Determination of Lead Isotope Ratios in Bullet Samples for Forensic Purpose by Single Collector High-resolution ICP-MS

The analytical conditions of a double-focusing sector field ICP-MS were optimized to determine the Pb isotope ratios accurately in bullet samples. The following three different types of mass bias corrections were examined to obtain accurate isotope ratios: i) an external correction with a NIST SRM981 standard, (ii) an internal correction with Tl as an internal standard (NIST SRM997), and (iii) both (i) and (ii). Method (i) gave the best result under the present experimental conditions. The relative standard deviations of lead isotopic ratios (208Pb/206Pb, 207Pb/206Pb) for bullet samples (N = 28) were as small as 0.034%, demonstrating that a high-precision lead isotope analysis was possible under almost comparable precision with a multiple collector sector field ICP-MS. It was indicated that this precision is suitable for distinguishing the origin of bullet samples; 87% of handgun bullet samples (N = 15) and 69% of rifle bullet samples (N = 13) were successfully discriminated from each other. The introduction of information concerning the trace element concentration obtained by the same ICP-MS into the lead isotope ratios will afford an almost complete discrimination of the bullet samples.