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

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Quaternary Science Reviews

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Geochemistry | Geology


An important method for dating Quaternary strata relies upon the chromatographic determination of D-to-L amino acid ratios (D/L) in geological materials, including fossil molluscs. However, workers in different laboratories have reported wide discordance in D/L measurements for the same molluscs, which they have attributed to differences in chemical and instrumental procedures and to various other analytical uncertainties. This paper considers yet other uncertainties in D/L measurement that potentially arise from the chromatograms themselves, namely: incomplete resolution of adjacent amino acid peaks, low signal-to-noise ratio, and ambiguity in the characterization of true baselines. Graphical methods of peak-size measurement (area- or height-based) yield highly inaccurate and discordant peak-size ratios when applied to marginal chromatograms, as demonstrated with a simulated chromatogram with severely overlapped peaks of known individual sizes. In contrast, accurate and precise peak-size ratios are recovered from this same chromatogram by first deconvolving the peaks using chemometric algorithms before applying the traditional graphical methods. This encouraging result in a near-worst-case scenario may represent a viable approach for recovering useful age information (relative or absolute) from important geological samples of marginal quality and/or quantity.