US Geological Survey, Reston, VA 20192, USA.
Rapid Commun Mass Spectrom. 2011 Aug 15;25(15):2209-22. doi: 10.1002/rcm.5095.
Accurate hydrogen isotopic measurements of keratin materials have been a challenge due to exchangeable hydrogen in the sample matrix and the paucity of appropriate isotopic reference materials for calibration. We found that the most reproducible δ(2)H(VSMOW-SLAP) and mole fraction of exchangeable hydrogen, x(H)(ex), of keratin materials were measured with equilibration at ambient temperature using two desiccators and two different equilibration waters with two sets of the keratin materials for 6 days. Following equilibration, drying the keratin materials in a vacuum oven for 4 days at 60 °C was most critical. The δ(2)H analysis protocol also includes interspersing isotopic reference waters in silver tubes among samples in the carousel of a thermal conversion elemental analyzer (TC/EA) reduction unit. Using this analytical protocol, δ(2)H(VSMOW-SLAP) values of the non-exchangeable fractions of USGS42 and USGS43 human-hair isotopic reference materials were determined to be -78.5 ± 2.3 ‰ and -50.3 ± 2.8 ‰, respectively. The measured x(H)(ex) values of keratin materials analyzed with steam equilibration and N(2) drying were substantially higher than those previously published, and dry N(2) purging was unable to remove absorbed moisture completely, even with overnight purging. The δ(2)H values of keratin materials measured with steam equilibration were about 10 ‰ lower than values determined with equilibration in desiccators at ambient temperatures when on-line evacuation was used to dry samples. With steam equilibrations the x(H)(ex) of commercial keratin powder was as high as 28%. Using human-hair isotopic reference materials to calibrate other keratin materials, such as hoof or horn, can introduce bias in δ(2)H measurements because the amount of absorbed water and the x(H)(ex) values may differ from those of unknown samples. Correct δ(2)H(VSMOW-SLAP) values of the non-exchangeable fractions of unknown human-hair samples can be determined with atmospheric moisture equilibration by normalizing with USGS42 and USGS43 human-hair reference materials when all materials have the same powder size.
由于样品基质中可交换的氢以及缺乏合适的同位素校准参考物质，准确测量角蛋白材料的氢同位素一直是一个挑战。我们发现，在环境温度下使用两个干燥器和两种不同的平衡水，用两组角蛋白材料平衡 6 天，可以测量到最具重现性的角蛋白材料的 δ(2)H(VSMOW-SLAP)和可交换氢的摩尔分数，x(H)(ex)。平衡后，将角蛋白材料在 60°C 的真空烘箱中干燥 4 天是最关键的。δ(2)H 分析方案还包括在热转换元素分析仪（TC/EA）还原单元的旋转架中的银管中穿插同位素参考水。使用该分析方案，确定了美国地质调查局 42 号和 43 号人发同位素参考物质的不可交换部分的 δ(2)H(VSMOW-SLAP)值分别为-78.5 ± 2.3‰和-50.3 ± 2.8‰。用蒸汽平衡和 N2 干燥分析的角蛋白材料的测量 x(H)(ex)值明显高于以前发表的值，并且即使进行过夜吹扫，干 N2 吹扫也无法完全去除吸收的水分。用蒸汽平衡测量的角蛋白材料的 δ(2)H 值比使用在线抽真空干燥样品时在环境温度下用干燥器平衡测量的值低约 10‰。当使用蒸汽平衡时，商业角蛋白粉末的 x(H)(ex)高达 28%。使用人发同位素参考物质校准其他角蛋白材料，如蹄或角，可能会导致 δ(2)H 测量产生偏差，因为吸收的水量和 x(H)(ex)值可能与未知样品不同。当所有材料具有相同的粉末尺寸时，可以通过与美国地质调查局 42 号和 43 号人发参考物质归一化，用大气水分平衡来确定未知人发样品不可交换部分的正确 δ(2)H(VSMOW-SLAP)值。
