28 Nov 2003
After more than 30 years of research, laser-induced breakdown spectroscopy may have finally found the applications to propel it from laboratory curiosity to field deployment. Oliver Graydon reports from the second European seminar on the technology in Crete.
From Opto & Laser Europe December 2003
The technique involves firing a high-energy laser pulse at a target, which results in a microplasma forming at the surface of the sample. The light emitted from the microplasma is then analysed and its spectral fingerprint reveals a wealth of information about the elemental composition of the target. The attractions of the approach are that it is potentially very sensitive, and the target needs no preparation.
Technology transformation Although LIBS has been around since the 1960s, for many years it was considered an esoteric technique that was unreliable and difficult to perform, especially outside of the lab. However, the advent of compact high-performance optical equipment - particularly Q-switched Nd:YAG lasers that emit millijoule pulses, and high-resolution broadband spectrometers - has changed the situation for the better.
"LIBS is an analytical detection technique and sensor technology that is undergoing a dramatic transformation in terms of hardware, software, and application areas," wrote three LIBS experts in their introduction to the October issue of Applied Optics, which was devoted to the topic. "Research worldwide in the area of LIBS has grown exponentially - these are indeed very exciting time for LIBS researchers."
Delegates at the recent EMSLIBS-II European symposium in Crete would probably agree. They heard how LIBS is now tackling an increasing number of "real-life" scenarios including mineral sorting, paper production, blast-furnace monitoring and civil engineering.
But perhaps the application that has attracted the most interest is homeland security, in which LIBS is considered a promising portable technology for detecting explosives, landmines, biological weapons and chemical contamination.
"As a technique we know it's fast, it works in real time and it is sensitive. A single laser shot can be used for material identification and only very small amounts of a target material are needed," Russell Harmon from the US Army Research Laboratory (ARL) told delegates. "We think that unlike most analytical instrumentation, LIBS will make its greatest impact in field applications."
"We are going to put a complete LIBS system on the back of a man," Roy Walters, director of R&D at Ocean Optics, explained. "And it needs to be a one-shot solution - we don't have the luxury of being able to hit the target repeated times."
The backpack-based system is designed to help soldiers distinguish land mines from harmless metal objects. A handheld probe fires laser pulses at the target object - such as the casing of a mine - and the light from the resulting plasma is captured by the probe. It is then fed through fibre-optic cables to a spectrometer in the user's backpack, where the signature of the light emission is checked against a database of known explosives and casing materials. Once the object has been identified, the result of the analysis is sent to the user's head-mounted display.
A prototype of the system has already been constructed and is based around two crucial components: a miniature Q-switched Nd:YAG laser from ALST, a Florida-based maker of laser rangefinders, that emits 35-50mJ pulses and is small enough to fit into the probe; and an Ocean Optics LIBS 3000 spectrometer that spans 200-980nm with a resolution of 0.1nm.
As well as detecting landmines, ARL is also interested in using LIBS to detect chemical and biological weapons. Both ARL and a group of researchers in France have recently independently demonstrated that LIBS can detect the presence of bacterial spores and distinguish them from harmless substances such as pollen and mould. The samples included simulated strains of anthrax and other dangerous bacilli.
"We're very optimistic about LIBS as a field-portable sensor. We feel that it's a sensor technology whose analytical powers in field applications are still largely untapped - there are a lot of really good ideas out there," commented Harmon. "Our ultimate objective is to have small robots on the field to do chemical and biological agent detection, or even landmine detection."
Future challenges As for the future of the technology, Harmon is quite clear on what still needs to be done: "We now need to establish reliable detection limits and identify approved experimental methods for quantitative measurements," he told delegates. "This is a challenge for the community at large if we are to move into the field-portable arena."
Another homeland security application for LIBS could be helping to fight crime and terrorism. Christopher Dockery and Scott Goode from the University of Carolina, US, have recently shown that LIBS analysis of skin samples from a person's finger can help to determine whether they have recently fired a gun. The technique searches for gunshot residue on the person's trigger finger. It still needs to be optimized to minimize false-positive and false-negative results.
