Quantitative Analysis of Li by Laser Induced Breakdown Spectroscopy

Lithium is currently difficult to analyze because it cannot pass through windows necessary in vacuum systems. However, analysis of Li is necessary, due to its petrological importance. Laser-induced breakdown spectroscopy (LIBS) is very sensitive to Li concentrations, and could possibly be a tool for Li analysis. The laser in LIBS ablates the surface of the specimen, generating a short-lived plasma, which puts the atoms and ions into an excited state. Photons are emitted when electrons return to a lower energy level, which are captured by a spectrometer, and finally displayed on a computer. In this study, standards for the Li calibration consist of synthetic glasses prepared from lithium tetraborate, sodium tetraborate, and pure SiO₂ powders that were placed in graphite crucibles in a furnace at 1100°C for 30 min. The standards were analyzed at a laser power of 91.5 mJ, and at a Q-Switch delay of –1.5 µm using an Ocean Optics LIBS 2500 system. Spectra were analyzed using multivariate analysis and the program Unscrambler. Multivariate analysis uses the entire spectrum to calibrate to each element, overcoming matrix effects that complicate single-element calibrations. The multivariate analysis resulted in a excellent calibration of known value vs. predicted value (r²=0.99988). To test this, a set of standard spectra not used in the calibration were analyzed; a plot of known vs. measured values yielded r²=0.927. Two lepidolites and two spodumenes of unknown Li concentrations were analyzed with the calibration, resulting in reasonable Li concentrations of 4.76 and 2.70 wt % Li (spodumene) and 3.68 and 4.33 wt % Li (lepidolite). These results suggest that LIBS can be developed to routinely analyze Li in rock and mineral samples using simple synthetic standards.