Stable isotope coded derivatizing reagents as internal standards in metabolite profiling.

Gas chromatography (GC) and liquid chromatography (LC) coupled to mass spectrometric (MS) detection have become the two main techniques for the analysis of metabolite pools (i.e. Metabolomics). These technologies are especially suited for Metabolite Profiling analysis of various metabolite groups due to high separation capabilities of the chromatographs and high sensitivity of the mass analysers. The trend in quantitative Metabolite Profiling is to add more metabolites and metabolite groups in a single method. This should not be done by compromising the analytical precision. Mass spectrometric detection comes with certain limitations, especially in the quantitative aspects as standards are needed for conversion of ion abundance to concentration and ionization efficiencies are directly dependent on eluent conditions. This calls for novel strategies to counteract all variables that can influence the quantitative precision. Usually, internal standards are used to correct any technical variation. For quantitation of single or just a few analytes this can be executed with spiking isotopically labeled standards. However, for more comprehensive analytical tasks, e.g. profiling tens or hundreds of analytes simultaneously, this strategy becomes expensive and in many cases isotopically labeled standards are not available. An alternative is to introduce a derivatizing step where the sample is derivatized with naturally labeled reagent, while a standard solution is separately derivatized with isotopically labeled reagent and spiked into the sample solution prior to analysis. This strategy, named isotope coded derivatization - ICD, is attractive in the emerging field of quantitative Metabolite Profiling where current protocols can easily comprise over hundred metabolites. This review provides an overview of isotopically labeled derivatizing reagents that have been developed for important metabolite groups with the aim to improve analytical performance and precision.