Environmental Forensic Application of Lead Isotope Ratio Determination: A Case Study Using Laser Ablation, Sector ICP-MS

2004:2: Millette

Environmental Forensics, 5(2):97-108

Abstract:

Laser-ablation sector field inductively coupled plasma mass spectrometry (ICP-MS) has been used for the in situ determination of concentrations and isotope compositions of Pb in an environmental sample in a form of layered paint chip. This study examines feasibility of using the powerful method of isotope ratio analysis in laser ablation sector ICP-MS as a routine and rapid, but very reliable, analytical instrument for fingerprinting sources of lead in environmental forensic investigations.

Significant lead concentrations were found in four of the six layers of a paint chip sample. The layers were analyzed for isotopes of lead (i.e., ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb) and two isotopes of mercury (i.e., ²⁰²Hg and ²⁰⁴Hg). In this study, Hg isotopes were measured for the purpose of interference correction for ²⁰⁴Pb isotope. Elemental data were obtained by ablation with Nd:YAG laser operating in the UV region (wavelength of 213 nm) manufactured by New Wave- Merchantek and isotopic analysis with a Finnigan MAT sector ICP-MS. A multi-element reference glass standard (NIST-612) was also analyzed for the same isotopes and gave stable and flat response for all isotopes of lead.

Four of the paint layers (C, D, E, F) contained significant amounts of lead and were used for isotope ratio measurements. An appreciable amount of Hg was found only in layer C. For layers D, E, and F, which had copious amount of Pb, the standard deviations for isotope ratios were: 0.492-1.347 for ²⁰⁶Pb/²⁰⁴Pb, 0.494-1.653 for ²⁰⁷Pb/²⁰⁴Pb, and 1.214-3.643 for ²⁰⁸Pb/²⁰⁴Pb. The lead isotope ratios for the layer with the highest Pb concentration, layer F, had a radiogenic signature similar to that of Mississippi Valley-type Pb-Zn mines in eastern Missouri. The lead isotope ratios of layer E are less radiogenic and had a signature similar to that of Pb-Zn-Ag mines of Sierra Madre in east central Mexico. The lead isotope ratio of the third layer, layer D, had a non-radiogenic signature.

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