Abstract
On‐line gas chromatography‐combustion‐isotope ratio mass spectrometry (GC‐C‐IRMS) is commonly used to measure isotopic ratios at natural abundance as well as for tracer studies in nutritional and medical research. However, high‐precision ^13^C isotopic enrichment can also be measured by liquid chromatography‐isotope ratio mass spectrometry (LC‐IRMS). Indeed, LC‐IRMS can be used, as shown by the new method reported here, to obtain a baseline separation and to measure ^13^C isotopic enrichment of underivatised amino acids (Asp, Thr–Ser, Glu, Pro, Gly, Ala, Cys and Val). In case of Val, at natural abundance, the SD(δ^13^C) reported with this method was found to be below 1‰ . Another key feature of the new LC‐IRMS method reported in this paper is the comparison of the LC‐IRMS approach with the conventional GC‐C‐IRMS determination. To perform this comparative study, isotopic enrichments were measured from underivatised Val and its N(O, S)‐ethoxycarbonyl ethyl ester derivative. Between 0.0 and 1.0 molar percent excess (MPE) (δ^13^C = − 12.3 to 150.8‰), the calculated root‐mean‐square (rms) of SD was 0.38 and 0.46‰ and the calculated rms of accuracy was 0.023 and 0.005 MPE, respectively, for GC‐C‐IRMS and LC‐IRMS. Both systems measured accurately low isotopic enrichments (0.002 atom percent excess (APE)) with an SD (APE) of 0.0004. To correlate the relative (δ^13^C) and absolute (atom%, APE and MPE) isotopic enrichment of Val measured by the GC‐C‐IRMS and LC‐IRMS devices, mathematical equations showing the slope and intercept of the curves were established and validated with experimental data between 0.0 to 2.3 MPE. Finally, both GC‐C‐IRMS and LC‐IRMS instruments were also used to assess isotopic enrichment of protein‐bound ^13^C–Val in tibial epiphysis in a tracer study performed in rats. Isotopic enrichments measured by LC‐IRMS and GC‐C‐IRMS were not statistically different (p > 0.05). The results of this work indicate that the LC‐IRMS was successful for high‐precision ^13^C isotopic measurements in tracer studies giving ^13^C isotopic enrichment similar to the GC‐C‐IRMS but without the step of GC derivatisation. Therefore, for clinical studies requiring high‐precision isotopic measurement, the LC‐IRMS is the method of choice to measure the isotopic ratio. Copyright © 2008 John Wiley & Sons, Ltd.