Single vessel procedure for acid-vapour partial digestion in a focused microwave: Fe and Co determination in biological samples by ETAAS Geórgia C. L. Araújo, a Ana Rita A. Nogueira,* b and Joaquim A. Nóbrega a a Departamento de Química, Universidade Federal de São Carlos, São Carlos, SP, Brazil b Embrapa Pecuária Sudeste, Caixa Postal 339, 13560-970, São Carlos, SP, Brazil Received 15th May 2000, Accepted 17th August 2000 First published as an Advance Article on the web 20th September 2000 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 ml of water or H 2 O 2 . The mixture was exposed to acid vapour stemming from 15 ml of concentrated HNO 3 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 21 HNO 3 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. Introduction Sample preparation methods have been improved to become appropriate for the many analytical techniques that have increased in sensitivity in the last decades. Emphasis has been placed on fast and contamination-free methods for the determi- nation of trace elements in order to increase the sensitivity obtained by the modern analytical techniques. Many plant studies involve total metal determination in different parts of the plants. Experimental designs usually require a large number of samples to be submitted to a tedious and dangerous acid- digestion preparation. This procedure represents one of the most time-consuming steps in the whole experiment. Using hot sand- baths or metal block heaters, the samples take several hours to digest, and the acid-fume evacuation is always a nuisance. 1 Spectrochemical techniques for trace analysis, such as electro- thermal atomic absorption spectrometry (ETAAS), inductively coupled plasma atomic emission spectrometry (ICP-AES) and mass spectrometry (ICP-MS), frequently require at least digestion of biological samples to obtain accuracy and preci- sion. 2 Among the classical sample preparation techniques, the popularity of microwave digestion has increased as it offers a great potential for speed and safety when used in a large number of chemical processes. Abu-Samra et al. 3 first reported the application of the microwave technique to wet digestion of biological samples as an alternative to conductive heating in trace elemental analysis. Interference from chemicals can be avoided by decreasing the contamination from the acids used for digestion of the solid samples. The procedures normally adopted are: (1) the use of acids specially purified by sub- boiling distillation; or (2) the use of such acids combined with sample digestion in a closed vessel at an elevated temperature and pressure. 4 The vapour-phase partial digestion procedure using inorganic acids has become an alternative procedure because of the possibility of simultaneous purification of the acid, which reduces the risks of contamination and allows lower detection limits and low analytical blanks. This methodology was previously proposed in closed systems with conventional heating. 5–8 High pressure/high temperature microwave-assisted vapour-phase partial digestion has been recently successfully applied to accelerate the acid oxidiation reaction with the sample. This approach was used for the determination of trace elements in biological and geological samples. 9,10 One common characteristic of these reports was the use of the vapour-phase digestion of small amounts of materials in a high pressure microwave before analysis by ICP-AES. In the present work, a partial vapour-phase acid digestion is proposed using a focused microwave oven operating at atmospheric pressure. Iron and Co extraction efficiency is evaluated by ETAAS. Iron is an essential nutrient for plants and animals, normally strongly linked with organic matter and not easily extracted. 11 Cobalt is also an essential element for mammals and is an important component of the B 12 vitamin, which is found in low concentrations and can be easily extracted in an acid medium. 12 Experimental Instruments and apparatus A SpectrAA-800 (Varian, Mulgrave Victoria, Australia) atomic absorption spectrometer equipped with a Zeeman background corrector and a GTA 96 graphite furnace was used for Co and Fe determinations performed at 242.5 nm and 248.3 nm re- spectively, with a slit-width of 0.2 nm. Pyrolytically-coated graphite tubes (Varian, Mulgrave Victoria, Australia) were used. A volume of 10 ml of sample was injected into the ETAAS by the autosampler. The results were obtained using peak area and the quantification was performed using analytical curves. The Co pyrolysis and atomisation temperatures were set as This journal is © The Royal Society of Chemistry 2000 DOI: 10.1039/b003872p Analyst, 2000, 125, 1861–1864 1861 Published on 20 September 2000. Downloaded by Empresa Brasileira de Pesquisa Agropecuaria on 13/08/2013 20:51:04. View Article Online / Journal Homepage / Table of Contents for this issue