¹⁴C Measurements Elucidate Isotopic Differences between Nails and Hair in Modern Humans

Abstract

In forensic sciences, radiocarbon found in modern human nails and hair is evaluated to determine the year of death. However, 14 C analyses presented herein of fingernails and hair from the same infant demonstrated 14 C values of hair that were lower than would be expected (e.g. depleted relative to the fingernails by at least 10‰). These results prompted a series of 14 C measurements on infant hair strands, fingernails, and infant shampoo, which suggested the presence of C contamination due to cosmetic products. To further evaluate these discrepancies, several hair strands and fingernail samples from multiple donors were collected, pretreated by several approaches, and measured using isotopic analysis (d 13 C, d 15 N, and C/N as well as 14 C accelerator mass spectrometry). SEM images of the surface of chemically pretreated hair strands were also taken to inspect the performance of the chemical pretreatments applied. Our 14 C and stable isotope results show that modern human hair is likely contaminated with fossil-fuel-derived carbon, which is found in most hair care products. Currently, the various chemical pretreatments available in the literature and presented herein show that it is not possible to completely remove foreign carbon contaminates. Thus, the human 14 C and d 13 C values between keratinous tissues (fingernails and hair) are not in agreement. From these observations, it becomes apparent that isotopic interpretations using human hair should be used with extreme caution and additional work is needed for its use in forensic and dietary research.

Improved radiocarbon analyses of modern human hair to determine the year‐of‐death by cross‐flow nanofiltered amino acids: common contaminants, implications for isotopic analysis, and recommendations

“…Previous time‐series experiments using 14 C‐AMS to address the deposition and accumulation effects of shampoo contaminants in scalp‐hair on an infant indicated that, once the contaminant was deposited into the hair, it remained for a period of several months . Since previous C/N ratios from Subject B were also anomalous (as mentioned above), we expected to see the same phenomenon, although all the keratinaceous samples for this study were taken 1 year apart.…”

“…Isotopic analyses and C/N determinations were carried out on fingernails and hair samples (scalp and body), and the results are listed in Table . Previously, the δ 13 C values from the scalp‐hair of Subject B were lower than those of the fingernail by 0.8‰ on average, while the atomic C/N values ranged from 4.0 to 4.6 (scalp to hair‐tip) with a total %C of 45.8–50.4 . An apparent loss on the %N was also observed, probably due to hair protein alterations at the molecular level, an issue already reported by others .…”

“…This procedural contamination was, however, observed on all the keratinized tissues undergoing CFNAA processing. Nevertheless, the single CFNAA procedure shows encouraging results compared with the conventional and specialized chemical treatments tested previously . The nanofiltration process applied here is an intermediate process of filtration between reverse osmosis (RO) and ultrafiltration (UF) that can reject particles in the approximate size range of 1 nanometer .…”

“…Fingernails, scalp‐ and body‐hair were sampled from a single human subject (subject B in De La Torre et al .) . The subject is a middle‐aged female, who has consistently used the same dark‐brown permanent coloring dye kits for over 3 years (same coloring formula and brand).…”

“…demonstrated that scalp‐hair strands were 14 C‐depleted (~5–13‰) compared with the subjects' fingernail reference, and that the C/N values appeared anomalous . Moreover, the 14 C offsets increased as the chemical treatments applied to hair strands evolved from conventional procedures (i.e., methanol/chloroform and acid/alkaline/acid) to those capable of stripping the hair surface . The fact that chemical compounds found in hair care products incorporate themselves deeply into the hair structure makes it difficult to distinguish and remove these products.…”