Single vessel procedure for acid-vapour partial digestion in a focused microwave: Fe and Co determination in biological samples by ETAAS
A single vessel procedure using a focused microwave oven is proposed for biological sample preparation with nitric acid vapour under atmospheric pressure. A laboratory-made PTFE support vessel equipped with four cups that received the samples was adapted to fit on the microwave glass vessel. Biological samples (30 mg) were directly weighed into these PTFE cups followed by the addition of 150 μl of water or H2O2. The mixture was exposed to acid vapour stemming from 15 ml of concentrated HNO3 placed in the bottom of the glass vessel. The acid vapour was formed at 115 °C and brought about the Co and Fe extraction in 10 and 60 min, respectively. The resulting suspension was diluted with 0.14 mol l−1 HNO3 to a final volume of 1.0 ml, shaken and centrifuged. The supernatant was analysed by electrothermal atomic absorption spectrometry (ETAAS) by placing the cups directly in the autosampler of the spectrometer. This system minimised contamination, and reagent and time consumption and was suitable for Co and Fe determination in biological materials. The accuracy of the proposed method was assessed by using certified reference materials and by comparison with the closed vessel microwave as a comparative technique. Cobalt and Fe recovery was around 82–99%. As an additional advantage, up to 6 samples can be simultaneously prepared in each vessel, thereby improving the sample throughput from 6 to 24, when a 6-cavity focused microwave is used.