Methods and limitations of stable isotope measurements via direct elution of chromatographic peaks using gas chromotography-Orbitrap mass spectrometry

doi:10.1016/j.ijms.2022.116848

International Journal of Mass Spectrometry

Volume 477, July 2022, 116848

https://doi.org/10.1016/j.ijms.2022.116848 Get rights and content

Highlights

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GC-Orbitrap enables molecular identification and precise isotope ratio measurements.
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Samples are introduced via gas chromatography without specialized modifications.
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¹³C/¹²C and ²H/¹H of organic analytes are characterized rapidly.
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Monte Carlo model reveals impact of GC peak characteristics and Orbitrap settings on isotope ratios.
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Strategies are recommended for optimizing isotope ratio measurements.

Abstract

The Thermo Scientific™ QExactive Orbitrap™ mass spectrometer combined with a Thermo Scientific™ Trace™ 1310 GC enables high-mass-resolution measurements of molecular isotopic structure (e.g., molecular-average, position-specific, and multiple substitution measurements), but thus far has employed non-traditional, slow sample introduction methods and long integrations lasting minutes or tens of minutes to optimize measurement precision. This study examines the performance of the Orbitrap for isotope ratio measurements of analytes eluting directly from the gas chromatograph (GC) as traditional GC peaks — i.e. eluting over a period of seconds and with rapidly varying signal intensities. Such a measurement holds potential for simultaneous compound identification and isotope ratio measurement of numerous analytes separated by GC within a single acquisition. We applied this “direct elution” measurement strategy to molecular and fragment ions of para-xylene, serine, and a mixture of polycyclic aromatic hydrocarbons (PAHs) at natural isotope abundances. We built a mathematical model and used a Monte Carlo simulation to evaluate how variations in data processing decisions and GC peak characteristics (e.g., peak shape and elution timing) affect the accuracy of the resulting absolute isotope ratios and sample-standard comparisons (δ values). These case studies inform our recommendations for applying direct elution measurements. The method is appropriate for systems with large position-specific, molecular, or multiply-substituted isotopic anomalies (e.g., isotopically labelled or extraterrestrial compounds), and for compounds that produce strong molecular ions. Precisions improve when experiments are designed to (1) target ions with relatively high mass spectral intensities, (2) optimize the number of ions and range of masses admitted into the Orbitrap, and (3) minimize the nominal resolution settings while still separating relevant isobars.

Graphical abstract

Introduction

Stable isotope ratios of natural samples are used to characterize chemical synthesis processes (e.g., to establish biogenicity), environmental conditions, and elemental budgets in natural cycles, and can be quantified at levels of natural abundance via mass spectrometry. Conventional isotope ratio measurements require an initial conversion of analyte into a simple molecular gas such as CO₂, H₂, or N₂ for isotope ratio mass spectrometry (“IRMS”) using specialized magnetic-sector instruments. The resulting isotope ratio therefore represents the average for an element across an entire molecule. These techniques sacrifice the study of variation at specific molecular positions, or "position-specific” isotope ratios, and measurements of clumped or multiply-substituted isotopologues — information that could improve existing interpretations or enable new applications — in order to achieve greater precision and simplify instrumentation and methods.

Orbitrap mass spectrometry is one technique that enables direct measurements of isotope ratios of higher molecular weight compounds without requiring conversion to simple gases, thus preserving clumped and position-specific isotopic information [[1], [2], [3], [4], [5]]. Orbitrap isotope ratio mass spectrometry has been used for tracing synthetically labeled compounds, using the natural isotope abundance profile as an aid to molecular identification [6], and constraining elemental isotope ratios [7,8]. These applications highlight the potential of Orbitrap mass spectrometry for geochemical studies, as the technology already serves an analogous role to simpler gas chromatography-MS (GC-MS) instruments. Further, the Orbitrap mass analyzer performs high mass resolution measurements (up to 240,000 full peak width at half maximum intensity, or FWHM, at 200 Da; Section 2.2) capable of distinguishing near-isobaric masses (i.e., resolving isotopic substitution of ¹³C versus ¹⁵N and ²H) and enabling it to function as a powerful tool for isotope ratio measurement [[1], [2], [3], [4]]. This set of capabilities raises the possibility that the Orbitrap could be capable of continuous flow isotope ratio measurements analogous to those now made by GC-IRMS [[9], [10], [11]], but with extended abilities to observe features of intramolecular isotopic structure.

