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Laser ablation - spark discharge - optical emission spectroscopy (LA-SD-OES) and Laser-induced breakdown spectroscopy (LIBS) are applied for the analysis of minor elements Manganese (Mn) and Chromium (Cr) in steel and for the chemical imaging of micro-patterned photoresist layers. In LA-SD-OES a weak laser pulse (typical energy 50 μJ to 5 mJ) triggers an electric discharge between a high voltage electrode and the laser ablated spot on the sample surface. Intense emission lines of neutral and singly-ionized atoms are observed with both spectroscopic techniques. However, lines of doubly-ionized atoms are detected with LA-SD-OES only. The line intensities of C III and Al III increase with decreasing laser pulse energy, surprisingly. The appearance of intense doubly-ionized lines indicates that the mechanisms of plasma excitation are different in LA-SD-OES compared to LIBS and that the combined plasma is not in local thermodynamic equilibrium at low laser energy. The calibration curves of Mn and Cr in industrial steel reveal steeper slope (higher sensitivity) for the Mn II and Cr II lines measured with LA-SD-OES and for the Mn I and Cr I lines measured with LIBS. Chemical imaging of the photoresist layer with LA-SD-OES at the atomic lines of C I and C III and the molecular band of CN reveals all details of the sample. Similar images are obtained for C I and CN using LIBS, however, no image contrast is obtained at the C III line. The very low pulse energy in LA-SD-OES enables soft sampling and may be beneficial also for the analysis and imaging of sensitive material. Unlabelled Image • Laser-triggered localized spark excitation and optical spectroscopy (LA-SD-OES) • Enhanced emission lines of singly and doubly ionized atoms with LA-SD-OES • Higher sensitivity for minor elements Mn and Cr in steel obtained with ionic lines • Chemical imaging of micro-patterned polymer using a C III emission line [ABSTRACT FROM AUTHOR]

The optical emission of plasma on industrial steel samples induced by Laser Ablation-Spark Discharge-Optical Emission Spectroscopy (LA-SD-OES) and by Laser-Induced Breakdown Spectroscopy (LIBS) is investigated and correlated to the volume of ablated steel material. The 36 steel samples investigated have an iron content C(Fe) above 94 wt%. The excitation energy in LIBS (laser pulse of 55 mJ) and in LA-SD-OES (laser pulse of 5 mJ and spark discharge of 50 mJ) is the same. In LA-SD-OES, the optical emission of plasma and the size of ablation craters are very similar for all samples and a linear calibration curve for Mn is measured (R2 = 0.99). In LIBS, however, a pronounced dependence of the plasma emission and of the crater volume on the steel matrix is observed and calibration curves show a strong cross-sensitivity to other elements such as Si (matrix effect). The hardness, grain size, and phase of steel samples are analyzed to correlate the matrix effect in LIBS measurements to a physical property of the specimen. The different behavior for LA-SD-OES and LIBS is probably due to different processes of sampling and plasma excitation. From our results we conclude that LA-SD-OES enables for the element analysis of industrial steel largely independent of composition and structure of samples while in LIBS the matrix effect has to be taken into account. [Display omitted] • Novel technique LA-SD-OES for the element analysis of steel. • Analyte emission intensities are independent of other elements in steel matrix. • Sampling in LA-SD-OES is independent of steel composition. • Linear calibration curves with high R2 for minor elements in steel. • LA-SD-OES overcomes strong matrix effect observed in LIBS of same samples. [ABSTRACT FROM AUTHOR]