Laser-induced breakdown spectroscopy analysis of sodium in pelletised pasture samples

Sodium concentration in plants inhibit shoot and root growth. Traditional wet-chemical methods of determining elemental concentrations require pre-treatment and leave unwanted by-products. Laser-induced breakdown spectroscopy is a technique that needs very little pre-treatment and produces no secondary waste. Laser-induced breakdown spectroscopy is used in this work to determine sodium concentrations in dried pasture samples. Temperature correction on the gathered spectra was attempted using Boltzmann and Saha-Boltzmann plots. These methods failed to deliver satisfactory results. Different combinations of internal reference standards were used on the Na I 818.326 nm line which resulted in an improved correlation with sodium concentration. Partial least squares regression was used on the gathered spectra to find emission lines that vary with the sodium concentration. Calcium, sodium, potassium and an argon line demonstrated high predictor weights. The Na I 818.326 nm, Na I 819.482 nm, K I 693.876 nm and K I 691.108 nm lines exhibited large dependence on sodium concentration. Building a calibration curve of sodium to potassium emission line intensity versus sodium to potassium concentration in the samples produced a correlation of R² = 0.918 and an error in prediction of 0.0254.