Laser-induced breakdown spectroscopy

A typical LIBS system consists of a Nd:YAG solid-state laser and a spectrometer with a wide spectral range and a high sensitivity, fast response rate, time gated detector. This is coupled to a computer which can rapidly process and interpret the acquired data. As such LIBS is one of the most experimentally simple spectroscopic analytical techniques, making it one of the cheapest to purchase and to operate.

The Nd:YAG laser generates energy in the near infrared region of the electromagnetic spectrum, with a wavelength of 1064 nm. The pulse duration is in the region of 10 ns generating a power density which can exceed 1 GW·cm⁻² at the focal point. Other lasers have been used for LIBS, mainly the Excimer (Excited dimer) type that generates energy in the visible and ultraviolet regions.

The spectrometer consists of either a monochromator (scanning) or a polychromator (non-scanning) and a photomultiplier or CCD detector respectively. The most common monochromator is the Czerny–Turner type whilst the most common polychromator is the Echelle type. However, even the Czerny-Turner type can be (and is often) used to disperse the radiation onto a CCD effectively making it a polychromator. The polychromator spectrometer is the type most commonly used in LIBS as it allows simultaneous acquisition of the entire wavelength range of interest.

The spectrometer collects electromagnetic radiation over the widest wavelength range possible, maximising the number of emission lines detected for each particular element. Spectrometer response is typically from 1100 nm (near infrared) to 170 nm (deep ultraviolet), the approximate response range of a CCD detector. All elements have emission lines within this wavelength range. The energy resolution of the spectrometer can also affect the quality of the LIBS measurement, since high resolution systems can separate spectral emission lines in close juxtaposition, reducing interference and increasing selectivity. This feature is particularly important in specimens which have a complex matrix, containing a large number of different elements. Accompanying the spectrometer and detector is a delay generator which accurately gates the detector's response time, allowing temporal resolution of the spectrum.

Because such a small amount of material is consumed during the LIBS process the technique is considered essentially non-destructive or minimally-destructive, and with an average power density of less than one watt radiated onto the specimen there is almost no specimen heating surrounding the ablation site. Due to the nature of this technique sample preparation is typically minimised to homogenisation or is often unnecessary where heterogeneity is to be investigated or where a specimen is known to be sufficiently homogeneous, this reduces the possibility of contamination during chemical preparation steps. One of the major advantages of the LIBS technique is its ability to depth profile a specimen by repeatedly discharging the laser in the same position, effectively going deeper into the specimen with each shot. This can also be applied to the removal of surface contamination, where the laser is discharged a number of times prior to the analysing shot. LIBS is also a very rapid technique giving results within seconds, making it particularly useful for high volume analyses or on-line industrial monitoring.

LIBS is an entirely optical technique, therefore it requires only optical access to the specimen. This is of major significance as fiber optics can be employed for remote analyses. And being an optical technique it is non-invasive, non-contact and can even be used as a stand-off analytical technique when coupled to appropriate telescopic apparatus. These attributes have significance for use in areas from hazardous environments to space exploration. Additionally LIBS systems can easily be coupled to an optical microscope for micro-sampling adding a new dimension of analytical flexibility.

With specialised optics or a mechanically positioned specimen stage the laser can be scanned over the surface of the specimen allowing spatially resolved chemical analysis and the creation of 'elemental maps'. This is very significant as chemical imaging is becoming more important in all branches of science and technology.

Portable LIBS systems are more sensitive, faster and can detect a wider range of elements (particularly the light elements) than competing techniques such as portable x-ray fluorescence. And LIBS does not use ionizing radiation to excite the sample, which is both penetrating and potentially carcinogenic.

LIBS, like all other analytical techniques is not without limitations. It is subject to variation in the laser spark and resultant plasma which often limits reproducibility. The accuracy of LIBS measurements is typically better than 10% and precision is often better than 5%. The detection limits for LIBS vary from one element to the next depending on the specimen type and the experimental apparatus used. Even so, detection limits of 1 to 30 ppm by mass are not uncommon, but can range from >100 ppm to <1 ppm.

From 2000–2010, the U.S. Army Research Laboratory (ARL) researched potential extensions to LIBS technology, which focused on hazardous material detection.[4][5] Applications investigated at ARL included the standoff detection of explosive residues and other hazardous materials, plastic landmine discrimination, and material characterization of various metal alloys and polymers. Results presented by ARL suggest that LIBS may be able to discriminate between energetic and non-energetic materials.[6]

In the 2010s, interest developed in LIBS that focused on the miniaturization of the components and the development of compact, low-power, portable systems. Interest from groups such as NASA and ESA - as well as the military - has furthered these developments. The Mars Science Laboratory mission brought ChemCam, a LIBS instrument, to the surface of Mars in 2012.

