The Mechanism Of The Signal Uncertainty Origination For Laser-induced Breakdown Spectroscopy

Laser-induced Breakdown Spectroscopy(LIBS)is considered as the “future superstar in analytical field” with great potential in the applications of in-situ or online quantitative chemical analysis in industrial production process.However,compared with other well-commercialized analytical techniques,LIBS has on e big disadvantage,that is,the measurement repeatability is poor due to large signal uncertainty,preventing LIBS to be fully commercialized as a mature analytical technique.In this thesis,the origins and influence factors of LIBS signal fluctuations were investigated.Time-resolved plasma morphological and spectral stability were studied,showing an increasing plasma fluctuation with delay time,while the spectra fluctuation firstly decreases then increases with delay time.By analyzing the relative contributions of plasma parameter fluctuations on spectra signal uncertainty,it was found out that the fluctuation of total number density in the plasma is the key factor influencing the signal fluctuation during the major plasma evolution process,while th e temperature fluctuation is dominant to the spectra signal fluctuation at the very early stage of the plasma evolution.In other words,during a normal LIBS measurement,the total number density fluctuation is the main reason bringing up the signal uncert ainty.Using consecutive plasma image acquisition technique on Ti alloy plasma,it shows that there is a critical moment when the upper part of the plasma dashes downward to the lower part of the plasma because of the counter-pressure from the shockwave on the plasma plume,bringing significant plasma morphological fluctuation,which leads to the severe plasma signal uncertainty at later stage.This process is influenced by different experimental conditions,such as sample matrix,laser energy and frequency etc.Lastly,the mechanism of stabilizing LIBS signal by cavity confinement was study by means of shadowgraphy and ICCD camera,showing the reflected shockwave pressing the plasma and modulate the plasma position is the main reason of cavity confinement s tabilizing the LIBS plasma signal.This work offers a fundamental guidance for further developing methods to mitigate the LIBS signal fluctuation,and to further improve LIBS quantification performance.