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How to Couple LC-IRMS with HRMS─A Proof-of-Concept Study.

Robert G H Marks1, Maik A Jochmann1, Willi A Brand2

  • 1Instrumental Analytical Chemistry, University of Duisburg-Essen, Universitätsstraße 5, 45141 Essen, Germany.

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Compound-specific stable isotope analysis (CSIA) using liquid chromatography (LC) can now identify unknown compounds. This new LC-IRMS and high-resolution mass spectrometry (HRMS) coupling confirms impurities in isotope standards, like pyroglutamic acid in USGS41 and USGS41a.

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Area of Science:

  • Analytical Chemistry
  • Isotope Analysis
  • Mass Spectrometry

Background:

  • Compound-specific stable isotope analysis (CSIA) is crucial for analyzing complex mixtures.
  • Traditional LC-IRMS loses analyte structural information during isotope ratio determination.
  • GC-IRMS-MS couplings exist for structural info, but LC-IRMS lacks this capability.

Purpose of the Study:

  • To develop and optimize a novel LC coupling for simultaneous IRMS and HRMS analysis.
  • To enable direct identification of unknown or coeluting species in LC-CSIA.
  • To address the purity of international isotope standards using the new technique.

Main Methods:

  • Coupling of liquid chromatography (LC) with simultaneous isotope ratio mass spectrometry (IRMS) and high-resolution mass spectrometry (HRMS).
  • Optimization of instrumental conditions to achieve a consistent split ratio between IRMS and HRMS using a variable postcolumn flow splitter.
  • Application of the developed LC-IRMS-HRMS system to assess the purity of isotope reference materials.

Main Results:

  • Successfully established a reproducible LC-IRMS-HRMS coupling with consistent split ratios.
  • Demonstrated reproducible peak areas, isotope values, and retention time differences between IRMS and HRMS.
  • Confirmed substantial pyroglutamic acid impurities in USGS41 and its replacement material USGS41a.

Conclusions:

  • The developed LC-IRMS-HRMS coupling enables simultaneous isotopic and structural analysis, identifying unknown compounds.
  • This technique overcomes previous limitations of LC-IRMS by providing structural information.
  • Caution is advised when using USGS41 and USGS41a for isotopic calibration due to identified impurities.