Abstract:
Laser induced breakdown spectroscopy (LIBS) is a very simple method to determine the elemental composition of samples without any sample preparation. A laser pulse induces plasma at the sample surface, and the emission spectrum of atoms, ions or molecules is detected and analyzed. The emission of light is fully determined by the properties of the plasma. The parameters of the laser/matter interaction and of the plasma expansion are therefore pertinent to study and optimize the LIBS signal. Thus, many LIBS studies have conducted in the past to demonstrate the influence of the laser wavelength, the laser fluence (i.e., the energy per pulse and the focusing conditions), the pulse duration, and the pressure and the composition of the surrounding atmosphere in which the plasma expands.

Problem statement
The quantitative analysis of the samples without the requirement of sample preparation is an important area of research. Laser induced breakdown spectroscopy (LIBS) is used for quantitative analysis and detection of trace elements in different solids, liquids and gases without the need for sample preparation. In LIBS, a flash lamp or pump laser energizes a pulsed laser. A beam from the pulsed-mode laser is then optically focused onto a moving target sample. The sample absorbs enough energy to exceed its ionization energy threshold (i.e. , Breakdown threshold), which leads to the formation of a plasma at the sample surface. The ions inside the plasma are excited into higher energy levels, where rapid heating, vaporization and ionization continue. As the hot plasma cools, electrons return to the ground state or, a less energetic state, by emitting photons of light. The photon emission from the entire plasma, or a portion thereof, is imaged into a spectrometer that spectrally resolves the photons. The photons, are then, detected by a high resolution spectrograph to detect the atomic and ionic emission transitions. These transitions once identified are used for the identification of the species present and their intensities for the quantitative determination of the elements present in the sample.

Objectives of the Project:

1. Identification of the metal alloys, by studying the emission spectrum of plasma generated as a result of an intense laser beam. 
2. Further LIBS can also be used for the analysis of hazardous environmental pollutants.
3. This study has led to the training of manpower in Pakistan.

Methodology

The project was aimed at the qualitative and quantitative analyses of metals and non-metals using Laser-induced breakdown spectroscopy. The breakdown was produced by a high energy Nd;YAG pulse laser. The spectral emission resulting due to the relaxation of constituents’ excited elements has been carried out to the entrance slit of the monochromator with the help of optical fiber. The light being separated here was recorded by a multichannel analyzer based on gated CCD detector, installed at the exit of the monochromator. All the equipment is controlled by a PC, interfaced through a GPIB card. The plasma generated at the sample surface is recorded with the help of a fast response spectrometer. The recorded spectra have been analyzed using NIST data base. The relative intensities of the peaks give the information about the constituents of the samples.

Results and Discussions
Laser induced breakdown spectroscopy (LIBS) was applied to the different verity of marbles gathered from various areas of Pakistan various spectrums of spatial evolution of the marble plasma  was produced by the fundamental (1064nm), second (532) harmonics of Q-switched  continues Nd-YAG laser. The interaction of laser with material generated plasma on the surface of the sample material. The emission spectra of different marble samples had been recorded by using spectrophotometer. The elemental composition and their relative abundance in each sample are found to be quite different which make it different physically too.

The sample of sunny grey marble was investigated by LIBS. The spectrum was taken by setting scan to average at 3 with LIBS experimental setup under atmospheric conditions. The emission spectrum consists of singly and doubly ionized spectral lines of calcium, iron, aluminum, strontium and carbon.
The most prominent peaks of calcium was at transitions 3p64s---3p64p, 3p63d4s---3p63d4p, 3p63d4s---3p64s5p. The peaks of Al with transition 3s4d--- 3s6f at 586.14nm and Sr at 644.21nm was also observed.
The analysis of the emitted element specific spectral lines permits the direct determination of the atomic composition of marbles with well known and tabulated NBS (NIST) database. This measuring principle allows the determination of main and trace elements in marble and of any element within a few seconds.

Conclusions:
LIBS prove to be very much effective by comparison to other analytical techniques,  in which removal of sample from the object is compulsory, as in optical microscopy, X-ray diffraction XRD, Fourier-transform infrared Spectroscopy FTIR and SEM coupled to energy dispersive X-ray analysis EDX. this is  a major advantage of LIBS  that, unlike the above techniques which require removal of sample from the object, LIBS can be provide  rapid  important depth profiling information. It has been found that the major building block of marbles in Pakistan is calcium, while iron takes its position as second in this race. Some minor traces of strontium, carbon and aluminum are also found. The spectral lines were singly and doubly ionized. It can be concluded that the Laser Induced Breakdown Spectroscopy is a very efficient technique for elemental analysis of unknown samples.

Recommendations:
The technique is recommended to be used for the quantitative analysis of all kinds of samples.