Using Orbitrap mass spectrometry to assess the isotopic compositions of individual compounds in mixtures

The isotopic compositions of individual chemical species are routinely used by the geochemical, environmental, forensic, anthropological, chemical, and biomedical communities to elucidate the conditions, sources, and reaction pathways of the molecules in question. Mass spectrometric methods of measuring isotopic compositions of individual compounds generally require that analytes be pure to yield precise, accurate results. Yet most applications examine materials that are mixtures of multiple components. Various methods of chemical purification, e.g., chromatography, are used to isolate analytes from mixtures prior to mass spectrometric analysis. However, these techniques take time and specialized equipment and are not always desirable, feasible, or even possible (e.g., in field- or space-deployable instrumentation). Here we report on the use of Orbitrap-based Fourier-transform mass spectrometry to perform isotope ratio measurements of single species within mixtures delivered to the mass spectrometer without prior chromatographic separation. We demonstrate that instrument biases (attributed here to space charge effects) within the Orbitrap mass analyzer can cause the measured 13C/12C ratio of a molecular ion to spuriously decrease relative to the ratio measured for the same ion in a pure analyte by an amount that is proportional to the proportions of the additional components. We then recommend several strategies by which this effect can be mediated such that accurate isotope ratios can be obtained.