Within an Orbitrap mass spectrometer, molecular and fragment ions form in the ion source, are accelerated through the instrument, and can be selectively filtered by mass using a quadrupole prior to injection into the Orbitrap mass analyzer (Fig. 1A). The mass analyzer measures the image current produced by oscillating ions (the “transient”). The relative abundances of molecular and/or fragment ions are retrieved via a fast Fourier transform of this time-dependent image current, which yields the mass-dependent frequency and relative intensity of each ion species contributing to the transient, and thus constrains the mass to charge (m/z) ratios and proportions of those species [2,12]. This enables the Orbitrap to recover precise and (with adequate standardization) accurate single and multiply-substituted (i.e.,“clumped”) isotopic compositions of molecules, at the per-mil-level fidelity required to study natural isotopic variations [2]. Combined with the Orbitrap's potential for molecular identification, such data can augment compound-specific isotope ratio measurements.

To optimize Orbitrap mass spectrometry for analysis of isotope ratios in natural samples, previous studies have developed techniques interfacing the Orbitrap mass analyzer with a gas chromatograph (GC; [1,2,13]), as well as with liquid-medium sample introduction, electrospray ionization and secondary collisional fragmentation (e.g., [5,7,14]). For GC Orbitrap measurements in particular, relatively high precision has been achieved by “peak capture”: trapping an analyte in a reservoir configured within the GC oven and gradually releasing it to the ion source over minutes to hours in order to obtain precision for replicate analyses, in some cases approaching ∼0.10‰ [1,2], and thus ideal for samples with small isotopic variations. An obvious drawback to the peak capture method is that only a single peak from each gas chromatogram can be selected, so separate GC runs are required for each potential analyte.

Measuring isotope ratios by “direct elution” from the GC allows one to observe isotopic compositions of several or all compounds in a mixture as they exit the GC column, without any trapping steps. This approach sacrifices precision because the duration of Orbitrap observation is limited by the duration of analyte elution from the GC but provides the opportunity to quickly study numerous different compounds in one experiment. Further, the highest levels of precision obtained in previous studies are not necessary for all applications and could be unnecessarily time consuming when isotopic variations are large.

Here, we explore the precision and accuracy achievable using a direct elution method of isotopic analysis on Thermo Scientific™ Q Exactive Orbitrap™ mass spectrometer coupled to a Thermo Scientific™ Trace™ 1310 GC instrument. We use three examples to examine how direct elution studies can provide meaningful isotope-ratio measurements and their limits of precision: (1) para-xylene (p-xylene), (2) derivatized serine reference standards, and (3) two polycyclic aromatic hydrocarbons (PAHs; pyrene and fluoranthene) in a meteorite sample and in terrestrial standards. In each case study, we highlight how optimizing chromatographic peak shape and strategically selecting Orbitrap parameters (see glossary of terms, Table 1) can improve the number of observed ion counts and enable more rapid, accurate, and precise measurements of samples for compound-specific, position-specific and clumped-isotope applications. We apply these parameter selections to a numerical model of chromatographic peak elution and the Orbitrap data collection system to demonstrate the minimal effect that our proposed data processing approach has on measuring isotope ratios of a single analyte and sample-standard comparisons under typical experimental conditions, and present examples of scenarios that could produce imprecise or inaccurate values. Overall, we conclude that isotope ratio measurements using the direct elution method achieve coarser levels of precision than the highest-performance peak-trapping methods, but are still adequate for many applications and have the additional advantage of using a commercially available instrument platform.

Section snippets

Standards and preparatory chemistry

The following analytes were prepared for GC-Orbitrap isotope ratio measurement: para-xylene, variably ¹³C-labeled and derivatized serine standards (SERC0 and SERC1), and polycyclic-aromatic hydrocarbons (PAHs). Full details of standard sources and preparatory chemistry are described in the supplemental material. The molecular average carbon isotopic composition of serine and PAH standards were characterized by combustion Elemental Analyzer-Isotope Ratio Mass Spectrometry (EA-IRMS) at Caltech.

