Development of a new method for Pb isotopic analysis of archaeological artefacts using single-collector ICP-dynamic reaction cell-MS

Abstract

A new methodology has been developed for Pb isotopic analysis of different kinds of archaeological artefacts. The method has been optimized for bone tissue, soil, amphorae and lead objects dating from the Roman Era, and consists of 3 key steps: (i) sample digestion with quantitative Pb recovery, optimized using the certified reference materials NIST SRM 1400 Bone Ash, BCR CRM 141 Calcareous Loam Soil and BCR CRM 142 Light Sandy Soil, (ii) quantitative isolation of the pure Pb fraction and (iii) isotope ratio measurement with a quadrupole-based ICP-mass spectrometer, equipped with a dynamic reaction cell (DRC). For Pb isolation, an extraction chromatographic column, containing a resin with a Pb-selective crown ether (4,4′(5′)-di-tert-butylcyclohexane-18-crown-6) purchased from Eichrom Technologies, was used. The Pb fraction can be obtained in a pure form and in a quantitative way after a relatively fast process, the columns can be reused and no Pb isotopic fractionation occurs on the column. The isolation process has been validated using NIST SRM 981 Common Lead, and by application to samples with a known isotopic composition. For isotope ratio measurements, the use of Ne as a collision gas in the DRC allowed an external precision below 0.17% RSD for the ^XPb/²⁰⁴Pb ratios (where X = 206, 207, 208) and below 0.09% RSD for the ²⁰⁷Pb/²⁰⁶Pb, ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁷Pb ratios to be obtained. The accuracy of the measurement protocol was validated by comparing the results thus obtained with the corresponding MC-ICP-MS and TI-MS values. Pb isotope ratios for the certified reference materials used are given. The method was also demonstrated to work for a totally different kind of (complex) sample (lichen), and thus it is expected that the method can be used for Pb isotope ratio analysis of a wide variety of materials.