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A novel procedure to determine elemental ratios by laser-induced breakdown spectroscopy is presented. This procedure, which we name optimized calibration (OC-LIBS), is a hybrid between empirical methods like calibration curves or chemometrics and the theoretical calibration-free LIBS method (Ciucci, A.; Corsi, M.; Palleschi, V.; Rastelli, S.; Salvetti, A.; Tognoni, E. Appl. Spectrosc. 1999, 53, 960-964) and seeks to reduce the high sensitivity to matrix effects seen when using, e.g., calibration curves by employing the mathematical framework of the calibration-free LIBS method. The OC-LIBS procedure is calibrated using 22 different certified powdered reference samples, spanning numerous different rock types as well as a large variation in the elemental composition. Using the OC-LIBS procedure, the compositional ratios between Mg and the elements Al, Ca, Fe, K, Na, Si, and Ti are calculated using an absolute minimum of sample preparation. A correlation between the reference and calculated values of R² ≥ 0.91 and a median relative prediction error ranging between 9.5% and 33% are achieved, despite this diverse set of samples and limited sample preparation. With total data processing times below 1 s, the OC-LIBS procedure allows for all the unique features of LIBS to be utilized, including the ability to provide very fast realtime in situ analyses. [ABSTRACT FROM AUTHOR]