Carbon position-specific isotope analysis of alanine and phenylalanine analogues exhibiting nonideal pyrolytic fragmentation

Recent advances in gas chromatography combustion-isotope ratio mass spectrometry (GCC-IRMS) has made compound-specific isotope analysis routine, but reports on position-specific isotopic analysis are still scarce. Online GC-pyrolysis (Py) coupled to GCC-IRMS is reported here for isolation and isotopic characterization of alaninol and phenethylamine, analogues of alanine and phenylalanine, respectively. Ideally, pyrolytic fragments will originate from unique sites within the parent molecule, and isotope ratios for each position within the parent can either be measured directly or calculated from fragment isotope ratios without substantially degrading the analytical precision. Alaninol pyrolysis yielded several fragments, of which CO and CH4 were used for isotope ratio calculations. Isotope labeling experiments showed that CO derived entirely from the C-(1) position, while all three positions of alaninol contributed to CH4 (29.0 +/- 0.3% from C-(1), 3.6 +/- 0.2% from C-(2), and 66.9 +/- 1.1% from C-(3)). We demonstrate iterative use of mass balance to calculate isotope ratios from all positions despite the nonideal positional fidelity of CH4. Pyrolysis of phenethylamine generated benzene and toluene fragments. Benzene derived entirely from C-(ring), and toluene was proportionately formed from C-(3) and C-(ring). Relative intramolecular isotope ratios (Delta delta C-13) were calculated directly from delta C-13 of fragments or indirectly by mass balance. Though the C-(3) isotope ratio was calculated from the benzene and toluene fragments, propagation of errors showed that the final precision of the determination was degraded due to the small contribution that C-(3) makes to toluene. Samples of each amino acid from four different vendors showed natural variability between sources, especially at the C-(1) position of alaninol (range of Delta delta C-13 - 50 parts per thousand). The average precision was SD(Delta delta C-13) < 0.20 parts per thousand for directly measured positions of alaninol and phenethylamine. The precision of indirectly measured positions was poorer (SD(Delta delta C-13) = 0.94 parts per thousand for alaninol, 6.54 parts per thousand for phenethylamine) due to propagation of errors. These data demonstrate that GC-Py-GCC-IRMS data can be used to extract high-precision isotope ratios from amino acids despite nonideal positional fidelity in fragments and that natural intramolecular variability in delta C-13 can be used to distinguish different sources of amino acids.