Measurement of the incorporation or conversion of infused stable isotope enriched metabolites in vivo such as amino acids plays a key role in metabolic research. Specific routes are frequently probed in knockout mouse models limiting the available amount of sample. Although less precise as compared to combustion-isotope ratio mass spectrometry (C-IRMS), gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS) techniques are therefore often the method of choice to measure isotopic enrichment of target metabolites. However, under conditions of metabolic depletion, the precision of these systems becomes limiting. In this paper, studies were performed to enhance the sensitivity and precision of isotope enrichment measurements using LC-MS. Ion-statistics and resolution were identified as critical factors for this application when using a linear trap mass spectrometer. The combination with an automated pre-column derivatization and a carefully selected solvent mix allowed us to measure isotopic enrichments down to 0.005% at plasma concentrations as low as 5 μmol/l, an improvement by a factor of 100 compared to alternative methods. The resulting method now allowed measurement of the in vivo conversion of the amino acid arginine into citrulline as a marker for the production of nitric oxide in an in vivo murine endotoxemia model with depleted plasma levels of arginine and citrulline.
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