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Basics of Laser Induced Breakdown Spectroscopy (LIBS)
and it’s Applications

Dr Manoj Kumar Srivastava

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Assistant Professor, Department of Physics

Army Cadet College, IMA, Dehradun-248007

[email protected]

 

Laser
Induced Breakdown Spectroscopy (LIBS) has become a portable technique for
elemental analysis of any sample with bare minimum or no sample preparation.
This is a real time analysis technique which does not require any reagents to
be mixed with sample. LIBS can identify the constituent materials of a sample,
even if these are present in traces. With
the help of LIBS, the contamination in the either state of matter
(liquid, solid or gas) can be
traced.

 LIBS associates two joint but distinct processes
1, 3. First
is the laser ablation and second, the analytical process of plasma state due to continuous exposure of the investigating sample by the laser source. The Laser ablation evaporates the extremely small amount of target sample because of high power laser falling on it. The ablation process is also studied by the researchers in detail
2. Once
plasma is formed, the second process does the spectral analysis of the plasma formed. When the electron and material density in the plasma exceeds certain threshold, an upward spray of the sample in the gas phase occurs. This plume becomes so optically thick that ablation stops when analytical plasma state is reached. This state is reached sooner in case of longer wavelength laser source. 

Modern LIBS system comprises of a pulsed laser (Q-switched),
optics for focussing the laser output on the target material and gather the
light emitted from the target in the plasma state. The gathered emitted
radiations are then fed to spectrometer connected to a computer which gives
online real time spectrum of the received radiation.

Fig 1: Schematic Diagram of LIBS (drawn from web)

Courtesy: Google
images

 

 

Working of the LIBS system

A Q-switched laser is used 3 to produce ultra short output
pulses of extremely high power of the order of several Giga-watts. Commonly,
Nd-YAG laser with output wavelength 1064 nm is used to achieve this high power
output.

When such a high power laser output falls on the target (solid,
liquid or gas) 4.The exposed extremely small portion of it, first melts and
attains the plasma state i.e. it is broken into its charged constituents.
During the first few microseconds after the fall of the laser pulse, the plasma
energy is prominently ‘White light’. These are the radiation produced by
decelerating charged particles due to collision with other charged particles
also known as Bremsstrahlung or braking radiation. After these broadband
emissions for few micro seconds, the electromagnetic radiations come out whose
spectral analysis is to be carried out with spectrophotometer. Hence, there
required a ‘Gate’ to allow only wanted radiations to the spectrophotometer
avoiding the initially produced broadband radiations. The optimum ‘gate time’ to
turn on the spectrometer is 1-2 micro seconds after the plasma initiation. Because, upto this time the dominant radiations are the Bremsstrahlung radiations. Afterwards for few hundred micro seconds, the line spectra is emitted which is to be fed to spectrometer connected to a computer for spectrum displaying.
LIBS can process a sample even from a distance up to 100m 8.

The temperature raised due to laser at the plasma state is from approximately 5000K to 20000K.Qualitative analysis of the spectrum observed in the spectrometer
6 reveals
the different contents of the target material. The identification of different elements present in the spectra is achieved by comparing with the spectra of known elements. Spectra of different elements can be found from the literature.
These are also
available online
now a days.

Quantitative analysis of the target sample is completed with the help of the software loaded in the computer which basically reads the relative intensities of the spectral lines corresponding to different frequencies for different elements. The standard quantity of elements already fed in the computer is used by the software to compare.

Applications
of LIBS

Different applications of LIBS 7,8 include analysis of soil and minerals mining, oil exploration, space exploration9 (Rovers sent on the Mars by NASA was equipped by LIBS device for efficient analysis of the crust of the Mars).
Environmental monitoring 10 is done real time by measuring the air and water pollutants contents using LIBS. In medical for diagnosis of cancerous cells, viral and bacterial infections, bacteria type detection of different bio hazards, anthrax and other air borne infectious diseases.

After 2001, the different military applications of LIBS were
noticed which are useful to fight against enemies. The portability of LIBS
system actually promoted the use of such devices to examine the contents and
contaminations in solid liquid and gases. People have started using this
technology for detecting explosives 12, biological weapons 11 and chemical
contamination .US Army Research Lab ARL started a comprehensive programme to
study different military applications of this technology. They developed
portable LIBS system for field measurements of potentially hazardous substances
and devices 17. The portable LIBS device works real time and can monitor the
chemical contamination, environmental pollutants etc both quantitatively and
qualitatively.

Counterfeiting the precious stones is a big problem for Gem industry. It has also been observed that such precious gems are used in funding the terrorist activities. With the LIBS technology, we not only reveal the purity but also come to know about from where that gem has been extracted by looking at the composition, the mining area can be located.
There are
possibilities to explore geographic origin of food, clothing, narcotics and laundered money with the help of this technology by determining the trace impurities.
With recent
development in broadband spectrometers and chemo-metric analysis techniques, the identification of melts has become easier with LIBS.

