Slope ratio calibration for analysis of plant leaves by laser-induced breakdown spectroscopy
A new calibration strategy based on the increase of the ablated mass with the number of accumulated laser pulses on a single solid calibration standard is proposed for laser-induced breakdown spectroscopy (LIBS). Under suitable experimental conditions, the emission intensity is directly proportional to the analyte amount in the ablated sample mass, which, in turn, is proportional to the number of laser pulses. Thus, two ablated mass functions are generated by plotting the analyte emission intensities as a function of the number of accumulated laser pulses in the test sample or in the calibration standard (either a CRM or a reference sample). The analytes mass fractions in the test samples are determined from the ratio of the corresponding slopes. The feasibility of this approach, named as slope ratio calibration (SRC-LIBS), was demonstrated by determinations of macro- (Ca, Mg, and P) and micronutrients (B, Mn, Cu, Fe, and Zn) in plant leaves in the form of pressed pellets. In order to minimize drawbacks related to analytes microheterogeneity, the pressed pellets were sampled at 30 different sites for each number of laser pulses. Calibrations performed within 5 and 30 laser pulses per site yielded linear correlation coefficients higher than 0.99 for all the analytes. The accuracy of the proposed calibration strategy was assessed from the analysis of five CRMs, as well as comparison with the results obtained for sugarcane leaves by ICP OES after microwave-assisted acid digestion, with agreement at the 95% confidence level. Results were more accurate than those determined by the single-point calibration strategy and by external calibration with a set of CRMs. The proposed approach is then a fast, simple, and practical calibration strategy, by requiring only a single standard for quantitative analysis by LIBS.