Differentiation of bullets by spark source mass spectrometry

A study has been made to demonstrate the capability of spark source mass spectrometry (SSMS) for differentiating bullets on the basis of elemental composition. Twenty-six elements are commonly found in bullet lead at the 0.1-ppm level and above. Only twelve of these prove to be generally useful for quantitative characterization. The precision of analysis with SSMS using photographic detection is about 10% relative standard deviation. The accuracy obtained in this study is considerably worse than this, about 30% average error. In spite of such large errors, SSMS is shown to be far superior to the more precise method of neutron activation analysis. The reason, of course, is the large number of elements determined by SSMS. Bullets do not have uniform elemental compositions within a box or lot. Instead, there are usually two or three distinct groups of bullets within a given box. Three Winchester boxes representing lots manufactured a few weeks apart showed common groups among them. All other boxes had unique groups of bullets, indicating that the method is a promising alternative to the conventional method of identifying bullets by physical markings.