Recent developments in LIBS have seen the introduction of double-pulsed laser systems.[8][9] For double-pulse LIBS one distinguishes between orthogonal and perpendicular configuration. In perpendicular configuration the laser fires twice on the same spot on the specimen with a pulse separation in the order of one to a couple of tens of microseconds. Depending on pulse separation, the second pulse is more or less absorbed by the plasma plume caused by the previous pulse, resulting in a reheating of the laser plasma leading to signal enhancement. In orthogonal configuration a laser pulse is fired parallel to the sample surface either before or after the perpendicular pulse hits the specimen. The laser plasma ignited in the surrounding medium above the surface by a first pulse causes (by its shock wave) an area of reduced pressure above the specimen into which the actual plasma from the sample can expand. This has similar positive effects on sensitivity like LIBS performed at reduced pressures. If the orthogonal laser pulse is delayed with respect to the perpendicular one, the effects are similar as in the perpendicular configuration. Timing electronics such as digital delay generators can precisely control the timing of both pulses.

Both double-pulse LIBS as well as LIBS at reduced pressures aim at increasing the sensitivity of LIBS and the reduction of errors caused by the differential volatility of elements (e.g. hydrogen as an impurity in solids). It also significantly reduces the matrix effects. Double-pulsed systems have proven useful in conducting analysis in liquids, as the initial laser pulse forms a cavity bubble in which the second pulse acts on the evaporated material.

LIBS is one of several analytical techniques that can be deployed in the field as opposed to pure laboratory techniques e.g. spark OES. As of 2015, recent research on LIBS focuses on compact and (man-)portable systems. Some industrial applications of LIBS include the detection of material mix-ups,[10] analysis of inclusions in steel, analysis of slags in secondary metallurgy,[11] analysis of combustion processes,[12] and high-speed identification of scrap pieces for material-specific recycling tasks. Armed with data analysis techniques, this technique is being extended to pharmaceutical samples.[13][14]

For an optically thin plasma composed of a single, neutral atomic species in local thermal equilibrium (LTE), the density of photons emitted by a transition from level i to level j is[19]

${\displaystyle I_{ij}(\lambda )={\frac {1}{4\pi }}n_{0}A_{ij}{\frac {g_{i}\exp ^{-E_{i}/k_{B}T}}{U(T)}}I(\lambda )}$

where :

• ${\displaystyle I_{ij}}$ is the emission rate density of photons (in m⁻³ sr⁻¹ s⁻¹)
• ${\displaystyle n_{0}}$ is the number of neutral atoms in the plasma (in m⁻³)
• ${\displaystyle A_{ij}}$ is the transition probability between level i and level j (in s⁻¹)
• ${\displaystyle g_{i}}$ is the degeneracy of the upper level i (2J+1)
• ${\displaystyle U(T)}$ is the partition function (in s⁻¹)
• ${\displaystyle E_{i}}$ is the energy level of the upper level i (in eV)
• ${\displaystyle k_{B}}$ is the Boltzmann constant (in eV/K)
• ${\displaystyle T}$ is the temperature (in K)
• ${\displaystyle I(\lambda )}$ is the line profile such that ${\displaystyle \int _{-\infty }^{\infty }I(\lambda )d\lambda =1}$
• ${\displaystyle \lambda }$ is the wavelength (in nm)

The partition function ${\displaystyle U(T)}$ is the statistical occupation fraction of every level ${\displaystyle k}$ of the atomic species :

${\displaystyle U(T)=\sum _{j}g_{j}\exp ^{-E_{j}/k_{B}T}}$

Recently, LIBS has been investigated as a fast, micro-destructive food analysis tool. It is considered a potential analytical tool for qualitative and quantitative chemical analysis, making it suitable as a PAT (Process Analytical Technology) or portable tool. Milk, bakery products, tea, vegetable oils, water, cereals, flour, potatoes, palm date and different types of meat have been analyzed using LIBS.[20] Few studies have shown its potential as an adulteration detection tool for certain foods.[21][22] LIBS has also been evaluated as a promising elemental imaging technique in meat.[23]

In 2019, researchers of the University of York and of the Liverpool John Moores University employed LIBS for studying 12 European oysters (Ostrea edulis, Linnaeus, 1758) from the Late Mesolithic shell midden at Conors Island (Republic of Ireland). The results highlighted the applicability of LIBS to determine prehistoric seasonality practices as well as biological age and growth at an improved rate and reduced cost than was previously achievable.[24]

