Single particle laser-induced breakdown spectroscopy for elemental analysis of water and bioaerosols

Laser-induced breakdown spectroscopy (LIBS) is a selective and robust atomic emission spectroscopy method where a high intensity laser pulse thermally dissociates and excites a minute quantity of the sample material. In LIBS, all elements can be measured simultaneously which makes it useful in applications where fast multicomponent analysis is required. Trace element analysis technology based on LIBS was developed in this Thesis for monitoring of dissolved impurities in water. The concentrations of dissolved elements in industrial process water and effluent are measured for process steering and for emission management. At the moment, the chemical analysis of water is mainly done by manual sampling and laboratory analysis. Online measurement technology would contribute to the economical use of water, chemicals and energy in the process industry and further the safety of liquid emissions.Two novel methods for improving the sensitivity of LIBS of dissolved elements were introduced in this Thesis. In the presented techniques, aerosol particles generated from the sample solution are analyzed individually with LIBS. The main benefits of single particle LIBS are the high preconcentration of the sample and the absence of the liquid matrix which suppresses the LIBS signal. In the first instrumentation, the particle travels along a narrow carrier gas flow and its presence in the small sampling volume is determined by an optical sensor. In the second presented technique, the generated droplet is trapped in three dimensions using electrodynamic balance (EDB) technology and the residual particle is analyzed after the complete drying of the droplet. A detailed characterization of LIBS of precisely trapped particles was also conducted in this Thesis. Clear improvement with respect to reproducibility and sensitivity was achieved in this work compared with the direct LIBS analysis of water. The limits of detection for the constructed EDB-LIBS system were at 50 µg/l level for several transition metals and are among the lowest values obtained with LIBS based trace element analysis. The flexible requirements for the LIBS laser pulse energy and the speed of operation improve the applicability of the aerosol based sample preparation to the monitoring of industrial and natural waters. Moreover, the concurrent detection of dissolved elements and microbiological contamination of water was demonstrated by the combination of laser-induced fluorescence and single particle LIBS.