libs - umalaser.uma.es/wp-content/uploads/2017/05/paraweb23.pdf · now decades old, laser-induced...

42 OPTICS & PHOTONICS NEWS MAY 2017

C. David Chaffee

A Spanish-Italian team used an underwater LIBS setup from Applied Photonics (Skipton, U.K.) to analyze archaeological objects in the Mediterranean Sea at a depth of 30 meters.Getty Images; inset courtesy of J.J. Laserna, from S. Guirado et al., Spectrochim Acta B 74–75, 137 (2012)

LIBS Continues to Evolve

43 MAY 2017 OPTICS & PHOTONICS NEWS

Sed min cullor si deresequi rempos magnis eum explabo. Ut et hicimporecum sapedis di aut eum quiae nonem et adi.

Now decades old, laser-induced breakdown spectroscopy is still finding applications in novel areas, driven by technology, miniaturization and creative extension of the technique to new problems.


Yet in many ways, LIBS remains, in terms of its potential applications, a still-evolving technology, as optics and photonics researchers seek new ways to take advantage of its strengths, and to overcome some of its challenges. The miniaturization of LIBS equipment has opened opportunities for taking it into the fi eld. And researchers are looking at novel approaches that would extend the power of LIBS into detecting not just atomic compositions, but actual molecular species. Here, we take a look at LIBS and a few recent developments in its application.

How LIBS worksFundamentally, LIBS involves a pulsed laser, some sort of detector to gather the optical data, and a spectrom-eter and associated software to analyze those data. The process begins when a pulsed laser—commonly a neodymium:YAG laser—is focused on a target, typi-cally a patch less than 1 mm2 in area. That creates an irradiance on the order of 1 GW/cm2 or more, enough to ionize the small target into a plasma, with the elec-trons stripped from their nuclei. The optical signal in the plasma, which includes spectrographic information

Laser-induced breakdown spectroscopy (LIBS)—a form of atomic emission spectros-copy, in which the spectra are read from a tiny patch of ablated sample material, sus-pended in a laser-induced plasma—has

been around for a long time. The fi rst observation of a laser-induced plasma with potentially useful spec-tral information was reported in 1962, only two years after the laser’s initial demonstration. Nineteen years later, Tom Loree and Leon Radziemski introduced the modern form of the technique, and gave it its name, in a landmark 1981 paper in the journal Plasma Chemistry and Plasma Processing.

Since then, as the tools of the trade—pulsed lasers, detectors and spectrometers—have advanced, LIBS has evolved into its own scientifi c discipline. In the fi rst fi ve years of this decade, according to the Scopus scientifi c index, more than 4,000 papers were pub-lished or presented on the technology. And in terms of applications, LIBS has found its way across a vari-ety of disciplines—from geology to planetary science, from defense to food security, from environment to industry, from chemistry to biology.

1047-6938/17/05/42/8-$15.00 ©OSA

Basic design of a laser-induced breakdown spectroscopy system.Illustration by Phil Saunders

Neodymium:YAG laser

Delay generator/detector controller

Computer datadisplay

Focusinglens

Field sample

Plasma

Collection optics

Fiber opticinput

Spectrograph-CCD module

Focused laser pulse(irradiance > GW/cm2)

Field sample


As the tools of the trade—pulsed lasers, detectors and spectrometers—have advanced, LIBS has evolved into its own scientific discipline.

on the material ablated from the sample, is then col-lected, via a photomultiplier tube, a photodiode array, or a charge-coupled device (CCD), for spectroscopic analysis on a computer that ties specific spectral lines to atomic species in the plasma.

“Typically, the spectrometer will scan the vis-ible spectrum to analyze the spectral lines across the plasma,” says Alan Samuels, who has been working on LIBS-related research for more than a decade at the U.S. Army Edgewood Chemical Biological Center, Md., USA. “All of the spectral lines are emissions from the elements, so LIBS is fundamentally an elemental analysis tool and allows us to profile and characterize individual elements in the space where the plasma was created.” Frank De Lucia, a research chemist at the U.S. Army Research Lab in Aberdeen, Md., USA, adds that a spectrometer that can read the visible and near-infrared range “covers atomic emission lines from every element on the periodic table.”

