Calculation of the relative frequencies of stable isotope labelled and unlabelled molecules in a gas chromatographic peak using gas chromatography/mass spectrometry

A method is described for the calculation of stable isotope enrichments from the mass spectra of organic compounds obtained by gas chromatography/mass spectrometry. Correction factors for the natural abundance of stable isotopes account for the probabilities of observing more than one stable isotope atom in a molecule and these are particularly significant when analysing trimethylsilyl derivatives containing 29Si, 30Si and 13C. It was demonstrated that 15N-labelled molecules, as well as 13C-labelled molecules, elute earlier than their unlabelled analogues in a gas chromatographic peak, and thus enrichment values should be calculated from sample scans taken at regular intervals throughout the peak.