Compound-Specific Isotope Analysis of Human Hair: Predicting Behaviors and Biometrics beyond Dietary Factors

This study describes the use of amino acid quantitation and amino-acid-specific isotope ratio analysis of scalp hair of American individuals to predict soft biometric traits about the donors. The scalp hair of each donor was washed, dried, homogenized and acid hydrolyzed before analysis using liquid chromatography-isotope ratio mass spectrometry (LC-IRMS). A variety of statistical tests examined the relationships between the amino acid variables and biometric questionnaire responses, the latter of which could be assessed as continuous variables-in the case of age or body mass index (BMI)-or as categorical variables in the case of sex and alcohol consumption. Correlations between biometric factors and amino acid δ¹³C values were more significant after controlling for the extent of ¹³C in the subjects' diets. Multivariate analysis revealed that the sex of a donor could be correctly predicted with cross-validated accuracies of 80% using the isotope ratios and 89% using amino acid quantities. Using amino acid δ¹³C values or quantities, the age of a subject could only be predicted with an accuracy of ±27 years (95% CI). Hair treatments, such as chemical straitening and dying, did not have any measurable effect on the isotope ratios or quantities of amino acids in the hair. Unexpectedly, the δ¹³C values of sulfur-containing amino acids were significantly different between teetotalers and subjects who consumed alcohol daily. Further refinement of this study, including larger cohorts with controlled behavioral states or genetic factors, could provide helpful investigative leads in forensic casework.