Pacific and Atlantic coast mollusk shells: chromatographic amino acid racemization kinetics and interlaboratory comparisons
Date
2006
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Publisher
University of Delaware
Abstract
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 high temperature heating experiments 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), Ensis directus (Ed), Macoma nasuta (Mn) and Protothaca staminea (Ps), mollusks from the Atlantic and Pacific coasts, are used in this study since they are commonly used in aminostratigraphic studies. Mm, Ed, Mn and Ps were heated at 140ºC and 125ºC for varying times to induce different degrees of racemization. Samples were split three ways at the hydrolysis level for analysis on GC, RPLC and IELC. Ideally, the analysis of a single sample on each instrument should yield identical D/L values. However, GC A/I has an apparent analytical bias of 6.8% over RPLC A/I and 1.9% over IELC A/I. There is a 0.4% systematic difference between GC and RPLC for aspartic acid in which RPLC returns higher D/L values. For glutamic acid there is a 4.6% systematic difference between RPLC and GC in which RPLC returns higher D/L values. And RPLC valine D/L values are on average 24.4 ± 1.7% greater than valine D/L values determined by GC. Due to differences between D/L values determined by peak height and peak area, it has been determined that A/I should be measured via peak height on all analyzers to optimize their correlations. Aspartic acid and glutamic acid should be measured via peak area, and valine should be measured via peak height on RPLC and peak area on GC. Kinetic parameters for Mm, Ed, Mn and Ps have been determined for eight amino acids on GC and for IELC A/I. It is widely accepted that the rate of racemization (epimerization) of amino acids varies among species; thus, the apparent order of relative rates of racemization will differ. Alanine and phenylalanine are found to have fast racemization rates while proline and valine have slow racemization rates. Furthermore, based on heating experiments, Mm is considered a fast racemizer, Ed and Ps are intermediate racemizers and Mn is a slow racemizer. Hence, elevated temperature experiments provide useful insights into the interpretation of racemization data in Quaternary fossils.