View source for Laser-induced breakdown spectroscopy



[[Image:Libs.jpg|thumb|right|200px|'''Schematic of a LIBS system''' - Courtesy of US Army Research Laboratory]]
==Overview==
'''Laser-induced breakdown spectroscopy''' (LIBS) is a type of atomic emission [[spectroscopy]] which utilises a highly energetic [[laser]] pulse as the excitation source. LIBS can analyse any [[matter]] regardless of its [[physical state]], be it solid, liquid or gas. Even slurries, aerosols, gels, and more can be readily investigated. Because all [[chemical element|elements]] emit light when excited to sufficiently high temperatures, LIBS can detect all elements, limited only by the power of the laser as well as the sensitivity and wavelength range of the spectrograph & detector. Basically LIBS makes use of optical emission spectrometry and is to this extent very similar to arc/spark emission spectroscopy.

LIBS operates by focusing the laser onto a small area at the surface of the specimen; when the laser is discharged it [[ablation|ablates]] a very small amount of material, in the range of nanograms to picograms, which instantaneously generates a [[Plasma (physics)|plasma]] plume with temperatures of about 10,000–20,000 K. At these temperatures, the ablated material dissociates (breaks down) into excited [[ion]]ic and [[atomic]] species. During this time, the plasma emits a [[continuum]] of radiation which does not contain any useful information about the species present, but within a very small timeframe the plasma expands at [[supersonic]] velocities and cools.   At this point the characteristic atomic emission lines of the elements can be observed. The delay between the emission of continuum radiation and characteristic radiation is in the order of 10 µs, this is why it is necessary to temporally gate the detector.

LIBS can often be referred to as its alternative name: laser-induced plasma spectroscopy (LIPS). Unfortunately the term LIPS has alternative meanings that are outside the field of analytical spectroscopy, therefore the term LIBS is preferred.

LIBS is technically very similar to a number of other laser-based analytical techniques, sharing much of the same hardware. These techniques are the [[vibrational spectroscopy|vibrational spectroscopic]] technique of [[Raman spectroscopy]], and the [[Fluorescence spectroscopy|fluorescence spectroscopic]] technique of [[laser-induced fluorescence]] (LIF). In fact devices are now being manufactured which combine these techniques in a single instrument, allowing the [[atomic]], [[molecular]] and structural characterisation of a specimen as well as giving a deeper insight into physical properties.

==Design==

A typical LIBS system consists of a neodymium doped [[yttrium aluminium garnet]] (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 [[nanometre|nm]]. The pulse duration is in the region of 10 ns generating a power density which can exceed 1 GW·cm<sup>-2</sup> at the focal point. Other lasers have been used for LIBS mainly [[Excimer]] ('''Exc'''ited d'''imer''') type generating energy in the [[visible]] and [[ultraviolet]] regions.

The spectrometer consists of either a [[monochromator]] (scanning) or a [[polychromator]] (non-scanning) and a [[photomultiplier]] or [[Charge-coupled device|CCD]] detector respectively. The most common monochromator is the Czerny-Turner type whilst the most common polychromator is the Echelle type, even so the Czerny-Turner type can be (and is often) used to disperse the radiation onto 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 (chemical analysis)|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.

==Advantages==

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 [[wiktionary:Homogeneous|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 fibre 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.

The use of specialised optics or a mechanically positioned specimen stage can be used raster the laser 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 utilise [[ionizing radiation]] to excite the sample, which is both penetrating and potentially [[carcinogenic]].

==Disadvantages==

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 [[Parts per million|ppm]] by mass are not uncommon, but can range from >100 ppm to <1 ppm.

==Recent developments==

Recent interest in LIBS has focused on the miniaturization of the components and the development of compact, low power, portable systems. This direction has been pushed along by interest from groups such as [[NASA]], [[ESA]] as well as the [[military]]. The [[Mars Science Laboratory]] mission plans to bring a combined Raman/LIBS onto Mars.

Recent developments in LIBS have seen the introduction of double-pulsed laser systems. For double-pulse LIBS one distinguishes between orthogonal and perpendicular configuration. In perpendicular configuration the laser is fired 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.

Both double-pulse LIBS as well as LIBS at reduced pressures are aimed 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).<!-- Paper from Kagawa group would be suitable as reference --> It also significantly reduces the matrix effects. Double-pulsed systems are also proving 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. Recent research on LIBS is focusing on compact and (man-)portable systems. Industrial applications of LIBS are e.g. the detection of material mix-ups, analysis of inclusions in steel, analysis of slags in secondary metallurgy and high-speed identification of scrap pieces for material specific recycling tasks.

