Quantitative laser-induced breakdown spectroscopy based on corona equilibrium for estimating atomic composition in powdered milk

The quantitative analysis is an issue of laser-induced breakdown spectroscopy (LIBS). A standard method for quantitative LIBS is to apply the local thermodynamic equilibrium (LTE) to the laser-induced plasma with the electron density exceeding the McWhirter criterion. In this paper, we report the usefulness of the corona equilibrium for quantitative LIBS. We observed the optical emission spectra of the plasma produced by pulsed laser ablation of a powdered-milk target at various delay times after the irradiation of the Nd:YAG laser pulse, and we evaluated the relative excited-state densities of Mg, Ca, Na, and K. The relative excited-state densities were not explained by assuming LTE for the plasma, even though the electron density exceeded the McWhirter criterion. This means the difficulty in the estimation of the atomic composition by applying LTE. The inapplicability of LTE is due to the fact that the detailed balance between the rates of the electron impact excitation and deexcitation is not realized in the plasma. In contrast, the relative densities which were obtained from the optical emission spectrum observed at a long delay time of (t_mathrm{d} ge 25)µs after the laser irradiation was consistent with the corona equilibrium at electron temperatures of 0.8–1.5 eV. The present work indicates that applying the corona equilibrium to the low-density plasma which is observed at a long delay time can work as an alternative method for quantitative LIBS.