Abstract:
Analysis of the elemental composition of paint films is a commonly used approach to discriminate paint samples in a forensic context. A recent prosecution in New Zealand included comparison of the elemental composition of paint fragments found on fish that had been dumped at sea to the fishing vessel that was the suspected source. The suspect pled guilty before trial, so the science of comparing paint samples that had been exposed to seawater was not examined in a court of law. The goal of this project was to investigate whether immersion of a paint film in seawater would result in significant changes to the elemental composition of the paint film, and over what timescales such changes would occur. Different paints were investigated, including a white acrylic latex, two blue acrylic latexes, a white alkyd resin, and a white marine epoxy antifouling paint. Glass plates were coated with a layer of paint, and then immersed in synthetic or natural seawater for differing time periods. The elemental compositions of the paint samples were examined before and after immersion using both laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and laser induced breakdown spectroscopy (LIBS). Repeated ablation at a single point on a paint sample was used to provide an estimate of elemental depth profiles. The elemental compositions of the paint samples showed changes upon immersion in seawater, with distinct patterns of incorporation of different elements that varied with the paint used. Elements present in elevated concentrations in the seawater compared to the paint, especially Na, Mg, and Sr, were incorporated diffusively into the paint over time, while Li in the paint leached out into the seawater. The elements already in the paint samples in high concentrations generally did not change concentrations significantly. The depth profile estimates showed that incorporation of elements occurred from the side of the paint film exposed to seawater. The results obtained suggest that initially similar paint samples exposed to seawater for periods of time greater than 24-48 hours could become more difficult to discriminate from each other using these analytical techniques.