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

A microwave-assisted HNO3-HF digestion system was explored for the total di ssolution of biological plant and grain materials followed by multi-element al determination using ICP-MS, in order to improve the low recoveries of se veral elements observed in a previous study using a microwave-assisted nitr ic acid digestion system. NIST standard reference materials (SRMs), includi ng Apple Leaves (1515), Peach Leaves (1547), Wheat Flour (1567a), Rice Flou r (1568a), Tomato Leaves (1573) and Pine Needles (1575), were analyzed. App roximately 0.5 g of sample was digested in 5 ml of HNO3 and 0.1 ml of HF, w ith or without a subsequent digestion stage with boric acid. The matrix eff ect 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 sp ectral interferences on Co, As and Se are tabulated. The ICP-MS system was calibrated using external standards prepared in undigested reagent blanks w ith 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 fluorid es in the digestate (without boric acid) was not significant. The recoverie s for some silicon-bound elements, such as Al, Co, Cr, Ni, Th, U and V, wer e significantly improved compared with those from digestions with HNO3 alon e. Using the HNO3-HF digestion procedure, the ICP-MS results for 30 element s, 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 dissolve d solids in solution for ICP-MS analysis and allowed the determination of b oron.