Cassiterite fingerprinting by LA-ICP-MS

Abstract

Cassiterite (SnO₂) has been specified as a ‘conflict mineral’ by a U.S. Government Act. This requires all companies that use tin in their products to verify that this tin does not originate from conflict regions in the Democratic Republic of the Congo where an ongoing violent conflict is fuelled by those minerals. Possible strategies to meet these requirements are based on written documents which certify the origin of the minerals. Direct chemical analyses of cassiterite, with the aim to confirm the documented sources, support those strategies in the case of doubt of the available documents. The German Federal Institute for Geosciences and Natural Resources (BGR) developed a geochemical fingerprinting method for cassiterite based on LA-ICP-MS. Single grains from cassiterite concentrates are analysed by LA-ICP-MS. Samples in question are checked against a database where data from samples of known origin are stored. The Kolmogorov–Smirnov statistic is used to demonstrate that independent samples taken from the same location show empirical cumulative distribution functions of element concentrations which are more similar to each other compared to those of samples taken from different locations. This gives confidence that cassiterite fingerprinting based on a comparison of a cassiterite sample in question with a database can be successful. The Wilcoxon rank sum test is used to establish a ‘ranking of similarity’ between a sample in question and samples from a database. This is used to assess whether the declared origin of the sample in question is credible or not.