ANALYTICAL COMPARISONS USING ALLOISOLEUCINE/ISOLEUCINE FOR AMINO ACID GEOCHRONOLOGY

In the geochronological application of amino acid racemization/epimerization, amino acid D/L values can be determined from either peak height or peak area ratios. Peak area ratios more accurately represent relative abundances, particularly when peaks are asymmetric or distorted. However, when the D-peak is much smaller than the L-peak, peak height ratios usually exhibit less variability in duplicate analyses. D-Alloisoleucine/L-Isoleucine (A/I) is an important amino acid ratio since it is the only amino acid ratio that is separated by the three common analytical procedures currently available for racemization studies: gas chromatography (GC), reverse-phase liquid chromatography (RPLC) and ion-exchange liquid chromatography (IELC). Using A/I values from samples heated for varying intervals of time at 140ºC, each analytical method can be directly compared over the entire range of A/I values from 0.0 to ~1.2. By plotting GC, RPLC and IELC A/I ratios against each other, it has been found that when all ratios are measured using either peak height or peak area, the peak height comparisons resolve a much smaller difference between analyzers than the peak area comparisons. Additionally, when peak height and peak area ratios for two different instruments are plotted, the resultant difference between each analyzer increases. Thus, A/I values are the most similar among all three analytical techniques when measured via peak height. Furthermore IELC A/I (peak area) returns the lowest A/I values than any other method when using peak height or peak area. This is important to note since A/I values are traditionally reported using peak area. Therefore, based on this and the fact that A/I values are the most similar between GC, RPLC and IELC when measured using peak height, we suggest that convention should to be changed so that A/I values are reported using peak height.