A revision in hydrogen isotopic composition of USGS42 and USGS43 human-hair stable isotopic reference materials for forensic science

The hydrogen isotopic composition (δ²H_VSMOW-SLAP) of USGS42 and USGS43 human hair stable isotopic reference materials, normalized to the VSMOW (Vienna-Standard Mean Ocean Water)–SLAP (Standard Light Antarctic Precipitation) scale, was originally determined with a high temperature conversion technique using an elemental analyzer (TC/EA) with a glassy carbon tube and glassy carbon filling and analysis by isotope-ratio mass spectrometer (IRMS). However, the TC/EA IRMS method can produce inaccurate δ²H_VSMOW-SLAPresults when analyzing nitrogen-bearing organic substances owing to the formation of hydrogen cyanide (HCN), leading to non-quantitative conversion of a sample into molecular hydrogen (H₂) for IRMS analysis. A single-oven, chromium-filled, elemental analyzer (Cr-EA) coupled to an IRMS substantially improves the measurement quality and reliability of hydrogen isotopic analysis of hydrogen- and nitrogen-bearing organic material because hot chromium scavenges all reactive elements except hydrogen. USGS42 and USGS43 human hair isotopic reference materials have been analyzed with the Cr-EA IRMS method, and the δ²H_VSMOW-SLAP values of their non-exchangeable hydrogen fractions have been revised:

where mUr = 0.001 = ‰. On average, these revised δ²H_VSMOW-SLAP values are 5.7 mUr more positive than those previously measured. It is critical that readers pay attention to the δ²H_VSMOW-SLAP of isotopic reference materials in publications as they may need to adjust the δ²H_VSMOW–SLAP measurement results of human hair in previous publications to ensure all results are on the same isotope-delta scale.