Comparison of liquid chromatography-isotope ratio mass spectrometry (LC/IRMS) and gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/IRMS) for the determination of collagen amino acid δ13C values for palaeodietary and palaeoecological reconstruction

Results are presented of a comparison of the amino acid (AA) delta C-13 values obtained by gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/IRMS) and liquid chromatography-isotope ratio mass spectrometry (LC/IRMS). Although the primary focus was the compound-specific stable carbon isotope analysis of bone collagen AAs, because of its growing application for palaeodietary and palaeoecological reconstruction, the results are relevant to any field where AA delta C-13 values are required. We compare LC/IRMS with the most up-to-date GC/C/IRMS method using N-acetyl methyl ester (NACME) AA derivatives. This comparison involves the analysis of standard AAs and hydrolysates of archaeological human bone collagen, which have been previously investigated as N-trifluoroacetyl isopropyl esters (TFA/IP). It was observed that, although GC/C/IRMS analyses required less sample, LC/IRMS permitted the analysis of a wider range of AAs, particularly those not amenable to GC analysis (e. g. arginine). Accordingly, reconstructed bulk delta C-13 values based on LC/IRMS-derived delta C-13 values were closer to the EA/IRMS-derived delta C-13 values than those based on GC/C/IRMS values. The analytical errors for LC/IRMS AA delta C-13 values were lower than GC/C/IRMS determinations. Inconsistencies in the delta C-13 values of the TFA/IP derivatives compared with the NACME- and LC/IRMS-derived delta C-13 values suggest inherent problems with the use of TFA/IP derivatives, resulting from: (i) inefficient sample combustion, and/or (ii) differences in the intramolecular distribution of delta C-13 values between AAs, which are manifested by incomplete combustion. Close similarities between the NACME AA delta C-13 values and the LC/IRMS-derived delta C-13 values suggest that the TFA/IP derivatives should be abandoned for the natural abundance determinations of AA delta C-13 values. Copyright (C) 2011 John Wiley & Sons, Ltd.