Aside from the military sector, it's hard to predict which commercial applications will take off first. At least three separate firms now sell LIBS systems: Thermo-Electron has its ARL LaserSpark, Pharmalaser has the PharmaLIBS system and APTI (now a part of BAE Systems) has the Tracer 2100.
Pharmalaser is trying to break into the pharmaceuticals market with its LIBS set-up, whereas prior to the BAE takeover APTI had sold systems to the oil industry for checking the composition of drilled samples. Thermo-Electron says that it has several instruments in the field at various European blast furnaces, where they are used to analyse the quality of the metal. It also claims to have taken an order from Siemens and "a famous Swiss watch maker" for materials characterization applications.
However, all of these systems have one thing in common - they are large, expensive instruments that are designed to make high-precision quantitative measurements. To date only a few systems have been sold, due partly to their high price-tag (at least $100,000, or €84,000) and partly to problems convincing potential customers that the technique is as accurate and reproducible as competing analytical methods such as mass spectrometry.
Many of the delegates who attended EMSLIBS-II seem to believe that the key to the commercial success of LIBS is to move towards cheaper field-based solutions that perform qualitative, rather than quantitative, measurements.
"I don't think that LIBS is going anywhere until the price goes right down," concluded Walters. "It needs to falls below the $50,000 mark."
The potential of LIBS: expanding applications Here's a snapshot of just some of the recent applications for LIBS that were presented at EMSLIBS-II:
Process control at blast furnaces The exhaust gases from several blast furnaces in Germany, Finland and Spain have been monitored with a LIBS system built by the Fraunhofer Institute for Laser Technology in Aachen, Germany. The on-site tests lasted up to 10h and measured levels of Na, K, Zn, Pb, C, N, O, H, Ca and Fe at gas temperatures and pressures of up to 180°C and 4 bar.
Checking the quality of building repairs Scientists from BAM, a German research institute for building materials, say that LIBS is a useful tool for checking the quality of concrete repairs. Analysis of spectral lines at 920 and 840nm reveals the presence of sulphur or chloride contamination that may lead to future corrosion. The team says that it can make measurements up to 25mm deep with a resolution of 2mm.
Optimizing paper coatings A team from the University of Jyväskylä in Finland has developed a LIBS system for analysing the properties of paper coatings. The system is now being used on a regular basis by local paper mills to check the distribution of a paper's coating weight, fillers and binders.
Hair tissue mineral analysis The Instituto per i Processi Chimio Fisici (IPCF) in Pisa, Italy, has applied LIBS to hair tissue mineral analysis. It checked hair from 11 people for evidence of heavy metal poisoning. Although the presence of metals can be detected, it says that quantitative analysis is difficult because the dominant spectral lines in hair are also found in air.
Investigating the ancient Egyptians Scientists at the National Institute of Laser Enhanced Science (NILES) at Cairo University have used LIBS to investigate the lifestyles of the ancient Egyptians. Enamel from 150 teeth, taken from living individuals as well as Egyptian mummies in the old (3400 BC) and middle kingdoms (2140 BC), was analysed. The study showed higher levels of lead and aluminium in the teeth of the mummies, which was probably caused by ancient techniques for making pottery and jewellery, and purifying water.
Environmental monitoring Javier Laserna's research group at the University of Malaga has developed an open-path LIBS system that has a range of up to 80m. The portable system is small enough to fit into a small van. Trials at a blast furnace in Spain showed that it was possible to monitor metal contamination in the leaves of the trees surrounding the factory.
Archaeology and culture heritage conservation A portable LIBS system built by the FORTH Institute in Crete has analysed archaeological remains found during excavations on the island. The system was used to determine the composition of samples of ancient pottery, jewellery and other metal objects. A fibre-coupled version has also been used to study the composition of pigments in old paintings.
Mineral sorting Field tests in Florida have recently shown that LIBS can distinguish between types of phosphate rock that cannot easily be identified by traditional optical techniques such as luminescence or fluorescence sorting. The developer of the system, Israeli firm International Laser Technologies, is now patenting and commercializing the technology.