Evaluation of ammonium fluoride for quantitative microwave-assisted extraction of silicon and boron from different solid samples
A novel, simple, efficient and environmentally friendly closed-microwave-assisted extraction (MAE) method of silicon and boron from a variety of industrial and environmental samples using ammonium fluoride as an extractant was developed. This method avoids handling the corrosive and toxic HF and prevents the potential risk of analyte loss due to the creation of volatile SiF4 and BF3 in the presence of HF. Atomic absorption spectrometry and inductively coupled plasma optical emission spectrometry were employed for the subsequent analysis of the resulting supernatant for determination of Si and B, respectively. Certified reference material BCR®-032 Natural Moroccan Phosphate Rock (phosphate fertiliser) was taken to optimise the extraction parameters such as the sample amount, extraction temperature and time and the volume of the extractant. The optimum extraction parameters evaluated using a fractional factorial design were as follows: 50 mg of the sample extracted with 5 mL of 100 g L−1 NH4 F for 15 min at 180°C. The optimised MAE procedure was successfully applied to nine different matrix reference materials intended primarily for validation of methods for determination of components in fertilisers, sludge, plants and fly ash. The obtained results were in a good agreement with the certified or comparative values with an overall precision better than 10% in all cases. The proposed method is recommended for fast and reliable preparation of samples with silicon content <8.2% (w/w). However, further decreasing the sample mass to 10 mg enabled the quantitative extraction of silicon from fly ashes at levels of 23% (w/w).
