Fs/ns-dual-pulse orthogonal geometry plasma plume reheating for copper-based-alloys analysis

Plasma plume emission spectroscopy signal enhancements between 12- and 280-fold were obtained in air at atmospheric pressure by reheating the fs-laser ablation plume (energy 0.75 and 3.0 mJ) with a 45 mJ ns-pulse in orthogonal geometry. The emission enhancements induced by the double pulse configuration (DP) at various inter-pulse delay times and distances of the second laser beam from the target surface were investigated for copper-based-alloy standards. Temporal surveys of the plasma plume temperatures induced by both fs-single pulse (fs-SP) and DP placed at a fixed distance of 0.5 mm from the target surface were carried out. Several copper-based-alloy standards were employed for drawing Zn calibration curves by using either fs-SP or DP configurations and considering Cu as internal standard. The experimental data show that, for high Zn contents, the fs-SP set-up is affected by a self-absorption phenomenon so that a deviation from the assumed calibration single linear response is observed and two linear regressions are considered. Conversely, it has been observed that the DP configuration is not affected by any self-absorption effect and provides an improvement of the Zn limit of detection (LOD) but worse calibration linear regressions than the fs-SP. Thus, the DP scheme can increase the analytical sensitivity of fs-SP and, furthermore, its process can be supposed to be independent from the matrix composition even for largely different Zn contents of the Cu-based-alloy standards used.