USGS42 and USGS43: Human-hair stable hydrogen and oxygen isotopic reference materials and analytical methods for forensic science and implications for published measurement results

Because there are no internationally distributed stable hydrogen and oxygen isotopic reference materials of human hair, the U.S. Geological Survey (USGS) has prepared two such materials, USGS42 and USGS43. These reference materials span values commonly encountered in human hair stable isotope analysis and are isotopically homogeneous at sample sizes larger than 0.2 mg. USGS42 and USGS43 human-hair isotopic reference materials are intended for calibration of δ2H and δ18O measurements of unknown human hair by quantifying (1) drift with time, (2) mass-dependent isotopic fractionation, and (3) isotope-ratio-scale contraction. While they are intended for measurements of the stable isotopes of hydrogen and oxygen, they also are suitable for measurements of the stable isotopes of carbon, nitrogen, and sulfur in human and mammalian hair. Preliminary isotopic compositions of the non-exchangeable fractions of these materials areUSGS42(Tibetan hair)δ2HVSMOW–SLAP=−78.5±2.3‰(n=62)   and   δ18OVSMOW–SLAP=+8.56±0.10‰(n=18)USGS43(Indian hair)δ2HVSMOW–SLAP=−50.3±2.8‰(n=64)   and   δ18OVSMOW–SLAP=+14.11±0.10‰(n=18)Using recommended analytical protocols presented herein for δ2HVSMOW–SLAP and δ18OVSMOW–SLAP measurements, the least squares fit regression of 11 human hair reference materials isδ2HVSMOW–SLAP=6.085δ18OVSMOW–SLAP−136.0‰δ2HVSMOW–SLAP=6.085δ18OVSMOW–SLAP−136.0‰with an R-square value of 0.95. The δ2H difference between the calibrated results of human hair in this investigation and a commonly accepted human–hair relationship is a remarkable 34‰. It is critical that readers pay attention to the δ2HVSMOW–SLAP and δ18OVSMOW–SLAP of isotopic reference materials in publications, and they need to adjust the δ2HVSMOW–SLAP and δ18OVSMOW–SLAP measurement results of human hair in previous publications, as needed, to ensure all results on are on the same scales.