Development of a microwave-assisted ultraviolet digestion method for biodiesel and subsequent trace elements determination by SF-ICP-MS

Abstract

A microwave-assisted ultraviolet digestion (MW-UV) method for biodiesel is proposed for subsequent determination of Ba, Co, Cr, Cu, Mn, Ni, Pb, Sr, and V by sector field inductively coupled plasma mass spectrometry (SF-ICP-MS). It is the first time that the combination of microwave and ultraviolet radiation is proposed for biodiesel pretreatment achieving high efficiency of digestion even when dilute acid and high sample mass were used. Variable concentrations of nitric acid (2, 4.5, and 7 mol L⁻¹), hydrogen peroxide (0, 1, and 2 mol L⁻¹), and sample mass (ranging from 500 to 1000 mg) were evaluated to optimize digestion (i.e., low residual carbon content and low residual acidity). Using the proposed MW-UV method, residual carbon content (RCC) of 19.2% and residual acidity (RA) of 17.4% were obtained for digestion of 950 mg of biodiesel using 7 mol L⁻¹ nitric acid. Samples were also digested by microwave-assisted digestion (MW-AD) using the same conditions and final digests were not suitable for analytes determination due to the high dissolved carbon content in solution (RCC about 27.4%). A study was performed in order to evaluate the maximum concentration of carbon that allowed accurate ⁵²Cr, ⁶⁰Ni, and ⁵¹V determinations without influence from polyatomic interferences by SF-ICP-MS. It was verified that interferences on ⁵²Cr occurred when C concentration was higher than 150 mg L⁻¹, requiring the use of medium resolution mode. The formation of polyatomic interference over ⁵¹V and ⁶⁰Ni species in low resolution mode was not observed varying the C concentration. Analysis of a certified reference material (CRM, HU-1 used oil) by the proposed method agreed with certified values (t-test, 95% confidence level). Analyte recovery studies were in the range of 95–108%.