Combined LIBS and Remote Raman Spectroscopy Instrumentation
A PIDDP Development Project
There are two instrumental techniques
that interrogate surface samples using high-intensity laser beams and use
spectrographs to disperse the signal over overlapping spectral ranges.
Both techniques require approximately the same spectral resolution.
These two techniques yield very complementary datasets: Raman spectroscopy
yields information about minerals and their polymorphs and also identifies
organic molecules from vibrational spectra, while LIBS yields detailed
information on elemental compositions, including many minor and trace elements.
Combining mineralogical and elemental
composition techniques into a single instrument will allow much more positive
and complete identification of samples than possible with a single technique.
For example, LIBS can identify samples of high Ca and C as likely to be
calcite, but Raman spectroscopy can determine whether the sample is calcite,
or actually aragonite. On the other hand, Raman spectroscopy mostly
distinguishes mineral structure. Cations are not determined directly—only
by relatively subtle shifts in wavenumber, potentially leaving the result
underconstrained. So while Raman spectroscopy can reasonably distinguish
cation composition in a 2-component solid solution, it cannot determine
compositions in complex solid solutions. In fact, the assumption
that a solid solution consists of only two components can lead to wrong
compositions. This can be important, for example, in the rhombohedral
carbonate family, where many solid solutions are possible (among calcite,
magnesite, siderite, ankerite, rhodochrosite, or smithsonite), or if an
olivine happens to contain calcium. LIBS gives unambiguous major
element compositions as well as yielding important minor and trace element
abundances. Raman spectroscopy also adds the capability to identify
key biomarkers.
The objective of this work is to build
a laboratory breadboard instrument combining remote Raman spectroscopy
with LIBS, without significantly degrading the science return from either
technique. Remote Raman spectroscopy has been demonstrated through
PIDDP development work by Paul Lucey and Shiv Sharma at the University
of Hawaii, while the LIBS has been developed at LANL. Starting in
2003, we are working together to develop a combined instrument.
LIBS labeled dolomite spectrum with inset of Raman dolomite
spectrum