Microwave-assisted digestion for trace elements analysis of tree nut oil: evaluation of residual carbon content

An inductively coupled plasma mass spectrometer method for estimating trace elements in tree nut oils has been developed which employs microwave digestion equipped with high pressure subassembly. Residual carbon content and residual acidity were determined to evaluate the efficiency of digestion. The best digestion efficiency was obtained using Easyprep (TM) system with 0.5 g of oil, 4 mL concentrated nitric acid, and 2 mL hydrogen peroxide, and a final temperature of 235 degrees C. Residual carbon content and residual acidity were estimated as 1.7% (corresponding to 852 mg L-1 of carbon in sample solution) and 7.5%, respectively, and recovery values ranged from 90.7% to 107.7%. Whereas, only 0.2 g of oil could be digested through conventional microwave system with 5 mL concentrated nitric acid and 2 mL hydrogen peroxide at 205 degrees C (residual carbon content = 3.2%). The developed method has been applied to determine nine trace elements (Cr, Mn, Fe, Ni, Cu, As, Zn, Cd, and Pb) in different categories of tree nut oils, and high content of Fe, Mn, and Zn were found in some of those oils. To the best of our knowledge, the present investigation is the first attempt to analyze trace elements in different categories of tree nut oils consumed in China.