that every spectrogram could have an internal standard for comparison.
We will evaluate the feasibility and desirability of incorporating this function
in the LRS and implement it if appropriate. Suppliers of fluorescence free
diamond chips have been identified, and lab tests are already proceeding at WU.