DETECTION OF HEAVY METALS AND OTHER CONTAMINANTS IN SOLID AND LIQUID SAMPLES USING LASER INDUCED BREAKDOWN SPECTROSCOPY

Abstract

In this work, Laser-Induced Breakdown Spectroscopic Technique was applied for detection of heavy metals and other contaminants in solid and liquid samples. This technique was applied for qualitative as well as quantitative measurement of elemental concentration present in test samples. A special chamber was designed and fabricated for analysis of solid samples in which provisions were made for insertion of sample, its rotation, evacuation, and introduction of various gases at different pressures. The LIBS system was calibrated and applied for determination of contaminants present in the samples in the pellet forms. Then the system was applied for sample collected from sites such as crude oil waste dump, greenhouse soil, oil spilled contaminated site, mine ore waste, iron slag, and chromium contaminated site from different locations of Saudi Arabia. The role of various binding material in the LIBS performance for trace elemental analysis of powder samples was also investigated. The influence of ambient environment such as pressure, temperature, nature of buffer gas on emission feature of solid sample was studied as well. Prior to LIBS application to the actual samples, the important parameters influencing LIBS performance were optimized. In this regard the effect of laser energy, distance between fiber optics and target material, limit of detection, precision, accuracy, repeatability, delay time were optimized. The concentrations detected with our LIBS system were verified by the conventional method such as ICP and the relative accuracy of the system for various elements was compared and found in the range of 0.03 - 0.6 at 2.5 % error confidence. Most of the elements under investigation showed maximum signal intensity at delay time between 4.0 and 5.5 μs and laser pulse energy between 20 mJ to 140 mJ. Prior to application of LIBS the plasma parameters such as temperature and electron density were also measured as these parameters play vital role in the sensitivity of LIBS system. The plasma temperature for iron slag sample was estimated using Boltzman plot of LIBS intensity for neutral iron lines and Stark broadening. The maximum plasma temperature and electron densities at 40 mJ energy was 5554 K and 1.46 x 1016 cm-3, respectively for these samples. In order to analyze the liquid samples different setups were developed and tested. The aim of fabricating the setup for liquid samples analysis was to keep the LIBS system simple, and to improve accuracy & limit of detection of elements under investigation. Calibration curves for metals of interest were drawn from standard solution of these metals in distilled water. Later, the system was applied for quantitative detection of contaminants in wastewater samples collected from paint industry, syringe manufacturing industry and dairy plant. The results obtained with our LIBS setup were comparable with that obtained from ICP. The experience gained through this work can be useful for the development of a portable LIBS system for on-line analysis of contaminated sites with heavy metals for improvement of the environment and controlling the pollution at industrial sites.