Optimization Methods of Laser Induced Breakdown Spectroscopy (LIBS) and the Self-Absorption Problem

Laser Induced Breakdown Spectroscopy (LIBS) is a well-established technique for determining elemental composition. Compared to conventional methods, its ability to analyze solids, liquids, and gases with little or no sample preparation makes it more versatile and ideal for field, multi-element, and in-situ analysis. The method can be certified for analytical applications of interest in environmental monitoring and quality control processes. Previous work has demonstrated that several different experimental parameters (e.g. laser wavelength, repetition rate, interaction geometry, surface conditions) may affect the effective analytical possibilities of the method. Therefore, many studies have been devoted to optimize such parameters in a way to enhance its figure of merits. In this thesis, LIBS is combined with a spark discharge to operate in a laser triggered spark discharge mode. This Spark Discharge Laser Induced Breakdown Spectroscopy (SD-LIBS) is evaluated for Al and Cu targets in air under atmospheric pressure. Significant enhancement in the measured line intensities and the signal-to-background ratios, which depends on the spark discharge voltage and the laser fluence, is observed in SD-LIBS when compared to LIBS alone for similar laser conditions. The measured line intensities increase with the applied voltage for both targets, and the ratio of the measured line intensity using SD-LIBS to that using LIBS is found to increase as the laser fluence is decreased. For Al II 358.56, such intensity enhancement ratio increases from ∼50 to ∼400 as the laser fluence is decreased from 48 to 4 J/cm2 at an applied voltage of 3.5 kV. Thus, SD-LIBS allows for using laser pulses with relatively low energy to ablate the studied material, causing less ablation, and hence, less damage to its surface. Moreover, applying SD-LIBS gives up to 6-fold enhancement in the SB ratio, compared to those obtained with LIBS for the investigated spectral emission lines. Self-Absorption Laser Induced Breakdown Spectroscopy (SA-LIBS) of compacted brass samples was introduced as a new approach whose principle stems from the well-known Atomic absorption Spectroscopy (AAS) technique. Two closely produced plasmas were generated; one acts as the light source analogous to AAS while the other acts as the analyte thus we let the plasma probe itself. Temporal development of Cu spectral lines was initially obtained in order to examine the lines which are more susceptible to self-absorption. SA-LIBS showed that Cu 324 and 327 nm are significantly subject to self-absorption while Cu 330 nm is not, a result that agrees with those reported in literature. Non-linear calibration curves were also obtained, a problem which was previously demonstrated in several reports for brass samples and was attributed to the difference in ablation rate. Internal standardization of emission lines is a method that results in linear calibration curves. Assuming that the concentration of the absorbing species follows a Gaussian distribution, we tried to solve the familiar Beer-Lambert\u27s law for our specific experimental condition. Linear calibration curves of the logarithm of the absorption and absorbing species concentration or equivalently exponential relationship between them have been obtained. The plasma temperature was about the same for both Zn and Cu species, which is reasonably accepted under the assumption of LTE