Actually getting at those spectral lines—and thus at the sample composition—amid the signals from the “hot soup” of the plasma, however, requires an in-depth understanding of how the plasma pulse evolves in time. When the laser first ablates material from the sample, the ablated material, continuing to absorb optical energy from the laser pulse, initially expands at a supersonic rate (producing a loud, audible snap), in a luminous plasma plume at temperatures of 10,000 K or more.

During this period, the optical signal is dominated by white-light, continuum emission from the plasma. When the laser pulse ends, the expanding plasma begins to cool down, and more discrete spectral lines, with wavelengths tied to emissions from specific atomic species, start to predominate. “What you are witnessing in the LIBS emission,” says Samuels, “is the continuum emission [from the plasma], and atomic lines emitted by an entire set atoms caught in the plasma.”

Thus getting a usable signal requires that the spec-troscopic analysis be time-delayed by a period ranging from 1 ns (for femtosecond lasers) to 1 µs (for nano-second lasers), to avoid the continuum radiation and capture the discrete spectral lines as the plasma cools. Indeed, the development of time-gated electronics and,

later, time-gated CCDs to accomplish that delay was a crucial step in making LIBS practical.

Finding LIBS’ sweet spotDavid Cremers, a researcher who has worked on LIBS since 1981, notes that the microplasma created by LIBS in fact acts as a point detector for the chemical composi-tion, given the tiny surface area and the resulting small ablated mass (micrograms) in a single shot. Cremers, who did LIBS research at Los Alamos National Lab, N.M., USA, from 1981 to 2004 and has more recently focused on LIBS as a principal researcher and fellow at Applied Research Associates (ARA), Albuquerque, N.M., USA, also observes that, because the microplasma is formed by focused laser light, the power density incident on the sample can strongly affect the emission spectrum.

Cremers stresses, too, that LIBS’ analytical accuracy and precision are limited for some types of samples. “Typically, LIBS cannot match the analytical accuracy of lab-based methods,” he says, “that require careful sample preparation and that conduct analyses under highly controlled conditions.” As a result, while it can yield important data in many situations, there’s a cer-tain amount of subjectivity in interpreting those data. Thus, Cremers doesn’t see LIBS as replacing standard, lab-based analytical techniques characterized by high accuracy and precision.

It is when one leaves the lab and goes into the field, however, that LIBS starts to shine. LIBS requires little or no sample preparation, and can provide in situ, real-time or near-real-time analysis, even in hostile, remote or unusual environments. Also, since it affects only a tiny part of the sample, it’s less destructive than many other analytical tools. It’s thus often the method of choice for applications requiring rapid analysis (such as in-the-field geological prospecting), high-speed mate-rial identification (such as metal or plastics sorting for recycling), remote investigation of targets not readily accessible (such as nuclear reactors), or stand-off inter-rogation of waste sites, explosives and other hazardous situations at several meters’ distance or more.

Because of those advantages, LIBS has cropped up in some unexpected places. Perhaps its most


celebrated use has been in the ChemCam instru-ment on the NASA Curiosity Rover, where it has been used to analyze the soil and geology of Mars (see “Zapping Mars,” OPN, January 2013, p. 26). In a somewhat more earthbound example, scientists from Los Alamos were dispatched in 2002 to the Winter Olympics in Salt Lake City, Utah, USA, where they used LIBS to analyze bobsled runners for the presence of unauthorized coatings. The “unusual” spectra found on the runners of some sleds led to their elimination from the competition.

More recently, Cremers and ARA colleague Rosalie Multari, along with Joanne Dupre and John Gustafson of New Mexico State University, have worked to extend LIBS into the realm of food safety, demonstrating the ability to detect pathogenic strains of E. coli and Salmonella in a wide variety of foods and on food-processing sur-faces, as well as in water. “We have also demonstrated the ability to detect pesticides in rendered oils,” Multari adds. Because it offers rapid, near-real-time analysis, she suggests, LIBS could “be easily employed in food-manufacturing facilities or in water-processing facilities to detect specific pesticides or other poisons, using target-specific detection algorithms.”

