Gas Chromatographic Techniques in Metabolomics
High chemical diversity and abundances ranging from trace to millimolar levels still constitute at times insurmountable challenges in the comprehensive analysis of metabolites in biomedical specimens. Nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS) hyphenated with separation techniques such as liquid chromatography (LC), gas chromatography (GC) and capillary electrophoresis (CE) are the most frequently used techniques for both targeted and discovery‐driven metabolomics. Of the separation techniques, comprehensive two‐dimensional gas chromatography (GC×GC) offers the highest peak resolution and capacity, and in combination with MS lower quantification limits in the submicromolar concentration range are realized. Moreover, electron ionization (EI), the most prominent ionization technique for GC‐MS, is highly reproducible, facilitating the generation of mass spectral libraries for routine metabolite identification. However, GC analysis often requires a derivatization prior to analysis and not all metabolite derivatives are recorded in the libraries available. Consequently, metabolite identification is still a major challenge. To identify unknown metabolite signals, soft ionization techniques in combination with high‐resolution MS are employed to determine the accurate mass of the quasi‐molecular ion. The latter is used to calculate elemental formulae that can be fed into metabolite databases for a putative identification or used for the interpretation of EI spectra.