A COMPARISON OF AMINO ACID RACEMIZATION KINETICS USING GAS CHROMATOGRAPHY, REVERSE-PHASE LIQUID CHROMATOGRAPHY AND ION-EXCHANGE LIQUID CHROMATOGRAPHY

Three different methods can be used to quantify the extent of amino acid racemization present in geological samples: gas chromatography (GC), reverse-phase liquid chromatography (RPLC) and ion-exchange liquid chromatography (IELC). Data obtained from any method can be used for geochronological and paleoclimate purposes. Each method resolves the diastereomer D-alloisoleucine/L-isoleucine (A/I), thus by conducting a high temperature heating experiment the three methods can be directly compared. Furthermore, since GC and RPLC also resolve enantiomers, kinetic data can be obtained for multiple amino acids. Mercenaria mercenaria (Mm) and Protothaca staminea (Ps), venerids from the Atlantic and Pacific coasts, are used in this study since they are commonly used in aminostratigraphic studies. Mm and Ps were heated at 140ºC for varying times to induce different degrees of racemization. Samples were split three ways at the hydrolysis level and then analyzed on GC, RPLC and IELC. Theoretically, the analysis of a single sample on each machine should yield identical A/I values. However, RPLC has an apparent analytical bias of +5-6% compared to IELC for A/I values determined using both peak height and area. GC A/I area ratios are 16% higher than RPLC and 21% higher than IELC; whereas peak height GC A/I values are 8% higher than RPLC and 9% higher than IELC. The activation energy (Ea) for epimerization of GC A/I in Mm shells is 29.2 kcal/mol, which is consistent with Ea values for other species in previous studies (Goodfriend et al., 1996; Miller, 2000; Bakeman et al., in progress). The Ea for other amino acids (alanine, valine, leucine, pheylyalanine, glutamic acid) in Mm resolved on GC are all between 29.1 and 29.9 kcal/mol. Ea on GC in Ps include phenlyalanine (29.9 kcal/mol) and aspartic acid (31.0 kcal/mol). RPLC and IELC Ea for both Mm and Ps are pending. It is widely accepted that the rate of racemization (epimerization) of amino acids varies among species; thus, the order of relative rates of racemization will differ. The order of relative racemization, from fastest to slowest, in Mm is: Ala > Phe > A/I > Glu > Leu > Val. Whereas, in Ps the order of relative racemization is: Phe > Ala > A/I > Leu > Glu > Val. Hence, elevated temperature experiments provide useful insights into the interpretation of racemization data in Quaternary fossils.