Ultraviolet radiation combined with microwave-assisted wet digestion of Antarctic seaweeds for further determination of toxic elements by ICP-MS

Abstract

In this work, the use of ultraviolet radiation combined with microwave-assisted wet digestion (MW-UV) was applied for digestion of seaweed samples and further determination of As, Cd and Pb by inductively coupled plasma mass spectrometry (ICP-MS). In the proposed method, UV radiation was generated in situ by electrodeless Cd discharge lamps inserted into digestion quartz vessels. This approach increased the digestion efficiency allowing lower consumption of acids. The feasibility of using diluted acid solution (0.5 to 7 mol L⁻¹ HNO₃) was evaluated for relatively higher sample masses (up to 800 mg). The efficiency of digestion was evaluated taking into account the residual carbon content and residual acidity in digests. Under the selected conditions, it was possible to digest up to 700 mg of sample using a nitric acid solution as diluted as 2 mol L⁻¹ HNO₃ allowing an efficiency of digestion higher than 77% (considering the total C content in the sample) and acidity as low as 0.19 mol L⁻¹ that is convenient for ICP-MS measurements. The accuracy of the proposed MW-UV method was evaluated by the digestion of two certified reference materials of aquatic plant (BCR 060 and BCR 670) and by comparison with the results obtained after digestion by microwave-assisted wet digestion in a high pressure system using concentrated HNO₃ (MW-AD). The results obtained by the proposed method did not present difference (t-test, 95% confidence level) with the certified values and with results obtained after seaweed sample digestion using the MW-AD method. Using 700 mg of sample, the limit of detection was 0.005, 0.001, and 0.012 μg g⁻¹ for As, Cd, and Pb, respectively. A clear advantage of the proposed method over classical approaches is that only diluted solution was necessary and it is possible to digest a relatively high sample mass, that is important to minimize the generation of laboratory residues and to improve the limits of detection, respectively.