Optimisation of microwave assisted digestion of sediments and determination of Sn and Hg

Abstract

The determination of Sn by flow injection–hydride generation–quartz furnace atomic absorption spectrometric (FI–HG–QFAAS) was optimised following different experimental designs. The best conditions were: 0.2% HCl (v/v), 0.5% NaBH 4 (w/v) and the furnace temperature 875 °C. Under those conditions, the limit of detection was 0.17 ng dm −3 and a precision of 5.3% was obtained. One of the aims of this work was to optimise the closed vessel microwave assisted digestion (MAD) of sediments for the determination of Sn and Hg in the same extract using the analytical conditions previously optimised for Sn in the FI–QFAAS. The optimisation of the MAD of sediments was accomplished following a D-optimal design, including the composition of the HCl–HNO 3 mixture, the pressure and irradiation time. However, we could not determine tin in the extracts due to the formation of foams, the optimisation of the digestion conditions were taken from the FI–cold vapour (CV)–QFAAS measurements of mercury. The optimum conditions were: 2.1 bar of pressure during 10 min of irradiation and two local optima composition of the acid mixtures: 80% HCl–20% HNO 3 and 60% H 2O–20% HCl–20% HNO 3. The determination of mercury in sediments was validated with the CRM-580. In order to determine Sn in sediments the solutions from the same D-optimal design were analysed using an ICP-MS and the digestion conditions were optimised for Sn and for other 8 metals. In this case the same optimal conditions were obtained (2.1 bar and 10 min) but different acid mixture composition 20% HCl–80% HNO 3. The determination of Sn and the other metals in sediments was validated using two other CRMs (PACS-2 and SGR-1).