LIBS has also found its way under water. In a 2011 proof of concept, a research team led by chemist J. Javier Laserna of the University of Malaga, Spain, used a cus-tom LIBS setup from the Skipton, U.K., firm Applied

Photonics to analyze archaeological materials to a depth of up to 30 meters in the Mediterranean Sea. The LIBS setup included a handheld laser probe, tied by a 45-m flexible fiber optic cable to a shipboard, suitcase-sized unit, which included the data collection, spectroscopy and analytical modules. An auxiliary air compressor allowed the team to apply an air flux before laser abla-tion, to create a sample-air interface and prevent sea water from gumming up the analysis.

Using the setup, the team was able to analyze archae-ological samples, including bone, bronze and pottery, from marine environments completely under water, establishing LIBS as a field tool for underwater analy-sis. More recently, Laserna’s team has demonstrated underwater standoff LIBS, at a distance of 0.8 m, actually propagating the laser pulse and analyzing the plasma spectrum through the water. The team believes that the work “opens a new horizon to the LIBS technique.”

Miniaturizing LIBSSome additional interest in LIBS has stemmed from the continual miniaturization of the technologies that constitute it. As solid-state pulsed laser systems and spectrometers (such as those pioneered by Ocean Optics) have become more compact, and as advances in micro-electronics have allowed improved system integration, LIBS has gotten a smaller footprint—which itself could open new application areas.

NASA/JPL-Caltech

Artist’s conception of the Curiosity rover’s ChemCam instrument in action. ChemCam uses

LIBS to determine the chemical composition of various materials on the martian surface.


Perhaps its most celebrated use has been in the ChemCam instrument on the NASA Curiosity Rover, where it has been used to analyze the soil and geology of Mars.

A group led by Cremers, for example, has devel-oped a wand instrument that has detected uranium and radiological materials, 20 different explosives, submerged metal objects and—when coupled to a high-resolution spectrometer—even uranium isotopes. Applied Photonics, the U.K. firm, meanwhile, offers a number of LIBS units that include a handheld laser/optical collector, tied fiber optically to a portable con-sole unit containing spectrometers, laser power supply and control electronics.

Other companies are also working on increasingly miniaturized LIBS product lines. One, SciAps, based in Woburn, Mass., USA, has developed several handheld, pistol-grip LIBS analyzers that package the full comple-ment of LIBS instruments in one handheld unit—and that, according to the company, can be used to identify elements from the entire periodic table. When the trig-ger is pulled, the instrument uses a 5-6-mJ solid-state diode laser that creates pulses on the order of 1-2 ns, and passes the pulses through a lens to focus the beam down to a zone around 50 microns in diameter. That focusing produces sufficient power density to create the plasma, spectra from which are then captured by a compact on-board spectrometer using a time-gated CCD and analyzed using on-board software. A 2016 field study in Applied Spectroscopy demonstrated the potential of the instrument to provide instant results in field-based geochemical investigations.

From atoms to moleculesAnother potentially exciting area now growing out of LIBS, according to Army researcher Alan Samuels, involves using long-wavelength infrared (LWIR) analysis, in the 8-to-12-micron range—which opens the door to revealing not only a material’s atomic com-position, but also its molecular composition. This is possible in LIBS because some compounds that have not been atomized, at the fringes of the plasma, con-tinue to exist in molecular form. As a result, LIBS can be combined with another technique, laser-induced thermal emission (LITE), to further mine the data in the plasma plume by capturing the IR region. Samuels calls LIBS LITE (also known as IR-LIBS) a “frontier research area.”

“We are using LIBS as a means to an end,” says Samuels. By looking at the periphery of the plasma event, and focusing on thermal emissions there, the technique can tease out molecular emissions from contaminants, he notes, without the need to unscramble these spectral fingerprints from “the cluttered background.”

