Different sample preparation methods for the analysis of suspension fertilizers combining LIBS and liquid-to-solid matrix conversion: determination of essential and toxic elements

Abstract

A novel calibration method was developed for the determination of macronutrients in suspension fertilizer samples by laser-induced breakdown spectroscopy (LIBS). The fertilizer samples were mixed with 10% m/v polyvinyl alcohol (PVA) and dried for approximately 2 h at 50 °C, resulting in a solid polymer film with the immobilized liquid sample. This study describes the use of LIBS and 12 normalization strategies to determine Cu, K, Mg, Mn and Zn as well as As, B, Ca, Cd, Cr, Fe, Na, P and Pb in suspension fertilizers by inductively coupled plasma optical emission spectrometry (ICP OES) and inductively coupled plasma mass spectrometry (ICP-MS) analyses. LIBS equipment parameters were chosen following optimization with a Doehlert design. The results clearly demonstrate a good agreement between the LIBS predicted concentrations and the reference concentrations determined by ICP OES after microwave-assisted acid digestion (AOAC 2006.03 Official Method). Correlations of R² = 0.9958, 0.9489, 0.9992, 0.9968 and 0.9809 for Cu, K, Mn, Mg and Zn, respectively, were obtained. Significant levels of potentially toxic elements such as 10 mg L⁻¹ As, 4.0 mg L⁻¹ Cd, 8.0 mg L⁻¹ Cr and 20 mg L⁻¹ Pb were detected after ICP-MS analysis. The results with low standard errors of prediction for LIBS (from 0.02 to 0.06%) indicate that this proposed suspension sample preparation procedure exhibits significant potential for the development and implementation of methods for determining essential nutrients in suspension fertilizers and other samples for which mineralization is difficult.