Abstract 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 instrumentation, both of which could potentially be obviated via the use of ultra-high-resolution mass spectrometry. 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 (MS) 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 in the presence of non-analyte-derived ‘contaminating’ species to spuriously decrease relative to the 13C/12C ratio measured for the same ion in a pure analyte. We observe that the decrease in 13C/12C is proportional to the relative concentrations of the additional ‘contaminating’ components. We then recommend several strategies by which this effect can be mediated such that accurate isotope ratios can be obtained.

中文翻译：

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使用轨道阱质谱法评估混合物中单个化合物的同位素组成

摘要 地球化学、环境、法医、人类学、化学和生物医学界通常使用单个化学物种的同位素组成来阐明相关分子的条件、来源和反应途径。测量单个化合物的同位素组成的质谱方法通常要求分析物是纯的，以产生精确、准确的结果，但大多数应用程序检查的材料是多种成分的混合物。使用各种化学纯化方法，例如色谱法，在质谱分析之前从混合物中分离分析物。然而，这些技术需要时间和专门的仪器，这两者都可以通过使用超高分辨率质谱法来避免。在这里，我们报告了使用基于 Orbitrap™ 的傅里叶变换质谱法对交付到质谱仪 (MS) 的混合物中的单个物种进行同位素比测量，而无需事先进行色谱分离。我们证明了 Orbitrap 质量分析器内的仪器偏差（此处归因于空间电荷效应）会导致在非分析物衍生的“污染”物质存在的情况下测量的分子离子的 13C/12C 比相对于 13C纯分析物中相同离子的 /12C 比值。我们观察到 13C/12C 的降低与额外“污染”成分的相对浓度成正比。然后，我们推荐了几种可以调节这种效应的策略，以便可以获得准确的同位素比率。