Microwave digestion of plant and grain standard reference materials in nitric and hydrofluoric acids for multi-elemental determination by inductively coupled plasma mass spectrometry

A microwave-assisted HNO3-HF digestion system was explored for the total dissolution of biological plant and grain materials followed by multi-elemental determination using ICP-MS, in order to improve the low recoveries of several elements observed in a previous study using a microwave-assisted nitric acid digestion system. NIST standard reference materials (SRMs), including Apple Leaves (1515), Peach Leaves (1547), Wheat Flour (1567a), Rice Flour (1568a), Tomato Leaves (1573) and Pine Needles (1575), were analyzed. Approximately 0.5 g of sample was digested in 5 ml of HNO3 and 0.1 ml of HF, with or without a subsequent digestion stage with boric acid. The matrix effect for boron was evaluated for an ICP-MS system and signal enhancement was observed for all the elements tested. Potential spectral interferences in ICP-MS with HNO3-HF, boron and biological matrices are discussed and the spectral interferences on Co, As and Se are tabulated. The ICP-MS system was calibrated using external standards prepared in undigested reagent blanks with In as an internal standard. It was found that with a low but sufficient amount of HF in the digestion, the possible precipitation of metal fluorides in the digestate (without boric acid) was not significant. The recoveries for some silicon-bound elements, such as Al, Co, Cr, Ni, Th, U and V, were significantly improved compared with those from digestions with HNO3 alone. Using the HNO3-HF digestion procedure, the ICP-MS results for 30 elements, Al, As, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, Ni, P, Pb, S, Sb, Se, Si, Sr, Th, Ti, Tl, U, V and Zn, agreed well with the certified values in leaf and grain SRMs. The recoveries were mostly within the range 85-115%. Hence, the use of boric acid in the digestion was not necessary, which simplified the procedure, minimized the content of the total dissolved solids in solution for ICP-MS analysis and allowed the determination of boron.