There are
numerous medical applications of LIBS have been reported in the field of
diagnostics 13, 15.

Laser ablation technique is used for surface cleaning and restoration. Advantages of this technique over other mechanical chemical cleaning include the ability to automate, selectivity and a high degree of precession. The impurity not only be removed but with laser ablation a real time detection of surface components can also be revealed.
With the
surface components, the partial or complete removal of the impurities can be detected. This can also remove unwanted coatings form the surfaces.

Another interesting application of LIBS may be explored in the detection and mapping of latent finger prints
10 because
trace amount of explosives residue can be transferred from finger tips to surfaces who has handed the explosives. This may enable us to locate the individual who had handled the explosive material.
LIBS has also
its applications in identification of one’s remains during suspected acts and detection of gunshot residue.

 

Conclusion

The broadband spectral analysis now available can identify all the elements of the periodic table. Broadband LIBS spectra along with chemo-metric analysis can be of great importance in days to come. This will provide an intensive and bulk of information about the samples.
Though there are challenges infront of researchers to develop this emerging
field but of course LIBS has contributed a lot in the field of detection
technology in last three decades leaving behind many other old technologies.
There is a extremely bright future of Bio-LIBS technology 11, 14, 15, 16 in
medical field. The LIBS technology is presently undergoing
significant advances both in the components (lasers, spectrometers, optical
delivery systems) as well as in advanced data processing. New laser devices are
being developed to be smaller, lighter and less expensive than the current
systems. There are other applications reported of this emerging technology and
many more to be explored.

 

References and Suggested
readings:

Singh, J., and
Thakur, S., 2007, Laser-Induced Breakdown Spectroscopy.1st edition
(Amsterdam: Elsevier).
C.R. Phipps
(ed.), Laser Ablation and Its Applications. Springer Series in
Optical Sciences, vol. 129 (Springer, New York, 2007)
David
A. Cremers & Leon J. Radziemski. Handbook
of Laser-Induced Breakdown Spectroscopy (London: John Wiley &
Sons, 2006)
D. Bäuerle, Laser
Processing and Chemistry, 3rd edn. (Springer, Berlin, 2000)
 A. Miotello, P.M. Ossi (eds.), Laser-Surface
Interactions for New Materials Production: Tailoring Structure and
Properties. Springer Series in Materials Science, vol. 130 (Springer,
Berlin, 2010)

 

Aragon, C., and
Aguilera, J.A., 2008, Characterization of laser induced plasmas by optical
emission spectroscopy: A review of experiments and methods. Spectrochimica
Acta B, 63, 893–916

 

Sergio
Mussazzi et al. Laser Induced Breakdown Spectroscopy: Theory and
applications (Springer Heidelberg)

 

Andrzej
W. Miziolek et al. Laser Induced Breakdown Spectroscopy: Fundamentals  and applications (Cambridge
University Press)

 

K. Knight, et.al, 2000 Characterization
of Laser-Induced Breakdown Spectroscopy (LIBS) for Application to Space
Exploration, Appl. Spectrosc. 54, 331.

 

Martin, M.Z.,
Labbe, et.al 2007, High resolution applications of laser-induced breakdown
spectroscopy for environmental and forensic applications. Spectrochimica
Acta B, 62, 1426–1432.

 

Gottfried, J.L., et.al 2008, Standoff detection
of chemical and biological threats using laser induced breakdown
spectroscopy. Applied Spectroscopy, 62, 353–363.

 

Jennifer L. Gottfried et.al 2009 ‘Laser-induced breakdown spectroscopy for
detection of explosives residues: a review of recent advances, challenges,
and future prospects’ Analytical and Bioanalytical Chemistry, Vol. 395, pp. 283–300
 Pouzar, M., et.al 2009, LIBS analysis of crop
plants. Journal of Analytical Atomic Spectrometry, 24,
953–957
Singh, V.K., Singh, V., Rai, et.al, 2008,
Quantitative analysis of gallstones using laser-induced breakdown
spectroscopy. Applied Optics, 47, G38–G47.

 

Morel, S., Leone, M., Adam, P., and Amouroux, J.,
2003, Detection of bacteria by time-resolved laser-induced breakdown
spectroscopy. Applied Optics, 42, 6184–6191

 

 Bogue,
R.W., 2004, Boom time for LIBS technology. Sensor Review, 24,
353–357.

 

De Lucia, F.C., Jr, Samuels, et.al. 2005, Laser induced
breakdown spectroscopy (LIBS): a promising versatile chemical sensor
technology for hazardous material detection. IEEE Sensors Journal, 50,
681–689.

 

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