Using lasers to detect explosives and hazardous waste

In a lab, a technician is inserting a fragment of a toy intoa sample case, inserting it into a machine, and pressing a button. He insertsone fragment after another—each test takes only a few seconds. The paint onsome of the toy fragments are testing positive for lead.

Another technician, this time at an abandoned industrialfacility, is collecting samples of concrete, metal and building materials tobring back to an analysis lab. He's part of a team looking for berylliumcontamination. Beryllium is a light metal—number four on the periodictable—that is extremely poisonous to living things. He'll collect hundreds ofsamples in just a few days. Analyzing each one will require less than a minute.

None of these scenes is science fiction anymore. And eachone is an example of green chemistry—chemical analysis that is quick andresults in no chemical waste generation. The military application can save thelives of soldiers in combat. The other applications can keep our children safefrom toys and materials illegally coated with lead paints, and workers fromsuffering the effects of chemical poisoning.

The key is a technology developed by scientists at theDepartment of Energy's Lawrence Berkeley National Laboratory using laserablation—aiming a laser beam at the material sample to be tested, causing atiny amount of it to be vaporized so that spectroscopic (optical and mass)analysis techniques and unique computer software can analyze the sample inseconds.

Rick Russo, a scientist in Berkeley Lab's EnvironmentalEnergy Technologies Division began studying the physics of laser ablation 28years ago. "I was interested in the fundamental physics of what happenswhen you fire a laser at a solid sample, and ablate the material into a vapor.There's so much physics involved that it's phenomenal," he says.

More than 200 publications, nine patents, and an R&D 100award later, Russo and his research group are the pioneers of a new chemicalanalysis technology based on laser ablation. The two most common approaches tolaser ablation chemical analysis are LIBS, (laser induced breakdownspectroscopy) in which the light from a tiny plasma is directly measured andrelated to the chemical element and its concentration, and LA-ICP-MS (laserablation inductively coupled plasma mass spectrometry), in which tiny particlesfrom the ablation are measured. On average, LIBS provides part-per-millionsensitivity whereas LA-ICP-MS provides parts-per-billion sensitivity. Bothapproaches allow the measurement of the entire chemical composition of a sampletarget using a single laser pulse. Although he never entered his field tocreate a practical application, Russo realized that LA-ICP-MS and LIBS havemany possible uses in the world.

In 2004, he created a company, with the help of SmallBusiness Innovation Research grants, called Applied Spectra to bring thetechnology, and its many potential life-saving applications, to themarketplace. It sold its first LIBS device in 2008. The company's corporateoffices are in Fremont, California,a community 30 miles south of Berkeley, in theeastern part of Silicon Valley. It also has asmall manufacturing facility in Aberdeen, Maryland. The jobs created by thecompany are "green" jobs, since the technology is a waste reducingone.

"Most of the samples in the world that we want toanalyze are solid," says Russo. "The conventional method of analysisrequires an entire analysis infrastructure that is based on dissolving thesesolids in strong acid so that the resulting liquid can be analyzed usingstandard methods. This is very dirty, generates a lot of chemical waste, andrequires a lot of labor and time."

In contrast, laser ablation requires no acid dissolution andso generates no hazardous wastes. "Laser ablation is greentechnology," says Russo. "Besides eliminating chemical waste, itsaves energy. My goal is to change the paradigm for the way that chemicalanalysis is done."

Laser ablation is very fast, allowing technicians to conductmore tests on samples in real time. And, laser ablation testing is cheaper—onlya technician is needed to operate the tool. The technician's job is to placethe sample and operate the machine; each test takes less than 30 seconds. NoPhDs are needed.

At the start of the laser pulse until one nanosecond later,violent evaporation takes place at the material’s surface. From one nanosecondto one microsecond (1,000 ns) after the end of the pulse, the high-temperatureplume expands outward. From one microsecond to 1,000 microseconds (1millisecond) after the end of the pulse, the heat leaks away through radiativeheating and the plume cools.

Remote military explosives detection

Russo's basic research has been funded for many years by theU.S. Department of Energy's Office of Basic Energy Science. But his applicationresearch for laser ablation has been funded by many other offices, such as theNuclear Non-Proliferation and Security Agency of DOE, which was interested inlaser ablation's potential for quick identification of nuclear wastes atnuclear manufacturing sites, and the Department of Defense, which was lookingfor a way to remotely and quickly identify explosive residues that mightprovide tell-tale hints of car bombs and other terrorist weapons.

