Microwave digestion of plant and grain reference materials in nitric acid or a mixture of nitric acid and hydrogen peroxide for the determination of multi-elements by inductively coupled plasma mass spectrometry

The closed-vessel microwave digestion of four plant standard reference materials (SRMs) and two grain reference materials (RMs) in nitric acid or in a mixture of nitric acid and hydrogen peroxide was explored for the direct determination of 26 elements, Al, As, B, Pa, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mo, Ni, Pb, Sb, Se, Sn, Sr, Ti, TI, Th, U, V and Zn, by ICP-MS. In 2.5 or 5 ml of HNO3 with or without the presence of 2 ml of H2O2, 0.5 g of grain or plant sample was predigested overnight at room temperature, then at an elevated temperature of 120-165 degrees C with a pressure limit of 13.8 bar (200 psi) for 20 min. The presence of H2O2 helped to maintain a higher temperature under the pressure Limit and reduced the carbon content in the digestates; but its impurities hampered the ICP-MS analysis for certain elements at low levels. The ICP-MS system was calibrated by the method of external standards prepared in reagent blank solutions with In as the internal standard. Interferences from the sample matrices were eliminated, corrected or reduced by subtracting the blank signals, selecting suitable isotopes and applying the appropriate interference correction equations. Using this method, nearly all of the predigestion spike recoveries for the 26 elements were within 90-110%. For the grain RMs studied including Corn Bran and Wheat Flour, the majority of the recoveries for most elements studied were within 85-115%. For the plant SRMs studied including Pine Needles, Tomato Leaves, Apple Leaves and Peach Leaves, the majority of the recoveries were within 90-115% for the determination of As, B, Ba, Ca, Cd, Cu, Mg, Mn, Mo, Pb, Sr, and Zn, within 70-100% for Al, Co, Cr, Fe, It, Sb and V, but were 40-80% for Ni, Th, Ti and U. The low recoveries were caused by the siliceous materials in these samples which were not decomposed during digestion.