Para-xylene

Electron-impact ionization of p-xylene produces both a strong molecular ion (m/z = 106.168 Da) and a second major tropylium fragment ion formed by loss of the methyl group (91.144 Da; these two are subsequently referred to as the 106 and 91 ions, respectively; Fig. 2A). Para-xylene peaks typically eluted over periods lasting ∼20 s above baseline, but the scans that were included in the signals we integrated for isotope ratio measurement typically sampled a time frame of 6 s due to the exclusion

Discussion

The direct elution method on the GC-Orbitrap enables high mass resolution isotope ratio measurements that are precise to per-mil levels and accurate when standardized. Precision and accuracy of isotope measurements using the direct elution method can be improved by optimizing the properties of peaks eluted from the GC and the settings of the Orbitrap mass spectrometer, consistent with findings from past studies [5,[26], [27], [28]]. The following paragraphs discuss effects that can be

Conclusion

This study demonstrates an optimized method for quick isotopic analysis of complex mixtures via Orbitrap mass spectrometry. We present results from experiments and a chromatographic model to highlight the ability of this method to produce accurate and precise isotope ratios for singly- and doubly-substituted isotope anomalies, including position-specific isotopic properties, at precisions sufficient for studies of samples that bear high-amplitude isotopic signatures. In all case studies, our

Author contributions

Sarah S. Zeichner: Methodology, Validation, Formal analysis, Investigation, Data curation, Software, Writing, Visualization, Supervision, Project administration. Elise B. Wilkes: Methodology, Validation, Investigation, Writing, Formal analysis, Software, Funding acquisition. Elle Chimiak: Methodology, Validation, Formal analysis, Investigation, Writing. Amy E. Hofmann: Validation, Investigation, Writing, Funding acquisition. Alexander Makarov: Conceptualization, Investigation, Writing (Review &

Funding

Funding for SSZ was provided by NSF GRFP and a NASA Emerging Worlds grant to AEH (grant number 18-EW18_2-0084). This work was supported in part by grants from NASA Astrobiology Institute (grant number 80NSSC18M094 to ALS and JME), the Agouron Institute (grant number AI-F-GB54.19.2 to EBW), and Caltech's Center for Environmental Microbial Interactions (CEMI, to EBW, ALS, and JME). A portion of this work was performed at the Jet Propulsion Laboratory, which is operated by the California Institute

Data and materials availability

All RAW data and code is available within online repositories, and cited in the main text or the Supplementary information.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Elliott Mueller, Gabriella Weiss, Tim Csernica, and Kate Freeman for helpful feedback and enlightening discussion on topics ranging from methodology to data processing. Additionally, we thank Guannan Dong, Peter Martin, Max Lloyd, Andreas Hilkert, Kostya Ayzikov, and Caj Neubauer for their contributions to the development of Orbitrap data analysis software. Finally, we express our gratitude to Nami Kitchen and Fenfang Wu for their invaluable support on instrumentation and

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Citation Excerpt :
Recently, people have paid more attention to garlic, and garlic-based products have become one of the most widespread healthy foods in the world today. Gas chromatography is a chromatographic separation and analysis method that uses gas as the mobile phase (Zeichner et al., 2022). It is suitable for the qualitative and quantitative analysis of volatile organic compounds.
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Methods and limitations of stable isotope measurements via direct elution of chromatographic peaks using gas chromotography-Orbitrap mass spectrometry

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Standards and preparatory chemistry

Para-xylene

Discussion

Conclusion

Author contributions

Funding

Data and materials availability

Declaration of competing interest

Acknowledgements

Geochem. Cosmochim. Acta

Int. J. Mass Spectrom.

Int. J. Mass Spectrom.

Cell

Int. J. Mass Spectrom.

Org. Geochem.

Org. Geochem.

Earth Planet Sci. Lett.

J. Chromatogr. A

Semin. Perinatol.

Geochem. Cosmochim. Acta

Icarus

Scanning the isotopic structure of molecules by tandem mass spectrometry

Int. J. Mass Spectrom.

Stable isotope analysis of intact oxyanions using electrospray quadrupole-orbitrap mass spectrometry

Anal. Chem.

Preliminary figures of merit for isotope ratio measurements: the liquid sampling-atmospheric pressure glow discharge microplasma ionization source coupled to an orbitrap mass analyzer

J. Am. Soc. Mass Spectrom.

Initial benchmarking of the liquid sampling-atmospheric pressure glow discharge-orbitrap system against traditional atomic mass spectrometry techniques for nuclear applications

J. Am. Soc. Mass Spectrom.

Picomolar-scale compound-specific isotope analyses

Rapid Commun. Mass Spectrom.

Compound-specific isotopic analyses: a novel tool for reconstruction of ancient biogeochemical processes

Isotope-ratio-Monitoring gas chromatography-mass spectrometry

Anal. Chem.