A test to identify cyanide origin by isotope ratio mass spectrometry for forensic investigation

Cyanide is one of the common poisons in murders. When cyanide has been used, to identify the origin of cyanide may be necessary in the forensic investigation. We have examined the possibility of distinguishing different commercial cyanide samples through the δ13C and δ15N values and developed a protocol for the isotope analysis of cyanide extracted from several matrices as food and medicine. Several cyanide precipitates were tested for the isotope analysis. The results show that cupric ferrocyanide Cu2[Fe(CN)6] is the most appropriate precipitate for the analysis. Thirteen batches of KCN and nine batches of NaCN chemicals were randomly chosen from different suppliers. The cyanides were converted to cupric ferrocyanide and then analysed by isotope ratio mass spectrometry coupled to elemental analysis (EA-IRMS). The isotopic signature of the commercial samples varied from −51.96 to −25.77‰ for δ13C and from −4.51 to +3.81‰ for δ15N, highlighting the potential of applying EA-IRMS technique to identify cyanide from different batches and sources. The influence of the cyanide extraction and isolation from spiked matrix on the isotopic analysis was also studied. Three matrices: orange juice, yogurt drink and a medicine were tested. In many cases, the isotopic analysis results obtained from the original cyanides precipitates and those isolated from the matrices showed a good accordance, especially for δ15N. In some matrices, the 13C analysis was interfered by co-precipitates. With carefully elaborated working protocol, determining the isotope ratio of N and C in cyanide by EA-IRMS is a promising method for forensic investigations.