UV-irradiation and MW-digestion pre-treatment of Port wine suitable for the determination of lead isotope ratios by inductively coupled plasma mass spectrometry

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An UV-irradiation pre-treatment for the determination of Pb isotope ratios (IRs) in Port wine by ICP-MS has been developed. Optimised conditions: mixtures of 20 ml Port wine and 120 µl 30% H2O2 (150∶1 mixture) are UV-irradiated (mercury high pressure vapour lamp, 1000 W) in PTFE-capped silica tubes for 1.5 h; the sample is then filtered and 10 times diluted with a 0.5% HNO3 solution. The procedure was compared with two other implemented methods: (1) UV-irradiation for 0.5 h of 10 ml mixture of 1∶1 (v∶v) Port wine and 30% H2O2, and dilution up to 50 ml with 0.5% HNO3 solution; and (2) high pressure MW-digestion of 1.5 ml aliquots of Port wine with 150 µl concentrated HNO3 and 1.5 ml 30% H2O2, and dilution up to 15 ml with de-ionised water. All these procedures were found to be suitable for determination of Pb IRs in Port wines. As the UV-irradiation of 150∶1 mixture of wine –H2O2 has the advantage of requiring lower amounts of chemicals and providing larger volumes of solution for analysis of Port wine, it was selected for further studies. The method was tested for eight different samples of Port wine. A precision study showed that the standard deviation associated with the overall procedure (sample pre-treatment and subsequent determination) was mainly due to the ICP-MS determinations, the sample pre-treatment giving only a small contribution. Precisions (relative standard deviation) of about 0.3% for 207Pb+/206Pb+ and for 208Pb+/206Pb+ and about 0.8% for 204Pb+/206Pb+ have been routinely obtained and shown to be sufficient to differentiate natural variations of the Pb isotope abundances in Port wine samples.