High speed laser-induced breakdown spectrometry for scanning microanalysis

For the first time laser-induced breakdown spectrometry, with laser pulse repetition frequencies of up to I kHz, with single pulse evaluation is realized for scanning microanalysis of macroscopic samples. The tightly focused beam of a diode-pumped Q-switched Nd: YAG-laser is scanned with a step size of 20 mum across a sample surface. The emission spectrum of each laser-induced plasma generated by a single laser pulse of 2 mJ energy is guided to a spectrometer of the Paschen-Runge type to detect the line radiation of up to 24 elements simultaneously covering a spectral range from the vacuum ultraviolet to the near infrared. The time resolved spectral signals linked to the respective detector channel are processed to generate maps of the spatial distribution of elements in the sample surface. In a measuring field of typically I x I cm(2) 250 000 measurements are performed within I I min. The spatial resolution achieved, in terms of the diameter of the crater produced by a single laser pulse in steel samples, is better than 15 mum. The experimental system is applied to investigate inclusions in steel samples. These inclusions influence the quality of steel grades for the production of spring steel, thin sheets and wires. They consist of oxides, nitrides or sulfides. Correlating the maps of different elements allows us to identify the type of inclusion, this is demonstrated for aluminium oxide, aluminium nitride and manganese sulfide.