Chapter 1. State-of-the-art in LC–MS Approaches for Probing the Polar Metabolome

Metabolomics continues to grow rapidly but its coverage of the metabolome is still highly dependent on the selected analytical technique. Technological maturity has not yet reached the level that is now evident in genomics. Metabolomics protocols are still in the phase of low levels of standardization, debate, application, and harmonization among researchers making interlaboratory comparisons difficult. In particular, the analysis of the complement of polar metabolites is probably the most demanding area for LC-based profiling methods. Polar hydrophilic metabolites are important in many basic biochemical pathways (e.g., central carbon metabolism), and consequently, their comprehensive, accurate, and efficient determination is of considerable importance. However, polar molecules are not easy to analyze by reversed-phase (RP) LC methods, and for this reason sample preparation by derivatization is often required. Alternative modes, such as HILIC (hydrophilic liquid interaction chromatography), ion exchange LC, etc., offer alternative approaches. Despite significant investment by the community in technologies to obtain a general solution to this demanding task none is entirely satisfactory, and research in this area remains very active. Here, we describe how liquid chromatography combined with mass spectrometry is currently being applied to probe the polar metabolome.