“When we started our research,” Samuels contin-ues, “we began with the hypothesis that there would be molecules that are local to but not inside the plasma that could be excited into higher vibrational states.” Upon doing the first experiments, his team found that vibrational emissions from intact molecules were indeed evident—a conclusion supported by experiments that

Portable LIBS instruments from Applied Photonics (left) and SciAps (right). Applied Photonics Ltd.; SciAps


compared the emission features with the infrared spec-tra of the compounds.

“We have essentially charted new ground in this scientific discipline,” Samuels says. “We initially thought that we may be breaking up these molecules into fragments, and then they were recombining back into molecular species. However, now we are confident that we are looking at molecules that were not broken up, since it is unlikely that the parent molecule would necessarily re-form after being bro-ken apart this way. They were excited into higher vibrational states, presenting infrared signatures with the intact molecule.”

Samuels and others suggest that the ability to capture information on molecules, rather than just elements, could spur numerous applications. “There is absolutely an opportunity there,” says Army researcher Frank De Lucia. “While LIBS is a strong technique for understand-ing transition metals, it is not as strong with regard to understanding organics. LWIR might give you more information about organic materials.”

De Lucia notes that, given current concerns about terrorism, the approach would open the door to use LIBS LITE for finer-scale study of biohazards and poten-tial chemical-warfare agents. Samuels adds that the shift from atoms to molecules could find use in a wide range of environmental settings, such as monitoring

hazardous-waste “Superfund” sites and looking for compounds from petroleum fracking that have made their way into water supplies.

Clayton Yang of the Battelle Eastern Science and Technology Center in Aberdeen, Md., USA, who has worked with Samuels both while at Battelle and in earlier work at the U.S. National Institute of Standards and Technology, observes that his team was also able to use the longwave signature to identify “energetic materials like ammonium chloride and pharmaceu-ticals like aspirin and Tylenol.” The capability, says Yang, could make LIBS LITE a powerful analytical tool. For example, what he calls “conventional LIBS” can tell you that a plasma contains carbon, hydrogen, nitrogen and oxygen. LIBS LITE can tell you if the same four elements are, say, in the molecular configuration of the explosive RDX.

Laszlo Nemes, a molecular spectroscopist with the Hungarian Academy of Sciences, Budapest, has been working with Hampton University, Va., USA, and the Battelle Institute to use LIBS LITE for remote sensing of inorganic and organic substances for civilian and battlefield situations. In particular, Nemes has focused on the technique’s ability to detect carbon fullerenes, or “buckyballs.” He likens the advances coming out of longwave LIBS to the impact of infrared observation in astronomy.

“In both laboratory and celestial plasmas (such as the stars themselves), the observational objects are the same—whether conventional optical observations or infrared or radio or X-ray observations are made,” notes Nemes. “But the extension of the observational wavelength/energy scale relative to visible/UV observa-tions has revolutionized astronomy and astrophysics. The application of infrared detection to plasmas will surely follow a similar path,” he says, suggesting that it could lead to “an entirely new branch of LIBS.”

Challenges and opportunitiesOne challenge in getting that new LIBS LITE discipline off the ground lies in the mercury-cadmium-telluride (MCT) arrays that, at present, constitute the detector of choice for the technique. Those arrays, according to Samuels, are very expensive to make and must be cryogenically cooled. He sees those costs coming down, however, with further development of new detector tech-nologies based on quantum dots and other approaches, as well as in different configurations involving single-element MCT systems linked in linear arrays. And, on

In a 2016 study in Applied Optics, Army researcher Alan Samuels and colleagues applied infrared LIBS (“LIBS LITE”) to a variety of inorganic materials—including gunpowder, the spectrum of which is shown here. The emission spectra obtained at early delay times are dominated by continuum emission, but at later delay times, peaks associated with molecular species such as NO3 (at 7.35 µm) and SO4 (at 9.1 µm) become apparent.C. Yang et al., Appl. Opt. 32, 9166 (2016)


“Conventional LIBS” can tell you that a plasma contains C, H, N and O. LIBS LITE can tell you if the same four elements are, say, in the molecular configuration of the explosive RDX.

the cryo-cooling front, Samuels says there are “ways to use thermo piles or multi-stage thermo-electric coolers to get the performance that you need.”

