One-point calibration of Saha-Boltzmann plot to improve accuracy and precision of quantitative analysis using laser-induced breakdown spectroscopy

Abstract

Calibration-free laser-induced breakdown spectroscopy (CF-LIBS) has recently gained attention due to its potential to overcome matrix effects on quantitative analysis using LIBS. However, the efficiency of CF-LIBS may be compromised by uncertainties in the experimental and spectroscopic parameters. To minimize these uncertainties and improve quantitative analysis, the one-point calibration (OPC) method was recently introduced to empirically correct the emission line intensities in a Boltzmann plot. In this work, we used the OPC method on a Saha-Boltzmann plot to cover a large energy range and to obtain more precise and accurate plasma temperature and electron density (Ne) values than were obtained with a Boltzmann plot using OPC method. For the analysis, we prepared two sets of sodium chloride (NaCl) samples: one with graphite and another with calcium carbonate (CaCO3). We observed matrix effects when we evaluated the calibration curves of carbon line intensity as a function of carbon concentration. Corrections by the OPC method to the Boltzmann or Saha-Boltzmann plots minimized the matrix effects and also guaranteed that the calculated plasma temperature were the same for the primary elements of each sample, suggesting that they were in local thermodynamic equilibrium. When the OPC method was applied to the Saha-Boltzmann plot, we obtained an uncertainty of less than 0.5% for plasma temperature and Ne; an r2 value of 0.994; and a root mean square relative error (RMSRE) of 8% for C concentration. Meanwhile, the traditional OPC method resulted in an uncertainty of 2.3% for plasma temperature and Ne and a validation with r2 of 0.977 and RMSRE of 13% for C concentration. These results showed that the OPC method efficiently improved the quantitative analyses possible with the Saha-Boltzmann plot using CF-LIBS.