Amino acid geochronology is based largely on the extent of racemization in fossils, as measured by the ratio amounts of D- and L-isomers. Here we report a new, fully automated reverse phase HPLC procedure for sample and precise stereoisomeric separations. At least nine pairs of DL-amino acids are separated with baseline resolution in 75 min using commercially available reagents and equipment. By optimizing precolumn derivatization, we attained compound detectability in the sub-picomole range, sufficient for milligram-size molluscan samples. Analytical reproducibility for nine DL ratios in four fossils spanning a broad range of ages averages 7% (n = 14-28) Asp and Glu DL ratios are the most consistently well resolved and reproduced, with analytical variations of 2 and 3%, respectively. Ratios in three fossil mollusc samples analyzed by the new method and measured previously by GC-based laboratories overlap in 17 out of 18 cases, when considering the ± 1 sd analytical errors and ± 1 sd inter-laboratory variation. To determine the hydrolysis procedure that minimizes induced racemization while maximizing amino acid recovery, we hydrolyzed seven powdered molluscan fossils of different ages and genera for 0-48 h at 110°C. Concentrations of most amino acids reached a stable plateau after 6-8 h. For young samples, in which faster-racemizing amino acids are targeted (especially Asp), a hydrolysis time of 6 h minimizes induced racemization while attaining nearly complete amino acid recovery. For older samples, 22 h at 110°C is preferred.
ASJC Scopus subject areas
- Earth-Surface Processes