Comparison of Different Digestion Procedures for Elemental Determination in Plant Samples by Inductively Coupled Plasma-Mass Spectrometry

The overall aim of this study was to identify the influence of different digestion methods for the determination of trace elements by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) in various plant samples. Four digestion procedures including HNO₃-HClO₄-HF at normal pressure, HNO₃-H₂O₂-HF in a pressurized vessel, microwave-assisted digestion with HNO₃-H₂O₂, and HNO₃-HClO₄-HF after dry ashing were applied for the decomposition of plant samples. Quantitative analysis for the 27 elements in three Certified National Reference Materials GBW 07603, GBW 07604 and GBW 07605 was measured by using ICP-MS. It was demonstrated that the results of some volatile elements (Hg, B, etc.) were significantly lower than certified concentration values, which was consistent with these elements being volatile at high temperatures during ashing. Using a large amount of reagents in normal pressure digestion pollutes the environment, and results in a high blank value. The efficiency of digestion was promoted by using HClO₄ in a normal pressure system. However, HClO₄ could not be removed completely nor the interferences to the determination of V and As by the complex ion with Cl^-. Using HF in digestion at normal pressure and in a pressurized vessel improved recoveries of the REEs, Be, Ti, Sb and U etc. However, the B and Hg volatilized during the procedure of removing HF with heating. A rusty steel vessel caused high blank values of Ni and Cr. Although none of the methods were good enough to provide accurate determinations for all elements in the Certified Reference Materials, a relatively simple method of HNO₃ and H₂O₂, digested in a microwave oven yielded the most acceptable results with a precision of less than 10% RSD and relative error of -4.6%-13.6%, except for Be, Ti, Sb, Bi and REEs.