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 bi ological sample preparation with nitric acid vapour under atmospheric press ure. A laboratory-made PTFE support vessel equipped with four cups that rec eived the samples was adapted to fit on the microwave glass vessel. Biologi cal samples (30 mg) were directly weighed into these PTFE cups followed by the addition of 150 mu l 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 degrees C and brought abou t the Co and Fe extraction in 10 and 60 min, respectively. The resulting su spension 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 atom ic absorption spectrometry (ETAAS) by placing the cups directly in the auto sampler of the spectrometer. This system minimised contamination, and reage nt and time consumption and was suitable for Co and Fe determination in bio logical materials. The accuracy of the proposed method was assessed by usin g certified reference materials and by comparison with the closed vessel mi crowave as a comparative technique. Cobalt and Fe recovery was around 82-99 %. As an additional advantage, up to 6 samples can be simultaneously prepar ed in each vessel, thereby improving the sample throughput from 6 to 24, wh en a 6-cavity focused microwave is used.