Calibration strategies for determination of the In content in discarded liquid crystal displays (LCD) from mobile phones using laser-induced breakdown spectroscopy (LIBS).

Mobile phones are one of the fastest growing types of electronic waste disposed of world-wide. One of the main components in these devices is the LCD (liquid crystal display) panel that contains conductive electrodes made of indium tin oxide. A large amount of In, which is categorized as a critical raw element, has been used to manufacture indium tin oxide films. This study applies laser-induced breakdown spectroscopy (LIBS) for the analysis of LCD samples from mobile phones in order to determine the In content. Both conventional univariate calibration and non-traditional calibration using different transition energies (emission lines), named multi-energy calibration (MEC), were assessed. To evaluate the accuracy of the results, Method EPA 3052 was performed for acid digestion of the samples using microwave-assistance, and the In content was determined by ICP OES. Indium concentrations ranged from 35 to 47 mg kg for all samples evaluated. The results showed the best accuracy for LIBS methods after the spectra were normalized by the carbon line at 193.09 nm. The univariate-LIBS model showed a standard error of calibration (SEC) about 10-fold lower than the samples' concentration, LOD and LOQ of 0.3 and 1.0 mg kg, respectively. MEC proved to be a fast and efficient alternative for direct solid analysis, and In concentrations were determined by LIBS using only two calibration pellets. The LOD and LOQ for MEC-LIBS method were 2.1 and 7 mg kg, respectively.