The Study Of Laser-Induced Breakdown Spectroscopy

Laser-Induced Breakdown Spectroscopy (LIBS) is a relatively new powerfulspectrochemical analytical technique. Compared with conventional spectroscopictechniques, LIBS offers many well-known advantages such as its versatility, nosample preparation, non-contact optical nature, simplicity and rapid analysis and soon. However, the interaction of focused laser radiation with materials is socomplicated that the plasma formation is still not explained satisfactorily so far.Previous works have demonstrated that the characteristics of laser-induced plasma notonly depend on laser beam properties (pulse duration, laser power and wavelength),but also on the physical and chemical properties of the sample, as well as the sampleenvironment. For the practical application of this technique, the characteristics ofLIBS need further study.Firstly, the present conditions and theoretical foundation of LIBS were given.Then, the data processing methods, variations of the plasma parameters (line intensity,electron temperature and electron number density) with the laser power density andtheir spatial distributions were studied. At the same time, three quantitative analysismethods (traditional calibration curve, internal standardization method andcalibration-free method) were analyzed based on a set of six standard samples ofaluminum alloy. Using LIBS, the elemental compositions of the Qin Mountain rock,water scale, egg shell, CuCl2, copper and brass, oneyuan coin and wujiao coin werestudied and their plasma parameters were calculated also. The following works weredone in this thesis.The physics of plasma relevant to laser-induced breakdown spectroscopy hasbeen discussed. The relationship of the ratio of signal to background emissionwas analyzed and the method of improving SNR was provided. In order tocalculate and subtract the continuum emission more precisely, the polynomialfunction was used to fit the background. An iterative Boltzmann plot method wasused to calculate the laser-ablated plasma temperature, and the electron numberdensity in plasma was determined based on Stark broadening.To study the effect of the laser irradiance on the plasma parameters, the aluminum alloy was irradiated with different laser power densities (3.7,6.3,9.7,12.4,15.9,19,21.9GW/cm2). The emission curves showed a rapid linear increase at lowpower densities, and started to bend when the6.3GW/cm2was reached with aflat region. When the laser power densities above9.7GW/cm2, a new increase ofthe emission intensities was produced. The laser-supported detonation wave atlow power densities and the laser-supported radiation wave at high powerdensities were used to explain the behavior of the laser-induced plasma,respectively. The electron temperature and electron density increased morequickly when the laser power density changed from15.9GW/cm2to21.9GW/cm2than from3.7GW/cm2to15.9GW/cm2.All the signals increased rapidly near the target surface, and reached themaximum at a distance about1.5mm from the target surface, then decreased withthe increasing of the distance. A slight decrease of temperature both at the plasmaedge and close to the target surface was observed, as was consistent with thespatial variation of the line emission intensity. The electron density at the distanceof0.1mm above the target surface was approximately0.29*1017cm-3anddecreased to about0.17*1017cm-3at the distance of4.0mm.Based on qualitative and quantitative analysis, the laser-induced plasma satisfiedLTE and optically thin models within experimental errors.With a set of six standard samples of aluminum alloy, calibration curves werepresented for samples containing Si, Fe, Cu, Mn, Mg, Ni, Zn, Ti using twoquantitative analysis methods (traditional calibration curve, internalstandardization method). Based on experimental results, the limits of detection(Wt%) of trace elements were0.0412,0.0894,0.0601,0.0172,0.0334,0.0477,0.0196and0.0050, respectively. At the same time, the calibration-free methodwas also studied.The composition of Qin Mountain rock was studied using a1064nm pulsedNd:YAG laser for the first time. Elements Ca, Mg, Cu, Fe, C, Na, Si and Al wereidentified qualitatively. The electron temperature and electron density wereinferred to be16835K and2.86*1018cm-3. Elements Ca, Mg, C, Si and Al wereidentified in the water scale by the analysis of the plasma spectra. The plasmatemperature4793K was inferred using the iterative Boltzmann plot method withten neutral calcium lines, while the electron density6.1*1018cm-3was obtainedfrom the Stark broadening of the profile of Mg I285.21nm. Based on the plasma spectra of egg shell, elements iron, oxygen, silicon, carbon and calcium wereidentified, and its parameters were7250K and6.1*1016cm-3, respectively.Using LIBS, the CuCl2, copper and brass, one-yuan coin and wu-jiao coin werealso studied.