A new detection system for laser induced breakdown spectroscopy based on an acousto-optical tunable filter coupled to a photomultiplier: Application for manganese determination in steel

The development of a new detection system for laser induced breakdown spectroscopy (LIBS), based on a collinear quartz acousto-optical tunable filter (AOTF) for the ultraviolet spectral region coupled to a photomultiplier, is described. It was used in conjunction with a 1064 nm, 5 ns pulse duration neodymium-doped yttrium aluminium garnet (Nd:YAG) laser source and also employed a radio-frequency signal generator to control the AOTF and a digital delay generator to delay the start of the detection in relation to the instant of the application of the laser pulse. The detection system was optimized for highest detectivity for the manganese peak at 293.9 nm while analyzing a steel sample by LIBS. The resulting signal to background ratio at the optimal conditions of 2 µs delay time, 40 µs integration time gate and 110 mJ pulse energy was similar to that of a commercial echelle-intensified charge-coupled device (echelle-ICCD) detection system. The new detection system was then employed for manganese determination in steel samples, taking the emission signals at just 15 wavelengths, 5 related to the above mentioned manganese peak, another 5 to background emission around 296.0 nm and the others to the iron peak at 297.3 nm (internal standard). The resulting analytical curve for manganese, obtained using 5 samples in the concentration range of 0.214 to 0.939% w/w, presented a correlation coefficient of 0.979 for an exponential regression function. The relative errors of predicting the manganese concentrations, using the calibration curve, for 2 samples, containing 0.277 and 0.608% w/w, were 20.7 and − 1.9%, respectively.