Practical considerations in the gas chromatography/combustion/isotope ratio monitoring mass spectrometry of 13C‐enriched compounds: detection limits and carryover effects

Abstract

This paper describes a methodological investigation of the use of gas chromatography/combustion/isotope ratio monitoring mass spectrometry (GC/C/IRMS) for the compound‐specific stable isotope analysis of ¹³C‐enriched compounds. Analysis of two ¹³C‐enriched fatty acid methyl esters, possessing δ¹³C values of ∼500‰, at a range of concentrations, demonstrated that detectable responses, i.e. chromatographic peaks, could be observed in the 45/44 output even when the compound was present in such low abundance that no peak was observed in the m/z 44 ion chromatogram. A limit of detection, defined as the point at which the signal‐to‐background ratio was equal to 3, was calculated for two compounds and for both ion chromatograms. The limit of detection in the 45/44 chromatogram was found to be ca. 30 pg injected for methyl ¹³C‐hexadecanoate and ca. 20 pg injected for methyl ¹³C‐octadecanoate, whilst, in the m/z 44 ion chromatogram, detection limits were ∼180 and ∼200 pg, respectively. The δ¹³C value recorded for the analytes was found to be both inaccurate and imprecise below 5 ng of each component injected, although this would not represent a significant drawback in qualitative tracer‐type experiments. In a further study of co‐injected mixtures of labelled (∼500‰) and unlabelled (natural abundance, −20 to −30‰) fatty acid methyl esters a significant within‐run carryover effect was observed, where the isotope values recorded for compounds eluting immediately after enriched components were significantly affected. Whilst this would not affect qualitative results, quantitative data for mixtures containing enriched compounds should be considered with caution. The standards employed in this investigation were enriched to ∼500‰ in ¹³C; however, these effects would probably be accentuated at higher levels of labelling and with other elements. The limit of detection work demonstrated the potential of GC/C/IRMS as a highly sensitive and selective detector with many possible applications. Copyright © 2003 John Wiley & Sons, Ltd.