Overcoming peak overlap and low signal intensity in chromatography: Implications for amino acid geochronology of Quaternary shell materials

An important method for dating Quaternary strata relies upon the chromatogr aphic determination of D-to-L amino acid ratios (D/L) in geological materia ls, including fossil molluscs. However, workers in different laboratories h ave reported wide discordance in D/L measurements for the same molluscs, wh ich they have attributed to differences in chemical and instrumental proced ures and to various other analytical uncertainties. This paper considers ye t other uncertainties in D/L measurement that potentially arise from the ch romatograms themselves, namely: incomplete resolution of adjacent amino aci d peaks, low signal-to-noise ratio, and ambiguity in the characterization o f true baselines. Graphical methods of peak-size measurement (area- or heig ht-based) yield highly inaccurate and discordant peak-size ratios when appl ied to marginal chromatograms, as demonstrated with a simulated chromatogra m with severely overlapped peaks of known individual sizes. In contrast, ac curate and precise peak-size ratios are recovered from this same chromatogr am by first deconvolving the peaks using chemometric algorithms before appl ying the traditional graphical methods. This encouraging result in a near-w orst-case scenario may represent a viable approach for recovering useful ag e information (relative or absolute) from important geological samples of m arginal quality and/or quantity. (C) 1999 Elsevier Science Ltd. All rights reserved.