High-Resolution Direct Infusion-Based Mass Spectrometry in Combination with Whole ¹³C Metabolome Isotope Labeling Allows Unambiguous Assignment of Chemical Sum Formulas

A new strategy for direct infusion-based metabolite analysis employing a combination of high-resolution mass spectrometry and 13C-isotope labeling of entire metabolomes is described. Differentially isotope labeled metabolite extracts from otherwise identically grown reference plants were prepared and infused into a Fourier transform ion cyclotron resonance mass spectrometer. The derived accurate mass lists from each extract were searched, using an in-house-developed database search tool, against a number of comprehensive metabolite databases. Comparison of the retrieved chemical formulas from both, the 12C and 13C samples, leads to two major advantages compared to nonisotope-based metabolite fingerprinting: first, removal of background contaminations from the result list, due to the 12C/13C peak pairing principle and therefore positive identification of compounds of true biological origin; second, elimination of ambiguity in chemical formula assignment due to the same principle, leading to the clear association of one measured mass to only one chemical formula. Applying this combination of strategies to metabolite extracts of the model plant Arabidopsis thaliana therefore resulted in the reproducible identification of more than 1000 unambiguous chemical sum formulas of biological origin of which more than 80% have not been associated to Arabidopsis before.