Analysis of molecular isotopic structures at high precision and accuracy by Orbitrap mass spectrometry

• Published in: International journal of mass spectrometry 2017-11, Vol.422 (C), p.126-142
• Main Authors: Eiler, John, Cesar, Jaime, Chimiak, Laura, Dallas, Brooke, Grice, Kliti, Griep-Raming, Jens, Juchelka, Dieter, Kitchen, Nami, Lloyd, Max, Makarov, Alexander, Robins, Richard, Schwieters, Johannes
• Format: Article
• Language: English
• Subjects: Clumped isotope; High precision; Isotope ratio; Natural abundance; Orbitrap; Position specific
• ISSN: 1387-3806; 1873-2798
• DOI: 10.1016/j.ijms.2017.10.002

[Display omitted] •Natural abundance isotope ratio measurements using an Orbitrap mass analyzer.•Sub-per-mil precision and comparable accuracy with appropriate standardization.•Site specific and clumped isotope analyses.•Suitable for analysis of sub-micromolar peaks eluted from a GC or LC. Several technologies are being developed to examine the intramolecular isotopic structures of molecules (i.e., site-specific and multiple substitution), but limitations in sample size and type or (for mass spectrometry) resolution have so far prevented the creation of a general technique. We have now demonstrated the capacity for precise and accurate study of molecular isotopic contents and structures by Fourier transform mass spectrometry, using instruments containing a Thermo Scientific™ Orbitrap™ mass analyzer, here the Thermo Scientific Q Exactive GC™ and Q Exactive HF™ instruments. Orbitrap mass analyzers achieve mass resolutions in the range ∼250,000–1M (FWHM) in the mass range of greatest interest to studies of molecular isotopic structure, 50–200amu. This allows for resolution of many nearly isobaric interferences for compounds containing H, C, N, O and/or S. In this paper we show that internal and external experimental reproducibilities of isotope ratio analyses using the Orbitrap analysis can conform to shot-noise limits down to levels of tenths of per mil (1SE), with similar accuracy when standardized to reference materials. Precision reaches ±0.015‰ for exceptionally long integrations. Such measurements do not call for modifications to the ion optics of the Q Exactive instruments, but do require specially designed sample introduction devices to permit sample/standard comparison and long integration times. The sensitivity of the Q Exactive instruments permit analysis of sub-nanomolar samples and quantification of multiply-substituted species. Site-specific capabilities arise from the fact that mass spectra of molecular analytes commonly contain diverse fragment ion species, each of which samples a specific sub-set of molecular sites.