Microwave-assisted digestion using diluted nitric acid for further trace elements determination in biodiesel by SF-ICP-MS

Abstract

A microwave-assisted digestion (MW-AD) using diluted nitric acid and hydrogen peroxide as auxiliary reagent was proposed for the first time for biodiesel for further trace elements determination by sector field inductively coupled plasma mass spectrometry (SF-ICP-MS). Biodiesel samples were digested under high-pressure closed vessels and the concentration of nitric acid (1, 2, 3.5, 7, 9.3 and 14molL−1) and volume of hydrogen peroxide (0, 1, 1.5, 2 and 3mL) were evaluated in order to obtain high efficiency of digestion. The efficiency of biodiesel digestion was determined by residual carbon content and residual acidity in final solutions. Up to 700mg of biodiesel were completely digested using 7molL−1 nitric acid and 2mL of hydrogen peroxide in high-pressure quartz vessels assisted by microwave radiation at 900W during 60min (included 20min for cooling step). Residual carbon content and residual acidity in digests obtained after MW-AD using optimized conditions were lower than 9 and 4%, respectively. After sample digestion, Ba, Cr, Cu, Mn, Mo, Ni, Pb, Sr, V and Zn were determined by SF-ICP-MS and analytes recoveries were in the range of 94 to 109%. Accuracy was evaluated using certified reference material (used oil) and agreement higher than 94% was achieved. Although biodiesel digestion could be performed using concentrated nitric acid, the use of diluted acid was preferable in view of the low reagent consumption and low blanks values, achieving better limits of detection (LODs) that is important aspect to green chemistry recommendations for trace elements analysis.