Microwave-assisted digestion using diluted acids for toxic element determination in medicinal plants by ICP-MS in compliance with United States pharmacopeia requirements

Abstract

The United States Pharmacopeia (USP) has introduced new requirements for toxic element determination in medicinal plants. However, the digestion step is still challenging since sample preparation should provide digests fully compatible with the determination technique and suitable for application in routine analysis. The proposed method was developed for medicinal plant digestion using diluted nitric acid solutions for further determination of As, Cd, Hg and Pb by inductively coupled plasma mass spectrometry (ICP-MS) in compliance with USP chapter 2232 (Elemental Contaminants in Dietary Supplements) requirements. Sample masses up to 500 mg of Passiflora incamata L., Paulinia cupana, Maytenus ilicifolia and Peumus boldus were digested in closed vessels using HNO₃ solutions (2 to 14.4 mol L⁻¹) and also by the dry ashing method recommended in current pharmacopoeias for comparison of results. As, Cd and Pb were determined by ICP-MS and Hg by flow injection cold vapor generation (FI-CVG) coupled to ICP-MS. Accuracy was evaluated using spikes and certified reference materials (CRMs) of plants (peach leaves, NIST 1547 and olive leaves, BCR 062). Medicinal plant samples were digested efficiently using diluted HNO₃ solutions (4 mol L⁻¹) and agreement between 96 and 103% with CRM values was obtained for all analytes. The limits of detection (0.004 to 0.02 μg g⁻¹) were lower than those recommended by the USP (0.5 to 1.5 μg g⁻¹). Significant losses in dry ashing were observed for Pb, while Hg was completely lost during digestion. Using the proposed microwave-assisted digestion method, up to eight samples could be simultaneously digested using diluted HNO₃ solution (4 mol L⁻¹) that provides digests with a carbon content lower than 320 mg L⁻¹. In this sense, reagent consumption and waste generation were minimized and these features are in agreement with the green chemistry recommendations.