New insights on the potential causes affecting the emission spectra variability in standoff LIBS
I. Gaona, J. Moros and J.J. Laserna, Journal of Analytical Atomic Spectrometry, 2013, 28, 1750 – 1759
Interest in the use of laser-based sensors operating in standoff mode is increasing due to the wide range of options offered in the evaluation of distant targets. However, despite this high potential, the performance aspects of any sensor for analysis at a distance are curtailed as compared with its use in situ. The present work addresses a sensitive topic in standoff laser-induced breakdown spectroscopy (ST-LIBS) which is the larger variability observed within the emission signals registered from faraway targets. A field deployable LIBS sensor has been used to ascertain how atmospheric propagation affects the laser pulses delivered to a distant target and the emitted light from the plasma plume created. In this way, the extent of the contribution of the alterations of each optical pathway to the signal uncertainty in ST-LIBS have been isolated. As has been experimentally verified, the amount of laser energy reaching the target remains constant, whatever the distance. In contrast, the laser beam cross section is distorted during its travel towards the target; an alteration that becomes ever larger as the distance to the target increases. This circumstance leads to further and random deterioration of the irradiance, thus resulting in plasma events notably differing in their intensity, which, in turn, have proven to be in direct correlation with the intensities and variability of the spectral responses collected by the system. The positional displacement of the laser-induced plasmas has no impact on the sensitivity and uncertainty of the standoff LIBS signals.