Despite being a relatively new technique, Laser-Induced Breakdown Spectroscopy (LIBS) has had a great impact and has found several applications in the elemental analysis of a wide range of samples over the last twenty years.
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The most common techniques for the elemental analysis of different materials include atomic absorption spectroscopy (AAS), inductively coupled atomic emission spectroscopy (ICP-AES), and X-ray fluorescence spectroscopy (XRF). Such methods often require complex sample preparation, solvent consumption, waste production, and an extended analysis time.
Conversely, LIBS is an emission spectroscopic method that has started to play a vital role since it offers several advantages over the other more conventional techniques, with no or little sample preparation and the possibility to conduct rapid analysis both in the laboratory and in the field.
How Does LIBS Work?
As the name might hint, LIBS uses a laser, commonly Nd: YAG (Neodymium doped Yttrium Aluminum Garnet), to generate plasma following the excitation and ionization of the atoms in the sample.
The laser with high energy and short pulse duration (c.a. 5 ns) is focused on the sample surface, causing the ablation of a small amount of material (100 ng-1 mg) that then undergoes fusion vaporization and plasma formation.
The laser initiates and sustains the plasma formation, which has a temperature of around 10,000K. The plasma promotes the ground-level electrons in the atoms of the sample to higher excited energy levels.
When the atoms return to their lower state level, they emit energy at specific wavelengths, characteristic of the underlying electronic transitions. The emission lines are transmitted to a spectrometer via an optical fiber, ultimately generating a spectrum that is finally interpreted.
Advantages and Disadvantages of LIBS
There are many pros that may conclude that LIBS is a more convenient technique than others. Nevertheless, some disadvantages must be taken into account. To summarize, some of the critical strengths of LIBS are listed below:
- LIBS is very versatile and can be applied to both solid, liquid, and gas samples
- It allows for direct sampling without pre-treatment. Very often, LIBS analysis can be done with no sample preparation. For homogeneous samples, mechanical separation, or some polishing, are the only actions needed. If the sample composition is too variable from spot to spot, analysis can be obtained by a raster scanning pattern
- LIBS is non-destructive and only requires a minimal amount of material
- Measurements are high-speed and are performed within a fraction of a second
- Analyses can be done even without having direct access to the sample
- Multiple elements are analyzed simultaneously
On the other hand, the limitations include:
- Limited sensitivity. The limit of detection (LOD) falls in the range of part per million (ppm), as opposed to other techniques where the LOD is of the order of part per billion (ppb)
- Self-absorption. Quantitative analysis is negatively affected when the colder atoms surrounding the plasma absorb the emissions from hotter regions, reducing the spectral intensity
- Matrix effect. The properties of the sample (both physical and chemical) can affect the ablation, composition of plasma, plasma temperature, and signal
Some advanced solutions, such as new instrumental configurations and extra excitation sources, have been proposed to overcome the limitations mentioned above. For instance, the sensitivity improves with double-pulse LIBS, where a second laser pulse is triggered after the first pulse by re-exciting the region where the plasma is generated.
Nanoparticle enhanced LIBS (NELIBS) is another strategy for enhancing sensitivity. The presence of gold or silver nanoparticles deposited on the sample can induce a local increase in the intensity of the incident laser beam by several orders of magnitude. This approach has great potential in applications such as analyzing metallic alloys, glass, and metals in liquid solution.
Recent Applications of LIBS
LIBS can find applications in many different fields, particularly in the earth sciences, with the analysis of minerals and soils. The concentrations of soil nutrients (Mg, Ca, P, etc.) are extremely relevant in agriculture and the determination of environmental contaminants (e.g., Pb, Cd, As).
The determination of gold and silver in mineral ore samples in the field is of great relevance to the mining industry. Although there are challenges associated with the analysis not being representative of the entire sample, multiple LIBS measurements can overcome this limitation and provide a more accurate determination.
LIBS has also been applied in space. NASA's Curiosity rover – designed to explore the climate and geology of Mars – is equipped with a LIBS system to investigate the elemental composition of rocks and soil, particularly looking at their H emission signal at 656.6 nm.
Although still little explored, some studies show the potential of LIBS in food analysis. LIBS can, in fact, be a fast and straightforward technique for honey authenticity certification. Honey can be adulterated with sugar cane or corn syrups, but verification of honey authenticity is a challenging task due to its complex composition. It is usually verified by costly, complex, and time-consuming analysis.
Adulterants should influence the LIBS spectral profile of honey. The analysis of a sample set made of 6 pure honeys, two sweetener syrups, and 228 adulterated honeys (236 samples in total) effectively-identified and quantified adulterants in honey. The method required only 3 min per sample and a small volume (1 mL) of honey and can potentially be applied for analysis of other food of high density, such as condensed milk.
There is a lot of ongoing research trying to overcome some of LIBS's limitations. Nevertheless, the low cost of the instrumentation and the possibility of both laboratory and field analysis – thanks to portable instruments – that can be performed by non-experts contribute to making LIBS a very attractive technique.
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- Fabre, C. (2020). Advances in Laser-Induced Breakdown Spectroscopy analysis for geology: A critical review. Spectrochimica Acta Part B: Atomic Spectroscopy, 166, 105799.10.1016/j.sab.2020.105799
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