Laser-induced breakdown spectroscopy (LIBS) is an extremely versatile spectroscopic technique which can be used to analyze the composition of almost any material. It is a favoured technique in situations where rapid analysis is required, as most samples do not need any preparation, and the full spectrum of elements present in a sample is reported in a single experiment.
How Does LIBS work?
The operating principle of LIBS is quite simple. An intense, highly focused laser pulse is fired at a sample, creating a small plume of plasma consisting of electronically excited atoms and ions. As these atoms decay back into their ground states, they emit characteristic wavelengths of light.
The emitted light is picked up by lenses and/or a fibre optic system on the instrument, and focused onto a detector such as a CCD. The spectrum of the emitted light is then analyzed for the presence of atomic emission lines to indicate which elements are present.
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Figure 1. LIBS analyzes material by firing a laser at the sample to create a plasma, then capturing the spectral profile of the constituent atoms. Image Credit: NASA.
A number of different types of laser can be used for LIBS, as the most important parameter for a successful reading is the spot size of the laser on the material - this typically has to be on the order of micrometers. The most common laser used are excimers and Nd:YAG lasers. The laser pulse used to create the plasma is very short - typically 5 - 20 nanoseconds long. Some LIBS equipment includes filters to remove the incident laser wavelength from the received light. This is rarely necessary, however, as there is most often a significant gap between the laser blast and the spectral lines becoming visible.
Immediately after the laser pulse, the light received by the spectrometer is white, as the plasma bubble is small and very hot. The conditions in the plasma break down the vast majority of chemical bonds, leaving the constituent elements as free atoms, which are often ionized. As the plasma expands and cools, the ions and atoms relax from their excited states, emitting the useful spectral lines - these can typically be detected around 1 µs after the initial laser pulse. To make sure that the reading is as clear as possible, the detector is usually shut off until this point.
What is LIBS used for?
LIBS is versatile in application, as the samples rarely need any preparation at all. It is popular for in situ analysis on manufacturing lines, and field work where less robust analytical equipment is impractical. Some common applications include:
- Environmental monitoring (e.g. soil contamination)
- Rapid material characterization in R&D
- Analysis of teeth and bones in forensic studies
- Analysis of vessels, tubing and radioactive waste in the nuclear industry
- Characterization of minerals and ores in mining exploration
LIBS on the Curiosity Mars Rover
A Laser-Induced Breakdown Spectrometer plays a crucial role as part of the "ChemCam" instrumentation on the Mars Rover "Curiosity". The rover's official title is the "Mars Science Laboratory" - its primary mission is to analyze the soils and rocks of Mars for signs of organisms which may have inhabited the planet millions of years ago.
Due to the unique capabilities of LIBS to analyze most materials without sample preparation, and even, with the appropriate optical equipment, at a distance, it will be used to determine areas in the vicinity of the rover which are of sufficient interest to analyze further with the other onboard equipment.
The LIBS system will also be able to gather readings from places the other equipment cannot reach, for example rocky outcrops and overhangs. The micrometre-scale focus of the laser will also allow individual strata within rocks to be analyzed and compared.
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Figure 2. An artist's impression of Curiosity analyzing a Martian rock using LIBS. Image Credit: NASA.
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