Silicon determination by microwave-induced plasma optical emission spectrometry: Considerations and strategies for the use of tetrafluorboric acid and sodium hydroxide in sample preparation procedures

Abstract

The use of tetrafluoroboric acid and sodium hydroxide were investigated for the total decomposition of plant tissues and fertilizers for Si determination by microwave-induced plasma optical emission spectrometry (MIP OES). The accuracy was estimated by comparing the results of the proposed method with those of energy dispersive X-ray fluorescence (ED-XRF). The optimized HBF4 volume required to decompose 100 mg of sample was estimated to be 0.2 mL (48% w v−1). The Si recovery obtained was 99%, and the limit of detection was 0.03 g kg−1. Concerning the reactions and the possible products formed, after digestion 70% of the Si was found as fluorosilicic acid. Besides this, hydrofluoric acid was formed. High dilution factors, the addition of excess boric acid, or the use of a sample introduction system manufactured from inert components can be adopted to avoid damage to the MIP OES components and contamination of the analytical blank. The introduction of air in the N2 plasma was assessed to reduce the matrix effect noted due to high sodium concentration in the alkaline digestion, and improved accuracy was achieved. The Si recoveries obtained by applying this method varied from 100 to 102%, and the limit of detection was 0.4 g kg−1.