Optimization of micro-Laser Induced Breakdown Spectroscopy analysis and signal processing

Abstract

As a result of continuing instrumental development (Echelle spectrometer and ICCD detectors), micro-Laser Induced Breakdown Spectroscopy analysis may become an increasingly recognized analytical technique for determining elemental compositions of geologic materials. Best conditions of time resolution conditions (delay and time acquisition window) are estimated with respect to the collection geometry of optical plasma emission of our system. It turns out that the level of the Bremsstrahlung continuum emission is weak in the first tens of nanoseconds after the laser excitation pulse. The enlargement of the emission lines is identified in the first 100 ns but remains comparable to the spectral resolution of our system. Thus, results show that time-resolved conditions are not necessarily required to perform elemental analysis at the micrometric scale using LIBS, contrary to macro-LIBS. This suggests potential improvements of micro-LIBS analysis (sensitivity and spectral resolution) using non-intensified CCD connected with the laser pulse. However, in order to improve the detection of weak signals obtained with an ICCD detector, working at high gain, a new method of signal processing on two-dimensional Echelle images has been developed. This method, based on the comparison of two 2D images, allows the identification of group of pixels (particles) that can be considered as representative of actual signals. This methodology applied to LIBS spectra eliminates the majority of noise peaks, allows the identification of weak signals which were almost impossible to extract from the noise, and does not alter the intensity ratio between emission lines. This method overcomes the poor dynamics of ICCD used at maximum gains and opens new possibilities in micro-LIBS analysis.