==See also==

*[[Laser]]
*[[Spectroscopy]]
*[[Atomic spectroscopy]]
*[[Raman spectroscopy]]
*[[Laser-induced fluorescence]]
*[[List of surface analysis methods]]

==External links==
{{external links|August 2007}}
*[http://www.unizar.es/janzano LIBS Research group. University of Zaragoza Spain)] Development of sorting plastics with LIBS and other applications.
*[http://www.imi.cnrc-nrc.gc.ca/Carrefour_d_informations/Factsheets/LIBS_e.html NRC/IMI LIBS Research group] Our group is devoted to on-line and in-line application of LIBS to industrial process.
*[http://libs.lanl.gov/ LIBS Planetary Science Applications Website]
*[http://www.arl.army.mil/main/main/default.cfm?Action=247&Page=247 Military Application of LIBS]
*[http://www.chem.physics.kth.se/research/research_l_projects.htm Laser Spectroscopy Projects - LIBS] Includes a Flash animation of remote LIBS
*[http://www.whoi.edu/cms/files/amichel/2007/2/060330-48_18747.pdf Evaluation of LIBS for under-water applications]
*[http://cer.ucsd.edu/REPORTS/UCSD-CER-05-03.pdf  Single Particle LIBS measurements]
*[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B7GWS-4KSD8FG-1&_user=10&_coverDate=01%2F31%2F2007&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=fd3ffba6d6ddba2ec9d38fe5b13e51f7 Also called LIPS - a Combustion Research example]

==References==

*{{cite journal
|author= Lee W. B., Wu J. Y., Lee Y. I., Sneddon J.
|title=Recent applications of laser-induced breakdown spectrometry: A review of material approaches 
|journal=Applied Spectroscopy Review
|volume= 39 
|pages=27-97
|year= 2004 
|doi=10.1081/ASR-120028868
}}
*{{cite journal
|author= Noll R.,Bette H., Brysch A., Kraushaar M., Mönch I., Peter L., Sturm V.
|title=	Laser-induced breakdown spectrometry – applications for production control and quality assurance in steel industry
|journal= Spectrochimica Acta Part B
|volume= 56 
|pages=637-649
|year= 2001
|doi=10.1016/S0584-8547(01)00214-2
}}

==Further reading==
*David A. Cremers & Leon J. Radziemski. ''Handbook of Laser-Induced Breakdown Spectroscopy'' (London: John Wiley & Sons, 2006) ISBN 0470092998
*Andrzej W. Miziolek, Vincenzo Palleschi, Israel Schechter. ''Laser Induced Breakdown Spectroscopy'' (New York: Cambridge University Press, 2006) ISBN 0521852749

*B. Gornushkin, K. Amponsah-Manager, B.W. Smith, N. Omenetto, and J.D. Winefordner.   “Microchip Laser Induced Breakdown Spectroscopy: Preliminary Feasibility Investigation” Applied Spectroscopy 2004, 58(7), 762-769.

*K. Amponsah-Manager, N. Omenetto, B.W. Smith, I.B. Gornushkin, and J.D. Winefordner. “Microchip Laser Ablation of Metals: Investigation of the Ablation Process in View of its Application to Laser Induced Breakdown Spectroscopy” JAAS 2005, 20(6), 544-551.

*C. Lopez-Moreno, K. Amponsah-Manager, B.W. Smith I.B. Gornushkin, N. Omenetto, and J.D. Winefordner. “Quantitation of Low-alloy Steel Samples by Powerchip Laser Induced Breakdown Spectroscopy” JAAS 2005, 20(6), 552-556.

*H. Bette, R. Noll. "High-speed laser-induced breakdown spectrometry for scanning microanalysis" J. Phys. D: Appl. Phys. 2004, 37, 1281–1288

*H. Balzer, M. Höhne, R. Noll, V. Sturm. "New approach to monitoring the Al depth profile of hot-dip galvanised sheet steel online using laser-induced breakdown spectroscopy" Anal Bioanal Chem 2006, 385, 225–233

*V. Sturm, L. Peter, R. Noll. "Steel analysis with laser-induced breakdown spectrometry in the vacuum ultraviolet" Appl. Spectroscopy 2000, 54, 1275–1278


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[[Category:Scientific techniques]]
[[Category:Spectroscopy]]

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