LAMIS uses the energy of a high-powered laser beam to ablate a tiny spot on a sample, creating a plasma plume for spectroscopic analysis that reveals chemical elements and their isotopes. (Image courtesy of Applied Spectra, Inc.) |
At some point this year, after NASA’s rover Curiosity has
landed on Mars, a laser will fire a beam of infrared light at a rock or
soil sample. This will “ablate” or vaporize a microgram-sized piece of
the target, generating a plume of ionized gas or plasma, which will be
analyzed by spectrometers to identify the target’s constituent elements.
Future Mars rovers, however, will be able to do even more. Researchers
with the U.S. Department of Energy’s Lawrence Berkeley National
Laboratory (Berkeley Lab), in collaboration with Applied Spectra, Inc.,
have developed an advanced version of this laser technology that can
also analyze a target’s constituent isotopes. This expanded capability
will enable future rovers for the first time to precisely date the
geological age of Martian samples.
Rick
Russo, a scientist with Berkeley Lab’s Environmental Energy
Technologies Division and a pioneer in laser ablation spectroscopy, led
the development of LAMIS—for Laser Ablation Molecular Isotopic
Spectrometry. As with the earlier Laser Induced Breakdown Spectroscopy
(LIBS) technology being used on rover Curiosity,
the basic premise is to use the energy of a high-powered laser beam
focused to a tiny spot on the surface of a sample to create a plasma
plume for analysis. Each species of atoms or ions within the plasma will
emit light with signature spectral emission peaks. However, whereas
LIBS only measures the optical emission spectra of atoms and ions, LAMIS
measures the emission spectra of molecules and molecular ions. This
enables LAMIS to identify the specific isotopes of a chemical element
within the plasma plume.
“Relative
to atomic emission, molecular spectra can exhibit significantly larger
isotopic shifts due to the contributions of the vibrational and
rotational motion in the molecule,” Russo says. “The trick is to be
patient and wait for the hot atoms and ions in the plasma to collide and
merge with the ambient environment to form an oxide, or a nitride or
fluoride, and then collect the molecular light emissions.”
Russo
and his research group have been using LAMIS to study isotopes of
strontium, an alkaline earth metal commonly found in geological and
natural materials. Although strontium’s major isotopes are stable
(strontium-90 being a notable exception), the percentage of strontium-87
will naturally increase over time as a result of the decay of
radioactive rubidium. Comparing the ratio of strontium-87 to
strontium-86 is a standard tool for age dating in geochronology,
oceanography and archeology. The ratio of these strontium isotopes is
also used to date the origin of historic or forensic samples. Currently,
the standard means of measuring strontium isotopic ratios is by mass
spectrometry technologies that involve time-consuming, labor-intensive
laboratory sample dissolution work with an extensive array of
instrumentation. This sample dissolution work generates substantial
chemical waste. LAMIS offers a green chemistry alternative that is
faster, less expensive and can be carried out from across vast
distances.
From left, Alexander Bol’shakov, Xianglei Mao and Rick Russo are part of the research team that developed LAMIS, a green chemistry laser spectroscopy technology that can be operated across vast distances. (Photo by Roy Kaltschmidt, Berkeley Lab) |
“LAMIS
is not yet as sensitive or precise as mass spectrometry but unlike mass
spectrometry it does not require chemical dissolution sample
preparation, vacuum chambers and a laboratory infrastructure,” Russo
says. “All we need is a laser beam and an optical spectrometer and we
can perform real-time isotopic analyses of samples at ambient pressures
and temperatures.”
LAMIS
represents what may be the only practical means of determining the
geochronology of samples on Mars or other celestial bodies in the Solar
System. Current age estimates of such bodies suffer from uncertainties
in the billions of years. That said, LAMIS also has many important
applications here on Earth. Strontium isotope ratios have been a focus
in the field of medicine for both treatment and diagnostic purposes.
Measuring these ratios can also provide valuable information about
atmospheric chemistry. They also can be used to trace the origins and
movements of early humans. But perhaps the most immediate and important
application of LAMIS will be in nuclear forensics aimed at
non-proliferation and terrorism.
“Uranium
and plutonium, like every chemical element, has a spectral signature
that’s as unique as every human’s DNA or fingerprint,” Russo says. “With
LAMIS, we can factor in isotopic ratios, giving us an additional level
of identification that could be critical.”
For
example, “yellow cake,” the powdered concentrate made from uranium ore
that is a main ingredient of nuclear fuel, can also be used to fabricate
a nuclear weapon. Measuring the elemental composition of yellow cake is
one way of identifying the geographic locale where the yellow cake was
produced, but because uranium ore is ubiquitous to our planet’s surface,
being able to also measure isotopic ratios in a sample of yellow cake
can be a huge advantage for pinpointing where it originally came from.
“The
natural ratio of uranium-235 to uranium-238 is defined by the geology
of our planet,” Russo says. “If you find a modified ratio in a sample
then you know someone has been enriching that uranium. Other isotopic
ratios within a nuclear reaction chain also can tell you how a nuclear
weapon was made and where it might have originated.”
Much
of this research was done in collaboration with Applied Spectra, a
company Russo created in 2004 with the help of Small Business Innovation
Research grants, to bring laser ablation spectroscopy technology to the
marketplace.
Artist’s concept of Rover Curiosity with LIBS technology firing a beam of infrared light at Martian rock for spectroscopic analysis. (Image courtesy of NASA) |
“The
next step is to improve the sensitivity and precision of LAMIS,” Russo
says. “Our immediate target is parts-per-million, which should be
relatively easy for us to reach, but ultimately we want to get to
parts-per-billion sensitivity, which will be a challenge. However, 50
years ago, the parts-per-billion sensitivity of today’s mass
spectrometry technologies would have been thought impossible.”
Russo
and his colleagues have described their work on LAMIS in several
papers, including one in which the technique was shown to be effective
for measuring isotopes of boron. The most recent paper appeared in the
journal Spectrochimica Acta Part B.
The paper is titled “Laser Ablation Molecular Isotopic Spectrometry:
Strontium and its isotopes.” Co-authoring this paper were Xianglei Mao,
Alexander Bol’shakov, Inhee Choi, Christopher McKay, Dale Perry and
Osman Sorkhabi. Co-author Bol’shakov with Applied Spectra, says his
company is eager to commercialize this technology.
“We envision multiple applications for LAMIS in industry, medical diagnostics, nuclear safeguarding and other areas,” he says.
Support
for this research came from the Defense Threat Reduction Administration
of the U. S. Department of Defense, DOE’s National Nuclear Security
Administration, and NASA through Applied Spectra, Inc.
More information about the research of Rick Russo and his group