In a larger sense, conventional LIBS itself needs to overcome a number of challenges in its quest to press into new applications. “The biggest challenge the LIBS industry is facing is the lack of ‘matched’ instruments,” according to Rosalie Multari, the former ARA researcher and current co-owner of Creative LIBS Solutions LLC. “Each LIBS instrument is unique, even if the design is kept the same, because of component variation.” For LIBS to go mainstream, Multari says, minimizing instrument-to-instrument variation will be crucial, so that “detection programming can be deployed on dupli-cate instruments with minimal adjustment.”

Dr. Sudhir Trivedi, director of research and develop-ment at Brimrose Technology Corp., who has worked on LIBS for more than a decade, characterizes it as “a young technology” with a long way to go until the full benefits can be reaped. “There are steps we need to take as a community to make it more rugged, more compact and more sensitive,” he says.

“We, as a community, have not gone through the process of optimizing the timing, the laser pulse, the gating of the emission signal, etc., to generate maxi-mum signals,” adds Samuels. “Nor have we done very much on the signal-processing front. We have not yet applied a sophisticated processing method. A lot of that is due to the clutter created by the plasma.” He notes that the fundamental lower limits of detection that can be achieved for an elemental identification in a LIBS experiment also remain unclear.

One place that has seen some progress is chemomet-rics, the statistical and modeling process by which the optical signals are identified and measured. Yet more can be done. Samuels points out that there is still room to improve the algorithms and better exploit signatures of interest, through better application of existing tools such as principal-components analysis and support vector machines.

“We like to look at various ways of analyzing cova-riance and analyzing and identifying the presence of signatures of interest,” says Samuels. “With more

sophisticated approaches, you should be able to tease out the right information.” LIBS researchers also con-tinue to seek better spectral libraries for comparing the results of their measurements and thereby presump-tively identifying constituents with more confidence, even in the face of a weak signal.

Yet, as this brief survey has suggested, LIBS has already come a long way since its beginnings decades ago—and the ongoing effort to solve these research puzzles could well open up still other applications and opportunities. That’s quite a prospect for a technology that has already made it to Mars. OPN

C. David Chaffee ([email protected]) has authored two books, more than 25 market research reports, and numer-ous magazine, newspaper and newsletter articles on photonics and optics.

References and Resourcesc T.R. Loree and L.J. Radziemski. “Laser-induced break-

down spectroscopy: Time-integrated applications,” Plasma Chem. Plasma Process. 1, 271 (1981).

c S. Guiardo et al. “Chemical analysis of archeological materials in submarine environments using laser-induced breakdown spectroscopy: On-site trials in the Mediterranean Sea,” Spectrochem. Acta B 74–75, 137 (2012).

c R. Noll. Laser-Induced Breakdown Spectroscopy: Funda-mentals and Applications (Springer-Verlag, 2012).

c M. Baudelet and B. Smith. “The first years of laser-in-duced breakdown spectroscopy,” J. Anal. At. Spectrom. 28, 624 (2013).

c D.A. Cremers and L.J. Radziemski. Handbook of Laser-Induced Breakdown Spectroscopy, 2nd ed. (Wiley, 2013).

c R.A. Multari et al. “Detection of biological contaminants on foods and food surfaces using laser-induced break-down spectroscopy (LIBS),” J. Agric. Food Chem., 61, 8687 (2013).

c F.J. Fortes et al. “A study of underwater stand-off laser-induced breakdown spectroscopy for chemical analysis of objects in the deep ocean,” J. Anal. At. Spectrom. 30, 1050 (2015).

c B. Connors et al. “Application of handheld laser-induced breakdown spectroscopy (LIBS) to geochemical analysis,” Appl. Spectrosc. 70, 810 (2016).

c C.S.-C. Yang et al. “Time resolved long-wave infrared laser-induced breakdown spectroscopy of inorganic energetic materials by a rapid mercury–cadmium–tel-luride linear array detection system,” Appl. Opt. 55, 9166 (2016).

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