Russo and his co-workers tested a military prototype fieldversion of the LIBS system at the Yuma Proving grounds in 2008. The detectorwas able to discriminate with 85 percent accuracy whether more than 100 samplescontained residues of several types of explosives from between 30 to 50 meters(90 to 150 feet) away, or whether the composition was of materials such asrock, wood, metal, plastics or, in one case, food materials—salami and cheese.

Applied Spectra has developed several commercial versions ofthis tool, and is planning to release a hand-held, and transportable versionlater in 2009.

Detecting lead and other heavy metals

With so many news stories recently about imported toys andother products painted or otherwise contaminated by lead, testing labs andconsumer safety authorities need faster, accurate ways of determining whichproducts to be concerned about.

Many European and Asian nations have adopted regulationsrequiring the removal of hazardous materials from new and to-be-recycledelectronic components called Restrictions on Hazardous Substances (RoHS) andWaste Electrical and Electronic Equipment (WEEE). More stringent U.S.regulations are also being looked at. Lead, cadmium, mercury, hexavalentchromium, and organic materials PBB and PBDE are among the dangerous materialsthat these regulations are aiming to remove from the products we use every day.Research has demonstrated that LIBS testing is more accurate than today's X-rayfluorescence technology for quick screening of these materials.

Laser ablation-based testing can also be used in a varietyof other "green" and "green policing" applications. Forexample, one manufacturer of electronic components is using it to check on thesoldering of electronic components to ensure that they are free of lead, whichmany countries now require. It can also analyze the purity of silicon used insolar photovoltaic panels for quality control.

Assisting with detection of wastes at contaminated sites

Those performing the clean-up of old mining sites will findLIBS technology useful in identifying the contaminants at the sites,information that can guide waste management experts determining the bestmethods to use to clean up and isolate what's present.

Russo has been discussing the possible use of LA-ICP-MS andLIBS methods to assist in hazardous waste reduction programs at several of theDepartment of Energy's National Laboratories, including the Idaho NationalLaboratory, and the Lawrence Livermore National Laboratory. In Livermore, program managers are conductingabout 4,000 to 5,000 tests for beryllium contamination per year, although morethan 90 percent of those tests turn up negative. The testing takes time, andgenerates its own hazardous waste. LA testing would require less than 30seconds per test, far less than what the current test requires, and wouldgenerate no waste.

Some of the research also has been oriented toward using LAto detect residues of nuclear weapons development for use in nuclearnon-proliferation programs.

What happens during laser ablation

Thanks to years of study, Russo's group now has timeresolved images, analyses of ablated material, and a mathematical model of thelaser ablation process. At the start of the laser pulse until one nanosecondlater, violent evaporation takes place at the material's surface (see figure 1).From one nanosecond to one microsecond (1,000 ns) after the end of the pulse,the high-temperature plume expands outward. From one microsecond to 1,000microseconds (1 millisecond) after the end of the pulse, the heat leaks awaythrough radiative heating and the plume cools. At the end of this interval,about one millisecond after the end of the pulse, the vapor plume drops to theboiling temperature of the sample, and condenses back to its solid form—albeitas a tiny dusting of particles (nanoparticles). The formation of nanoparticlesin these laser ablation plumes was the basis of Smalley's discovery ofbuckeyballs from graphite.

In LIBS, the plasma emission from the ablated sample isgathered using special optics. A spectrometer analyzes the white light emittedfrom the plasma, separating the light into its colors (wavelengths). Everysample has a unique spectral signature thanks to its chemical components. AnICCD (intensified charge coupled device) camera records the signature,converting the light into pixels of information.

A variation on LIBS, called LA-ICP MS (laser ablationinductively coupled plasma mass spectrometry), the ablated material itself, inthe form of fine particles, can be gathered by a stream of a carrier gas suchas argon, and heated to a high temperature in order to be converted to a plasma(atoms stripped of their electrons). The chemical composition of the plasma isthen analyzed using a mass spectrometer.

LIBS testing is extremely sensitive. It can detect materialsat concentrations of the tens of parts per million. LA-ICP MS is even moresensitive, capable of detecting chemicals at the parts-per-billion level.

Russo says that "We understand the fundamental physicsof it, and we better understand which parameters to use to make the mosteffective measurements with laser ablation." His group's research servesas an example of fundamental research to better understand a natural phenomenonthat led to an unexpected, but useful application in the industrial andcommercial world.