• Spectroscopy
• Atomic spectroscopy
• Raman spectroscopy
• Laser-induced fluorescence
• List of plasma (physics) articles
• List of surface analysis methods
• Laser ablation
• Photoacoustic spectroscopy

• Lee, Won-Bae; Wu, Jianyong; Lee, Yong-Ill; Sneddon, Joseph (2004). "Recent Applications of Laser-Induced Breakdown Spectrometry: A Review of Material Approaches". Applied Spectroscopy Reviews. 39 (1): 27–97. Bibcode:2004ApSRv..39...27L. doi:10.1081/ASR-120028868. ISSN 0570-4928.
• Noll, Reinhard; Bette, Holger; Brysch, Adriane; Kraushaar, Marc; Mönch, Ingo; Peter, Laszlo; Sturm, Volker (2001). "Laser-induced breakdown spectrometry — applications for production control and quality assurance in the steel industry". Spectrochimica Acta Part B: Atomic Spectroscopy. 56 (6): 637–649. Bibcode:2001AcSpe..56..637N. doi:10.1016/S0584-8547(01)00214-2. ISSN 0584-8547.

• Andrzej W. Miziolek; Vincenzo Palleschi; Israel Schechter (2006). Laser Induced Breakdown Spectroscopy. New York: Cambridge University Press. ISBN 0-521-85274-9.
• Gornushkin, I.B.; Amponsah-Manager, K.; Smith, B.W.; Omenetto, N.; Winefordner, J.D. (2004). "Microchip Laser Induced Breakdown Spectroscopy: Preliminary Feasibility Investigation". Applied Spectroscopy. 58 (7): 762–769. Bibcode:2004ApSpe..58..762G. doi:10.1366/0003702041389427. PMID 15282039. Archived from the original on 2013-04-15.
• Amponsah-Manager, K.; Omenetto, N.; Smith, B. W.; Gornushkin, I. B.; Winefordner, J. D. (2005). "Microchip laser ablation of metals: Investigation of the ablation process in view of its application to laser-induced breakdown spectroscopy". Journal of Analytical Atomic Spectrometry. 20 (6): 544. doi:10.1039/B419109A.
• Lopez-Moreno, C.; Amponsah-Manager, K.; Smith, B. W.; Gornushkin, I. B.; Omenetto, N.; Palanco, S.; Laserna, J. J.; Winefordner, J. D. (2005). "Quantitative analysis of low-alloy steel by microchip laser induced breakdown spectroscopy". Journal of Analytical Atomic Spectrometry. 20 (6): 552. doi:10.1039/B419173K.
• Bette, H; Noll, R (2004). "High speed laser-induced breakdown spectrometry for scanning microanalysis". Journal of Physics D: Applied Physics. 37 (8): 1281. Bibcode:2004JPhD...37.1281B. doi:10.1088/0022-3727/37/8/018.
• Balzer, Herbert; Hoehne, Manuela; Noll, Reinhard; Sturm, Volker (2006). "New approach to online monitoring of the Al depth profile of the hot-dip galvanised sheet steel using LIBS". Analytical and Bioanalytical Chemistry. 385 (2): 225–33. doi:10.1007/s00216-006-0347-z. PMID 16570144.
• Sturm, V.; Peter, L.; Noll, R. (2000). "Steel analysis with laser-induced breakdown spectrometry in the vacuum ultraviolet". Applied Spectroscopy. 54 (9): 1275–1278. Bibcode:2000ApSpe..54.1275S. doi:10.1366/0003702001951183.
• Vadillo, José M.; Laserna, J.Javier (2004). "Laser-induced plasma spectrometry: Truly a surface analytical tool". Spectrochimica Acta Part B: Atomic Spectroscopy. 59 (2): 147. Bibcode:2004AcSpe..59..147V. doi:10.1016/j.sab.2003.11.006.
• Doucet, François R.; Faustino, Patrick J.; Sabsabi, Mohamad; Lyon, Robbe C. (2008). "Quantitative molecular analysis with molecular bands emission using laser-induced breakdown spectroscopy and chemometrics". Journal of Analytical Atomic Spectrometry. 23 (5): 694. doi:10.1039/b714219f.
• В.Копачевский, В.Шпектор, Д.Клемято, В.Бойков, М.Кривошеева, Л.Боброва. (2008). "Количественный анализ состава тарных стекол анализатором LEA S500". Фотоника (in Russian) (1): 38–40.CS1 maint: multiple names: authors list (link)
• Noll, Reinhard (2012). Laser-Induced Breakdown Spectroscopy: Fundamentals and Applications. Berlin: Springer. ISBN 978-3-642-20667-2.

• NIST LIBS Database