USING STABLE ISOTOPES TO IDENTIFY CRIMINAL SUSPECTS

Identification of trace evidence from a suspect or victim is instrumental to accurately linking a suspect to a crime. Hair is a common type of trace evidence but is largely class evidence in that color and texture are not individualizing characteristics. However, because no two people have the same diet, and because the δ¹³C and δ¹⁵N stable isotopic composition of hair is diet dependent, the determination of δ¹³C and δ¹⁵N in hair has the potential to play a pivotal role in individualizing hair samples. δ¹³C is influenced by amount and type of C3 and C4 plants consumed, whereas δ¹⁵N is influenced by the trophic level of organisms consumed. This study investigates the ability to distinguish between individuals’ hair samples via their isotopic composition. Samples were obtained from a diverse group of 8 subjects in Massachusetts. Samples were washed in a (2:1) methanol: chloroform solution, subsampled into tin capsules, and analyzed via EA/IRMS. ANOVA and Tukey’s range test were performed on the resulting dataset using PAST 3.0 (Hammer et al, 2012). ANOVA suggested that both δ¹³C and δ¹⁵N had significant differences between means of individuals (p <1×10^-4). 43% of possible comparisons between individuals were significantly different in δ¹³C, and 50% were significantly different in δ¹⁵N. These results seem to indicate that, within a region, diet is the principal factor influencing the degree of individualization of hair samples and may have a valid application to forensic investigations. Future studies could include larger and more geographically diverse subjects, as well as other stable isotopes of interest, namely δ²H, δ¹⁸O, and δ³⁴S.